Phytoplankton and macroinvertebrate diversity and eco-exergy responses to rainfall diverge in semiarid reservoirs

Rainfall is a variable that drives community structure in aquatic ecosystems and is even more important in dry regions. Most studies have evaluated how the seasonal variation of rainfall influences aquatic communities. However, studies that evaluate the influence of spatial gradients of rainfall on aquatic organisms are scarce, especially regarding biological diversity and thermodynamic information. This study evaluated the response from diversity metrics and thermodynamic indices (eco-exergy and specific eco-exergy) in phytoplankton and benthic macroinvertebrate communities in 13 reservoirs distributed along a spatial gradient of rainfall in the Brazilian semiarid region. We hypothesized that the phytoplankton and benthic macroinvertebrate communities are congruent in response to the spatial gradient of rainfall in terms of diversity and thermodynamic indices in semiarid reservoirs. We expect to find higher values of biomass and the eco-exergy index under lower rainfall. To test our hypothesis, generalized linear models (GLMs) were used to relate the effects of rainfall changes on community structure. The eco-exergy and specific eco-exergy indices of the phytoplankton community were positively related to rainfall and reflected a greater investment in biomass production and genetic information (e.g., Chlorophyceae). As for the macroinvertebrate community, the reduction in rainfall promoted a greater investment in energy storage by generalist organisms (e.g., Melanoides tuberculata and some genera of Chironomidae). Thus, for a holistic evaluation of ecosystem health, the joint analysis of thermodynamic indices associated with an assessment of taxonomic composition, diversity and taxonomic richness is important, providing a more complete and multifaceted response. Additionally, thermodynamic indices better reflected the changes in both aquatic communities in response to rainfall, and were more sensitive than the diversity and richness indices. Consequently, the thermodynamic indices represent an efficient tool in the assessment of aquatic community responses to climate change, even when these communities respond in an opposite manner.