Diversidade funcional do fitoplâncton em reservatórios em cascata e funções ecossistêmicas associadas

Abstract

Understanding the patterns of species distribution in the environment and what role it play in the ecosystem is one of the great challenges of ecology. Planktonic communities can be used as biological indicators in the monitoring of aquatic environments, since they signal changes in ecosystems through changes in their structure, composition and metabolism. The functional approach, based on the use of functional traits of the species as descriptors, signals in a more comprehensive changes in ecosystem functions, such as primary production and energy transfer to other trophic levels. Thus, the aim of this study was to explore how changes in the taxonomic composition of the phytoplankton community are reflected in changes in the functional traits of communities in cascading reservoirs. The main hypothesis was that the taxonomic and functional diversity of the phytoplankton community would be different between the three reservoirs and the spatial variation being more explanatory than the temporal variation for changes in the structure and composition of the phytoplankton. The study was carried out over 6 years (2013-2018) in three cascading reservoirs along the Paraibuna River, Paraíba do Sul river basin. There was no separation in the distribution of points by reservoirs in terms of environmental variables, so that these remained overlapping throughout the sampled period. However, seasonal variation was observed over the 6 years studied. The two most upstream reservoirs showed very similar characteristics in terms of taxonomic composition of phytoplankton and zooplankton. The last reservoir in the cascade, however, showed greater diversity for both groups. The trait ‘single-cell arrangement’ was the one with the greatest contribution in all reservoirs. Nevertheless, the presence of silica structure and mixotrophy had a considerable contribution. Functional richness also varied between reservoirs. Likewise, the relationship between the Shannon index and the CWM proved to be positive for the upstream reservoirs, indicating a trend between increasing diversity and the CWM. On the other hand, it was not possible to observe a trend in the reservoir downstream. The results showed that changes in the structure of the phytoplankton community altered the contribution of the functional traits of this community in ecosystems, with potential impacts on the upper trophic levels (zooplankton) and associated ecosystem functions. Still, there were changes in the structure and composition of the zooplankton community, indicating that the quality of the food resource can be a guiding factor for this community. Finally, spatial characteristics of the systems, such as their position in the cascade system, were a determining factor for the structure of the phytoplankton community.