Environmental and Hydrological Variation Influence Fish Size Spectra in the Upper Paraná River Floodplain

ABSTRACT Smaller individuals tend to be more abundant than larger individuals. Variations in this pattern can be assessed through size spectra analyses, which describe the relationship between abundance and body size. The parameters estimated from these analyses—midpoint height (expected abundance at the midpoint of the size class axis) and slope—can indicate the effects of natural environmental variability and anthropogenic impacts on biological communities. In this study, we analysed a long‐term dataset on fish communities in the Upper Paraná River floodplain to assess temporal variations in size spectra slopes and midpoint heights, as well as their responses to varying levels of impact from damming, variation in water level, and different proxies of productivity (total phosphorus, total nitrogen, and chlorophyll‐ a concentrations). We also tested interactions between levels of impact, productivity, and time. We found that size spectra slopes were more negative at less impacted sites, where the smallest size classes also occurred predominantly. Water level was positively correlated with the abundance of the fish community (represented by the midpoint height) and the occurrence of the smallest size class. Total phosphorus concentration was negatively correlated with the slope. Over time, we observed a reduction in the magnitude of the slope, related to a decrease in the abundance of the smallest size classes. Our results highlight the importance of floods for fish recruitment and suggest that size spectra estimates are not always consistent indicators of natural change or anthropogenic impact, as they may respond in complex ways to both sources of variation