How does the storage volume of semi-arid reservoirs change water quality and modulate the diversity of benthic macroinvertebrates?

In the current environmental crisis scenario, it becomes increasingly important to understand the hydroclimatic effects on the availability and quality of water resources and their impacts on aquatic biodiversity. The understanding of how these components are interconnected is necessary to support initiatives for monitoring and managing water resources and protecting biodiversity. In this study, we analyzed the hierarchical effects and dissociated the complex relationships between the storage volume of tropical semi-arid reservoirs on water quality and the diversity of benthic macroinvertebrates. We analyzed these nested hierarchical relationships using Structural Equation Modeling (SEM). Our results showed that the storage volume of the reservoirs exhibited a negative effect on total phosphorus (TP) but a positive effect on chlorophyll-a, and these parameters exhibited mediated effects on the richness and abundance of benthic macroinvertebrates. While TP had a positive mediated effect on abundance, chlorophyll-a exhibited a negative mediated effect on the abundance and richness of benthic macrofauna. Furthermore, DIN exhibited only a direct negative effect on richness and the Simpson's index. These results indicate that in periods of increased storage volume, there is a reduction in TP concentrations in water due to their dilution. However, despite this dilution, nutrient concentrations in ecosystems still favor primary productivity. We found that semi-arid reservoirs are ecosystems susceptible to environmental degradation, even in periods of increased storage volume due to the hydrological dynamics of the region. This causes the enrichment of nutrients in the reservoirs, with noticeable impacts on the diversity of benthic macroinvertebrates. These results highlight the need for adequate monitoring and management measures of strategic water resources in semi-arid regions to promote the health and integrity of reservoirs and the promotion of their ecosystem services.