Influência da dispersão e filtragem ambiental sobre a estruturação das comunidades planctônicas em uma cascata de reservatórios em um rio tropical.

Abstract

Planktonic communities play fundamental roles in aquatic ecosystems, being essential for the production and flow of matter and energy in these environments. This study evaluated the effects of a cascade of reservoirs on the mechanisms that shape the structure of different planktonic communities. Specifically, it was expected that: i) The species richness of planktonic communities will decrease downstream of the Tietê River cascade; ii) Along the trophic gradient between reservoirs, upstream and downstream, the environmental filtering process will be more evident for larger organisms, while communities of smaller organisms will not be structured by dispersal limitation processes. Environmental DNA samples were collected in the Barra Bonita (BB), Promissão (PR), Nova Avanhandava (NO), and Três Irmãos (TI) reservoirs, located along the Tietê River, during 6 campaigns. The results showed that, in general, reservoirs with an intermediate trophic level were the most diverse when considering total plankton richness. Although all the planktonic communities studied here are small organisms with a high capacity for passive dispersal, those with larger components, such as phytoplankton and cyanobacteria, protozooplankton and zooplankton, were considerably influenced by space and time, that is, by the influence of the species-sorting mechanism (environmental filtering). On the other hand, bacterioplankton responded as expected for small organisms, being essentially structured by the mass effect mechanism (without dispersal limitation). A redundancy analysis showed that the hydrological gradient and the degree of trophic state influenced the assembly of the communities, although the effects of environmental variables were specific to each community. It is concluded that the structuring of planktonic communities along a cascade of reservoirs has a variable contribution between dispersal and environmental filtering mechanisms, in addition to an important contribution from the trophic state, with considerable dependence on the size of the organisms.