Microinvertebrates form an important trophic link between basal carbon resources and higher predators such as waterbirds and fish. These tiny creatures have a wide-ranging diet including; algae, bacteria, other microinvertebrates and detritus, subsequently becoming either prey for larger animals or detritus. We used stable isotope analysis to explore the diet of freshwater microcrustaceans in the Macquarie Marshes. Data were taken from drought and flood years, channels and floodplains, ranging from 2006 to 2012. Ostracods consumed terrestrial vegetation carbon resources and cladocerans consumed aquatic vegetation carbon resources in channels during drought. As resources expanded, and inundated floodplain connected to channels, resource consumption expanded.
Ostracod carbon stable isotope ratios (δ13C) ranged from -15.8 to -14.3‰ in drought indicating a preference for terrestrial plants (C4 plants) or water couch. During floods, ostracod δ13C ratios expanded and ranged from -22.6 to -13.3‰. Cladoceran δ13C ratios ranged from -29.6 to -23.4‰ in drought indicating a preference for the consumption of C3 algae and plants i.e. aquatic wetland plants. During flood Cladoceran δ13C ratios expanded and ranged from -34.1 to -21.6‰, with floodplain values towards the terrestrial range. Similar nitrogen stable isotope ratios (δ15N - Cladocera= 0 to 7.2; Ostracoda= 0.3 to 8.7) indicate that both groups are at a similar trophic level i.e. one is not consuming the other. Understanding the link between microinvertebrates and carbon resources can inform the adaptive management of floodplain river systems, especially decisions about the duration of both dry phases and reconnection events.