Quantifying precipitation, N:P ratio, and river properties
We calculated the mean monthly precipitation (MMP) from 2005 to 2021 for each month. The MMP data was obtained from the WorldClim 2.0 database (http://www.worldclim.org) at a 1-km² spatial resolution. To quantify N:P ratio in the water, we took water samples during the fish sampling to measure total phosphorus and total nitrogen (μg l−1). Total nitrogen was converted to nitrate following the protocol of Valderrama (1981) and analyzed as NO3− by the standard sodium salicylate (Müller & Wiedemann 1955). Total phosphorus (P) was measured according to Valderrama (1981). The water discharge was measured using data available from the water volumes discharged by Salto Grande, a dam located approximately 200 km upstream from the sampling sites, while turbidity (expressed in NTU) was measured using a turbidimeter (LaMotte, Chestertown, MD, USA).