Unravelling the influence of cattle stocking rate on the macroinvertebrate community of freshwater wetlands subjected to hydrological modifications in three hydroclimatic periods.

The Paraná River Delta in South America, a large wetlands macromosaic, faces threats from climate change, human activities like livestock intensification, and hydrological modifications driven by the construction of water management infrastructure to prevent flooding in productive lands. Macroinvertebrates, essential for wetland health, are affected by cattle-induced changes in water quality, nutrient enrichment, and trampling, posing challenges to the ecosystem's ecological balance and long-term survival of these organisms. In this study, we analyzed the impact of two categories of cattle stocking rates (low and high) on the taxonomic and functional structure of the aquatic macroinvertebrate community in freshwater marshes. In addition, we compare the influence of cattle stocking rate on macroinvertebrates in natural and modified freshwater marshes, and, finally, the effect of cattle stocking rate in three contrasting hydrometeorological periods: a drier, a humid, and an extreme drought period. Samplings were conducted in 16 freshwater habitats of the Lower Paraná River Delta, examining variables such as temperature, pH, dissolved oxygen, coliforms, and nutrient concentrations. Macroinvertebrates were collected and functional and taxonomic metrics were estimated. Statistical analyses, including ANOVA and Kruskal-Wallis tests, were conducted to evaluate the effects of cattle stocking rates, hydrological modifications, and hydrometeorological periods on macroinvertebrate metrics and environmental variables. RDA, PERMANOVA, and SIMPER analyses explored the relationships between assemblage composition and environmental factors. High stocking rate altered the community structure, modifying its composition and decreasing the density, taxonomic and functional richness. Moreover, hydrological alterations exacerbated these negative impacts of cattle overstocking in macroinvertebrates. Under severe drought conditions, only tolerant species can survive cattle overstocking conditions. Our findings provide relevant insight into the ecological risks associated with cattle overstocking in natural and modified freshwater marshes and underscore the need to control cattle stocking rates in extreme drought to avoid loss of ecological functions.