Fatty acids reveal aquaculture and drought effects on a large tropical reservoir

Fatty acids (FAs) and their metrics have been used to detect and assess the impacts of urbanization and agriculture on aquatic ecosystems. Here, we investigated whether seston FAs are also useful to characterize and understand early-stage aquaculture impacts in a large tropical reservoir (Furnas Reservoir, SE Brazil). We tested the hypothesis that single FAs, as well as selected FA metrics in the seston fraction, are efficient markers of net-cage fish farming effects. In general, fish farming had only minor effects on standard water chemical variables, mainly small increases in ammonium, nitrate, and dissolved organic nitrogen concentrations. By increasing concentrations of several polyunsaturated FAs, early-stage fish farming improved sestonic food quality in the more oligotrophic branch of the reservoir under drought conditions. However, in general, increases in concentrations of bacterial FAs, due to fish farming, suggested organic matter (OM) subsidies from non-ingested and non-assimilated fish feed. In the more eutrophic reservoir branch, seston FA profiles suggested that fish farming caused an increase of low-quality food resources, such as cyanobacteria. Thus, background impact levels may determine the biochemical responses of tropical reservoirs to fish farming. Higher contributions of potentially sewage-derived and bacterial FAs during drought conditions, especially at reference sites of the more oligotrophic branch, suggested that drought shifted OM inputs towards anthropogenic sources, thereby overwriting land-use related differences between reservoir branches and homogenizing their environmental conditions. In conclusion, FA variables were useful to evaluate and understand environmental conditions, as well as the effects of early-stage fish farming and drought, and should be considered in impact assessments in tropical lentic ecosystems.