The Recent Victorian Drought and its Impact Without precedent?

MARCH 2013 RIRDC Publication No. 12/040

The Recent Victorian Drought and its Impact Without precedent?

By Keely Mills, Peter Gell and Peter Kershaw

March 2013

RIRDC Publication No. 12/040 RIRDC Project No. PRJ-005440

© 2013 Rural Industries Research and Development Corporation. All rights reserved.

ISBN 978-1-74254-481-6 ISSN 1440-6845

The Recent Victorian Drought and its Impact. Without precedent? Publication No. 12/040 Project No. PRJ-005440

The information contained in this publication is intended for general use to assist public knowledge and discussion and to help improve the development of sustainable regions. You must not rely on any information contained in this publication without taking specialist advice relevant to your particular circumstances.

While reasonable care has been taken in preparing this publication to ensure that information is true and correct, the Commonwealth of Australia gives no assurance as to the accuracy of any information in this publication.

The Commonwealth of Australia, the Rural Industries Research and Development Corporation (RIRDC), the authors or contributors expressly disclaim, to the maximum extent permitted by law, all responsibility and liability to any person, arising directly or indirectly from any act or omission, or for any consequences of any such act or omission, made in reliance on the contents of this publication, whether or not caused by any negligence on the part of the Commonwealth of Australia, RIRDC, the authors or contributors.

The Commonwealth of Australia does not necessarily endorse the views in this publication.

This publication is copyright. Apart from any use as permitted under the Copyright Act 1968, all other rights are reserved. However, wide dissemination is encouraged. Requests and inquiries concerning reproduction and rights should be addressed to the RIRDC Publications Manager on phone 02 6271 4165.

Researcher Contact Details

Peter Gell Centre for Environment Management, School of Science, Information Technology & Engineering, University of Ballarat, VIC 3353, Australia.

[email protected]

In submitting this report, the researcher has agreed to RIRDC publishing this material in its edited form.

RIRDC Contact Details

Rural Industries Research and Development Corporation Alan Davey, Senior Research Manager Level 2, 15 National Circuit BARTON ACT 2600 PO Box 4776 KINGSTON ACT 2604

Phone: 02 6271 4100 Fax: 02 6271 4199 Email: [email protected]. Web: http://www.rirdc.gov.au

Electronically published by RIRDC in March 2013 Print-on-demand by Union Offset Printing, Canberra at www.rirdc.gov.au or phone 1300 634 313

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Foreword

Understanding climate change is a critical challenge for managers of natural landscapes and the industries that are supported by them. Southern Australia sits in a climate zone that can respond greatly to variations in effective rainfall. Recent years attest to the impact of severe drought, and flood, brought about by shifts in the major atmospheric circulation patterns that affect the southern half of our continent. The region is also subjected to increasing temperatures, and climate models reveal a high prospect for drier conditions through this century.

The capacity to detect subtle, long term trends in climate, such as the incidence of drought, is challenged by the high level of natural variability in our climate. Analyses that rely largely on memory, or even the instrumental record, are limited in the extent to which they can qualify recent climate extremes. This can be overcome by extending the instrumental record many centuries, and even millennia, from archives such as tree rings, speleothems, corals and the sediments that accumulate regularly over time. This provides the context, and benchmark, against which the conditions most familiar to us, the present and recent past, can be compared. In preparing for the future, the most appropriate measures are informed by an understanding of the risk of extremes and the present trajectory and rate of change.

This is especially critical for all involved in rural industries, from those engaged in writing the policy settings, to those making decisions of land use at the individual farm level.

This project has sought out the sediment records from lakes that are most likely to be responsive to the past variations in effective moisture, such as past drought events, and analysed them to quantify climate variation over thousands of years to the highest practical resolution. It is clear that there have been several severe, long lasting droughts over the last 5000 years, and that they have resulted in considerable change to the ecology of lakes. This report considers the evidence that, at the millennial scale, the present state of lakes is highly unusual, if not unprecedented.

While there is strong evidence that the recent condition of the lakes is, at least in part, a consequence of direct catchment disturbance, this report presents evidence that the level of effective rainfall over the last 40 years, and particularly since 1997, is unusual, and possibly unprecedented, relative to the last 5000 years.

With the strong evidence provided by this report at hand, it is very difficult to imagine a future where the trajectory of existing industries, current rural practices and the delineation of food-producing regions within our continent, does not change dramatically.

This report is an addition to RIRDC’s diverse range of over 2000 research publications and it forms part of our Dynamic Rural Communities R&D program, which aims to enhance the capacity of rural communities to manage economic, social and environmental change.

Most of RIRDC’s publications are available for viewing, free downloading or purchasing online at www.rirdc.gov.au. Purchases can also be made by phoning 1300 634 313.

Craig Burns Managing Director Rural Industries Research and Development Corporation

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About the Authors

Peter Gell Peter Gell is the Associate Dean for Research within the School of Science, Information Technology and Engineering and the Director of the Centre for Environmental Management at the University of Ballarat. Peter has 25 years’ experience as a palaeoecologist including projects examining the impact of forest harvesting in East Gippsland and of land and water use on wetlands of the Murray Basin. He is currently examining the long-term human impact on Australian ecosystems and the impact of people and climate change on the world’s lake ecosystems. He has published numerous articles, produced over 20 industry reports and lead projects understanding climate and waterway change in Brazil, France, China and across Australia. Using short term palaeoecology he has established environmental baselines, conducted biomonitoring to assess wetland and stream water quality over time, and geochemical and macro fossil analysis to infer the source of nutrients, sediments and changing trophic structure, macrophyte and invertebrate biota. He has a broad understanding of climate and climate change, hydrology, wetland biota and water sediment chemistry. His strength is his capacity to integrate these disciplines to understand river and wetland functioning in the past, present and future.

Peter Kershaw Peter Kershaw is currently a Professor Emeritus and Director of the Centre for Palynology and Palaeoecology in the School of Geography and Environmental Science at Monash University. Since 1970 he has published over 150 peer-reviewed papers and 16 co-authored books or journal special issues and supervised 25 PhD students in the general area of the vegetation and environmental history of Australasia and South-east Asia, with particular emphases on peatland and rainforest dynamics, the timing of arrival and impact of indigenous people, biomass burning and climate change and variability. This research was supported continuously by the Australian Research Council for over 30 years. He is Vice-President of the International Quaternary Association's Palaeoclimate Commission and is an Editor-in-Chief of the Elsevier journal Palaeogeography, Palaeoclimatology, Palaeoecology.

Keely Mills Keely Mills is a Post-Doctoral Research Associate within the Centre for Environmental Management at the University of Ballarat. Since her Honours research in 2003 she has been involved in the analysis of sediment records for biological proxies in order to understand past changes environmental changes. Her research focus for her PhD involved the use of fossilised algae (diatoms) from lake sediments in Uganda to reconstruct past changes in rainfall and human impacts as well as the development of quantitative diatom models (transfer functions) to reconstruct changes in water chemistry parameters in a variety of lake systems. In 2009 Keely moved to Australia to undertake research on the diatom records from the volcanic lakes of western Victoria in order to understand drought history over the last 2000 years.

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Acknowledgments

This study was funded by the Rural Industries Research and Development Corporation (RIRDC; 2009-2011) and Land Water Australia (LWA; 2008-2009). This research would not have been possible without the assistance and skills of a number of researchers in Australia, New Zealand and the United Kingdom.

First and foremost, sincere thanks are extended to Dr Merna McKenzie (Monash University) who completed the pollen counts on three of the lake sediment cores contained within this report (Lakes Colac, Purrumbete and Burn) and Tara Lewis (PhD Candidate, Monash University) for her macrofossil identification and counting of samples from Lakes Colac, Modewarre and Burn. Two students from Monash University are also sincerely thanked for their efforts in counting pollen from sediment records that have been included in this research - Ben Price (Tower Hill Main Lake) and Kim Parker (Lake Modewarre). Much of the pollen analyses would not have been possible without the assistance of Dr Ursula Pietrzak and Nicole Pancer in the preparation of many of the samples.

Chronological analyses on the lake sediment cores were completed by Professor Peter Appleby, University of Liverpool, UK (210Pb and 137Cs analyses) and Dr Fiona Petchey and Dr Alan Hogg (AMS radiocarbon dating), Waikato Radiocarbon Dating Laboratory, The University of Waikato, New Zealand.

We would also like to thank Rosie Grundell, Phuong Doan and Giri Kattel (University of Ballarat) for their assistance with the fieldwork as well as the various landowners who gave us full access to their properties and shared with us stories of their local lakes. The Department of Sustainability and Environment are also thanked for the grant of permit number: 10004720.

Finally thanks are due to Dr John Tibby (University of Adelaide) and Associate Professor Stuart Pearson (UNSW@ADFA) for their stimulating discussions and support during the last 3 years.

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Contents

Foreword ...... iii

Executive Summary ...... ix

Introduction ...... 1

Objectives ...... 7

Study Site ...... 9

Western Victorian Volcanic Plains...... 9 Geology ...... 9 Climate and Vegetation ...... 10 Study lakes ...... 13

Methodology ...... 15

Sample collection ...... 15 Sample analysis ...... 15 Chronological analyses ...... 15 Plant and charcoal macrofossil preparation and data representation ...... 18 Statistical analyses ...... 18

Results ...... 20

Lake Colac ...... 20 Lake Purrumbete ...... 30 Tower Hill Main Lake ...... 37 Lake Modewarre ...... 43 Lake Burn...... 51 Lake Rosine ...... 56

Discussion ...... 57

Recommendations ...... 68

References ...... 69

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Tables

Table 1. Fallout radionuclide concentrations from Lake Colac ...... 21

Table 2. 210Pb chronology of the Lake Colac sediment core ...... 21

Table 3. Results of radiocarbon dating for Lake Colac ...... 22

Table 4. Fallout radionuclide concentrations from Lake Purrumbete ...... 31

Table 5. 210Pb chronology of the Lake Purrumbete sediment core...... 31

Table 6. Results of radiocarbon dating for Lake Purrumbete ...... 32

Table 7. Fallout radionuclide concentrations from Tower Hill Main Lake ...... 38

Table 8. Results of radiocarbon dating for Tower Hill Main Lake ...... 39

Table 9. Fallout radionuclide concentrations from Lake Modewarre ...... 44

Table 10. 210Pb chronology of the Lake Modewarre sediment core ...... 45

Table 11. Results of radiocarbon dating for Lake Modewarre ...... 46

Table 12. Fallout radionuclide concentrations from Lake Burn ...... 54

Table 13. 210Pb chronology of the Lake Burn sediment core ...... 54

Figures

Figure 1. Trend in annual total rainfall across Australia, 1970-2010 (mm/10 years) ...... 2

Figure 2. Annual rainfall anomaly in the southern hemisphere since AD 1900 ...... 11

Figure 3. Annual mean surface temperature anomaly in the southern hemisphere since AD 1900 ...... 12

Figure 4. Map showing the location of the study lakes in western Victoria ...... 14

Figure 5. Fallout radionuclides from Lake Colac ...... 23

Figure 6. Radiometric chronology of the Lake Colac sediment core showing the CRS model...... 23

Figure 7. Age model for the Lake Colac sediment cores derived from the 210Pb and 14C dates ...... 24

Figure 8. Lake Colac dry land pollen stratigraphy ...... 26

Figure 9. Lake Colac aquatic pollen stratigraphy ...... 26

Figure 10. Lake Colac macrofossil record ...... 28

Figure 11. Lake Colac diatom stratigraphy ...... 29

Figure 12. Fallout radionuclides from Lack Purrumbete ...... 33

Figure 13. Radiometric chronology of the Lake Purrumbete sediment core showing the CRS model ...... 33

Figure 14. Age model for the Lake Purrumbete sediment cores derived from the 210Pb and 14C dates ...... 34

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Figure 15. Lake Purrumbete dry land pollen stratigraphy ...... 36

Figure 16. Lake Purrumbete aquatic pollen stratigraphy ...... 37

Figure 17. Lake Purrumbete diatom stratigraphy ...... 37

Figure 18. Fallout radionuclides from Tower Hill Main Lake ...... 40

Figure 19. Age model for the Tower Hill Main Lake sediment cores derived from the 14C dates ...... 40

Figure 20. Tower Hill Main Lake dry land pollen stratigraphy ...... 41

Figure 21. Tower Hill Main Lake aquatic pollen stratigraphy ...... 41

Figure 22. Tower Hill Main Lake diatom stratigraphy ...... 43

Figure 23. Fallout radionuclides from Lake Modewarre ...... 47

Figure 24. Age model for the Lake Modewarre sediment cores derived from the 210Pb and 14C dates ...... 47

Figure 25. Lake Modewarre dry land pollen stratigraphy ...... 48

Figure 26. Lake Modewarre aquatic pollen stratigraphy ...... 48

Figure 27. Lake Modewarre macrofossil record ...... 50

Figure 28. Lake Modewarre diatom stratigraphy ...... 50

Figure 29. Fallout radionuclides from Lake Burn ...... 52

Figure 30. Radiometric chronology of the Lake Burn sediment core showng the CRS model ...... 52

Figure 31. Lake Burn dry land pollen stratigraphy ...... 55

Figure 32. Lake Burn aquatic pollen stratigraphy ...... 55

Figure 33. Lake Burn macrofossil record ...... 56

Figure 34. Relationship between radiocarbon and calendar ages (De Vries effect) ...... 58

Figure 35. Diatom chronozones from the 4 lakes from western Victoria ...... 60

Figure 36. Responses of a lake system to an environmental driver ...... 61

Figure 37. Regional trends in change across western Victoria over the last 2000 years ...... 63

Figure 38. Comparison of diatom-inferred conductivity over the last 2000 to the Lake Keilambete curve ...... 64

Figure 39. Deviation from the average diatom-inferred conductivity of the four lakes analysed in this research...... 66

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Executive Summary

What the report is about

This research has sought out the lakes that are most likely to be responsive to the past variations in effective moisture, such as past drought events, and analysed them in the highest practical resolution allowing regional changes in rainfall to be inferred, as well as assessing the resilience of many of our wetland ecosystems to future climate and anthropogenic stress.

Who is the report targeted at?

The research contained within this report will be of relevance to researchers in the field of climate change research and environmental studies, as well as being important for policy makers and catchment management authorities. For many farmers and members of the public, this research will provide some insight into the long-term trends in rainfall and drought history in an area of Victoria that is particularly sensitive to rainfall variability.

Where are the relevant industries located in Australia?

This research is immediately relevant to producers and policy makers in western Victoria. The research itself is also of interest to the wider scientific community, and the data will be incorporated into regional and national-scale climate models, thus widening the significance of this research.

Background

In line with many parts of eastern Australia, Western Victoria has been suffering from a prolonged (> decade) dry spell that has had enormous economic, social and environmental impacts. However, it is uncertain whether such an event is a natural component of long term, natural, climatic variability or whether it has been brought on, or exacerbated by, regional European land use or by human induced global warming.

Aims/objectives

This research will document variability in effective rainfall over the last 2000 years from six key sites using contiguous high-resolution diatom analysis. In combination, the nature of aquatic plant response to climate and lake level and quality change will be documented using fossil remains (e.g. seeds, leaves, pollen). Of particular interest will be the rate of recovery to past climate perturbations.

Specifically, the three main objectives of this research were to:

• Document the salinity and nutrient history, and ephemerality, of key Western Victorian lakes, thereby determining any precedents for the present Western Victorian drought.

• Document the history of aquatic plants at these sites in response to climate change.

• Document regional and catchment changes to determine their contribution to lake history.

Methods used

The majority of this research was laboratory based. Sediment cores were collected from six lakes across western Victoria and were analysed for a number of elements, including: 210Pb, 137CS and 14C, to build a reliable chronology that allows the assignation of calendar ages to the sediment samples allowing sites to be compared, and changes observed to be compared, with other records from the region. The main bulk of the analyses included looking at fossil pollen, macrofossils and algae

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(diatoms) contained within the lake sediment archive. The various proxy records were analysed using a range of specialist statistical programs and the derived information was used to answer the objectives as laid out above.

Results/key findings

Sediments from a range of lakes in the landscape provide an ideal set of temporal and spatial scales to study climate change and lake ecosystems. The ability to simultaneously understand time (such as long-term dynamics) and space (a number of lakes with differing characteristics across a landscape) is becoming increasingly important in (palaeo) limnological studies. This is a particularly pertinent approach when realising that not all lakes respond to external (e.g. climate) forcing in a similar way. Whilst climate change has been shown, in the broadest sense, to manifest as general trends across regions and continents (and beyond), these are only really apparent and addressed in long-term (e.g. millennial) studies of lake sediments.

The reason for the variation in system responses to shared external forcing almost certainly arises from the complexity of lake ecosystems in their response to these drivers. Lakes have several levels of filters that allow them to respond uniquely. A lake’s response to external forcing is governed by factors such as its morphometry, chemistry, local hydrology (e.g. groundwater) and ecology.

Regional reconstructions of past changes in rainfall, generally exhibit similar changes. There are three key time periods that appear in the data: AD 550-700, AD 1300-1500 and AD 1900-present. In the two earlier episodes all lakes exhibit changes in conductivity and lake level that suggest regional drying events may be responsible for the observed fluctuations. In fact, shallow Lake Colac and, surprisingly the deep Lake Purrumbete, provide some of the best evidence for a long-lived, pre- European drought event in south-eastern Australia.

Droughts have serious and detrimental effects on not only the environment, but also the society and economy of the region. The impact of the ‘Big Dry’ on south-eastern Australia is known to be unprecedented in the historical past. It has been well documented that previous droughts that have occurred during European settlement have been of a similar duration (e.g. 1936-1945 drought), however, the most recent drought has been of a greater intensity, and is linked to rising temperatures over recent decades. In terms of Australia’s modern climatic setting, droughts are not unusual and are a natural characteristic of climate variability, however under a future of global warming, these droughts are likely to become more frequent, of longer duration and of greater intensity into the future.

The response of a lake to any given drought event, in the past or in the future, will largely depend on the condition of that system. The more stressed and disturbed a lake is by current climate and human impacts, the more this will severely impact its ability to cope with extra or enhanced stresses. Similarly, this is something that must be taken into consideration when compiling management plans for many lake systems; A single agenda will not suit every lake system, and serious consideration must be given to current and past events and responses in order to manage these fragile ecosystems into the future. The role of palaeoecological analyses in these circumstances provides an excellent, and unrivalled, tool in management planning.

Recommendations

Agricultural producers should read from the report that there is considerable, independent support for claims from instrumental data that the climate in southern Australia is changing. Evidence from the large lakes shows that the drying of the regional climate has its origins over 100 years ago. However, given the long term context provided herein, they should recognise that, since European settlement, there has been a significant, additional driver of climate that has taken water balance deficit to extreme levels at the five thousand year scale, into unprecedented states. They should also recognise that landscape development has had an opportunity cost, in the degradation of wetland condition, to

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an unprecedented state. Producers should recognise the need to ‘tread more lightly’ on the landscape and to consider participation in landcare and other restoration agents. Producers should also recognise the risk to ‘business-as-usual’ practise in the form of the trajectory of a drying climate and embark on diversifying their practises and economy to boost resilience in an uncertain climatic and economic future. This could include investing in activities that bring returns from carbon and biodiversity funding opportunities and in primary products that continue to provide income in future droughts.

Aside from the lessons for producers identified above, policy makers should recognise the latitudinal shift in rainfall zones with climate change, and quarantine zones of high effective rainfall for high return agricultural activities. They should implement structural adjustment support for industry to diversify practises, and make subsidies and support to restore landscapes more attractive and accessible. Policy makers should continue to support modelling of the impacts of climate scenarios at regional levels, and continue to support extension activities in the regions to encourage rural sectors to be best prepared for change and to more actively Care for the Country. Policy makers should also recognise that the combined impact of direct catchment change and a drying climate have degraded many, if not all, wetlands, and that trajectories of change suggest that this will continue to happen. So, investment in the protection and restoration of wetlands remains a priority and is probably, at present, insufficient to match the challenge.

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Introduction

In line with many parts of eastern Australia, Western Victoria has been suffering from a prolonged (> decade) dry spell (Figure 1) that has had enormous economic, social and environmental impacts. However, it is uncertain whether such an event is a natural component of long term, natural, climatic variability or whether it has been brought on, or exacerbated by, regional European land use or by human induced global warming. Whatever the cause(s), both the length and overall severity, as well as future frequency, are difficult to predict and, consequently, there is great uncertainty in the planning for the implementation of mitigation or adaptation measures. In contrast to other parts of eastern Australia, western Victoria is blessed with a number of lakes, many of them permanent or near permanent, whose sedimentary sequences retain proxy records of past climate and human impact. These records can allow assessment of the relative contributions of natural and human-induced climate variability, as well as changing land use, to be applied to an understanding of the present drought, relative to those of the past. The drought is of particular concern in south-western Victoria, as it is characterised by some of the most reliable rainfall in Australia and, consequently, has a long history of productive agriculture. This very reliability therefore, leaves the region’s agricultural economy and community particularly vulnerable to climate related stress, with many systems less adapted to poor rainfall years. For example, 2006-2007 wheat and barley yields were approximately 30% of average, with wheat yields argued to exceed those of the 1982-3 drought only due to better agricultural practices. The large rural cities of Ballarat and Bendigo, and many smaller regional centres, had been placed under stage 4 water restrictions, with plans for substantial water transfers from the Murray-Darling Basin being made. However, record-breaking high rainfall in late 2010 and early 2011 caused by a strong La Niña event, has in the short-term, alleviated much of this water stress. It is too early to state whether the recent drought has ‘broken’ as has widely been reported, and although water storages in the region are nearing capacity, the long-term average rainfall is still at a decadal low, and several more years of average rainfall conditions would be required to maintain the current water levels in south-west Victoria.

The lakes of the Volcanic Plains are internationally significant based on their ecological, social, recreational and scientific values. This has seen the establishment of the Western District Ramsar site which, in particular, notes the lakes’ rich diversity of flora and fauna. The lakes regularly support populations of over 20,000 water birds and several threatened plant species. The lakes are also suggested to be of particular value because they provide refuges during drought. The region is also internationally renowned as a concentration of sites that reveal long-term climate change, and more recently the Volcanic Plains have been designated as a Geopark (Kanawinka Global Geopark). Monitoring and palaeoecological studies have demonstrated that the lakes and wetlands of the region have undergone significant, climate-driven changes in hydrology and ecology over the past two decades (Barry et al., 2005). As a result of the ‘Big Dry’, many of the lakes in the region were dry for the first time in recorded history (e.g. Lake Terangpom, listed under Ramsar as permanently freshwater), while others experienced dramatic increases in salinity. For example, the salinity of Lake Bolac rose from 20 to 200 mS cm-1 in five months in 2006-07, decimating a $1M eel fishery. Up until late 2010 only three of the nine Ramsar-listed lakes contained standing water and none contained fresh water. Numerous stranded boat ramps now testify to the dramatic nature of recent change across the region. The high 2011 rainfall has led to a rapid refilling of many of these lake systems (although not quite to capacity), and many of the declared dry lakes (e.g. Lakes Colac and Modewarre) now contain water. Plans are being made to restock many of these ecosystems with fish species to enhance recreational fishing opportunities.

The recent observed declines in southern Australian rainfall are similar to those predicted to occur under some human-induced, climate change scenarios (Suppiah, 2004). By contrast, such shifts may have occurred also before European arrival, potentially providing ecosystems with a degree of resilience. For example, while water levels in Lake Bullenmerri have fallen >20 m since European

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Figure 1. Trend in annual total rainfall across Australia, 1970-2010 (mm/10 years) settlement, this fall has exposed c. 2000 year old trees suggesting a precedent for these events (Jones et al., 2001). Information such as this has been used to document the broad scale climate variability in western Victoria (e.g. Gell, 1998). However, it is not at sufficient temporal resolution to determine the nature and frequency of past arid phases. High-resolution proxies for temperature (Cook et al., 2000) derived from Tasmania suggest that late 20th Century warming is not outside the bounds of natural variability but this and other proxies reveal little of the history of aridity since they do not document moisture balance besides being distant from the western Volcanic Plains.

The sites lie within the winter rainfall belt of western Victoria. Rainfall within this climate zone is dominated by the passage of westerly wind systems across the southern ocean. Despite its apparent distinctiveness, this region represents a bellweather for the climate of other parts of southern Australia. Suppiah et al. (2004) show that annual rainfall in south-central Victoria has a very high correlation coefficient (r>0.9) to annual rainfall across Victoria. The strength of this relationship exceeds that of most other regions indicating that the proxy rainfall data generated here will be relevant to natural resource managers across a broad geographic region.

Lake Sediment Archives One of the most widespread proxy records of climate change in western Victoria, and across eastern Australia is obtained from lake sediments. These archives have the potential to provide chronologically structured, ecologically integrated and (in some instances) continuous records of both local and/or regional environmental change (Anderson and Battarbee, 1994). Lakes are excellent sensors of environmental change, and sediments that accumulate in a climatically sensitive lake can provide a continuous, high-resolution record of past climate variability (Battarbee, 2000). These lake sediment archives are the principal source of information on the climate history of western Victoria.

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Crater and maar lakes, in particular, have the potential to provide some of the best high resolution sedimentary records of environmental change (Williams et al., 1993), as these lakes tend to have small, well defined catchments and simple basin morphology (Lamb et al., 2000).

Despite the potential for the extraction of high-resolution records over the last 2000 years from many of these lake sequences, these archives remain underexploited, with the focus of the majority of palaeolimnological investigations in western Victoria biased towards the longer Holocene records, and in many instances restricted to the ‘type-sites’ of Victorian climate (e.g. Lake Keilambete, Bowler, 1970).

Crater and maar lakes are numerous in western Victoria. These small, often closed-catchment lakes are widely held to present ideal conditions for analyses of short-term events on regional and local scales. Such lakes situated in small volcanic crater basins are generally ideal sites for recovery of reliable palaeoclimatic records, as many are thought to have a simple hydrogeological setting, thus simplifying the relationship between lake history and climate history (Gasse et al., 1995; Verschuren, 2001). However, in some cases groundwater may complicate this relationship by keeping the lakes fresh during dry periods or delivering salts into many systems.

Closed lake basins

Hydrologically closed lakes are sensitive to climatic fluctuations if net groundwater fluxes are ~0 (Fritz, 1990). One of the main characteristics of closed basin lakes is their capacity to undergo major fluctuations in lake level and lake chemistry in response to changes in effective moisture (= P-Et); the difference between the input precipitation (P) and outputs evaporation and evapo-transpiration (Et) in response to seasonal, inter-annual or long-term climatic fluctuations (Street-Perrott and Harrison, 1985; Laird et al., 1996). Variations in effective moisture, and hence water balance, are often reflected in the ionic strength and composition of lake waters through dilution and evaporative concentration of dissolved salts (Battarbee et al., 2001).

As a result the physical, chemical and biological properties of the lake and its sediments change through time; thus sediment cores from these lakes provide an opportunity to reconstruct past water chemistry. These records can also be used to infer periods of increased or decreased effective moisture, providing valuable information on climatic variability in terms of temperature and/or rainfall. However, the hydrological sensitivity which causes closed basin lakes to respond to moderate, short-term rainfall variability also makes them prone to intermittent or complete desiccation, resulting in interruption or partial loss of the climate records which they accumulate.

In palaeoclimatic studies of closed basin lakes, diatom-based conductivity (or salinity) transfer functions have provided a key tool for generating-high resolution, quantitative data (Fritz, 1990; Fritz et al., 1991; Laird et al., 1996). Quantitative climate reconstructions in Australia that use biological proxy indicators are currently focused diatom-based inferences of lake water conductivity (salinity), ionic composition (Gell, 1997) and nutrients (Tibby, 2004). Both conductivity and ionic composition reflect net precipitation. As a result, the relationship between conductivity and diatom distributions and abundance enables fossil assemblages from sediments to be interpreted in terms of palaeosalinity and, therefore, indicate past precipitation-evaporation gradients (Gasse et al., 1995; Gell, 1997).

There are several other local factors (e.g. groundwater, irrigation, diversion, deforestation, agriculture) that can affect the hydrochemistry of closed basin lakes, and thus it is important that lakes are chosen carefully to minimise these factors, therefore allowing reconstructions of past water chemistry from proxy indicators (e.g. diatoms) to provide an independent method of quantifying past changes in effective moisture (Gasse et al., 1995). However, a fundamental problem with diatom- based conductivity inferences is that the relationship between salinity changes and climatic forcing is indirect and complex (Gasse et al., 1997; Fritz, 2008). In regions, such as western Victoria, where it is impossible to avoid the impacts of humans on many ecosystems, a multi-lake approach across a landscape of similar geology and climate may help differentiate changes in the sediment record that

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are driven by broader-scale atmospheric forcing from those driven by anthropogenic impacts within a lake catchment.

Groundwater exerts local and regional controls on closed-basin lake level changes (Almendinger, 1990). If lake hydrology is dominated by groundwater inflow, aquifer residence time and the geographic source area for groundwater recharge buffer a lake’s response to climatic variability. In lakes where groundwater outflow is important, removing water and solutes, the response of lake salinity to surface evaporation is dampened (Sanford and Wood, 1991). Similarly, lake salinity can be increased by inflows of saline groundwater.

Even when closed lakes do show long-term salinity variations proportional to climatic fluctuations, the relationship is often non-linear (Verschuren, 2002). In higher-resolution studies of fossil assemblages, sudden transitions between saline and fresh conditions may be apparent (Verschuren et al., 2000), which may be unduly interpreted as a rapid climate transition, rather than a gradual trend exacerbated by threshold scenarios (Verschuren et al., 2000).

Lake basins with outflow In outflow lake systems, past changes in climate may be reconstructed through the use of lake level changes; during periods of aridity, lake levels decrease, and if lake levels drop below the outflow threshold, there may also be a rise in salinity. Under wetter conditions, lake levels rise and will overflow (depending on the height of the outflow system). Whilst the potential climate signal recorded within freshwater outflow lakes is often less sensitive compared to closed-basin (and saline) systems, studies on such systems have shown climate reconstructions are still possible (Stager et al., 2005). Furthermore, freshwater systems are likely to provide continuous sediment records (as they are likely to be buffered towards complete desiccation) and avoid many of the preservation issues associated with closed-basin lakes.

Conductivity and habitat changes have been successfully reconstructed from many open lake systems, even in out-flowing (or overflowing) conditions. In addition to this, studies indicate that habitat changes can occur as a result of water level fluctuations in the lake basin (which can be inferred using the abundance of shallow water diatom species), indicating that habitat group changes based on depth distributions (i.e. benthic vs. planktonic) can produce reliable evidence of water level change within lake systems (Wolin and Duthie, 1999; Stager et al., 2005). In the sediment record, planktonic species colonising open water are assumed to increase in abundance during periods of higher lake levels, whilst benthic taxa will decrease, and vice versa. Ratios based on the abundance of planktonic versus benthic species can be successfully used to reconstruct lake level changes (Gasse et al., 1989, Barker et al, 1994; Stager et al., 2005; Stone and Fritz, 2004). It has been shown that this relationship of planktonic species and higher water levels can be complicated in the recent past due to human activity in lake catchments. Catchment changes due to clearance and agriculture can increase the amount of nutrients delivered to the lakes, causing blooms in planktonic taxa and perhaps erroneously leading to an inferred lake level rise (Wolin and Duthie, 1999). It should be noted that lake level changes linked to climate forcing can result in other signals being apparent in diatom assemblages. For example, there may be evidence of changes in the stability of the column (increasing stability with increasing water depth), and the susceptibility of the water column to turbulence, nutrient and thermal conditions (Wolin and Duthie, 1999).

Diatoms in Palaeoclimatic and Palaeoenvironmental Reconstructions Diatoms are microscopic, unicellular algae that have been used extensively in the reconstruction of past environmental changes, especially in lakes (Stoermer and Smol., 1999; Battarbee, 2000; Mackay et al., 2003). Diatom analysis is an unrivalled tool for quantitative reconstructions because diatoms (a) are extremely sensitive indicators of lake water chemistry, (b) tend to occur in high numbers in modern and sedimentary sequences, allowing sound quantitative analyses and (c) many diatom taxa

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are often cosmopolitan (Richardson, 1968; Gasse, 1986; Round et al., 1990). Diatoms comprise the principal proxy used in this thesis.

Diatoms have long been used as proxy indicators to reconstruct Holocene environmental changes. During the last two to three decades, the application of diatoms in Holocene environmental reconstructions has increased considerably (Stoermer and Smol, 1999). Diatoms are excellent indicators of water chemistry and contemporary studies use quantitative multivariate techniques to reconstruct past climate variables both directly and indirectly by the comparison of fossil assemblages to modern analogues (Barker, 1990). Direct approaches include the reconstruction of surface water temperature (Rosen et al., 2000) and air temperature (Korhola et al., 2000), although debate over such reconstructions is rife (Anderson, 2000). Indirect approaches reconstruct changes in the major chemical parameters such as dissolved organic carbon (DOC; Pienitz et al., 1995), pH (Psenner and Schmidt, 1992) and the reconstruction of salinity (Fritz, 1990; Fritz et al., 1991; Gasse et al., 1995; Gell, 1997; Laird et al., 1996; Verschuren et al., 2000; Ryves et al., 2002). Diatoms have also proved to be a useful proxy in non-climate related studies, e.g. in reconstructing anthropogenic eutrophication of lakes using total phosphorus (TP; Bennion et al., 1996; Lotter, 1998; Bradshaw and Anderson, 2001).

In closed basin system, diatoms are highly sensitive to changes in conductivity (Hecky and Kilham, 1973; Gasse et al., 1983), and shifts in fossil assemblages can be quantitatively interpreted in terms of past conductivity (as a proxy for climate) through the use of a transfer function (ter Braak, 1987). In arid regions of Africa, Australia, USA, Spain and West Greenland, diatom models (transfer functions) have been successfully developed and used to reconstruct quantitatively changes in salinity as a direct proxy for effective moisture and to infer from fossil assemblages the nature of climate change during the late Holocene (e.g. Fritz et al., 1993; Cumming and Smol, 1993; Cumming et al., 1995; Gasse et al., 1995; Wilson et al., 1996; Gell, 1997; Reed, 1998; Davies et al., 2002). It should be noted that the use of closed-basin, saline lakes can also be problematic in environmental reconstructions. Poor preservation as a result of silica dissolution can occur in all lake systems; however saline systems are particularly vulnerable (Barker, 1990; Barker et al., 1994; Ryves et al., 2001; Ryves et al., 2006; Ryves et al., 2009) and as such the accuracy of environmental reconstructions will always be limited by the quality of the raw data employed. Differential preservation in saline lakes caused by the dissolution of biogenic silica can affect the composition of the microfossil assemblage (Ryves et al., 2001; Ryves et al., 2009). In extreme cases entire assemblages can be destroyed; in other systems dissolution can be incomplete, biasing the species composition towards more resistant taxa and potentially compromising the value of such records when inferring ecological and climatological changes (Ryves et al., 2006; Ryves et al., 2009). Therefore, understanding the losses associated with poor preservation is fundamental when assessing the quality of palaeoenvironmental inferences (Ryves et al, 2006).

Although the relationship between conductivity (salinity) and climate is complex, and depends on an array of factors including the geological, hydrological, climatological setting and the morphology of the lake (Laird et al., 1996), the consistency between diatom records and other independent evidence for past climates is promising (Gasse et al., 1995; Gell, 1997; Gasse et al., 1997; Verschuren et al., 2000; Stager et al., 2005; Ryves et al., 2011).

Qualitative and quantitative reconstructions of past lake levels (often related to climatic changes) of open lakes, are possible using knowledge of diatom habitat preferences, and the physical and chemical preferences of individual diatom species. Lake level changes can also be inferred through the use of diatom species associated with turbulent, or upwelling hydrological regimes. For example, the genus Aulacoseira contains species which require strong turbulence in order to remain suspended (Talling, 1966; Pilskaln and Johnson, 1991). During low water levels, the increased wind-driven mixing and associated increases in nutrient upwelling can favour this genus (Wolin and Duthie, 1999). Differences in the Aulacoseira species present (e.g. A. ambigua vs. A. granulata v. angustissima) may

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also provide information on turbidity and water clarity, which can be related to the amount of suspended sediment present in the water column as well as the degree of mixing and productivity.

Palaeoclimates of the Western Plains The lakes of the western Victorian Volcanic Plains have been intensively studied by palaeoecological researchers. Records from the region have been the subject of pollen (e.g. Dodson, 1974), diatom (Gell et al., 1994), ostracods analysis (De Deckker, 1982) and sedimentological studies (Bowler and Hamada, 1971). The majority of the records from the region are restricted to the Holocene (last 10,000 years), though several do extend back to the early-Pleistocene (Wagstaff et al., 2001).

The Last Glacial Maximum (LGM, 20,000-15,000 years BP) and its associated climate has been recorded in the region in Lake Bullenmerri (Dodson, 1979), Tower Hill (North-West Crater, D’Costa et al., 1989), Terang (Kershaw et al., 1991), Wangoom (Harle et al., 2004) and Lake Surprise (Builth et al., 2008).The vegetation during this period is one of a semi-arid, grassland-steppe environment (Builth et al., 2008), almost devoid of trees (Kershaw, 1998) and temperature estimates suggest that the region was 4-6 °C cooler than present (Chappell, 1991; Kershaw, 1998). Due to the lack of modern analogues in the modern Australian context, it is difficult to reconstruct past rainfall during this period, though some estimates suggest that mean annual rainfall would have been around 250 mm (Horton, 1984).

Immediately following the LGM, temperatures in the region gradually increased, and vegetation evolved to a grassland environment. This period of time also saw the spread of Eucalyptus from highland refuges to the lower altitudes (Kershaw, 1998). Whilst precipitation at this time increased, effective precipitation also decreased as a result of the increasing temperatures, and studies from Tower Hill (D’Costa et al., 1989) and Lake Bullenmerri (Dodson et al., 1979) attest to persistent aridity until c. 12, 000 years BP.

The Holocene climate in western Victoria was wetter than the preceding Pleistocene interglacial. This increased availability of water in the landscape led to the formation of many of the lakes on the Volcanic Plains and this is reflected in the age of the sediment records contained within (6000-9000 years old). During the early to mid-Holocene, open grassland was replaced by Eucalyptus and Casuarina woodland. Amongst these dense forests would have existed tall open forests, dominated by Pomaderris. Effective precipitation was at a maximum and many of the lakes were overflowing at the time (as recorded in the Lake Keilambete sediment record [Bowler, 1981; Chivas et al., 1986]). From c. 5000 years BP, lake levels declined from the earlier high-stands, but vegetation remained largely unchanged until the arrival of Europeans (Kershaw, 1998). It appears the water balance in a number of lakes in western Victoria responded to increasing rainfall c. 2000 years ago, leaving a large number of drowned trees in the catchments of Lakes Bullenmerri, Gnotuk and Keilambete (Gill, 1971). It is postulated that this climate was continued until c. 1840-1860, after which time lake levels in the region declined once more (Jones et al., 2001), leading to the emergence of the previously drowned trees in the 1960s. The current climatic conditions experienced in western Victoria are the driest they have been for the last 2000 years, and it is suggested that up until recently, the precipitation: evaporation ratio (effective rainfall) across the region was similar to that of the LGM (Roger Jones, pers. comm.).

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Objectives

Western Victoria has produced many of the most reliable palaeoclimate records in Australia, with over twenty sites studied by the applicants over the last three decades. In general, those studies have focused on longer-term and lower resolution climate reconstruction than is the focus of this proposal. However, experience derived from those studies allows the selection of a combination of those sites most likely to yield high quality, high-resolution records of climate change (Lake Purrumbete, Tower Hill Main Lake). These are complemented from a selection of lakes that yield records of slightly lower resolution but which have significant economic, social and environmental value (e.g. Lakes Colac, Modewarre, Rosine, Burn) in which the on-ground and in-lake effects of past drought are measured.

This research documents variability in effective rainfall over the last 2000 years from six key sites using contiguous high-resolution diatom analysis. In combination, the nature of aquatic plant response to climate and lake level and quality change is documented using fossil remains (e.g. seeds, leaves, pollen). Of particular interest is the rate of recovery to past climate perturbations.

Specifically, the three main objectives of this research were to:

• Document the salinity and nutrient history, and ephemerality, of key Western Victorian lakes, thereby determining any precedents for the present Western Victorian drought.

• Document the history of aquatic plants at these sites in response to climate change.

• Document regional and catchment changes to determine their contribution to lake history.

This research utilises a multiproxy palaeolimnological approach, using analyses of diatoms, sediment properties and fossil pollen. High-resolution (sub-decadal), multiproxy investigations of lake sediment cores based on biotic assemblages (e.g. diatoms and pollen) and sedimentary variables (loss-on- ignition) potentially provide independent lines of evidence when reconstructing past climate and environmental changes (Lotter, 2003). Given the variation in response rates and sensitivity between proxies, a multiproxy approach provides a powerful means to reconstruct past environments, whilst a multi-lake approach assists in the identification and separation of local (catchment-scale) and regional effects (Fritz, 2008). The data collected during this research will be used to examine questions of spatial and temporal heterogeneity of climate change in the context of growing human impacts on the landscape over the last 2000 years.

Proxy records of past climatic conditions can provide a long enough time series to establish patterns of climate variability (Laird et al., 1996), and inform our understanding of past climate change and variability (Bowler and Hamada, 1971; Laird et al., 1996; Verschuren et al., 2000) as well as giving an insight into the responses of lakes and biota to future climate change, and changes that arise due to human activity. There has been a long history of such research in Australia with programmes such as CLIMANZ and SLEADS (Salt Lakes, Evaporates and Aeolian DepositS).

The Western Plains of Victoria are characterised by a large number of permanent and ephemeral saline and freshwater lakes, and many of these systems have been the subject of a number of palynological and palaeolimnological studies (Jones et al., 2001; Kershaw et al., 2004). There are a number of key sites in Victoria that are used as the basis of understanding climate across the region (such as the classic lake level curve of Lake Keilambete; Bowler, 1970). Whilst a type site for climatic fluctuations in western Victoria over the last 8000 years, when reconstructions are compared to other sites in the region, differences in the palaeo-records can occur (Fritz et al., 2000). Although the study of Jones et al. (1998) on Lakes Keilambete, Gnotuk and Bullenmerri has demonstrated general agreement between effective precipitation records, it also highlighted a number of

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inconsistencies in reconstructions between sites (Jones et al., 1998). The differing sensitivities of lakes to various forcing factors (e.g. climate, anthropogenic impacts) are not unusual, though it can have considerable impacts when attempting to reconstruct past climate and environmental changes (Webster et al., 2000).

Diatoms (unicellular algae) have been selected as the primary indicator in this study since they have short life cycles and a high degree of sensitivity to water quality that make them suitable to reconstruct climate variability at sub-decadal scales. Contiguous, sub-centimetre samples were analysed for fossil diatoms with diatom inference models used to calculate past salinity (Gell, 1997) and nutrients (Tibby, 2004), allowing the frequency, and duration, of past wet and dry phases to be reconstructed. At each site, and particularly at the high resolution sites, reconstructed salinity in the European period are compared to the long-term effective rainfall record to assess the sensitivity of individual sites to climate forcing. Pollen analysis, undertaken at a similar resolution, is used to complement the diatom studies by providing supporting information about terrestrial, and aquatic, vegetation responses to climate variation. In part, the application of pollen analysis will allow us to determine the extent to which diatom-related changes are the result of internal or external (climate- related) forcing. Pollen analysis also helps to identify some key points in the early post-European period, a technique that is useful in the process of checking the climate sensitivity of the lakes against the historical record.

Records of aquatic plants have been derived from plant macrofossils preserved in the lake sediments. The fluctuations through time of key aquatic plants (and some invertebrates) in response to climate in the fine resolution sites, and climate and catchment change in the response sites, has been documented. Understanding the fate of aquatic plants in these lake systems is important since they play a pivotal role in providing food and habitat, maintaining biodiversity and, to some extent, regulating water quality in many Western Victorian lakes. Of particular interest to the NRM managers is whether these plants typically recover from climate perturbations such as prolonged drought once conditions improve.

The data was collected to address the question as to whether or not these wetland systems are continuing to operate within the range of past wetland experience (natural variability) or whether human land use changes have acted, perhaps in tandem with climate, to generate unprecedented wetland states. This provides the unique opportunity to assess whether the present climate is unusual, whether these lakes are impacted relative to the long-term record, and so test whether present land-use practices are sustainable relative to past, present or future climates.

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Study Site

Western Victorian Volcanic Plains The volcanic plains stretch from Melbourne to the South Australian border in a belt averaging 100 km wide. The majority of the volcanic activity and deposited material was derived from eruptions which occurred between 2 and 4.5 million years ago. Sporadic activity has also continued through the Pleistocene into more recent times.

There are a number of saline and freshwater lakes in western Victoria. Williams (1981) offers a comprehensive review of all studies on these systems that predate 1979. However, the studies reviewed mainly concern small, highly-saline, shallow and ephemeral lakes, or large, deep, permanent maar lakes. Few studies had, at this time, been conducted on the large, shallow, permanent and only moderately saline (salinity < 60 g 1-1) lakes that dominate the landscape in this region. The lakes of western Victoria vary considerably; despite the fact they receive their salts from the same source, waters of neighbouring lakes can be of contrasting salinity and ionic proportions. The chemistry of many of these lake systems does not rely solely on the underlying geology but also on hydrologic factors including lake catchment area ratio, and whether a lake system is open or closed using hydraulics and the influence of regional groundwater. Lakes in the region are largely fresh where there is surface outflow (such as Lake Purrumbete), and large surface water lakes (Lakes Colac, Modewarre) in shallow pans are prone to frequent overflows and are largely fresh to brackish, with a tendency to become more saline during drier periods.

Geology The Western Volcanic Plains lie to the south of the Western Uplands, extending towards the coast in the west and the Otway Ranges in the southeast (Joyce, 2003). Streams extend from the uplands and have led to the formation of lakes and swamps in lower lying areas. Volcanic activity in the region began c. 5 million years ago, with activity continuing to c. 4000 years ago. Many of these eruptions have disrupted natural drainage lines forming further lakes and swamps in the region. The general geomorphology of the region is undulating plains composed of thin lava flows overlying a Tertiary marine plain (Joyce, 2003). A number of lava shields and scoria cones mark the likely sources of the flows. The lower parts of the landscape have a large number of lakes, which are subject to seasonal drying.

There are three types of volcanic activity that characterise the Victorian Volcanic Plains; lava volcanoes, scoria cones and maar lakes. Maar lakes are amongst the most common form of volcanic crater in western Victoria, with around 40 of these craters located between Colac and Port Fairy (Joyce, 2003). These volcanoes have large, circular craters up to 2 km in diameter and often contain lakes. These volcanoes are formed by explosive forces and are often phreatomagmatic in origin (Hutchinson, 1957; Crumpler and Aubele, 2001). Phreatomagmatic explosions occur when molten magma interacts with either groundwater, or water contained within sedimentary rocks. The water is superheated and explodes with force (Crumpler and Aubele, 2001) causing debris and ash to be deposited within the locality of the crater. The lighter ash material is deposited in relation to the prevailing winds, and, it can be noted that the majority of maars in western Victoria have thicker ash deposits on their eastern sides, reflecting the dominant westerly wind direction (Joyce, 2003).

The lakes located in the region are often, erroneously, referred to as crater lakes. True crater lakes are defined by Timms (1992) as those lying within the summit of non-active volcanoes, of which Lake Surprise is a type-site in western Victoria. The majority of the lakes on the volcanic plains belong to

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the category of maars (Timms, 1992) and are formed by single volcanic explosions as a result of the phreatomagmatic interactions. Examples of these lake types on the western plains include Lakes Keilambete, Bullenmerri and Purrumbete and usually form some of the deepest lakes in the region (Timms, 1992). Large, shallow pans (such as Lakes Colac and Modewarre) form in depressions and collect surface runoff as a result of changes in the natural drainage patterns as a result of volcanic activity.

Climate and Vegetation The climate of western Victoria can be described as ‘Mediterranean’ with hot dry summers and cool, wet winters. However, western Victoria exhibits a range of climates from the humid Otway Ranges to the arid northwest. Average annual rainfall totals range from 600 mm and 1600 mm, and the majority of rainfall occurs during the winter months (June-August). The total rainfall received declines latitudinal with distance from the coast (wetter regions) and western Victoria has a lower rainfall at its northern margin, with the western uplands receiving less than 600 mm per year. Significant rainfall events can occasionally occur between March and October although this largely depends on the dominant atmospheric system. These latter rainfall events are intricately linked to weather patterns associated with the Indian Ocean Dipole (Smith et al., 2000). The westerly winds in the region control the rainfall received in western Victoria, and the prevalence of these winds is driven by the Southern Annular Mode (SAM; BOM).

As well as the influence of SAM, rainfall variability across south-eastern Australia is influenced by El Niño – Southern Oscillation (ENSO; Nicholls, 1988). The Southern Oscillation Index (SOI) is calculated as the difference in mean sea level pressure between Tahiti and Darwin. When the SOI is assigned negative values, El Niño is the dominant climate phase, whilst sustained positive values indicate a La Niña phase. In south-eastern Australia El Niño is related to below average rainfall and prolonged drought conditions (Nicholls and Kariko, 1993) which may be enhanced depending on the influence of the atmospheric phenomenon of the Indian Ocean Dipole. The mean annual rainfall recorded in the latter half of last century has been greater than the first half; (Figure 2) however since 1970 there has been a general decline in decadal rainfall averages (Figure 2). The most recent period has experienced a serious decline in rainfall, with below average rainfall for more than a decade coinciding with an El Niño-dominated phase, leading to ‘The Big Dry’ across Victoria and more broadly, south-eastern Australia.

As with rainfall, the temperature gradient in Victoria is also strongly latitudinal, with temperatures increasing further inland. Generally, January and February are the hottest months, with temperatures exceeding 25°C and July is the coldest month with a minimum of 5°C (average). Mean annual temperature anomalies in Victoria show an increase since 1980 (Figure 3). This increase in temperatures has led to a concomitant increase in evaporation and has increased the severity of recent droughts compared to past droughts with similar rainfall totals (Ummenhoffer et al., 2009).

Native regional vegetation reflects the underlying, contrasting geology which in turn influences soil properties, as well as reflecting the variation in climate. Soils derived from basalt, tend to be heavily- textured, dark cracking clays which inhibit the formation of dense tree and shrub growth, as a result open savannah woodlands prevail and grasslands dominate in drier areas (D’Costa et al., 1989). Tertiary and some Quaternary sediments have given rise to a variety of soils that support open forests or woodlands dominated by species of Eucalyptus with Casuarina prominent, especially near the coast. Much of the native vegetation in the region has been cleared or severely modified for agricultural purposes (D’Costa et al, 1989) and as a result much of the vegetation in the study region is composed of grassland and introduced species with a large proportion of the region now dominated by agriculture.

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Figure 2. Annual rainfall anomaly in the southern hemisphere since AD 1900

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Figure 3. Annual mean surface temperature anomaly in the southern hemisphere since AD 1900

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Study lakes The location of the lakes under investigation is shown in Figure 4. A brief description of each site is given below.

Lake Colac

Lake Colac is located in western Victoria, north from the township of Colac and forms part of the landlocked Corangamite basin formed by early Pleistocene and Tertiary volcanic activity. This volcanic activity blocked the natural outflow of Lake Colac’s two tributaries, Deans Creek and Barongarook Creek to the south of Lake Colac, redirecting the rivers into the basin (Colac Otway Shire Council, 2002). Lake Colac is relatively shallow and has a water surface area of 2,668 ha. The maximum depth of the lake is 2.4 m (Lidston, 1993), although the lake was declared dry in early 2008 and was dry at the time of sampling. Much of the catchment is made up of volcanic materials, which have low permeability, leading to high runoff. Compared to the nearby lakes, Colac is relatively fresh. However, it is still considered ‘brackish’ as described in the 1976 Land Conservation Council Victoria ‘Report on the Corangamite Study Area’. As well as receiving water from its tributaries, rainfall and domestic run off makes up a high proportion of the total annual inflow, and changes in land-use have affected runoff in the catchment. Lake Colac has been characterised as being a partially closed system, with surface outflow restricted to extremely wet periods. It also has low catchment relief and fluctuating water levels (Slater and Boag, 1978). In most years, the water balance in Colac is influenced by rainfall and evaporation, and, to some extent groundwater seepage.

Lake Purrumbete

Lake Purrumbete forms the deepest of the group of lakes under investigation and is one of the deepest lakes on mainland Australia. Lake Purrumbete is also classed as only one of three ‘true’ crater lakes located within the Victorian Volcanic Plains. It is postulated that Purrumbete formed as a single point eruption (Timms, 1976). However the lake itself occupies two basins, likely as a result of the original lake overflowing into a second depression. The lake has been recorded as fresh since European settlement in the 1830s (Timms, 1976) and it has been noted that, following heavy rainfall events, acid, turbid water enters the bays of Purrumbete before being mixed and incorporated into the basin. Lake Purrumbete has a maximum diameter of c. 2.8 km and a maximum depth of c. 45 m (Timms, 1976). It is currently classed as a fresh, clear water, alkaline, eutrophic lake (D’Costa et al., 1989). As with much of the region, the vegetation surrounding the lake has been heavily modified by European land use, though there are stands of remnant native vegetation. Ecological studies of Lake Purrumbete began in the early 1960s and continue into the present (e.g. Hussainy, 1969; Yezdani, 1970; Timms, 1976; Mitchell and Collins, 1987; Tibby et al., in press).

Tower Hill Main Lake

Tower Hill is a large volcanic complex located 2 km from the coast at the southern margin of the volcanic plains of south-western Victoria. It is one of the largest known maars with a maximum diameter of 3.2 km and was formed by phreatomagmatic eruptions. Subsequent activity formed a scoria cone complex within the maar. Lake and swamp sediments have accumulated within the main lake that almost surrounds the scoria cone complex and within, and between, a number of the scoria cones (D’Costa et al, 1989).

Tower Hill was heavily impacted with the arrival of Europeans, with major clearance of native woody vegetation within the crater between 1850 and 1860. In addition to this, the natural outflow of Tower Hill was blocked in 1857, which raised water levels and led to more seasonal lake level variations (D’Costa, 1989). More recently, efforts have been directed at revegetating the catchment, with some limited success. Tower Hill is one of the more permanent saline wetlands in western Victoria. Water

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Figure 4. Map showing the location of the study lakes in western Victoria

levels in Tower Hill Main Lake have been documented to be high for most of the second half of the 19th century (Edney, 1984). The water level is known to have dropped in 1854, during the 1870s,1890s and between 1914 and 1946. After 1946, the water levels increased, but only to a maximum of c. 1.5 m (Edney, 1984). At the time of the sampling in January 2010 the lake was only 10 cm deep.

Lake Modewarre

Lake Modewarre is the eastern-most lake in the region under investigation. The lake itself is surrounded by subdued topography. The lake was declared dry in early 2009 and was still dry in November of the same year when coring was undertaking. When full, extensive beds of red milfoil spread over the western half of the lake, providing excellent cover for fish stocks, including native galaxias and the introduced trout. However, recent conditions have led to high levels of nutrients in the lake. This combined with shallow water as a result of the recent drought, led to an increased number of algal blooms and subsequent fish kills (DPI, 2010).

Lake Burn

Lake Burn is located immediately to the east of Lake Colac and sits within private property. The lake itself is surrounded by a nature reserve, consisting of swampy vegetation and reeds. Immediately outside the boundary, the land surrounding the lake is intensively farmed. At the time of sampling the lake was around 80 cm deep and the lake water was clear, with an abundance of submerged aquatic macrophytes covering the lake bed. The lake level is known to respond to changes in lake level and an older shoreline depicting a higher lake level in the recent past could be seen around the edge of the lake.

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Methodology

Sample collection Sampling was undertaken during number of field excursions between November 2008 and February 2011. During this fieldwork, 6 piston cores and multiple Russian core sediment sequences were collected from 6 lakes across the western Victoria. All of the shorter core sections (up to 80 cm long) were collected using a piston corer (80 mm diameter) from the centre of the lake, which is assumed to be the point of greatest sediment accumulation, dependent on the basin morphology. This method of choosing a coring site was straightforward in those lakes that contained shallow water (1-3 metres) at the time of sampling (i.e. Lake Burn and Rosine). For the deeper Lake Purrumbete (c. 41 m) cores were extracted from the shallower margins (c. 6 m), as for very deep lakes, unless major fluctuations in water depth (in terms of tens of metres) has occurred over the period of interest, the diatom flora contained within the sediments would not necessarily show a response; by targeting the littoral areas, changes in the diatom assemblages will occur with fluctuations of only a few metres. Many of the lakes at the time of sampling were essentially ‘dry’ (i.e. Lakes Colac and Modewarre) or contained only a few centimetres of water (i.e. Tower Hill Main Lake). This posed a major impediment to reaching the area of highest deposition. In all cases, with the lake sediments still being water-logged in the upper sections, the cores from these sites were restricted to only 50-100 metres off shore.

Unconsolidated, or water-logged upper sediments were collected using the piston core, and this core was extruded in the field or in the laboratory at contiguous 2 mm intervals, (constituting an unprecedented sampling resolution for palaeoenvironmental studies in Australia), and sediment samples were placed into labelled sample bags and refrigerated (4 °C) at the University of Ballarat until required for further analysis. The collection of the unconsolidated sediments using a piston core results in the collection of an undisturbed sediment sequence; and in the case of the lakes that contained water was extremely important to preserve the sediment-water interface, ensuring the top of the sediment was successfully collected. To collect longer core sequences, sediments were collected using a Russian peat-type corer (D-section corer, 50 cm long, 5 cm wide; Belokopytov and Beresnevich, 1955; Jowesy, 1966). Multiple cores were taken until, in all cases except Tower Hill Main Lake, basal clays were reached (sediment sequences of 80-3.3 metres in length). All Russian cores were kept intact and placed in half drain-pipes and wrapped in cling-film immediately after collection. As with the piston core samples, the cores were stored in dark refrigeration until required for analyses.

Sample analysis: Chronological analyses

Lead-210, Caesium-137 and Radon-226 Recent sediment samples from five of the six long core sequences were analysed for 210Pb, 226Ra and 137Cs by direct gamma assay (Appleby et al. 1986; Appleby, 2001). The analyses were carried out using Ortec HPGe GWL series well-type coaxial low background intrinsic germanium detectors (Appleby et al. 1986). 210Pb was determined via its gamma emissions at 46.5keV, and 226Ra by the 295keV and 352keV γ-rays emitted by its daughter isotope 214Pb following 3 weeks storage in sealed containers to allow radioactive equilibration. 137Cs was measured by its emissions at 662keV (Appleby et al. 1992).

Radiometric dates for each core were calculated using the constant rate of supply (CRS) and constant initial concentration (CIC) 210Pb dating models where appropriate (Appleby and Oldfield, 1978), and

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compared with stratigraphic dates determined from the 137Cs record. The best chronologies for the five lakes analysed were determined using the procedures described in Appleby (2001). As atmospheric fallout of 210Pb is known to be much lower in Australia compared to the northern hemisphere and equatorial areas, samples in most cases were restricted to the upper 10 cm of each sediment core and were taken at 0.4 cm intervals, with increasing resolution towards the top of the core.

All of the cores were sub-sampled, dried and weighed at the University of Ballarat. The 210Pb, 226Ra, and 137Cs were carried out at Liverpool University Environmental Radioactivity Laboratory (under the supervision of Prof. Peter Appleby).

Radiocarbon (Carbon-14) Five of the six longer core sequences from western Victoria were dated using AMS 14C dating of bulk sediment samples. Ideally terrestrial macrofossils are preferentially used to construct a chronology as they are less likely to suffer from the effects of ‘old carbon’ sourced from the catchment and incorporated into lake sediments. Unfortunately, in almost all lakes sampled, there was a lack of suitable terrestrial macrofossils available for dating. In order to undertake high-resolution analyses, bulk sediment provided the only suitable solution, as more sediment is required to sieve and pick macrofossils, which would have led to a poorly resolved chronology when compared to the biological proxies. At least four dates were obtained for five of the core sequences to establish a full chronology for the sediment sequences.

All of the samples for radiocarbon dating, submitted to the radiocarbon laboratory, were subject to a physical and chemical pre-treatment. Any visible physical contaminants were removed prior to the chemical treatment. Chemical treatment of the samples involves an Acid-Alkali-Acid (AAA) treatment. All of the samples were washed using hot HCl. The samples were then rinsed and treated with multiple hot NaOH washes. The NaOH insoluble fraction was treated with hot HCl, filtered, rinsed and dried. The results were reported as conventional radiocarbon years before present (BP, relative to AD 1950). Calibrated ages were derived from 14C dates using the OxCal program (v4.1.7; Bronk Ramsey, 2009) using the southern hemisphere calibration curve.

After sub-sampling and drying of samples at the University of Ballarat, all of the samples for radiocarbon dating were prepared and analysed at the University of Waikato’s Radiocarbon Laboratory (New Zealand) under the direction of Dr Fiona Petchey and Professor Alan Hogg.

Organic content

Loss-on-ignition (LOI) was used to estimate the organic content (Corg) and carbonate content (CO3) of the sediment samples from the lakes included in this study. LOI provides an approximate measure of the organic content of the lake sediments (Dean, 1974). Known sample weights (typically in the range of 1-2 g) of the lake sediment were placed in a crucible and dried overnight at 105 °C. The samples were reweighed to estimate the water content. The sediments were then placed in the furnace and kept at 550 °C for 2 hours. The resulting weight loss following heating at this temperature derives values of ‘loss-on-ignition’ (a proxy for the sample Corg). The ashed samples were then used to estimate the carbonate content of the sediment by heating the sediment to 925 °C for 4 hours. The amount of carbon dioxide lost in the process can be used to determine the original carbonate content of the sediment (cf. Dean, 1974). Whilst simple, this analysis often overestimates the Corg and CO3 as other components, such as interstitial water in clays can contribute to the mass loss (Snowball and Sandgren, 1996; Leong and Tanner, 1999). The error associated with the analytical technique can pose challenges with interpretation (Heiri et al., 2001).

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Sedimentary diatom analysis The standard technique of diatom analysis follows the method as set out by Battarbee (1986). Approximately 0.5 g of wet weight material per sample was used for the analysis, to which 30% hydrogen peroxide (H2O2) was added to oxidise the organic material present and heated on a hotplate to 90 °C for 4 hours, topping up the H2O2 when necessary to prevent the samples from boiling dry. In the case of extremely organic-rich sediment, the samples were left in cold H2O2 for 24 hours to prevent the samples from boiling over when heated. Following the oxidisation process, 10% hydrochloric acid (HCl) was added to the samples to dissolve any carbonates present and to neutralise the sample. Samples were washed four times after the chemical treatments to remove any salts that had formed during the preparation process. The samples were allowed to settle for 24 hours between each wash. After washing the samples were diluted and placed on coverslips and allowed to dry for approximately 48 hours. These strewn slides were mounted in Naphrax (refractive index of 1.73), and at least 300 valves per sample were counted in parallel transects under oil-immersion phase-contrast light microscopy (LM) at x1000 magnification on a Zeiss research microscope.

A variety of general (e.g. Krammer and Lange-Bertalot, 1986-1991; Patrick and Reimer, 1966, 1975; Germain, 1981) and region-specific floras (e.g. Gasse, 1986; Sonneman et al., 2000) were consulted, and valves identified to species level where possible. The dissolution of the diatom valves was assessed using a four-scale system (pristine and dissolved; cf. Ryves et al., 2001). This ratio varies from 0 (all valves partly dissolved) to 1 (perfect preservation). All data was recorded manually on count sheets and then transferred into an electronic database to allow further analysis.

Pollen analysis 1 cm³ sub-samples were taken from 2mm slices at regular intervals where possible from each core for pollen and charcoal analysis. Due to the time taken to count pollen relative to diatoms, fewer samples were selected for analysis but, for comparative purposes, they were from slices also used for diatoms. Preparation involved sediment dispersal in 10% sodium pyrophosphate, treatment with potassium hydroxide to remove organic matter, sieving through mesh sizes of 180μm and 7μm to remove unwanted large and small fragments respectively, treatment with hydrochloric acid to remove carbonates, acetolysis ( a mixture of acetic anhydride and sulphuric acid) to reduce cellulose matter and darken grains to make them easier to identify, treatment with hydrofluoric acid to dissolve silicate material and heavy liquid (specific gravity 2.0) separation of any remaining mineral matter from the pollen residue using sodium polytungstate. A known amount of exotic Lycopodium spores was added to each sample at the beginning of the preparation process, to allow for pollen and charcoal concentrations to be calculated. Prepared samples were mounted on microscope slides for identification and counting of pollen grains and charcoal.

Pollen and charcoal counts were undertaken using either an Olympus model BHB or a Zeiss Axiorod compound light microscope at x 600 magnification. Sample counts were continued until a total of 100 grains of major regional south-east Australian dry land taxa (D’Costa and Kershaw 1997) had been recorded. All pollen taxon percentages were calculated in relation to this major pollen taxon sum to prevent taxa such as Chenopodiaceae that can be derived from locally growing as well as regional plants, and local aquatics, distorting regional vegetation reconstructions. Microcharcoal particles, greater than 5µm maximum diameter, were counted with Lycopodium spores along selected transects and expressed as particles per cm3 or as charcoal/pollen sum ratios.

Data for each record were entered into the one or both of the data graphing and analytical programs TILIA (Grimm, 2004) and C2 (Juggins, 2003) and portrayed on two pollen diagrams, one for mainly dry land pollen taxa and charcoal and the other for predominantly aquatic taxa. To facilitate diagram comprehension, the following actions were undertaken. Many taxa with minor representation and which would not add significantly to interpretation have been excluded from the diagrams

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Individual taxa have been colour coded according to their major habit or ecology (purple for introduced taxa, red for woody plants, orange or fuchsia for woody or herbaceous taxa, yellow for herbs, green for ferns, dark blue for aquatic macrophytes and light blue for aquatic algae).

Each diagram has been zoned independently based on variation in representation of pollen sum taxa and using a stratigraphically constrained hierarchical classification technique, CONISS, contained within TILIA (Grimm, 2004).

Plant and charcoal macrofossil preparation and data representation Generally, macrofossil analysis is aimed at providing a comprehensive picture of local plant assemblages and requires large volumes of sediment. However, this aim was not achievable here because of the very limited material available due the high-resolution nature of the project and sampling equipment available. Instead, macrofossils were examined from those sites where macrophytes appeared to be significant in order to both complement the pollen flora, in that some plants are never or seldom represented by pollen and improve plant identification in that greater morphological variation in macrofossils than pollen can allow more refined taxonomic resolution. Material for examination derived from the sieving of samples for pollen analysis. A range of remains of plant and animal organisms were recorded but only those from plant macrophytes, including macrocharcoal particles, are considered here. A focus was placed on seeds for which there had been developed recently, an interrogative database for western Victoria (Lewis, in press).

Statistical analyses

TILIA The results of pollen analysis on each lake are presented as two pollen diagrams prepared using TILIA (Grimm, 2004); one for predominantly aquatic taxa and one for mainly dry land pollen taxa and charcoal. A moisture index, based on percentages of predominantly freshwater taxa (Myriophyllum, Pediastrum and Botryococcus) and saline tolerant taxa (Chenopodiaceae, Myriophyllum muelleri and Ruppia), is included on the aquatic pollen diagram. Each diagram was zoned independently, on either all aquatic taxa or pollen sum taxa, using a stratigraphically constrained hierarchical classification technique, CONISS, contained within TILIA (Grimm, 2004).

C2 For all stratigraphic diatoms, all diatom species >5% or >10% in any sample are shown as percentage abundance and ordered according their down-core weighted averaging abundance (species ordered in terms of their occurrence in the core). The cores are plotted against depth and dates are presented alongside the y-axis. The dissolution index (F-index), total concentration of diatoms (x106 valves g-1 dry sediment), reconstructed salinity, DCA axis 1 sample scores and testate amoeba counts (as a testate amoeba:diatom ratio) and Chaetoceros:diatom ratios are also displayed. The diatom zones presented are based on the output of ZONE. Diatom-inferred conductivity is also presented and these curves were generated using an existing diatom transfer function (European Diatom Database; EDDI).

Zone The stratigraphical diatom data from each core were divided into assemblage zones using optimal sum of squares partitioning (Birks and Gordon, 1985) by the program ZONE (version 1.2; Juggins, 2002).

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Canoco 4.5

Principal Components Analysis (PCA); Correspondence Analysis (CA) and Detrended Correspondence Analysis (DCA)

Indirect ordination analyses were carried out using CANOCO 4.5 (ter Braak and Šmilauer, 2002) to identify the dominant trends within the data. Initially a Detrended Correspondence Analysis (DCA; Hill and Gauch, 1980) with detrending by segments, and down-weighting of rare species, was used to explore the main patterns of taxonomic variation among sites and to estimate the compositional gradient lengths of the first few DCA axes. The diatom percentage data were transformed using log transformation in an attempt to reduce clustering of abundant or common taxa at the centre of origin (Leps and Šmilauer, 2003). The gradient lengths allow the determination of the most appropriate response model for further analysis. If the gradients were sufficiently long (>1.5 s.d.), it indicated that numerical methods based on a unimodal response model were most appropriate (e.g. [Detrended] Correspondence Analysis; CA or DCA; ter Braak and Prentice, 1988). Where gradient lengths were <1.5 s.d. a linear response model (e.g. Principal Components Analysis; PCA) was deemed the most appropriate.

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Results

Lake Colac

Radiometric dating

Lead-210 Activity

Total 210Pb activity in the upper sediments of Lake Colac reached equilibrium with that of the supporting 226Ra at a depth of around 10 cm (Figure 5a). Unsupported 210Pb activity (Figure 5b) declined irregularly with depth. Concentrations were relatively high in the top 2 cm of the core, with a maximum value of 65 Bq kg-1 in the surface sample. Below 2 cm they fell sharply to a value of around 16 Bq kg-1, but then remained relatively constant at this value down to a depth of 9.5 cm, below which there was a further abrupt decline. In all samples below 10 cm unsupported 210Pb concentrations were close to, or below, the limit of detection (Table 1).

Artificial Fallout Radionuclides

Caesium-137 concentrations (Figure 5c) were very low, and significantly above the minimum level of detection only in the upper 3.5 cm of the core. The highest value (7.6 Bq kg-1) occurred in the topmost sample. There was no indication of a subsurface peak recording the high levels of 137Cs fallout from the atmospheric testing of nuclear weapons in the early 1960s.

Core Chronology

Lead-210 dates were calculated using the Constant Rate of Supply (CRS) model, placing 1964, the year of peak nuclear weapons fallout in the southern hemisphere, at a depth of about 6 cm. Taking the 210Pb results at face value, the mean post-1950 sedimentation rate is calculated to be 0.06 g cm-2 y-1 (0.14 cm y-1). The 210Pb data below 6 cm is not reliable, but does suggest significantly lower sedimentation rates in the first half of the 20th century. The deepest sample, which has significant unsupported 210Pb concentrations (9-9.4 cm), is dated to the early part of 20th century. The pre-1950 sedimentation rate is calculated to be 0.02 g cm-2 y-1 (0.04 cm y-1). These results, shown in Figure 6 and given in detail in Table 2, are reasonably consistent with the 14C dates, which suggest a long-term accumulation rate during the past 500-600 years of around 0.04 cm y-1. The 210Pb dates also suggest a brief episode of more rapid accumulation in the early 1980s. In the absence of a good 137Cs record the 210Pb chronology should, however, be regarded with some caution unless supported by other evidence.

Radiocarbon Dating

Seven dates were obtained from the Lake Colac sediment core, and were all taken below the appearance of Pinus in the pollen record to ensure that the samples submitted would be old enough for the use of this dating technique. The samples submitted for radiocarbon dating comprised of bulk organic sediments. Preferentially terrestrial macrofossil material would be submitted for dating, but the low number of macrofossils in the Lake Colac core prevented the use of this material. The results of the radiocarbon dates are given in Table 3. Five of the seven dates obtained from Lake Colac fall in stratigraphic order (Figure 7), suggesting that whilst there is likely to be limited disturbance in the core sequence, some of the samples may be subject to contamination by old carbon (not unusual when relying on bulk sediment dates). The upper dates (25 and 28 cm) are similar in age and suggest that this depth is almost certainly dated to c. 500 cal. years BP (AD 1500). The results from these upper dates also lends support to the calculated sedimentation rates derived from the 210Pb analyses; using the pre-1950 sedimentation rate of 0.04 cm yr-1, AD 1500 would be placed at c. 26 cm. The basal dates

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Table 1. Fallout radionuclide concentrations from Lake Colac

Table 2. 210Pb chronology of the Lake Colac sediment core

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Table 3. Results of radiocarbon dating for Lake Colac

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Figure 5. Fallout radionuclides from Lake Colac

Figure 6. Radiometric chronology of the Lake Colac sediment core showing the CRS model.

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Figure 7. Age model for the Lake Colac sediment cores derived from the 210Pb and 14C dates

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(72 and 78 cm) again fall in stratigraphic order and suggest a basal core date of c. 5250 yr BP (BC 3250). The dates taken at 35 and 60 cm appear to be out of sequence with the remainder of the dates from this core. It should also be noted that these two dates were also submitted 2 years after the initial dates were obtained. It is likely that these samples have been subjected to contamination, and these samples have been excluded from the age model.

The most confusing date in the sequence is that at 55 cm. The steep change in the age model curve could suggest (1) a dramatic reduction in the sedimentation rate between 500-1000 cal. Yr BP; (2) the existence of a hiatus in the sediment core; (3) a dating error due to sample contamination. A dramatic reduction in sedimentation may occur if lake level is very low, where boggy conditions prevail, and sediment delivery to the lake system slows due to the lack of catchment runoff (under a more arid climate) and a reduction in the productivity of the lake system. Given that Lake Colac is in a very shallow lake basin, it is likely that a hiatus may exist in the Lake Colac core, the location of the sediment core in the lake-level (and hence climate) sensitive littoral area of the lake would make the site prone to complete drying during periods of aridity and decreased water levels. In order to fully understand the implications of this dated horizon, further samples above and below 55 cm were sent for radiocarbon dating to see if the date is ‘real’ (attributable to environmental conditions or a hiatus) or whether there was a contamination issue (such as the penetration of roots into this area of the sediment core during a drier climate which may have delivered younger carbon, resulting in a younger date).

Pollen There are extreme variations in taxon representation below about 35 cm in Lake Colac (Figures 8 and 9), a number of which may be the result of differential preservation rather than vegetation change. Consequently, the diagram is not presently zoned and is little considered below this depth. Perhaps the most significant change in the diagram occurs around 25 cm, the zone 3/2 boundary, with a switch from a macrophyte-dominated to an algal-dominated aquatic system. The dominant macrophyte is the submerged plant Myriophyllum, identified to the species M. salsugineum from recorded seeds, although values are exceeded by those of Rumex between 35 and 40 cm depth. Myriophyllum is replaced largely by the alga Botryococcus. The change may represent an increase in water depth although, considering that the maximum attainable depth of the lake is only about 2.5 m, it may be more related to a loss of visibility due perhaps to increased turbidity.

The event is dated, from two similar but inverted radiocarbon dates, to about 500 years ago. This is long before the arrival of Europeans, whose activities may have been considered a cause of the change in state of Lake Colac. However, it is a common phenomenon for radiocarbon ages to appear several hundred years too old in recent sedimentary records, possibly due to reworking and redeposition of carbon with intensification of human impact on the landscape, and this event may mark the earliest possible time of European arrival. There is, though, little indication of influence on dry land vegetation and unless some evidence emerges for special, early attention to the lake, then a more natural cause, such as an increase in effective rainfall, seems most likely. One possibility is that early cattle grazing, from about 1837 in this area, resulted in disturbance to the sediment and vegetation at least of the lake margin. Detection and counting of spores in the sediment, derived from fungi of cattle dung, could resolve this question.

A second possible initial impact of Europeans, or perhaps more specifically their agriculture, is at the subzone 2c/2b boundary, marked by a sustained increase in spores of the moist ground liverwort Anthoceros and dry land understorey Pteridium (bracken). The former may also indicate a fall in lake level, as also possibly suggested by the reduction in Botryococcus values, but continued cattle disturbance may have prevent a return to an aquatic macrofossil substrate. This period also witnesses the beginning of a sustained decline in pollen of Casuarinaceae, a common feature of early European impact due to the selection of its timber for fuel and construction. There is a marked peak in Asteraceae Tubuliflorae pollen just above the subzone 2c/2b boundary. It is possible that this relates

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Figure 8. Lake Colac dry land pollen stratigraphy

Figure 9. Lake Colac aquatic pollen stratigraphy to the Scotch Thistle weed infestation that accompanied the intensification of agriculture in the early 1850s and was subject of control in 1856. There is no doubt that agriculture was well established by the subzone 2b/2a boundary as exotics, in the form of shelter trees Pinus and Cypress Pine

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(Cupressaceae), and the pastoral weed Plantago, had arrived and begun flowering. Earlier occurrences of pine pollen are no doubt contaminant resulting from the method of sediment sampling and perhaps also pollen in the atmosphere, as pine pollen is a notorious contaminant. The competitive nature of the introduced Plantago is shown by the immediate decline in native Plantago.

The zone 2/1 boundary indicates the establishment of a landscape similar to that of today with extensive pine plantations and a more open tree cover demonstrated by high percentages of Poaceae as well as reduced eucalypt and Casuarina percentages. The most likely date is about 1920. Through the period though there are general reductions in disturbance taxa such as Plantago, Pteridium and Anthoceros that may reflect increasing conservation values. However, Asteraceae Liguliflorae percentages have increased and stay high in this period. Perhaps they indicate weeds of a more arable agricultural system this century. It is interesting that M. salsugineum has not re-emerged considering that conditions have been frequently dry during the last 80 years, yet Myriophyllum muelleri, an indicator of more saline aquatic environments, is conspicuous through zone 1. Its only previous representation of note is a peak within zone 3 and could indicate a dry period prior to European presence. However, the very different assemblages associated with the early and later phases of M. muelleri demonstrate the importance of factors other than water availability on aquatic succession. Through the whole zoned period, charcoal values are very low suggesting that fire has not been a major influence on vegetation change. High values at the base of the sequence may indicate concentration under periods of low or no other sediment deposition.

Macrofossils Samples for macrofossil analysis were much smaller than desired, due to high-resolution sampling of the sediment cores; however they were sufficient to provide an idea of the types of macro remains that were present. They were also sufficient to provide some taxonomic resolution to the pollen data.

The macrofossil record contained six identified vascular plant taxa. There exist another potential five plant species pending identification, though, as material was so sparse (due to sample size) and degraded (due to having undergone KOH treatment as part of the pollen preparation process and possibly preservation influences), they have not been identified at this point. Additionally one freshwater sponge species, and between 3 and 5 charophyte species, were present. The record also contained a number of invertebrate macrofossils including 5 or 6 cladoceran species, 1 bryozoan, at least 4 trichopteran (caddis-flies) taxa and a range of other beetles, true bugs, ostracod and mites. A summary diagram is presented with selected macrofossil taxa (Figure 10).

The presence of macrofossils in the lower part of the record adds some taxonomic refinement to the pollen data; species were able to be assigned to the aquatic pollen genera Myriophyllum salsugineum (Myriophyllum), Lepilaena cf. bilocularis (Lepilaena) and Azolla filiculoides (Azolla), and to the families cf. Chenopodium glaucum (Chenopodiaceae) and cf. Bolboschoenus medianus (Cyperaceae). The macrofossils also add ecological depth to the interpretation of the record, reflecting both the submerged lake vegetation (Myriophyllum salsugineum, Lepilaena cf. bilocularis and Azolla filiculoides) and those species growing at the lake edge (Juncus cf. pallidus, cf. Bolboschoenus medianus and cf. Chenopodium glaucum). Freshwater to brackish conditions are suggested by the plant species and their modern habitat preferences.

The presence of Daphnia sp. throughout the lower section of the record supports moderately fresh to brackish water and indicates that salinity levels were no greater than 5.8‰ during this period. In agreement, the charophytes present in the lower sediments Chara globularis/vulgaris type and Nitella sp. suggest freshwater fluctuating to brackish water with salinity no greater than 5‰. The disappearance of these taxa, along with all Trichopteran types and Cladoceran type 4, around 25 cm

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Figure 10. Lake Colac macrofossil record

could support the inferred drought and increase in salinity shown by the diatom record at this point (or may simply be an artefact of the small sample size).

The second main change occurs at 47.6-47.8 cm where there is a loss of three cladoceran types coincident with the appearance of Myriophyllum salsugineum, cf. Chenopodium glaucum, Azolla filiculoides and Lepilaena cf. bilocularis. This coincides with the inferred AD 1000 drought in the diatom record.

Diatoms The diatom stratigraphy shows much change in the condition of Lake Colac over the last 5550 years (Figure 11). Seven zones have been statistically identified in the core. Zonation was carried out using a statistically robust method (optimal sum of squares) which allows the assemblage data to be divided into groups based on the principle of dissimilarity.

The broadly tolerant Cyclotella meneghiniana is usually common. The base of the core (zone COL 1) supports an unusual population of aerophilous taxa suggesting a shallow, muddy substrate of in- washed littoral sediments. This scenario is supported by the testate amoebae: diatom ratio. Testate amoeba are small soil dwelling creatures and are often washed into lake sediments from the littoral areas. In this research testate amoeba scale are used to infer in-wash from the littoral areas, while they may also be used to infer the extent of littoral areas around the lake (which may occur during periods

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of lower lake levels). The halophilous taxon Amphora veneta is also present revealing a naturally brackish condition for Lake Colac. Throughout COL 2 (44 cm) the diatom flora is represented by taxa with varying salinity tolerance, but including some preferring fresh or oligosaline conditions e.g. Gomphonema affine, Nitzschia lacuum. The appearance of Amphora coffeaeformis and Navicula tenelloides in COL 3 marks an increase in salinity. From here progressive changes occur with small peaks in the eutraphentic Nitzschia palea, and increase in the nutrient-salinity planktonic form Actinocyclus normanii (COL 4). This latter change reveals an increase in depth, and salinity, possibly suggesting increased catchment contribution of salt, independent of drought. The section from 30-25 cm hosts the greatest representation of saline taxa, and particular those capable of withstanding hypersaline conditions (Amphora coffeaeformis, Gyrosigma spp. (particularly G. attenuatum) and Navicula tenelloides, as well as Amphora veneta, and a peak in the Chaetoceros: diatom ratio). The period represented in Col 4 (c. 27 cm) marks the best evidence for a major drying event, which is currently dated to AD 1500, prior to the arrival of Europeans and concomitant with the onset of the northern hemisphere’s climate phenomenon the ‘Little Ice Age’. Other, lesser saline events appear in zone COL 5 (21 and 11 cm). An increase in sediment flux and lake water turbidity may be reflected by the increase in Fragilaria spp. which peak after this AD 1500 salinity event.

The Chaetoceros: diatom ratio, and abundance of Actinocyclus normanii, increases through COL 6 suggesting increasing lake salinity. The abundance and diversity of salinity indicators increases in COL 7 (from 3 cm) and reaches a peak for the entire record in the middle of this zone. It includes the presence of the halobiontic Staurophora wislouchii. The plankter Cyclotella meneghiniana and tychoplanktonic Fragilaria spp. are largely lost.

The ordination analyses (DCA) particularly DCA axis 2 suggest unprecedented conditions in the Lake Colac system in the last 30 years than has been observed for the entire 6000 year record. This change in the statistical analyses is supported by a 3-fold increase in the diatom-inferred conductivity (a proxy for salinity).

Figure 11. Lake Colac diatom stratigraphy

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Lake Purrumbete

Radiometric dating

Lead-210 Activity

Total 210Pb reached equilibrium with the supporting 226Ra (Figure 12a) at a depth of about 11 cm. Unsupported 210Pb activity (Figure 12b) in this core declined irregularly with depth. In the upper sections concentrations increased with depth from relatively low value of 35 Bq kg-1 in the surface sample to a maximum value of 75 Bq kg-1 in the 7-7.6 cm section. Below this peak value, concentrations declined steeply with depth, falling below the minimum level of detection between 10.4-11 cm (Table 4).

Artificial Fallout Radionuclides

The presence of 137Cs (Figure 12c) in samples well below the 210Pb/226Ra equilibrium depth shows that this radionuclide is relatively mobile in these sediments. Although a well-defined sub-surface peak is not observed, the steep increase in 137Cs concentration, from quite low values below 12 cm to moderately high values in the 9-9.4 cm section, suggests that sediments at this depth probably date from the period of maximum 137Cs fallout in the early 1960s.

Core Chronology

Initial 210Pb dates calculated using the CRS model place 1964 at a depth of about 7.2 cm, significantly above the depth suggested by the 137Cs record. The discrepancy appears to be due to a substantial increase in the sedimentation rate, around 30 years ago, that was also associated with a substantial (but not proportionate) increase in the 210Pb supply rate. The abrupt disappearance of the unsupported 210Pb record below the 137Cs peak could also be an indication of a hiatus in the sediment record. Using the 137Cs date as a reference point the mean sedimentation rate during the period of rapid accumulation is calculated to be 0.05 g cm-2 y-1 (0.25 cm y-1). The gradient of the 210Pb activity versus depth profile, below its peak value in the 7-7.6 cm section, suggests that sedimentation rates prior to this increase were as little as 0.01 g cm-2 y-1 (0.04 cm y-1). Although this is consistent with the 14C dates, which suggest a long-term mean accumulation rate during the past 2000 years of around 0.04 cm y-1, assuming the 137Cs date to be valid it is not consistent with the inference that there can only be a relatively small time gap (at most 20 years) between the 210Pb peak at 7.3 cm and the 137Cs peak at 9.2 cm. It does therefore, seem likely that there was some kind of hiatus in the 210Pb record in the late 1950s or early 1960s. In view of these uncertainties a best chronology for this core has been constructed in the following way: dates of sediments above the 210Pb peak at 7.3 cm have been calculated according to the CRS model using the 1964 137Cs date at 9.2 cm as a reference point. Dates between 7.3 cm and 9.2 cm have been calculated using a calculated mean sedimentation rate for this period of 0.028 g cm-2 y-1 (0.13 cm y-1). Dates of sediments below 9.2 cm have been calculated using the slow pre-1963 sedimentation rate of 0.01 g cm-2 y-1 (0.04 cm y-1) indicated by the basal 210Pb calculations. The results are shown in Figure 13 and given in detail in Table 5. As with the Colac core, they too should be regarded with some caution unless supported by other evidence.

Radiocarbon Dating

Four radiocarbon dates were obtained for Lake Purrumbete (Table 6). At the time of sample submission, a pollen stratigraphy did not exist for the core, so samples for radiocarbon dating were extracted from below the samples used for 210Pb analyses. As with the samples from the Lake Colac core, all analyses were completed on bulk organic material, as there were limited terrestrial

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Table 4. Fallout radionuclide concentrations from Lake Purrumbete

Table 5. 210Pb chronology of the Lake Purrumbete sediment core

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Table 6. Results of radiocarbon dating for Lake Purrumbete

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Figure 12. Fallout radionuclides from Lack Purrumbete

Figure 13. Radiometric chronology of the Lake Purrumbete sediment core showing the CRS model

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Figure 14. Age model for the Lake Purrumbete sediment cores derived from the 210Pb and 14C dates

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macrofossils in the Lake Purrumbete samples. The results of the radiocarbon dates are given in Table 6. Three of the four dates obtained from Lake Purrumbete fall in stratigraphic order (Figure 14).

Again, as with Lake Colac there is one discrepancy in the data (at 90 cm), which corresponds to a sample sent with the second batch of radiocarbon dates two years after the initial dates were obtained. It is likely that this sample has been subject to contamination, and has been excluded from the age model.

The uppermost date from Lake Purrumbete (60 cm; c. 1800 cal. yr BP) is in stratigraphic agreement if the sedimentation rate calculated using the 210Pb dating is extrapolated back through time and once again, provides confidence in the age of these sediments. Similar to the results from Lake Colac, there seems to be a confusing date at 105 cm (c. 2000 cal. yr BP). Again, there is steep gradient in the age curve, once again suggesting either a change in sedimentation rate, a hiatus in the core sequence or contamination of the sample sent for analysis. With regards to Lake Purrumbete it is also possible to use existing dates obtained on an older core sequence. With similarities in the diatom stratigraphies of the new high-resolution core and older, lower-resolution studies, it may be possible to strengthen the chronology by using key changes in the diatom flora as a tie point between the cores.

Pollen The preliminary pollen stratigraphy for Lake Purrumbete is shown in Figures 15 and 16. Forty-five samples have been analysed at 1-2 cm intervals for the top 70 cm of the sediment record. Selected features of both the dry land and aquatic pollen records through the upper 67 cm of the core are shown. The dominance of Poaceae and relatively constant representation of Casuarina, Eucalyptus, Plantago (native) and Asteraceae indicates the regional presence of woodland vegetation through much the recorded period under relatively unchanging environmental conditions. The first presence of exotics together with a slight but sustained decline in Casuarina pollen at the zone P3/P2 boundary marks initial impact of Europeans on the landscape. The lack of any changes in the aquatic pollen record or in the diatom record suggests that impact did not extend to the lake environment. The most likely age for this initial impact would be around 1850 AD, much later than the age inferred from radiometric dating. More substantial impact of European settlement is marked at the zone P2/P1 boundary with substantially increased representation of both Pinus and Cypress pine, probably due to planting as both wind belts and plantations, and an increase in Poaceae and decrease in Plantago (native) that indicate mainly the replacement of native grasslands by introduced taxa. The pattern is very similar to that at Lake Colac suggesting a regional change in the early part of last century. Although this is rather earlier than suggested by lead 210, a hiatus in sediment accumulation from around 1920 to 1960 AD is possible though not likely. However, the event had a local as well as regional impact with marked declines in Typha and Pediastrum and an increase in Myriophyllum within the aquatics. There are also abrupt changes at the same depth with the zone PUR6/7 boundary in the diatom record. Interestingly, the reverse of the changes in aquatic pollen or spore taxa are evident at the zone P4/P3 boundary at a depth of about 54 cm and this is also a time of change in the diatom record. Causation is difficult to determine, but assuming that planktonic algae suggest deeper water than submerged Myriophyllum, conditions could have been driest in the record around 2000 years ago and within the last 50 years or so.

Diatoms The record from Lake Purrumbete (Figure 17) has been divided into 8 assemblage zones. Zones PUR 1 and PUR 2 are dominated by Tabularia fasciculata, Cocconeis placentula and Aulacoseira subarctica, as well as a continuing presence of Cyclostephanos dubius, suggesting a deep open water lake that may have slightly elevated nutrient levels (such as total phosphorus). At c. 90 cm there is a switch in the diatom assemblage and zones PUR 3 - PUR 5 are dominated by the oligotraphentic and

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Figure 15. Lake Purrumbete dry land pollen stratigraphy

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Figure 16. Lake Purrumbete aquatic pollen stratigraphy

Figure 17. Lake Purrumbete diatom stratigraphy

oligosaline planktic form Discostella stelligera. At the lower boundary of PUR 6, Tabularia fasciculata increases at the expense of D. stelligera followed by an increase in the Chaetoceros: diatom ratio which is further followed by an increase in testate amoebae:diatom ratio. This may reflect early catchment disturbance driven by people or climate. There is little else to distinguish this record (evident in relatively constant DCA axis 1 scores), until zone PUR 7 and PUR 8. Here there is an increase in Chaetoceros and greater terrestrial input (revealed by testate amoebae). In the diatom flora, the eurytopic and eutraphentic epiphytic forms Cocconeis placentula and Rhoicosphenia abbreviata increase. The uppermost zone (PUR 8) has an increase in the brackish form Staurosirella pinnata and the first arrival of several species into the record including brackish water taxa Bacillaria paxillifer and Achnanthidium hauckianum.

The DCA axis 1 scores reveal the flora in the uppermost zone are clearly outside the historical range of variability represented by the 150 cm long core. Whilst the salinity evidence is weaker than that at Lake Colac, it is clear that the last decade has created unprecedented conditions. This lake, being deeper and less responsive in terms of salinity, is apparently still sensitive to climate-driven change.

Tower Hill Main Lake

Radiometric dating

Lead-210 Activity

Total 210Pb was effectively in equilibrium with the supporting 226Ra in all samples analysed, down to a depth of 52 cm (Figure 18a). Unsupported 210Pb concentrations were below the level of detection. Traces of fallout 137Cs were detected in the uppermost 4 cm (Figure 18b), but in all deeper samples concentrations of this radionuclide were close to, or below the level of detection (Table 7). As a consequence, it was not possible to date this core either by 210Pb or 137Cs. The almost complete absence of these fallout radionuclides could be due to the loss of surface sediments shortly before coring e.g. by erosion or slumping, or to dilution by extremely high sedimentation rates.

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Radiocarbon Dating

Four radiocarbon dates were obtained from Tower Hill Main Lake (Table 8), and despite the issues with 210Pb dating of the sediments, the radiocarbon analyses returned dates that were in clear stratigraphic order (Figure 19). The radiocarbon dating suggests a relatively uniform sedimentation rate in the lake over the last 4500 years. The major problem with the Tower Hill sediments is, essentially, the ‘floating’ chronology, as it cannot be confirmed whether the top of the sample is indeed representative of the most recent time period. It may be possible to relate observed changes in the pollen and diatom assemblages to other published studies, or knowledge of the timing of changes in regional pollen assemblages to help constrain the upper core sediments. Linear extrapolation from the uppermost radiocarbon date (50 cm) would show a continuation of a relatively uniform sedimentation rate to the top of the core. Sediment was not lost during the coring process (the upper sediments were collected undisturbed using a Piston corer), however, due to the shallow nature of Tower Hill Main Lake (c. 10 cm at the time of sampling), there is a possibility that wind stress on this system could have caused the resuspension and redeposition of the upper-most sediment.

Pollen The pollen stratigraphy from Tower Hill main Lake is displayed in Figures 20 and 21. Samples for the upper 62 cm of the piston core have been analysed at 1-2 cm intervals. The impact of Europeans is demonstrated clearly only in the top 4 cm with high values for the exotic taxa Pinus, Cupressus and Brassicaceae. Considering the extent of disturbance to Tower Hill that includes the complete deforestation of the structure and damming of the outlet to the lake mid nineteenth century, followed by extensive revegetation early in the 20th century, there appears to have been little sediment accumulation.

Table 7. Fallout radionuclide concentrations from Tower Hill Main Lake

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Table 8. Results of radiocarbon dating for Tower Hill Main Lake

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Figure 18. Fallout radionuclides from Tower Hill Main Lake

Figure 19. Age model for the Tower Hill Main Lake sediment cores derived from the 14C dates

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Figure 20. Tower Hill Main Lake dry land pollen stratigraphy

Figure 21. Tower Hill Main Lake aquatic pollen stratigraphy

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It is possible that either the shallow lake has dried periodically inhibiting accumulation or that deposited sediments have been redistributed within the lake or perhaps removed during high rainfall events. The lack of significant representation of exotics before this depth suggests also that the accumulation process has been reduced or interrupted during at least the whole period of European settlement. The gross impact of Europeans on the native vegetation is demonstrated by relatively minor changes in regional taxa Poaceae, Eucalyptus and Casuarina but major increases in the salt tolerant Chenopodiaceae and planktonic algae Botryococcus and Pediastrum. Increases in both salt tolerant taxa and the freshwater taxon Pediastrum, as well as the overall substantial increase in aquatics, is difficult to understand and demands further investigation. In the preceding 1000 years or so, there is little evidence of sustained changes in dry land pollen that reflects domination of the landscape by Eucalyptus and Casuarinaceae forest with a grassy understorey although there is some minor indication of cyclicity, especially in Eucalyptus. Percentages of aquatic taxa as a whole are very low and suggest that the Main Lake was extensive and was almost totally lacking in macrophytes and non-siliceous algae.

Diatoms The Tower Hill diatom stratigraphy for the upper 87 cm of the sediment sequence is presented in Figure 22. The diatom preservation in Tower Hill is generally very good (> 0.7) and the dominance of samples by a small number of species has allowed for a quick count of this upper core sequence. Sub- samples for diatom analysis have been taken at 1 cm intervals (in 2 mm sections) to produce a skeleton diagram, the remaining 3.5 m of sediment are currently being prepared by a research assistant.

Similar to other lakes, there are several striking changes in the Tower Hill diatom record. Seven diatom assemblage zones have been identified (TH 1- TH 7). The core is dominated by benthic diatom species, likely reflecting the littoral nature of the core site, and the saline tolerant Amphora veneta is present throughout most of the core. The lower sections of the core (TH1 and TH 2) are dominated by Sellaphora pupula, Tabularia fasciculata, Anomoeoneis sphaerophora and Navicella pusilla. S. pupula is indicative of alkaline conditions with a high mineral content, the co-occurrence of N. pusilla supports an interpretation for high pH and saline conditions; TH 1 also has an abundance of Chaetoceros muelleri, suggesting a period of higher salinity and/or nutrient concentrations. In TH 3 there is a decline in T. fasciculata and N. pusilla and an increase in S. pupula, with T. fasciculata increasing through TH 4. Zone TH 4 also has testate amoeba scales present. Zones TH 5 and TH 6 are dominated by S. pupula and A. sphaerophora with increasing numbers of Amphora copulata. Navicella pusilla peaks and then declines throughout TH 6. The largest change in the diatom stratigraphy is observed in the transition from TH 6 to TH 7, where there is a major switch in the dominant species. The alkaline (and saline) tolerant Sellaphora pupula, A. sphaerophora and N. pusilla decline and almost disappear from the record entirely and these species are replaced by saline tolerant Opephora sp., Staurosira elliptica and Planothidium delicatulum. Chaetoceros muelleri also returns to the sediment record. The exact nature of this shift is unknown, but it may be that brine type or a change in the ionic concentrations of the lake waters may be causing this observed switch in the dominant species.

Statistical analysis of the changes in the diatom assemblage through time (DCA axis 1) suggests that there is a major change in the diatom assemblages in the zone TH 7 to diatom assemblages that are very different to the average diatom assemblage and this is likely a result of ‘current’ lake conditions (< 30 years) being unprecedented when compared to the palaeo-record.

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Figure 22. Tower Hill Main Lake diatom stratigraphy

Lake Modewarre

Radiometric dating

Lead-210 Activity

Total 210Pb activity significantly exceeded that of the supporting 226Ra only in the top 5 cm of the core (Figure 23a). The highest unsupported 210Pb concentration (100 Bq kg-1) occurred below the surface of the core, in the 1-1.4 cm sample (Figure 23b). Below this there was a slight decline, down to 72 Bq kg-1, in the 4-4.4 cm sample. In the deeper samples this concentrations fell abruptly to levels close to the limit of detection, suggesting a hiatus in the sediment record at around 5 cm (Table 9).

Artificial Fallout Radionuclides

Caesium-137 concentrations (Figure 23c) were low in all samples measured. The highest concentration occurred in the 4-4.4 cm sample, suggesting that sediments from this depth post-date the period of maximum fallout from the atmospheric testing of nuclear weapons in the early 1960s. The abrupt decline in 137Cs activity below 5 cm supports the suggestion from the 210Pb results of a hiatus in the sediment record at this depth.

Core Chronology

Although the abbreviated 210Pb record makes the dating of this core highly problematic, since the 210Pb inventory of the core (2488 Bq m-2) corresponds to a mean 210Pb supply rate (78 Bq m-2 y-1) greater than the estimated atmospheric flux. However, it does appear that sediments at the core site have been exposed to significant levels of atmospheric fallout and that the poor radiometric records are due to sedimentological factors. Simple application of the standard CRS and CIC dating models (Appleby -2 -1 and Oldfield, 1978) suggests a mean sedimentation rate of between 0.05-0.06 g cm y (0.07-0.08 cm y-1). Assuming a mid-1960s date for the maximum 137Cs concentration at 4-4.4 cm suggests a mean accumulation rate of around 0.073 g cm-2 y-1 (0.09 cm y-1). In the absence of better information, a reasonable estimate of the mean sedimentation rate is 0.065 g cm-2 y-1 (0.08 cm y-1). A chronology

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based on this result, given in Table 10, dates the hiatus at 5 cm to the early 1950s. In view of the poor nature of the fallout records, these dates should not be regarded as reliable unless supported by other evidence.

Radiocarbon Dating

Four radiocarbon dates were obtained for the Lake Modewarre core (Table 11), all of which fall in stratigraphic order (Figure 24). The dates do not suggest a linear sedimentation rate through time, but rather are indicative of at least one hiatus between 50 cm and 100 cm; the change in the angle of slope between the dates obtained at this depth suggest extremely low sedimentation rates between 2000- 6000 years before present. Given the problem of a potential hiatus in the 210Pb record during the last 60 years, it would not be unreasonable to assume that Lake Modewarre is prone to desiccation. This may also be attested to in the lack of well-preserved diatoms within the core.

Pollen Samples were prepared from two adjacent 2mm sediment slices at 2 cm intervals through the whole of the 72 cm piston core (Figures 25 and 26). Sampling was not extended into deeper sediments of the D- section core because pollen was becoming sparse and the sediment sandy. Through most of the record, that may extend to about 2000 years BP, the regional vegetation was composed of Eucalyptus and Casuarinaceae woodland or forest with a grassy and herbaceous ground cover. Through the period there was a gradual increase in Eucalyptus relative to Casuarinaceae until the topmost zone, LMP6,

Table 9. Fallout radionuclide concentrations from Lake Modewarre

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Table 10. 210Pb chronology of the Lake Modewarre sediment core

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Table 11. Results of radiocarbon dating for Lake Modewarre

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Figure 23. Fallout radionuclides from Lake Modewarre

Figure 24. Age model for the Lake Modewarre sediment cores derived from the 210Pb and 14C dates

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Figure 25. Lake Modewarre dry land pollen stratigraphy

Figure 26. Lake Modewarre aquatic pollen stratigraphy

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when percentages of both taxa collapsed, with Casuarinaceae virtually absent. Asteraceae Tubuliflorae values were also substantially reduced, leaving grassland totally dominant. The proposed date for this event is about 1950 and, judging from the concomitant sharp and sustained increases in the introduced trees Pinus and Cupressaceae, it marks the establishment of the present landscape. The previous zone, LMP-5, was also influenced by European settlement with a consistent presence of the three main exotics including an early peak of the weed Plantago that appears to have also replaced the native species of the genus. The notable peak in Pteridium, and also that of Asteraceae Liguliflorae, that extends into the surface zone, demonstrate also disturbance within the native vegetation that has been reduced to the present day, presumably with improved landscape management. Stability may also be suggested by very low fire activity in recent years.

Apart from the two topmost zones, the most distinctive zone in the dry land diagram is the basal LMP- 1 with very high values of Chenopodiaceae and significant representation of Apiaceae, Asteraceae Liguliflorae and a variety of ferns including tree ferns. The Chenopodiaceae representation suggests a very local presence on the swamp surface under dry saline conditions and this could be supported by the presence of Myriophyllum muelleri on the aquatic diagram and by deposition of more inorganic sediments and evidence for pollen corrosion. A disturbed stratigraphy is suggested by the ferns, Asteraceae Liguliflorae, Anthoceros and relatively high charcoal values that peak just above the zone.

Both dry land and aquatic diagrams suggest ameliorated conditions in LMP-2, dated to sometime between about 700 and 900 years ago with the near absence of saline indicators and together with good representation of the freshwater alga Pediastrum. After an intermediate phase, saline conditions on the swamp are marked in sub-zone LMP-3b by highest values of Myriophyllum muelleri for the record and the consistent presence of the equally saline taxon, Ruppia. Wetter conditions then appear to persist until the recent drought with a substantial value for Myriophyllum muelleri in the topmost sample. One unusual feature of the record is the almost total absence of any other Myriophyllum species except for high values during the early part of European occupation.

Macrofossils Macrofossils are recorded throughout much of this record, at least sporadically. Although there are insufficient records to give an indication of abundance of pollen taxa, there is good support for times of occurrence, and identity, of some pollen taxa including the Myriophyllum species, Ruppia and Chenopodiaceae while it is notable that both Ruppia and Lepilaena are represented by more than one species (Figure 27). The identity of the Chenopodiaceae species confirms the existence of local saltmarsh. There is greatest representation of the palynologically invisible macroalgae Nitella and Chara whose abundance is, at least occasionally, greatest in the lower half of the record. There is a similar representation of Daphnia spp. and, together, they suggest moderately fresh to brackish water where salinity levels were no greater than about 6% during this period. Also largely restricted to the lower half of the core are macrocharcoal particles, a pattern providing general support to the microfossil record of higher values at the base than the top. Lowest values are during the period of European occupation, suggesting that fire has never been a component of landscape management.

Diatoms Contiguous sub-samples for diatom analysis were taken at centimetre intervals (in 2 mm sections) for the top 70 cm. Samples at centimetre intervals are also being prepared for analysis over the remaining 2 m of core. Initially a skeleton outline of the diatom stratigraphy is adopted in the first instance. This allows an overview of the major changes in the core sequence, and then interesting areas of the core can be targeted for higher-resolution analyses (2 mm intervals). Once dating is obtained, the last 2000 years will be counted at 2 mm intervals in a similar fashion to Colac and Purrumbete.

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Figure 27. Lake Modewarre macrofossil record

Figure 28. Lake Modewarre diatom stratigraphy

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Unlike the other lake sediment records, Lake Modewarre has proved challenging for diatom analysis. There are many obvious sedimentary changes in the cores extracted from Lake Modewarre, but there is a slight problem with preservation issues of the diatom frustules in the top 70 cm of the core (F index <0.5). The poor preservation of diatom frustules is increasing the counting time per sample (only 2 slides per day as opposed to 4 [Colac] or 6 [Tower Hill]). Never the less, the diatoms that are in the samples can still be identified to species level, so interpretation of many of the changes will not be hindered. Similarly the issue of preservation itself contains a wealth of environmental and climate information that will add to the interpretation of the diatom record.

Thirteen samples have been counted from Modewarre (from the top 70 cm). It is anticipated that the preservation of the diatoms will improve down core, with many of the preservation issues observed in the upper record being related to recent drought and water level fluctuations.

A preliminary diatom stratigraphy from Lake Modewarre is shown in Figure 28. Whilst there are a limited number of samples from this sequence, there are several clear changes in the diatom stratigraphy. So far five assemblage zones have been identified, encompassing the major changes in the diatom flora.

The earlier part of the record (60-22 cm; MOD 1-MOD 3) contains samples dominated by Epithemia adnata, a species with a preference for alkaline waters and higher total phosphorus (TP) concentrations. The occurrence of Campylodiscus sp. in zone MOD 2 suggests a period of higher salinity (52 cm). There is a major switch in the diatom assemblage in the upper 20 cm, where E. adnata almost disappears from the record and is replaced by Cocconeis placentula and Amphora copulata. C. placentula is a widespread species that can occur in multiple habitat types and is not an ideal indicator species; however, its abundance may suggest unstable or fluctuating lake conditions which have allowed this species to thrive, especially in terms of fluctuating salinity as C. placentula can tolerate fresh to saline conditions. The latter species, A. copulata is a widespread species with a much lower TP optima than E. adnata. The appearance of Chaetoceros muelleri and Actinocyclus sp. in MOD 4 and MOD 5 suggests a transition to much more saline conditions. This is also reflected in the diatom-inferred conductivity which increases through these upper zones.

Statistical analysis of the changes in the diatom assemblage through time (DCA axis 1) suggests that, even with limited samples, that there is a major change in the diatom assemblages in the upper zones to lake conditions that are unprecedented in the palaeo-record.

Lake Burn

Radiometric dating

Lead-210 Activity

Lake Burn exhibits perhaps the best and most detailed 210Pb record of all of the sites analysed during this research. Total 210Pb activity still exceeded that of the supporting 226Ra at the base of the core (Figure 29a), limiting the period of time spanned by the core to no more than around 3-4 210Pb half- lives (66-88 years). Although unsupported 210Pb concentrations (calculated by subtracting 226Ra activities from the total 210Pb concentrations) vary irregularly with depth (Figure 29b), above 17 cm the overall trend is more or less exponential, suggesting no systematic change in sedimentation rate in recent decades. Non-monotonic features at a depth of 14.2 cm, and again in the surficial sample, may however indicate brief episodes of more rapid accumulation. The relatively flat trend in unsupported 210Pb concentrations between 17-25 cm may indicate a significantly different regime during this earlier period, though the absence of a 210Pb record below 25 cm makes any estimation of the sedimentation rate for this period highly problematic (Table 12).

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Figure 29. Fallout radionuclides from Lake Burn

Figure 30. Radiometric chronology of the Lake Burn sediment core showng the CRS model

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Artificial Fallout Radionuclides

The 137Cs activity versus depth record (Figure 29c) has a relatively well-resolved peak between 16- 20.4 cm that almost certainly records the 1964 fallout maximum from the atmospheric testing of nuclear weapons.

Core Chronology

210Pb dates calculated using the CRS dating model (Appleby and Oldfield 1978) place 1964 at a depth of 17.2 cm, in relatively good agreement with the depth suggested by the 137Cs record. Since the 1964 depth is most probably between 18-20.4 cm, a small correction to the 210Pb dates has been made to reflect this. The results, shown in Figure 30 and given in detail in Table 13, suggest that, apart from two brief episodes of rapid sedimentation, in the early 1970s and again in the past year or so, accumulation has been relatively uniform since at least the 1950s. Excluding the episodes of rapid -2 -1 sedimentation, the mean sedimentation rate during this period is calculated to be 0.11 ± 0.02 g cm y -1 (0.35 cm y ). For the period prior to 1950 the raw CRS model calculations suggest a substantially -2 -1 -1 210 lower sedimentation rate of around 0.06 g cm y (0.19 cm y ), though in the absence of the Pb record below 25 cm this figure is subject to a large level of uncertainty.

Pollen Pollen and charcoal were analysed from 2mm thick samples every two centimetres along the 63 m piston core and results are presented in dry land and aquatic pollen diagrams Figures 31 and 32). It appears that the sediment accumulation rate has been substantially higher than at other sites, although, as a consequence, the temporal record is very short, it does provide detail of changes that have taken place since around or slightly before the time of arrival of Europeans. The basal zone LBP-1 indicates a regional vegetation dominated by Eucalyptus woodland with a herbaceous understory, a shallow, probably brackish open water lake with submerged Lepilaena and some Ruppia and abundant planktonic Botryococcus and surrounded by extensive salt marsh. Fire may have been a regular feature.

Plantago was the first introduced plant recorded, at the base of zone LBP-2 dating to about 1850 from extrapolation of the lead210 ages, and appears have completely replaced native Plantago within about 40 years. This initial human impact seems also to have included the removal of both Eucalyptus and Casuarinaceae resulting in a more open canopied vegetation or the expansion of grassland. There are also marked changes in aquatic vegetation at the LBP-1/LBP2 boundary including sustained declines in Lepilaena and Botryococcus and a sharp increase in Rumex that seem likely related to human impact although a causal link is difficult to determine. There is also the beginning of a terminal decline in local salt marsh.

Zone LBP-2 represents the period between initial European impact and establishment of the current landscape around 1930 AD. It is marked by a high degree of variability including several peaks in charcoal, increased levels of Pteridium, initial representation of the exotic trees Pinus and Cupressaceae about one third and two thirds way through the zone respectively, and extreme values for the aquatics Lepilaena and Ruppia. Perhaps burning and clearing for agriculture resulted in an increase in landscape disturbance that altered nutrient status of the lake system.

There has been a great deal of stability in the landscape since the LBP-2/LBP-3 boundary with consistently values for most dry land taxa except for the clear dominant Poaceae, low charcoal levels and consistently low values also for most aquatic taxa except Ruppia that may indicate relatively saline lake conditions over the last 60 years or so. Despite the high resolution nature of the record,

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there is no evidence of any significant response to the recent decadal-scale drought suggesting perhaps a long term adaptation and short term insensitivity to such an event.

Table 12. Fallout radionuclide concentrations from Lake Burn

Table 13. 210Pb chronology of the Lake Burn sediment core

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Figure 31. Lake Burn dry land pollen stratigraphy

Figure 32. Lake Burn aquatic pollen stratigraphy

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Figure 33. Lake Burn macrofossil record

Macrofossils The macrofossil data, derived from the pollen sample residues, are presented in Figure 34. Within the macrophytes only Lepilaena and Characeae record above trace values, but Daphnia, Chironomids and macrocharcoal are well represented. High values for Lepilaena and Daphnia in the basal zone support the pollen evidence for a well-developed aquatic macrophytic vegetation. Macrofossils also add to the diversity of aquatic taxa and support to some extent the nature and pattern of representation of salt marsh and the fire regime derived from the pollen records.

Diatoms Preliminary diatom analysis suggests that preservation in a large part of the Lake Burn core is extremely poor and that where diatoms are preserved they are dominated by a single saline-tolerant species (Pinnuavis elegans). This may suggest that this lake is either unresponsive to fluctuations in water levels, or, due to the naturally brackish water in the system, that the lake system has not undergone a significant shift in salinity to force a change in the diatom flora. The response observed in Lake Burn may also be indicative of a lake system that is mediated by the input of brackish-saline groundwater, again accounting for the relatively unresponsive nature of this site.

Lake Rosine Lake Rosine was the sixth lake targeted for this research. Preliminary diatom analyses suggest extremely poor preservation of the diatom valves, and the site was not considered appropriate for further investigation.

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Discussion

Lake chronologies Lake-based chronologies of hydrological and climate changes in Australia continue to rely upon a combination of 210Pb and 14C dating. However a number of problems have been encountered using both techniques. In Australia the atmospheric fallout of 210Pb is much lower than in the northern hemisphere, with surface concentrations c. 60-90 Bq kg-1 (contrast to >400 Bq kg-1 in African lake sediment; Ssemmanda et al., 2005). Generally the 210Pb obtained do not contradict the pollen evidence for the arrival of Europeans in the sediment record (represented by the first appearance of Pinus pollen, c. AD 1880), though the exact age which should be assigned to this first appearance is under debate. The major problem associated with the 210Pb in this study is the extremely low concentrations, which means chronologies can only be calculated for the last 50-80 years (again contrast to c. 200 years in the northern hemisphere). Almost all of the 210Pb data showed a decline with depth. The most problematic cores were those obtained from Lake Modewarre and Tower Hill Main Lake. Modewarre had an extremely poor 210Pb record likely as a result of an hiatus in the core record around 40 years ago. In terms of Tower Hill, it is uncertain as to what happened to the 210Pb record, as 210Pb was absent in any appreciable quantities in the upper section of the core, but was present in small amounts all the way through the core (to 40 cm). Studies have shown that in some sediment profiles, 210Pb can be extremely mobile and can move downcore. However, further analyses of deeper core samples did not confirm this. Another likely explanation is the loss of the uppermost sediment profile (top 20-30 cm). Whilst this was not lost during the coring process (as the upper sediments were collected using a piston corer to prevent loss of the flocculent sediment-water interface) it is probable that these sediments have been ‘lost’ in situ, either through redeposition due to low lake levels, or the highly flocculent nature of the sediment and wind stress. It was noted at the time of core collection, that the darker sediments that were overlying the lighter sediments appeared to have ‘shrunk’ in size since the system was last cored in the 1980s and early 2000s.

One of the most pertinent problems in the radiocarbon dating of Australian lakes is the presence of a radiocarbon reservoir age associated with the dating of bulk material (Barton and Polach, 1980; Barton and Barbetti, 1982). Terrestrial macrofossils tend to yield more reliable ages as the plants 14 obtain their C from atmospheric CO2 (Verschuren, 2003). The dating of bulk sediments in some of the larger, and smaller, (crater) lakes in Australia has proved problematic. In many instances, bulk sedimentary material may contain considerable 14C from aquatic algae, which can overestimate the true age of the sediment. Aquatic algae derive their 14C from the dissolved inorganic carbon (DIC) from the lake water, and, in many closed-basin lakes, the long residence time can cause a reduction in the 14C/12C ratio relative to the atmosphere (Verschuren, 2003).

Terrestrial macrofossils can themselves produce problems with radiocarbon dating. For example, there may be significant delays in the burial of large terrestrial plant remains in offshore lake sediments due to the retention of these macrofossils in soils or near shore sediments. Furthermore, wood remains can survive for many years (decades; even centuries) before being delivered to the burial site. In this study, the majority of rejected dates were either charcoal (charred wood) or large wood, which, in the majority of cases, appeared to produce erroneously older ages. The exclusion of these dates is on the basis that these large wood fragments (>5 mm) may have been resident in the catchment (e.g. trapped in littoral vegetation) for a long period before being washed into the lake. Furthermore, some of the charcoal may have been produced through the incomplete burning of older wood material, thus producing an older age in relatively young sediments.

The chronological integrity of the lake sediments records presented here as archives of climate history is somewhat constrained by the analytical uncertainty associated with 14C-based age determination

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Figure 34. Relationship between radiocarbon and calendar ages (De Vries effect)

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and the additional uncertainty due to the occurrence of non-unique calendar ages (cf. Verschuren et al, 2000). The calculation of the calendar ages is particularly problematic during the last 1000 years due to the ‘de Vries effect’, which causes several plateaus as well as age reversals in the radiocarbon calibration curve (Figure 35). This results in multiple calibrated ages for single samples, many with almost equal probabilities. The ‘de Vries effect’ is a natural phenomenon often linked to variations in sunspot activity, which can cause problems with the precision of calibrated radiocarbon dates from AD 1450 to AD 1950 (Stuiver and Becker, 1993; Figure 35). There are two main perturbations (“de Vries effect”) in the 14C time scale during this period. The first is centred on 150 14C yr BP (a, on Figure 35), with a calibrated range interval of 285 years (AD 1665-1955 at two sigma error). The second is centred on 350 14C yr BP (b, on Figure 34, with a two sigma range of 200 years (AD 1450- 1600; Stuiver and Becker, 1993). In some instances these perturbations can cause limitations when resolving the actual ages of the sediments.

The use of exotic tree pollen can, and will continue to provide some of the best assessment of the accuracy of radiometric dating in many of these lake systems. The use of pollen analysis of introduced plant species in combination with a number of radiometric techniques has not only improved the value of this technique in chronological studies, but has also highlighted the absolute need for all future studies to consider the use of exotic pollen for dating purposes.

Coherence of signals between lakes - A regional synthesis Climate and environmental (e.g. catchment) changes are external drivers of lake dynamics; however uniform changes in climate across a region can produce a variety of responses in lake ecosystems. This is primarily due to the way in which these different ecosystems filter these signals and alter their expression (Magnuson et al., 2004). Understanding the temporal coherence of lakes at various spatial scales will provide insight into the factors influencing lake dynamics.

Sediments from a range of lakes in the landscape provide an ideal set of temporal and spatial scales to study climate change and lake ecosystems (Magnuson et al., 2004). The ability to simultaneously understand time (such as long-term dynamics) and space (a number of lakes with differing characteristics across a landscape) is becoming increasingly important in (palaeo) limnological studies (cf. Magnuson et al. 2004). This is a particularly pertinent approach when realising that not all lakes respond to external (e.g. climate) forcing in a similar way. Whilst climate change has been shown, in the broadest sense, to manifest as general trends across regions and continents (and beyond), these are only really apparent and addressed in long-term (e.g. millennial) studies of lake sediments.

All of the lakes analysed in this study were located within a narrow belt (c. 200 km stretch from west to east) and thus in close proximity to each other and were located within a similar geological and climatic regimes. The sedimentary records from all lakes highlight several differences in their functioning, which might be expected given the range of lake and catchment morphologies, size, and most likely, differing land use histories (cf. Magnuson et al., 2004; Ssemmanda et al., 2005; Ryves et al., 2011). However, despite this, there are several commonalities between the lakes in terms of the diatom assemblage zones. Most of the lakes differ in their diatom assemblages and response, yet independent zoning of the diagrams shows several significant and regionally important time zones (Figure 35).

The diatom records from the four cores analysed for this research demonstrate, in many instances, a remarkably consistent pattern of change in the sediment stratigraphy over the last 2000 years. Figure 36 also highlights the first appearance of Pinus in the cores and, it highlights how this corresponds to the ages of the sediments as assigned by extrapolation of the 210Pb dates. This draws attention to the problems associated with the dating of these sediments, but also some of the uncertainty as to the exact timing of Pinus appearance in the sedimentary record. It can be observed that immediately following the appearance of pine pollen in the sediments there is a seemingly immediate response noted in the diatom flora, with all sites showing a distinct change in the diatom assemblages. This may

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Figure 35. Diatom chronozones from the 4 lakes from western Victoria

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Figure 36. Responses of a lake system to an environmental driver

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attest to the first signal of European disturbance at each of these sites, especially in terms of physical effects on the water chemistry. The reason for the variation in system responses to shared external forcing almost certainly arises from the complexity of lake ecosystems in their response to these drivers. For example, a model presented by Magnuson et al. (2004) suggests that lakes have several levels of filters that allow them to respond uniquely. A lake’s response to external forcing is governed by factors such as its morphometry, chemistry, local hydrology (e.g. groundwater) and ecology. The model outlined by Magnuson et al. (2004) suggests that lakes may amplify, attenuate, delay or extend the climate signal (Figure 36).

With specific reference to the lakes used in this study, the morphometry and size of the lake basin is almost certainly one of the largest factors controlling the response of the lake ecosystem. Lakes Colac and Modewarre have very large, flat, shallow basins. The littoral and benthic zone is extremely important throughout the sedimentary record, given the shallow nature of the lake basin. Similarly, the littoral/benthic zone is the most dominant habitat in Tower Hill Main Lake, which again has a rather gently sloping catchment. Lake Purrumbete is seemingly the outlier when compared to the other 3 lakes, as this is the deepest site at 42 m. Although coring was undertaken in the littoral area (in 6 metres of water), the taxa present in the core are representative of the entire lake, with many planktonic species incorporated into (and in several cases, dominating) the sedimentary record. As a result of the Purrumbete being deeper, the system appears less responsive in terms of the number of switches in the diatom assemblages when compared to the three shallower lake systems. The depth, and continuing presence, of water at Purrumbete has likely buffered this system against some of the smaller climatic fluctuations.

All of the shallower lake systems share a large number of zone boundaries, suggesting that many of the changes observed in the records may well be as a result of broader-scale atmospheric forcing as opposed to localised catchment changes, especially in the sediments that pre-date the late 1770s. The most striking change in all of the sediment sequences occurs in the last 20-30 years, where there is a distinct change in the observed diatom assemblages (Figure 37). This change is also reflected in the ordination data from each site that shows a distinct decline in values towards the top of each core. The changes observed in each site are concomitant with the observed decline in rainfall across western Victoria over the last 30 years, and suggests that the current drought is extremely unusual in the 2000 year records from each site. This use of a regional, multi-lake study therefore allows the identification of regional climate events, from local-scale, in-lake processes.

This research aimed to document drought history in western Victoria through the undertaking of high- resolution analyses of pollen and diatoms from a series of lake sediment sequences. Comparing the coherence of the signals between the lakes analysed in this research was a straightforward undertaking when contrasted with comparing the high-resolution sequences obtained here to sequences that already exist in western Victoria. The comparison of the results obtained here to other studies is hampered somewhat by the lack of high-resolution records from the region. To try and draw regional conclusions, the results from the lakes included in this study were compared to the key climate site in western Victoria, and most well know lake level curve from Lake Keilambete.

In order to draw comparison, the reconstructed salinities from each of the four sites were used to compare to the lake level curve (Figure 38). The Lake Keilambete curve was derived from a series of sedimentological proxies, and whilst it would be useful to compare lake level curves from the 4 lakes in this study, the construction of lake level curves using diatom analysis can be complex, especially in areas that have undergone catchment changes as a result of human impacts within the lake catchment. The traditional method of constructing lake levels from diatom records involved compiling a ratio of planktonic vs. benthic species. This can be problematic as some, typically planktonic, diatoms can thrive in very low water levels if the nutrient levels are slightly elevated (as can be the case with human impacts in lake catchments), leading to an erroneously high lake level inference. The conductivity curves were taken to avoid this complication, though these records are not without their own problems (see below). The ability to compare a lake level curve to conductivity curves comes

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Figure 37. Regional trends in change across western Victoria over the last 2000 years

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Figure 38. Comparison of diatom-inferred conductivity over the last 2000 to the Lake Keilambete curve

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from the basic understanding that as lakes dry out, the water level is lowered and the salts evaporatively concentrate in the system, thereby raising the conductivity; as rainfall increases and lakes fill, they become more dilute (fresher).

The conductivity reconstructions from the 4 lakes and in comparison to Lake Keilambete, do, in the main, exhibit similar changes. There are three key time periods that appear in the data: AD 550-700, AD 1300-1500 and AD 1900-present. In the two earlier episodes all lakes exhibit changes in conductivity and lake level that suggest regional drying events may be responsible for the observed fluctuations. In fact, shallow Lake Colac and, surprisingly, the deep Lake Purrumbete provide some of the best evidence for a long-lived, pre-European drought event in south-eastern Australia; tentatively dated to a period concomitant with the onset of the Little Ice Age in the northern hemisphere. It has been suggested that in south-eastern Australia, the Little Ice Age would have manifested as a persistent El Niño phase (Turney and Palmer, 2007), causing drought conditions across the region.

There are a number of discrepancies between the records presented here, and indeed across south- eastern Australia. This highlights the clear need for additional research from the southern hemisphere, especially at high-resolution. Increasing the number of high-resolution studies from the Australasian region would allow the processes and drivers of change in the region to be identified (Turney et al., 2006), aiding future prediction of climate change and its impacts on these wetland systems.

Drought frequency, intensity and duration - The recent drought. Without precedent? Droughts have serious and detrimental effects on not only the environment, but also the society and economy of the region. The impact of the ‘Big Dry’ on south-eastern Australia is known to be unprecedented in the historical past (Murphy and Timbal, 2008). It has been well documented that previous droughts that have occurred during European settlement have been of a similar duration (e.g. 1936-1945 drought), however, the most recent drought has been of a greater intensity and is linked to rising temperatures over recent decades (Figure 3; Nichols, 2004; Ummenhoffer et al., 2009). In terms of Australia’s modern climatic setting, droughts are not unusual and are a natural characteristic of climate variability, however under a future of global warming, these droughts are likely to become more frequent, of longer duration and of greater intensity into the future (IPCC, 2007; Mpelasoka et al., 2008).

The high-resolution nature of the records from climatically sensitive lakes in western Victoria allow the most recent drought to be placed into a context that extends beyond European historical records and offers a much longer-term perspective of natural climate variability. It also allows the recent drought to be assessed in terms of its length and severity when compared to similar events in the last 2000 years. To try and understand drought history in western Victoria, the diatom record from each lake is presented as a deviation from the average conductivity of the entire record (Figure 39).

There are a number of problems associated with the data, which must be approached with caution. Primarily, given the discussion above, the chronologies of each core would need refining, with a larger number of dates obtained on each sediment core in order to be able to derive precise duration of past droughts; in addition to this there is the need for those samples that are dated to have been derived from terrestrial macrofossils rather than bulk sediment analyses. As a result of potential chronological issues, it may seem that some events are ‘mismatched’ but this could purely be a function of chronological control.

A second problem, which is clear in at least 2 of the four sites raises issues with regards to the use of diatom-conductivity models when exploring past environmental changes. The two records that highlight potential problems are Lake Purrumbete and Lake Modewarre. Lake Purrumbete shows an apparent freshening of the lake system in the last 30 years, however, it is know from monitoring data

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Figure 39. Deviation from the average diatom-inferred conductivity of the four lakes analysed in this research.

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that the lake itself has seen a decline in lake level and a change in the conductivity of the waters. This issue highlights some of the inherent problems with the use of diatom-based transfer functions when inferring past changes in water chemistry, and this will largely depend on the origin of the dataset. In this case, a large, combined modern training set was used to compare to the fossil diatom data and in some cases the water chemistry and habitat preferences of the diatoms included in the modern training set may not accurately reflect those in the fossil assemblage. This is especially true in Lake Purrumbete where the upper samples are dominated by an epiphytic diatom species (such as Cocconeis placentula) which has a broad tolerance for a range of salinities from fresh to saline – however, the presence of this species is clearly indicative of lower lake levels as the species habitat preference is attachment to submerged macrophytes. One of the other most commonly encountered issues with transfer functions is the exclusion of some diatom species from the modern data together. In the case of Lake Purrumbete the extremely saline species noted in the top 5 cm of the core (e.g. Fragilaria sopatensis and Opephora sp.) are not present in many inland lake datasets as they are species most often associated with coastal systems (Witkowski et al., 2000) as opposed to a typical inland freshwater or brackish system. With the removal of these species in the modelling, preference for reconstruction is given to other abundant species (a number of the Fragilariaceae group) which have relatively undefined habitat preferences and may have forced the data to show an apparent forcing to freshwater when all other qualitative evidence shows an increase in conductivity and a decrease in water depth. This is also apparent between Ad 1300 and 1900 when the system is dominated by Tabularia fasciculata that is given a high conductivity preference in the training set used, yet most other lakes exhibit a freshening of the lake systems at this time.

The second site, Lake Modewarre shows an almost consistently highly-saline lake system, with all conductivity deviations sitting above the sediment record average. In the case of this system, there are a limited number of samples included in the analyses as a result of poor diatom-valve preservation. This has most likely biased the remaining dataset to large, heavily silicified diatoms that were able to preserve under highly-alkaline conditions and low silica concentrations.

Despite some minor problems, all of the records do show a long history of drought. Most of the diatom records, and indeed the pollen records suggest that there have been dry periods every 500 or 1000 years. However, statistical analyses do not suggest such past events were as intense as the present conditions (Figure 39). It is important to note that every event in the sediment record (both within and between lakes) has a distinct signature recorded in the sediment archive. There is no clear single feature, or even a combination of signatures, that characterises these wet or dry phases. Within a lake, this difference occurs because no two climate events are ever the same, and small differences in drivers (change in temperature versus wind stress for example) would manifest as a variety of ecological responses. In addition to this, the antecedent conditions within the system would also define the response of the system to a future change. Differences in responses between lakes will largely depend on the characteristics of the lake in question: its depth, catchment size and water chemistry at the time of the event. Some systems may be particularly resilient to some drought events (such as the deep Lake Purrumbete, which would be buffered from major changes in water chemistry as a direct result of the depth of the system).

The response of a lake systems to any given drought event, in the past or those in the future will largely depend on the condition of that system. The more stressed and disturbed a lake is by current climate and human impacts, the more severely this will impact on its ability to cope with extra or enhanced stressors. Similarly this is something that must be taken into consideration when compiling management plans for many lake systems; a single agenda will not suit every lake system and serious consideration must be given to current and past events and responses in order to manage these fragile ecosystems into the future, and the role of palaeoecological analyses in these circumstances provides an excellent, and unrivalled, tool in management planning.

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Recommendations

Agricultural producers, land managers and policy-makers should read from the report that there is considerable, independent support for claims from instrumental data that the climate in southern Australia is changing. Evidence from the large lakes shows that the drying of the regional climate has its origins over 100 years ago. However, given the long term context provided herein, they should recognise that since European settlement there has been a significant, additional driver of climate that has taken water balance deficit to extreme levels at the five thousand year scale, into unprecedented states. They should further recognise that landscape development has had an opportunity cost in the degradation of wetland condition to new states, the need to ‘tread more lightly’ on the landscape and to consider participation in LandCare and other restoration instruments. They should also recognise the risk to ‘business-as-usual’ practices in the form of the trajectory of a drying climate and embark on diversifying their practises and economy to boost resilience in an uncertain climatic and economic future. This could include investing in activities that bring returns from carbon and biodiversity funding opportunities and by diversifying into primary products that continue to provide income in future droughts.

Aside from the lessons identified above, policy makers should recognise the latitudinal shift in rainfall zones with climate change and quarantine zones of high effective rainfall for high return agricultural activities. They should implement structural adjustment support for industry to diversify practises and make subsidies and support to restore landscapes more attractive and accessible. Policy makers should continue to support modelling of the impacts of climate scenarios at a regional level and continue to support extension activities in the regions to encourage the rural sector to be best prepared for change and to more actively care for the country. Policy makers should also recognise that the combined impact of direct catchment change and a drying climate have degraded many, if not all, wetlands; that their resilience has been lost, and that trajectories of change suggest that this will continue to happen. Currently investment in the protection and restoration of wetlands remains a priority and is probably, at present, insufficient to match the challenge.

To date $276,000 has been received to run this project. The researching institutions have invested in key research staff to undertake field and laboratory work, statistical analyses and activities directed at outcome adoption. This has comprised a full time Research Associate, two part-time Research Associates and casual field and laboratory assistants. It includes the in-kind contribution of the time of two research Professors to direct the research and lead fieldwork. There has been a considerable investment in sediment dating using internationally recognised laboratories in the U.K. and N.Z. Interpretation and adoption has been facilitated by support to attend and present at conferences, workshops and seminars including the hosting the inaugural Victorian Centre for Climate Change Adaptation Research Think Tank in Horsham under the theme of Adapting to Climate Change in Drylands as well as hosting the Palaeoclimates Relevant to NRM in the MDB and reporting to the Murray Darling Basin Authority.

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77 The Recent Victorian Drought and its Impact - Without precedent? By Keely Mills, Peter Gell and Peter Kershaw Pub. No. 12/040

In line with many parts of eastern Australia, Western Victoria has been suffering from a prolonged dry spell that has had economic, social and environmental impacts. However, it is uncertain whether such an event is a natural component of long term, natural, climatic variability or whether it has been brought on, or exacerbated by, regional European land use or by human induced global warming.

This research has sought out the lakes that are most likely to be responsive to the past variations in effective moisture, such as past drought events, and analysed them in the highest practical resolution allowing regional changes in rainfall to be inferred, as well as assessing the resilience of many of our wetland ecosystems to future climate stress.

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