This report presents findings from a collaborative project funded by the Commonwealth Environmental Water Office (CEWO) and led by Charles Sturt University (CSU), the NSW Office of Environment and Heritage (OEH) and the University of New South Wales (UNSW). The aim of this project was to monitor waterbird, nutrient and invertebrate responses to environmental water delivery in the Lowbidgee floodplain, in south-western NSW, in 2015-16 and 2016-17. The CEWO in conjunction with OEH delivered 5000 ML and 910 ML of Commonwealth and NSW environmental water respectively to four sites in the Western Lakes system (Hobblers Lake, Paika Lake, Cherax Lake and Penarie Creek). In addition OEH delivered 966 ML to Wagourah Lake, in March and April 2016. The aim of these watering actions was to create foraging habitat for dabbling ducks and shorebird species that autumn, winter and in the following spring.
We expected environmental water delivered in autumn would inundate previously dry habitats in the wetlands releasing and transporting nutrients that stimulate productivity and diversity of microinvertebrate and macroinvertebrate communities. The success of these watering actions depends on the response of these invertebrate communities which are important food sources for fish, frogs, and waterbirds. Microinvertebrates are the key prey in floodplain river food webs for filter-feeding ducks, and macroinvertebrates are important food sources for other ducks and shorebirds. However, due to cool temperatures with the watering event in autumn, we did not expect as large a response as if the watering action occurred in spring or summer. As the ecological outcomes for waterbirds from watering actions undertaken in autumn are uncertain, this monitoring project was initiated to assess waterbird and invertebrate responses to guide the adaptive management of future watering actions.
Our surveys showed that the delivery of environmental water in autumn can benefit a suite of waterbird species, with more than 33 species detected in our study, as well as high densities of diverse invertebrate prey. We observed increases in both waterbird abundance and diversity in response to the delivery of environmental water in the Lowbidgee Floodplain. Overall, numbers of dabbling and filter feeding duck were higher in wetlands that were dry prior to the delivery of environmental water compared to sites that were already wet. The influx of dabbling and filter feeding ducks coincided with high numbers of microinvertebrate and macroinvertebrate prey following the wetting of the previously dry wetlands. Nutrient levels and water quality supported these responses, although we did not detect a pulse in nutrients in April 2016 as predicted.
Our data showed that although spring is the preferred timing for wetland inundation, there are benefits from delivering environmental water to wetland habitats in autumn. Waterbird numbers pulsed at two of the three newly inundated wetlands, especially compared to the previously wet wetlands. Increasing the area of newly inundated wetland at the start of autumn and winter could enable managers to sustain habitat for waterbirds over winter. If wetlands are filled so they are drying down in spring, the shallow productive edge habitat would support high shorebird numbers. Our study also demonstrated that drying wetlands between environmental watering events triggers a greater response in invertebrate prey. Where possible watering strategies aimed to create feeding habitat for dabbling and filter feeding ducks and shorebird species should account for natural flooding and drying cycles to promote invertebrate food supplies.
By delivering environmental water during autumn months there are also other potential benefits, such as provision of foraging habitat for migratory shorebirds which migrate north during the February-May period. Where habitat is maintained into spring this can potentially provide habitat for migratory shorebird species on their return trip to Australia from August-October. The depth of water is also important for many dabbling duck and shorebird species that feed on the water’s edge as water depth determines the accessibility of invertebrate prey.