Amino acids in freshwater food webs: Assessing their variability among taxa, trophic levels, and systems

Abstract Although the amino acid composition of fishes and some marine invertebrates varies among taxa and systems, similar information is lacking for freshwater invertebrates. The objectives were to characterise and compare the amino acid composition among different aquatic species, dietary habits, and environmental conditions. Benthic macroinvertebrates from different functional feeding groups (FFG), bulk zooplankton, biofilm, and fishes representing 12 families (21 genera or species) were collected from temperate lakes in eastern Canada during the summers of 2013 and 2014. Fifteen protein‐bound amino acids, including thiols, were measured in whole invertebrates, biofilm, or fish muscle. We hypothesised that the amino acid composition will differ among species and systems. Multiple discriminant analyses revealed significant differences in the amino acid composition among species—based on varying percentages of cysteine (as cysteic acid) and histidine—and among FFG/trophic designations—based on histidine and lysine—where the primary consumers were more variable than the predators. Overall, the results suggest that patterns were based on phylogenetics, biological characteristics, and the FFG/trophic designations of biota. The within‐taxon variability in composition was also related to differences among lakes. Characteristics of their environment, including lake pH and the food web structure (abundance and composition of taxa), probably influenced their dietary habits and amino acid composition of diet. These results expand the currently limited knowledge of the biochemical composition of freshwater biota and provide impetus for further studies on nutritional values in predator‐prey relationships, trophic guilds, and the biomagnification of protein‐bound contaminants through food webs.