The Impact of Drying on the Ecology of the Menindee Lakes

"June 1999".

Project Number: Deflation Lake Ecology Study - F636 ew28a.

MDFRC item.

The Menindee Lakes, in far west NSW, are an integral part of the Murray-Darling Basin's water supply infrastructure. They are also an important natural resource, providing habitat for native fish and birds within the Darling River system. Since its completion in the late 1960s, the operation of the Menindee Lakes has involved the regulation of natural flows between the Darling River and each of the System's lakes. This has dramatically altered the regimes of wetting and drying and has, more recently, raised concerns regarding environmental health and sustainability of current management practices. As environmental health is a pre- requisite for achieving regional and national social, cultural and economic objectives managers need to incorporate environmental information in their management decision making framework. This report represents a first step towards this objective by providing some base line information on habitat/water quality, plankton and fish communities, and by identifying key ecological processes operating within the Menindee Lakes throughout a single drying event from August 1997 to August 1998. As lakes dried aquatic habitat was lost, habitat diversity decreased and water quality was modified. Changes in water quality were primarily due to evaporative concentration of suspended, dissolved and particulate matter. As lakes dried salinity, turbidity and total nutrient levels increased, filterable reactive phosphorus levels decreased, and the aquatic environment became less buffered to diel changes in temperature, dissolved oxygen and pH. Aquatic community composition and abundance vaned throughout the study period with 27 genera of phytoplankton, at least 44 genera of zooplankton, 20 genera of macroinvertebrates and 10 species of fish being identified. As drying progressed there was an increased likelihood of the development of blue-green algal blooms, especially during warm, calm periods. Although structural changes in terms of abundance and diversity of the phyto and zooplankton assemblages could be ascribed to the lake drying process, seasonal factors were also implicated. This highlights the potentially critical role of timing of dry and flood events. There were differences in the fish fauna of the ephemeral and more permanent lakes. Native fish species, including golden perch, bony herring and smelt, though present in all lakes were generally dominant in the more permanent lakes (Tandure, Menindee and Cawndilla). The small alien mosquito fish, however, dominated in the more ephemeral lakes (Malta, Balaka and Bijiji) prior to re-flooding. That smaller native fish species were poorly represented in the ephemeral lakes suggests that only the larger species/individuals were able to re-enter lakes with the flooding inflows. Carp were not excessive ly abundant during the dry phase. The carp population consisted primarily of large individuals, which were especial ly quick to re-invade the re-flooded shallow lakes, and utilize spawning habitat along the shallow margins. Regulation of water regimes has effectively decreased the frequency and duration of drying events within the Menindee Lakes system. This has ameliorated the development of physicochemical extremes and biotic interactions upon which many organisms are thought to depend upon as life-cycle cues. There was also some evidence to suggest that drying phases arc necessary to promote nutrient cycling and thus ecosystem productivity. Inappropriate regulatory strategies thus risk a reduction in net system productivity and the replacement or loss of species adapted to ephemeral habitats in favour of those more suited to permanent environments (e.g European carp, mosquito fish). Either risk reduces the system's resilience to change and its ability to support fish and water fowl populations. Long-term data is essential if the effects of water level changes are to be properly examined and understood in relation to system productivity, diversity and long-lived organisms such as fish.