Partitioning beta diversity of aquatic macrophyte assemblages in a large subtropical reservoir: prevalence of turnover or nestedness?

The identification of factors promoting biodiversity and, more specifically, the causes that contribute to beta diversity is a central and growing issue in ecology. In this study, we used long-term data (1999-2007, including a pronounced drawdown in 2000) to assess the beta diversity partitioning of aquatic macrophyte assemblages in five arms of the Itaipu reservoir. We partitioned beta diversity (beta(sor)) into two components: nestedness (beta(nes)) and species turnover (beta(sim)). To check whether environmental variables (electrical conductivity, water transparency, and fetch) were correlated with the components of beta diversity, we used a permutational multivariate analysis of variance. Over the study period (1999-2007), we observed high values of beta diversity and prevalence of spatial turnover of the species. They were correlated with all environmental factors studied, which demonstrated significant interactions. Dendritic-shaped arms, high shore development, local physical and chemical conditions of the water, and biotic interactions could be determining factors for the prevalence of spatial turnover of aquatic macrophyte assemblages. The drawdown promoted an enormous quantity of dead macrophyte biomass, specifically submerged species that preferentially colonized shallow margins of the reservoir. On the other hand, large open areas were available for colonization of ruderal species and seed bank germination. This dynamic led to an increase of beta diversity and turnover values in the year of the drawdown compared to the 1999 and 2001. This pattern remained until the end of the study, characterizing the aquatic macrophytes of the Itaipu reservoir as an assemblage with high beta diversity and prevalence of species turnover over nestedness, among and within arms.