Study area
This study was conducted in the Río Uruguay, a large South American river, encompassing an extension of 1,838 km from headwaters to mouth, and a basin area of about 350 km2 (Fig. S1). In this river, flood pulse is determined by precipitation in the upper two-thirds of its catchment drainage area, which increases during El Niño Southern Oscillation (ENSO) events (Krepper et al. 2003). This region is characterized by a subtropical climate with a mean annual temperature of 17.4ºC and mean precipitation of 1200 mm year-1. To provide a comprehensive spatial assessment, fish community monitoring was carried out at three different sites on the Río Uruguay, namely Fray Bentos, Nuevo Berlín, and Las Cañas (Fig. S1). These three monitoring sites are located on the littoral zone of the main channel of the Uruguay River along a distance of 39 km (Fray Bentos at 24km of Nuevo Berlín, and Nuevo Berlín at 15km of Las Cañas), thus it represents true independent replicates. Furthermore, to provide a comprehensive temporal assessment each river site was sampled four times per year (i.e., two samples were conducted in two different climatic seasons: autumn and spring) over 17 years (2005-2021). Importantly, in 2005, 2006, 2020, and 2021 only two annual samples at each river site were conducted (totaling 6 annual samples). In the remaining years (2007-2019) four annual samples were conducted (two samples per climatic season, totaling 12 annual samples). In total, 179 samples were collected over 17-years. The fish community was collected using the standard Nordic multimesh gillnet that presents 30 m-long nets with 12 sizes of mesh from knot to knot (5, 6.25, 8, 10, 12.5, 15.5, 19.5, 24, 29, 33, 43, and 55 mm, respectively). This sampling method allows different sizes of fish to be surveyed, which is crucial in food web studies. A detailed explanation of fish sampling is provided in the Appendix (Assessing fish community). All fishes were identified at the species level, and their abundances (number of individuals per unit effort - CPUE) were quantified.