The impact of catchment nutrients on algae has historically focussed on the effect on biomass and species composition, whilst the nutritional value of algae for higher trophic levels has received less attention. Nutrient sources have distinct nutrient forms, stoichiometry, organic matter and trace element composition, and are therefore likely to differ in their impacts on the nutritional composition of algae, e.g., sewage treatment plant (STP) effluent, as a typical point source, and soil erosion runoff, as a typical diffuse source. Nevertheless, conventional assessments often treat equivalent nutrient concentrations as ecologically comparable, leaving source-specific consequences for aquatic food webs poorly understood.
This study aims to determine whether algal nutritional quality can also be a measure of ecosystem health. We are conducting outdoor mesocosm experiments to investigate the impacts of a nutrient enrichment gradient from STP effluent and soil erosion runoff on the nutritional quality and community responses of algae in the subtropical Logan–Albert River, Australia. Benthic algae and phytoplankton were compared. An initial study showed that benthic algal primary productivity was very high, whilst phytoplankton productivity was limited by light availability. Diatoms dominated at low to intermediate nutrient concentrations, whereas cyanobacteria, green algae, or both dominated at high concentrations. Algal biomass, measured as chlorophyll-a and biovolume, increased with nutrient concentration. The fatty acid concentrations and composition, critical for animal growth and survival, were also examined, particularly the omega-3 long-chain polyunsaturated fatty acids EPA and DHA. Preliminary findings indicate that increases in algal biomass linked to higher nutrient availability were not necessarily accompanied by increases in nutritionally valuable fatty acids, although more analyses will determine whether there is a more nuanced response.