Microinvertebrates play a key role in freshwater river and wetland food webs. They graze on micro-organisms such as algae and bacteria and are a major food source for larval fish, waterbirds and frogs. Lowland rivers with large terminal wetlands such as the Gwydir in western NSW provide a range of river and wetland habitats with hydrologic variability that can promote a diversity of microinvertebrate communities that in turn sustain aquatic food webs. The delivery of environmental water has allowed us to explore the key hydrologic drivers of microinvertebrate density and community composition in the Lower Gwydir River and wetlands.
Microinvertebrates were sampled from pelagic and benthic environments in channel and wetland sites from 12 surveys between 2015 and 2018. Water quality, water column nutrients and macroinvertebrate density and composition were measured and linked to flow data and inundation environmental water events. Zooplankton communities responded to flow events differently in channel and wetland systems. In channel environments, flow acted as a hydrological disturbance and initiated taxonomic replacement and reduced microinvertebrate densities. In wetland systems, antecedent hydrologic conditions and length of inundation periods regulated the response of microinvertebrate density and diversity. Microinvertebrate density at all sites was positively correlated to high water column nutrients and water column chlorophyll a concentrations. Microinvertebrate diversity was generally lower in stable flow conditions of channel habitats. In contrast, higher diversity was recorded during environmental flow delivery to wetlands, highlighting that the role of longitudinal connectivity supplementing emergent communities from wetland inundation, and supported by high nutrient concentrations and algal communities.