Effects of oligotrophication on fish growth: Insights from a mesocosm experiment

Human activities can frequently affect aquatic ecosystems in irreversible ways, with flow regulation and water quality being major concerns. Dams can promote the retention of sediments and nutrients, increase water transparency, and reduce primary productivity, all of which characterize the oligotrophication process. Although the beneficial effects of oligotrophication in eutrophic systems are well documented, the oligotrophication process caused by river impoundment can negatively affect aquatic communities in non‐eutrophic systems. Our study aimed to assess the effects of oligotrophication on the specific growth rate of small‐bodied fish. We hypothesized that the fish‐specific growth rate would decrease under an oligotrophic scenario, due to bottom‐up processes. We performed a full‐factorial experiment with nine treatments, using three levels of inorganic turbidity crossed by three levels of nutrients, in mesocosms during 24 d. In each tank, we added plankton and 30 individuals of Moenkhausia forestii , a fish species with omnivorous feeding habits. The fish were weighed at the beginning and at the end of the experiment to calculate specific growth rates. We observed a significant interaction between inorganic turbidity and nutrients on the specific growth. The smallest reduction in growth rate occurred in treatments with intermediate turbidity and high nutrient levels (T2N3), and high turbidity and high nutrient levels (T3N3). Our results indicate that changes in inorganic turbidity and nutrients, coinciding with oligotrophication caused by river dams, would negatively affect fish growth, which could have severe implications for fishery production and irreversible impacts on aquatic ecosystems.