Flood pulses play a crucial role in the food-web processes of rivers by providing lateral connectivity between floodplains and the main channel. Inundated floodplains contain large amounts of terrestrial detritus and are hotspots for productivity of high-quality aquatic basal resources such as algal biofilms. Algae are a key source of omega-3 fatty acids that are essential for consumer growth. Collector species like decapods may be able to utilise flood conditions to quickly accrue biomass. However, the frequency of floodplain inundation within Australia’s Murray Darling Basin has decreased significantly over the past century. It’s unknown how the loss of these events have impacted the quality of decapod diets and the basis of their productivity.
Using stable isotope and fatty acid biomarkers, the relative dietary contributions of terrestrial detritus and algae were determined for the decapod species Paratya australiensis and Macrobrachium australiense before, during and after an overbank flow event. Stable isotope mixing models revealed diets consisted of predominantly detrital sources before and during the floodplain inundation event. After flow returned to bank-full levels, diets of both species shifted to a mix of algal biofilm and detrital sources. When the ratio of ∑ω3/∑ω6 fatty acids was used as a marker for the relative amount of aquatic vs terrestrial matter in decapod tissues, it indicated a mix of both resources by Macrobrachium after high flows while Paratya shifted to a predominantly algal.
This highlights that the trophic basis of production for decapods varies during floods, as they preferentially incorporate high-quality resources after events, perhaps which become available as nutrient-rich waters return to the main channel. Changes in the frequency of floodplain inundation due to flow regulation may alter the availability of high-quality resources for decapods. This could have implications for higher trophic levels, as decapods are important prey for larger aquatic consumers.