Assessment of correspondence between periphytic biofilm fatty acid profiles and measures of stream water quality

Abstract

Periphytic biofilms are rich in essential biomolecules, such as fatty acids (FAs), which play a critical role in supporting metabolic functions, including growth and reproduction. There has been increased interest in using biofilm FAs as a proxy for characterizing food webs, dietary sources, energy transfer, trophic relationships within ecosystems, and stream integrity assessment. However, new knowledge is required to validate the use of fatty acids as biomarkers of water quality. The objective of this study was to determine whether stream water quality degradation, in particular nutrient enrichment, affects the fatty acid composition of periphytic biofilms. We conducted a large-scale field study in 56 natural streams across Southern Quebec (Canada) in the summers of 2019 and 2020. The studied streams represented a water quality gradient as indicated by 5-year historical physico-chemistry data collected during the summer seasons. Overall, we did not observe strong relationships between nutrients or general water quality indices and periphytic biofilm fatty acid composition. However, proportions of total polyunsaturated FAs (PUFAs) and long-chain PUFAs (LC-PUFAs) gradually increased with nutrients and as a function of general water quality degradation. In addition, we observed decreased proportions of heterotrophic marker fatty acids and increased proportions of diatom marker fatty acids with increased nutrient concentrations and general water quality degradation. Our results suggest nutrient enrichment and water quality may shape stream periphytic biofilm fatty acid composition and thereby have an impact on the nutritional quality of the basal resources in natural streams. However, our findings suggest that, although periphytic fatty acid composition may represent a valuable tool to explore changes in the nutritional quality of basal resources as a response to stress, nutrient effects on FA composition may be masked or affected by interactions with multiple environmental factors. More research is thus required to effectively account for natural environmental variations before recommending and/or considering FAs as a complementary tool for routine stream biomonitoring.