The complexity of aquatic macrophytes and environmental variables as a filter of functional traits of aquatic invertebrates from a subtropical stream in southern Brazil

Aquatic macrophytes play a key role in structuring aquatic ecosystems due to their varying degrees of structural complexity and perform diverse functions for different organisms (e.g., refuge and food resources). Environmental variables can serve as filters by selecting organisms with characteristics adapted for establishment in these environments. The aquatic invertebrate community is the most frequent and abundant biotic component associated with these plants. In this study, we investigated whether macrophyte complexity and environmental variables affect the richness, abundance and functional traits of the aquatic invertebrate community across different seasons in a subtropical stream. We collected different species of macrophytes in the fall and spring of 2021 and defined the degree of structural complexity —low (C1), medium (C2) and high (C3)— based on biotype and plant biomass. We washed the plants under running water and identified the organisms at the lowest possible taxonomic level. Five functional characteristics were defined: size, life stage, reproduction, body shape and eating habits. We determined the richness, abundance, frequency of occurrence, density and frequency of functional traits of invertebrates, and calculated the Shannon-Weiner index, Simpson dominance and evenness. A total of 1650 individuals were recorded in the fall and 1228 in the spring. The lowest diversity, equitability and dominance were observed for C1 in autumn and for C3 in spring. Autumn showed a greater number of categories of functional traits than spring. The flow and width of the stream showed greater amounts of correlations with the functional traits. The different degrees of macrophyte complexities influence the metrics and functional traits; however, environmental variables act primarily on them.