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Assessing Change in Floodplain Wetland Condition in the Murray

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Assessing Change in Floodplain Wetland Condition in the Murray Anthropocene 8 (2014) 39–45 Contents lists available at ScienceDirect Anthropocene journal homepage: www.elsevier.com/locate/ancene Assessing change in floodplain wetland condition in the Murray Darling Basin, Australia Peter Gell a,*, Michael Reid b a Water Research Network, Federation University Australia, Ballarat, Victoria, Australia b Riverine Landscapes Research Laboratory, University of New England, Armidale, NSW, Australia ARTICLE INFO ABSTRACT Article history: Lowland Australian rivers and floodplains have been affected by agriculture and flow regulation for more Received 31 July 2014 than a century. Our capacity to understand the complex causes of ecosystem change is limited by the lack Received in revised form 10 December 2014 of historical records of ecosystem condition. Records of change over this critical period are available Accepted 11 December 2014 through analysis of sedimentary records. These provide benchmarks of the range of natural conditions Available online 18 December 2014 and, by providing a long time series of conditions, trends and trajectories of change. Over recent decades, 51 sediment records from billabongs and lagoons throughout the southern Murray Darling Basin have Keywords: been subject to palaeoecological analysis. The picture that emerges from this synthesis is that Wetlands ecosystems have undergone substantial ecological change in response to human activities. Diatom Murray River Salinity assemblages preserved in wetland sediments attest to salinisation in 34% of sites and increased nutrient Regime shift concentrations in 48%. More extensive is the impact of increased sediment flux with evidence for Sediments increased sedimentation rate, turbidity or macrophyte loss in 80% of sites. Intriguing differences exist in Nutrients the timing and nature of change experienced by aquatic ecosystems in different parts of the Basin. These patterns of ecosystem response suggest underlying differences in the resilience of these ecosystems to different anthropogenic stressors, which may result from contrasting hydrologic, geomorphologic and climatic contexts. This systematic compilation of the palaeoecological evidence of change in the aquatic ecosystems of the Basin sheds light on what are the principal drivers of change across the region and provides guidance as to how these systems can be best restored. ß 2014 Elsevier Ltd. All rights reserved. Introduction rivers’ waters were sought for irrigation agriculture from as early as 1888. When variable river flows brought calls for river The Murray Darling Basin is Australia’s largest, and among the regulation to ensure passage, the commissioning of weirs from twenty largest in the world, spanning 1.06 Â 106 km2. Human 1922 most advantaged the irrigators, with river transport having presence in the river basin has considerable antiquity with largely succumbed to that on land. By 1936 much of the lower evidence for occupation of Lake Mungo dated to before 42,000 River Murray was regulated and significant additions to the years B.P. (Bowler et al., 2003). While the junction of the Murray reservoir system were marked by the commissioning of the Hume and Darling Rivers hosted a relatively large population of Weir (1520 GL; upgraded to 3040 GL in 1961) in 1936 and indigenous Australians (Pardoe, 1998), the impact of human Dartmouth Dam (3800 GL) in 1980 (Ogden, 2000). populations increased substantially after the arrival of European The Murray Darling Basin extends from sub-tropical zones in settlers from the 19th century. From early in settlement the southern Queensland to temperate climates in the south. Its main catchment was used extensively for sheep and cattle grazing and watershed is the elevated, alpine to sub-alpine zones in the south- the main rivers were a focus for travel owing to the difficulty of east associated with the Great Dividing Range. To the west the traversing the land. For navigation trees were cut for firing engines effect of continentality, and prevailing sub-tropical high pressure and channels were cleared of woody debris. Early on stocking rates cells, ensures a drier climate. Here, low in the catchment, the main of sheep and cattle were high and impacts intense, particularly river channels pass through semi-arid, and even arid, climate along stock routes where animals were driven to markets. The zones. So, the main runoff is driven by cool season rainfall in the south-east, followed by snow melt, yet the northern parts of the catchment can receive warm season rainfall. Ultimately, river flow * Corresponding author. is impacted by high evaporation rates in the dry, western zone limiting the volumes that pass through the outlet to the sea in http://dx.doi.org/10.1016/j.ancene.2014.12.002 2213-3054/ß 2014 Elsevier Ltd. All rights reserved. 40 P. Gell, M. Reid / Anthropocene 8 (2014) 39–45 South Australia. In all, this climatic context ensures that the region magnitude of the influence of these drivers of change can be hosts the most variable runoff on Earth (McMahon and Finlayson, documented. 1992). Despite this, the Basin hosts sixteen wetlands registered under the Ramsar convention on wetlands of international Sediment records significance. River regulation and a very high water reserve were responses to a highly variable climate. The region is subject to the Long term records of change exist where the biological remains cycles of dry and wet associated with the El Nin˜o Southern of organisms are buried with sediments in still water environ- Oscillation, as well as the Indian Ocean Dipole, which can bring ments. Continuous sediment records that contain this evidence are significant drought phases and extensive floods. Most notable are more likely in sites which are perennially covered in water. The the Federation Drought (1907), the World War II drought and the climatic diversity of the Basin dictates that some areas are more recent Millennium Drought (1997–2009), considered the longest humid than others and the high climatic and runoff variability and deepest dry phase in European history (Gergis et al., 2012). The ensures that basins in many areas are not always filled with water. region is also impacted by multi-decadal climate variability So, sites which provide continuous records are focussed in the associated with the Pacific Decadal Oscillation with significant humid upper catchments and along the main channels where wet phases (1845–1898; 1946–1976), deemed flood-dominated overbank flows regularly refill wetlands. Critically too, many regimes, and extended dry phases (1898–1946; 1997–?) under intermittent floodplain wetlands have become permanent, their drought-dominated regimes (Warner, 1987). Notably, La Nin˜a water level linked to the height of the weir pool behind a nearby ensured that the early 20th century drought-dominated regime lock, and so provide sediment records mostly over the last century. was interrupted by the 1917 flood. One could speculate that, Central to the reconstruction of a long term history from similarly, the 2010–2011 floods were mere interruptions of an sediment is the establishment of chronology. As dead carbon can extended drought dominated regime that commenced in 1997, be exposed to air and degraded or metabolised radiocarbon particularly given rainfall deficits have continued across the region techniques are not straightforward, particularly where there are since 2011. Drying is likely into the future with south-eastern geological sources of old carbon. Also, 210Pb approaches to date Australia identified as a global climate hotspot at risk of substantial sediments are best where the proportion of aerial input of fine declines in wet (cool) season rainfall (Giorgi, 2006). sediment is highest. Clearly, floodplain systems naturally receive The waterways of the Murray Darling Basin are recognised as river borne sediments, episodically, creating challenges in being in a degraded condition (Norris et al., 2002; Davies et al., developing depth-age models in sediment sequences (Gell et al., 2012). Only the remote, unregulated streams of the arid north- 2005a). Further, luminescence dating relies on a known dosimetry west are considered in good ecological condition with the (Gell et al., 2007) and this varies if a lake is wet or dry. So, the remaining, intensively used systems considered degraded or even establishment of time lines, especially at around the time of known severely degraded. This state is closely associated with the level of first impact by European people from the 1830s, is challenging and abstraction and diversion of river flow, largely for irrigation few confidently dated sequences exist. Nevertheless, the use of a agriculture. Much infrastructure was funded to significantly suite of dating techniques, including exotic pollen, provides for increase water allocations through the flood dominated regime generalised chronologies which enable attribution of human of the post WW II period. This has left water users, and the impact; albeit not so precisely in time. environment, highly vulnerable to the recent drought, and in Traditionally, in Australia at least, fossil pollen was the main particular, a drought-dominated regime should the ‘drought’ biological indicator extracted from sediments. This focus was to persist beyond the recent La Nin˜a phase. In response to this water examine vegetation responses to long term climate cycles and was stress the local authority has implemented a highly contested infrequently applied to reconstructing the impact of industrialised Murray Basin Plan that dictates that 3200
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