Macroinvertebrates are an essential part of healthy, functioning aquatic ecosystems, providing essential ecosystem services that range from nutrient cycling to provision of food for larger aquatic organisms such as fish. Macroinvertebrates are frequently monitored in aquatic ecosystem assessments to understand the health of those ecosystems. In large lowland rivers, such as the Goulburn River, the macroinvertebrate communities tend to be dominated by species that favour relatively simple habitats and are able to tolerate moderate to poor water quality. Environmental flows delivered to these rivers are more likely to influence macroinvertebrate abundance and biomass than diversity. The aim of the macroinvertebrate monitoring program was to understand what combination of freshes and low flows are required to maximise macroinvertebrate abundance and biomass in the river.
The impacts and benefits of CEW on macroinvertebrate diversity and abundance was assessed using several techniques including artificial substrates, replicated edge sweep sampling (RESS) and a targeted assessment of crustacean diversity and abundance in bait traps. Artificial substrates and RESS sampling occurred before and after environmental flows at one site in the Lower Goulburn and a reference site in Broken River. Crustacean bait trap sampling was undertaken monthly at 3 sites for a period of 5 months.
Macroinvertebrate richness, abundance and large crustacean biomass increased in both the Goulburn and Broken rivers following natural winter/spring floods in 2016. Smaller environmental flows also resulted in increased macroinvertebrate biomass and abundance, although the effect was smaller when compared with natural events. Crustaceans seem to be particularly responsive to flows in the lower Goulburn River.
These indicators have contributed to a better understanding of how environmental flow delivery in the lower Goulburn River can affect the abundance and composition of macroinvertebrates and the lifecycle (reproduction and recruitment) of large bodied crustaceans. This has important implications for the river in terms of the services and functions provided by macroinvertebrates. The role of bank vegetation, macrophytes and biofilms play an important role in sustaining these populations, while it is likely large-bodied crustaceans are likely to be an important food source for other riverine species, especially Golden Perch.