The impact of drought on aquatic communities in the lower Darling River - M/BUS/241
MDFRC Technical Report
It is generally anticipated that river pools are core habitat for native fish and that during droughts isolated deep pools function as key refugia for aquatic communities, yet there is considerable pressure placed on these pools for irrigation, stock and domestic supply. The overarching objective of the project was to address the distinct lack of knowledge on how these systems function, and of their relative importance as refugia during drought periods by investigating the ecology of river pools as they become isolated, and subsequently reconnect due to regulated releases. Understanding processes that occur within these pools during periods of low- and no- flow, and during periods when flow is reinstated is vital to maintaining sustainable ecosystems. The project monitored conditions in three distinct, deep (>5m) pools situated above Harcourt and below Pooncarie (within and downstream of the areas involved in the 2004 lower Darling River Fish Kill respectively) between December 2006 and May 2008. The project has captured a summer-autumn low flow (250MLday-1), autumn-winter cease to flow, late winter reinstatement of low flow (<300MLday-1), spring-summer cease to flow, summer recommence to flow (peak flow = 4,500 MLday-1) period and return to base-flows. The two cease to flow periods resulted in the contraction of the wetted area of channel, and subsequent establishment of disconnected pools. This temporal sequence has generated a high quality data set that encompasses a wide range of flow and seasonal conditions. Surveys of fish communities demonstrated higher abundances of native fish in the shallow reaches compared to the deep pools. High recapture rates of marked fish suggest strong site fidelity for Murray cod. These data indicate that fish utilise the shallow reaches as preferred habitat, with deep pools primarily used as drought refugia. Within the refuge pools, maximum daily temperatures at the water surface were regularly in excess of 32 ◦C at all sites during summer periods, bordering on the recognised upper tolerance limit for native fish such as Murray cod. The data demonstates that minimum flow thresholds drive water quality, particularly thermal stratification and the establishment of hypoxic and anoxic conditions in deep pools. Although flows of 200-250 MLday-1 at Weir 32 are sufficient to maintain a diurnally mixed water column at Karoola, persistent periods of intense thermal stratification (Δt = > 6°C) occur at the downstream sites during both summer and autumn at this flow band. During stratified periods, conditions in the bottom section of the water column were frequently either hypoxic (low dissolved oxygen) or anoxic (no dissolved oxygen) for extended periods of time (>100 days). In contrast, hypoxic conditions were rarely recorded in the surface layer. Taking into account balancing the needs for maintaining security of supply, summer base-flows should be established and maintained that are sufficient to prevent the onset of persistent thermal stratification and subsequent anoxia at depth in refuge pools and weir pools for the entire reach of the lower Darling.The observation that robust fish communities exist in the sampled reaches, despite a combination of high temperatures and hypoxic and anoxic conditions at depth within the pools suggests that sufficient habitat is available to sustain native fish in these pools during droughts. However, the fish community is highly susceptible to disturbances. For example, the data associated with the re-instatement of flow following a summer no-flow period demonstrated that hypoxic conditions were present throughout the water column in the deep pools prior to and immediately following the arrival of the flow front. Periods such as this when the entire water column becomes deoxygenated pose a significant threat to aquatic communities and suggest that a tipping point for onset of anoxia throughout the water column was approached if not narrowly avoided. If it is necessary to reinstate flows during summer, extreme care should be taken in order to minimise the risk of extensive fish deaths.