The role of aquatic macrophyte complexity and heterogeneity in structuring fish assemblages

Abstract Aquatic macrophytes function as key habitat-structuring elements, providing habitat complexity and heterogeneity. This study evaluated the relationships between macrophyte density (proxy for complexity) and functional diversity (proxy for heterogeneity) and the abundance, taxonomic richness and mean total length of associated fish. Fish and plants were sampled in multispecific stands, resulting in a gradient of macrophyte density and functional diversity. Relationships between fish abundance and richness and macrophyte attributes were tested using generalized linear models (GLMs), while the relationship between fish size and macrophyte density was tested using a linear mixed model (LMM). Highest fish abundance occurred in macrophyte stands with intermediate density and functional diversity, showing significant hump-shaped responses, suggesting that intermediate levels promote a balance between habitat accessibility and resource availability. Fish total length decreased with increasing macrophyte density, indicating that the small interstitial spaces in complex habitats can restrict the movement and presence of larger-sized individuals. These results reinforce the role of macrophytes in habitat structuring, modifying the structure of fish fauna and filtering trait composition according to habitat complexity and heterogeneity. These findings also highlight the importance of using a combination of metrics, such as density and functional diversity, to comprehensively assess the dynamics of fish assemblages.