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.