Hydrobiologia DOI 10.1007/s10750-012-1163-4 CLIMATE CHANGE AND AUSTRALIAN WETLANDS Review Paper A legacy of climate and catchment change: the real challenge for wetland management Peter Gell • Keely Mills • Rosie Grundell Received: 6 December 2011 / Accepted: 6 May 2012 Ó Springer Science+Business Media B.V. 2012 Abstract Wetland managers are faced with an array capacity to recover from the recent ‘big dry’. These of challenges when restoring ecosystems at risk from sedimentary archives reveal most modern wetlands to be changing climate and human impacts, especially as many outside their historical range of variability. This approach of these processes have been operating over decadal– provides a longer-term context when assessing wetland millennial timescales. Variations in the level and salinity condition and better establishes the restoration challenge of the large crater lakes of western Victoria, as revealed posed by the impact of climate change and variability and over millennia by the physical, chemical and biological human impacts. evidence archived in sediments, attest to extended periods of positive rainfall balance and others of rainfall Keywords Australia Á Climate change Á Ecological deficit. The recent declines in the depth of these lakes condition Á Wetland response Á Acceptable change have been attributed to a 15% decline in effective rainfall since AD 1859. Whilst some sites reveal state shifts following past droughts, the response of most wetlands to Introduction millennial-scale climatic variations is muted. Regional wetland condition has changed comprehensively, how- The management of wetlands has largely been under- ever, since European settlement, on account of extensive pinned by the assumption that their hydrology and catchment modifications. These modifications appear to biodiversity are stationary (Pittock et al., 2010) and have reduced the resilience of wetlands limiting their this is reflected in the ecological condition assessment process of wetlands of international significance (Ramsar). Contemporary management requires ‘limits Guest editor: C. Max Finlayson / Wetlands and climate of acceptable change’ to be set (e.g. Newall et al., change: ecological outcomes and adaptation as shown by 2008) and this often relies on the identification of Australian case studies a baseline, or benchmark, condition against which present, or future, condition may be compared. Often, Electronic supplementary material The online version of this is the condition of the wetland as described when it this article (doi:10.1007/s10750-012-1163-4) contains supplementary material, which is available to authorized users. was listed under the convention, or from anecdotal evidence of some remembered condition of a past P. Gell Á K. Mills Á R. Grundell state. Whilst the forces that compromise this ‘natural Centre for Environmental Management, School of ecological character’ often relate to catchment scale Science, IT & Engineering, University of Ballarat, Ballarat, VIC 3353, Australia drivers of wetland change such as land clearance, river regulation and abstraction, secondary salinisation and 123 Hydrobiologia eutrophication, it is increasingly clear that anthropo- rims. At c. 3,000 years ago, drier conditions prevailed genic climate change is another driver of changing under a ratio of 0.7 driving these lakes into saline wetland condition. This has led to the concept of states (Fig. 2). Whilst wetter conditions prevailed stationarity being ‘declared dead’ on account of after 2,000 years ago, the last two centuries have been climate-driven changes to the hydroecology of aquatic marked by a sustained decline in water balance of systems (Milly et al., 2008). *15%. These key crater lakes, akin to rain gauges in Climates vary over a range of timescales. Orbital the landscape, reflect considerable hydrological var- forcing generates glacial–interglacial cycles that drive iability. Those that view wetland management with high-amplitude variations in sea level, temperature the benefit of such temporal context rarely consider and rainfall over c. 100,000 years. Through the systems to be stationary. In fact, from the vantage Holocene, southeast Australia has experienced con- point of the palaeoclimatologist, any number of siderable hydroclimatological variation (Mooney, baselines could be identified from the past, against 1997; Gell, 1998; Jones et al., 2001) as evident in which the present can be assessed. the records of effective rainfall archived in the The management of the catchment drivers of sediments of large crater lakes (Fig. 1c). The precip- wetland condition faces an additional challenge when itation–evaporation (P/E) values reached 1.2 through viewed from the long-term record of change. Again, it the mid-Holocene filling these lakes to their crater is usual for the natural ecological character of a wetland Fig. 1 Map showing sites discussed in the text: a location of the Murray- Darling Basin, b the wetlands of the Murray- Darling Basin and c the crater lakes of the Victorian Volcanic Plains 123 Hydrobiologia Fig. 2 Hydrological changes in three crater lakes of the and aerial photography (redrawn from Jones et al., 2001) and Victorian volcanic plains over different timescales. a The c modelled climate change for Lakes Gnotuk and Bullenmerri inferred water-level changes during the Holocene (last showing the P/E ratios (adapted from Jones et al., 2001). The 10,000 years) at Lake Keilambete, reflecting fluctuating water pre-European climate is situated along the lower lines of the levels as a result of natural hydrological changes in precipitation P–E matrix. From AD 1841, the modelled climate change and evaporation ratios (redrawn from Bowler, 1981), b historical progresses from the heavy lines within the bounded range of the water depths since AD 1860 for Lake Keilambete (estimated to arrows towards the current climate (0.79; refer to Jones et al., the nearest 1 metre) based on land survey data, depth sounding 2001 for further detail) to be presumed, and it is not unusual for this to represent eastern Australia has prompted many to ponder the a derived state, one that has already shifted owing to prospects for wetland ecosystems, and has generated a human impact on catchment processes (Tibby et al., plan for the Murray-Darling Basin to return 27–37% of 2007; Gell, 2010; Dick et al., 2011). Again, across flows to the environment (MDBA, 2010), the longer- southern and eastern Australia, few wetlands remain term view recognises that the condition of wetlands that unaffected from the impacts of direct catchment exist today, or even before the recent drought, has disturbance and most have drifted beyond their histor- departed greatly from ‘natural’ and that the manage- ical range of variability (e.g. Gell, 2012; Gell et al., ment challenge is greater than widely perceived. 2012; Tibby et al., 2012). Furthermore, several Ramsar- ‘‘Whither water management’’ (Milly et al., 2008, listed wetlands were severely degraded at their time of p. 573) indeed? listing, challenging the notion of their defined natural This review paper aims to look at past evidence for ecological character. So, whilst the recent ‘big dry’ in wetland change across southeastern Australia against 123 Hydrobiologia which management practices could be based. Current used to infer unknown environmental conditions to management practices rarely account for the legacy of species’ changes observed in sediment cores (see Mackay past changes in climate and catchment, which pose et al. (2003) for a detailed overview). many challenges and affect the functioning and resil- In the case of diatom autecology, while several ience of ecosystems to future changes and stressors. international species-environment calibration sets can be considered, several Australian transfer functions (Gell, 1997; Tibby & Reid, 2004; Philibert et al., 2006) Methods attest to the local rigour in these relations. Based on the evidence of responses of modern assemblages to water The evidence for this article synthesises that from a quality and habitat changes (Reid & Ogden 2009), wide range of published palaeoenvironmental and, diatom palaeolimnology provides a rare opportunity to particularly, palaeolimnological records of wetland trace the condition of a wetland through time, change in southeastern Australia. The review draws providing a retrospective means of assessing contem- upon case studies from 15 lakes and wetlands from porary wetland condition. across Victoria, South Australia and New South Similarly, sediment cores may also be analysed for Wales. Table 1 in Supplementary material includes fossil pollen which provides a record of local, and information for each of the lakes included in this study regional, terrestrial and aquatic vegetation. Pollen and a list of the references relating to the original analysis is often supported by the analysis of plant published study. This information can also be accessed macrofossils. In aquatic systems, the analysis of online via the OZPACS meta-database (http://www. macrofaunal remains (e.g. ostracods) can provide aqua.org.au/Archive/OZPACS/OZPACS.html). additional water chemistry information, and high- The palaeolimnological approach utilises sediment resolution analyses of the carbonates and major ions cores from wetlands that preserve natural archives of contained within the shells of ostracods can be used to change. These sediment records can be radiometri- infer changes in water balance. cally dated so that calendar ages to can be assigned to samples and