Assessing change in floodplain wetland condition in the Murray Darling Basin, Australia
Introduction
The Murray Darling Basin is Australia's largest, and among the twenty largest in the world, spanning 1.06 × 106 km2. Human presence in the river basin has considerable antiquity with evidence for occupation of Lake Mungo dated to before 42,000 years B.P. (Bowler et al., 2003). While the junction of the Murray and Darling Rivers hosted a relatively large population of indigenous Australians (Pardoe, 1998), the impact of human populations increased substantially after the arrival of European settlers from the 19th century. From early in settlement the catchment was used extensively for sheep and cattle grazing and the main rivers were a focus for travel owing to the difficulty of traversing the land. For navigation trees were cut for firing engines and channels were cleared of woody debris. Early on stocking rates of sheep and cattle were high and impacts intense, particularly along stock routes where animals were driven to markets. The rivers’ waters were sought for irrigation agriculture from as early as 1888. When variable river flows brought calls for river regulation to ensure passage, the commissioning of weirs from 1922 most advantaged the irrigators, with river transport having largely succumbed to that on land. By 1936 much of the lower River Murray was regulated and significant additions to the reservoir system were marked by the commissioning of the Hume Weir (1520 GL; upgraded to 3040 GL in 1961) in 1936 and Dartmouth Dam (3800 GL) in 1980 (Ogden, 2000).
The Murray Darling Basin extends from sub-tropical zones in southern Queensland to temperate climates in the south. Its main watershed is the elevated, alpine to sub-alpine zones in the south-east associated with the Great Dividing Range. To the west the effect of continentality, and prevailing sub-tropical high pressure cells, ensures a drier climate. Here, low in the catchment, the main river channels pass through semi-arid, and even arid, climate 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 is impacted by high evaporation rates in the dry, western zone limiting the volumes that pass through the outlet to the sea in South Australia. In all, this climatic context ensures that the region hosts the most variable runoff on Earth (McMahon and Finlayson, 1992). Despite this, the Basin hosts sixteen wetlands registered under the Ramsar convention on wetlands of international significance. River regulation and a very high water reserve were responses to a highly variable climate. The region is subject to the cycles of dry and wet associated with the El Niño Southern Oscillation, as well as the Indian Ocean Dipole, which can bring significant drought phases and extensive floods. Most notable are the Federation Drought (1907), the World War II drought and the recent Millennium Drought (1997–2009), considered the longest and deepest dry phase in European history (Gergis et al., 2012). The region is also impacted by multi-decadal climate variability associated with the Pacific Decadal Oscillation with significant wet phases (1845–1898; 1946–1976), deemed flood-dominated regimes, and extended dry phases (1898–1946; 1997–?) under drought-dominated regimes (Warner, 1987). Notably, La Niña ensured that the early 20th century drought-dominated regime was interrupted by the 1917 flood. One could speculate that, similarly, the 2010–2011 floods were mere interruptions of an extended drought dominated regime that commenced in 1997, particularly given rainfall deficits have continued across the region since 2011. Drying is likely into the future with south-eastern Australia identified as a global climate hotspot at risk of substantial declines in wet (cool) season rainfall (Giorgi, 2006).
The waterways of the Murray Darling Basin are recognised as being in a degraded condition (Norris et al., 2002, Davies et al., 2012). Only the remote, unregulated streams of the arid north-west are considered in good ecological condition with the remaining, intensively used systems considered degraded or even severely degraded. This state is closely associated with the level of abstraction and diversion of river flow, largely for irrigation agriculture. Much infrastructure was funded to significantly increase water allocations through the flood dominated regime of the post WW II period. This has left water users, and the environment, highly vulnerable to the recent drought, and in particular, a drought-dominated regime should the ‘drought’ persist beyond the recent La Niña phase. In response to this water stress the local authority has implemented a highly contested Murray Basin Plan that dictates that 3200 GL of water will be returned to the environment to restore its health. Depending on whether this water is redeemed through the purchase of water rights, or through water efficiency infrastructure, the provision of this environmental allocation will come at a cost of between $5.5B and $27.5B AUD (Wittwer and Dixon, 2013).
However, the provision of water is likely only one of the drivers of the degraded condition of the Basin's aquatic systems with the increased flux of salt, sediments and nutrients strongly implicated in the changing waterway condition. Little is known of the past variation in these drivers of change with water quality monitoring programmes commencing only from the mid-1900s. This instrumental data is pre-dated by the clearance of vegetation from the catchment, the initiation of intensive cropping, the widespread application of irrigation water, the regulation of flow and the running of many million head of grazing stock. Regional water tables and extensive erosion was noted by the 1930s. So, reference to the available data will not reveal the impact of these early drivers of waterway change. Evidence for these changes is evident, however, in the natural archives of the biological elements of wetlands and rivers that are preserved in continuous sediment sequences. These records of change have been a focus of recent research and the condition of many wetlands, relative to their unimpacted state, has now been revealed (e.g. Thoms et al., 1999, Gell et al., 2005a, Gell et al., 2005b, Fluin et al., 2007, Fluin et al., 2010, Reid et al., 2007). This synthesis of 51 such records reveals changes at a sub-catchment scale and so the timing and magnitude of the influence of these drivers of change can be documented.
Section snippets
Sediment records
Long term records of change exist where the biological remains of organisms are buried with sediments in still water environments. Continuous sediment records that contain this evidence are more likely in sites which are perennially covered in water. The climatic diversity of the Basin dictates that some areas are more humid than others and the high climatic and runoff variability ensures that basins in many areas are not always filled with water. So, sites which provide continuous records are
Natural ecological character
Many natural resource management instruments attempt to identify a benchmark condition against which to assess contemporary condition. In Australia the Sustainable Rivers Audit assessed the condition of rivers against expected, natural conditions (Davies et al., 2012). The Ramsar Convention on the protection of wetlands of international significance attempts to identify ‘natural ecological character’ which is often that described at the time a wetland is added to the register of sites, and the
Human impact
The impact of indigenous people on Murray River floodplains wetlands is not well revealed in the sediment records. While direct burning of wetland vegetation is invoked from charcoal records of sites elsewhere (Head, 1988, Mooney et al., 2011, Mills et al., 2013), the routine analysis of charcoal from floodplain wetlands remains an important area of future endeavour. In the absence of charcoal records, inference for the impact of early humans on wetlands is left to ethnohistoric and
Regime shifts
Wetlands are known to undergo abrupt changes in condition on account of the breakdown of stabilising forces that tend to direct the condition of wetlands to a stable state, enabling a greater variation in condition that may approach thresholds of ‘irreversible’ change (Scheffer et al., 1993). One such threshold relates to the light regime whereby aquatic plants, attached to substrates, receive less energy as turbidity impacts upon the light regime and their capacity to photosynthesise. Abrupt
Management implications
This synthesis has greatly strengthened knowledge of the degraded nature of the wetlands of the Murray Darling Basin by providing a comprehensive assessment of change relative to a long-term, and at times, pre-European settlement baseline of ecological condition. It has also expanded the spatial extent of the previously reported palaeolimnological changes (Gell et al., 2009) across the basin. While these records are skewed towards the more permanent wetlands in the basin that continuously
Conclusion
Palaeolimnological approaches shine a light on the nature and magnitude of human impact on the floodplain wetlands of the Murray Darling Basin. Many wetlands have been substantially changed on account of river regulation, abstraction, eutrophication, salinisation, acidification and declining light regime. Some changes occurred early in European settlement and, in many cases, changes were abrupt. The report card for the condition of the wetlands of the Murray Darling Basin, at least those
Acknowledgments
This synthesis relies on the records of change produced by many colleagues and students. The work of Kate Adamson, David Baldwin, Jennie Fluin, Rosie Grundell, Garry Heinitz, Ifteara Khanum, Fiona Little, Sorell Lock, Ralph Ogden, Martin Thoms, John Tibby and Brendan Walsh is greatly appreciated. The original studies were supported by Australian Research Council Linkage grants (LP0560552 and LP0667819) to PG and several grants through the Australian Institute for Nuclear Science and Engineering
References (48)
- et al.
Anthropogenic acceleration of sediment accretion in lowland floodplain wetlands, Murray-Darling Basin, Australia
Geomorphology
(2009) - et al.
The 10 Australian ecosystems most vulnerable to tipping points
Biol. Cons.
(2011) - et al.
Late Quaternary fire regimes of Australasia
Q. Sci. Rev.
(2011) - et al.
Alternative equilibria in shallow lakes
Trends Ecol. Evol.
(1993) A palaeoecological investigation of late Holocene environment variability and the impact of European settlement in the lower Murray Basin
(2002)- et al.
The European Union Water Framework Directive: opportunities for paleolimnology
J. Paleolimn.
(2007) - et al.
New ages for human occupation and climatic change at Lake Mungo, Australia
Nature
(2003) - et al.
A diatom species index for bioassessment of Australian rivers
Mar. Freshw. Res.
(2007) - et al.
Sustainable Rivers Audit 2: the ecological health of rivers in the Murray–Darling Basin at the end of the Millennium Drought (2008–2010). Summary
(2012) - et al.
When trends collide: the challenge of protecting freshwater ecosystems under multiple land use and hydrological intensification scenarios
Sci. Total Environ.
(2015)