Alternating drought and overbank flooding influence the availability and quality of basal resources in lowland river systems. Floodplain inundation stimulates benthic and water column primary production, potentially increasing the availability of algal resources rich in high-quality, long-chain polyunsaturated fatty acids (LC-PUFA). Decapod grazers and collectors are known to respond positively to flood events, but the role of food quality in driving these responses remains unclear. We assessed whether tissue fatty acid (FA) composition of the grazer Paratya australiensis and collector Macrobrachium australiense tracked resource quality during a major flood in a temperate Australian floodplain river. We characterised FA profiles during three flow components: prior to a major flood, after flows returned to low levels, and more than a year later. We hypothesised (i) that LC-PUFA concentrations would be more abundant in biofilm than detritus, (ii) LC-PUFAs proportion in biofilms would increase immediately following flow disturbance and floodplain inundation, and (iii) decapods would preferentially incorporate LC-PUFA rich biofilm resources.
Our results demonstrated that biofilms contained a greater proportion of LC-PUFA’s following the flood pulse, particularly of eicosapentaenoic (EPA, 20:5ω3) and docosahexaenoic (DHA, 22:6ω3) acid. However, LC-PUFA concentrations declined in all basal sources by the final sampling period. Decapod consumer FA profiles were most strongly correlated with algal FA signatures, suggesting preferential assimilation of higher quality algal resources. In contrast to expectations, detritus quality increased once the system returned to low flows, likely because of increased fungi and diatoms within leaf litter. These findings are consistent with trophic upgrading, whereby microbes enhance the nutritional value of low-quality organic matter. Changes in floodplain connectivity and the frequency of extreme hydrological events may have broader implications for riverine food webs by altering the quality of basal resources available to aquatic consumers.