Functional rather than taxonomic diversity reveals changes in the phytoplankton community of a large dammed river

Abstract

River damming reduces the dispersal rates of biota and filter species by altering the local environmental conditions. Although phytoplankton is a key biotic group to indicate changes in aquatic environments, the effects of river damming on the longitudinal distribution of phytoplankton communities are not widely addressed. Here we investigated the changes on the taxonomic and functional composition of the phytoplankton community in a large tropical dammed river. We expect compositional changes to be higher at taxonomic than functional level especially in the dam-free stretches of the river because running waters select species able to cope with the water turbulence. Furthermore, flow regulation allows species with different traits to colonize the increasingly lentic stretches resulting in higher functional diversity towards dams. Phytoplankton samples were taken in 210 sites, spaced by 5 km, along the whole river channel. Data were split at a reservoir level considering the sites within 30 km before and after each dam to analyze the effects of damming on the community composition. Compositional changes were assessed by estimating the degree to which each community differed from all other communities within the regional pool (community distinctiveness) considering taxonomic and functional composition separately. Functional diversity was estimated as the degree to which species within the same community differed from each other with respect to their trait composition (functional dispersion). We used community-weighted means to test how the different traits were filtered by river damming. Furthermore, we assessed the effects of local environmental conditions on the phytoplankton taxonomic and functional composition using boosted regression trees (BRT). We observed no effects of river damming on species dispersal. Functional turnover was low, suggesting that compositional changes occurred mainly among functionally similar species. However, there was a noticeable reduction of phytoplankton diversity that persisted for more than 100 km after one of the dams. Flow regulation allowed flagellated and mucilaginous species to colonize the dammed areas. After the dams, such species were filtered due to the increase of water mixing downstream, as evidenced by a hump-shaped trend in the functional diversity of the sites within 30 km distance of the dams. BRT analyses highlighted the role of nutrient input in the dammed areas for the community compositional change. This study highlights the importance of considering both taxonomical and functional diversity to describe the effects of human intervention such as damming on aquatic ecosystems.