Dam promotes downriver functional homogenization of phytoplankton in a transitional river-reservoir system in Amazon

Abstract

Dams disrupt the longitudinal gradient and decrease the natural connectivity of lotic systems and change the physical, chemical, and biological compartments of the river environment. Here, we investigated the effects of damming and seasonal variation in environmental conditions and richness, density, taxonomic and functional beta diversity patterns of phytoplankton, through dry and rainy seasons (of 2011 and 2012), among four zones (upriver, reservoir, dam, downriver) of the Tucurui dam (Tocantins River sub-basin, eastern Amazon, Brazil). Species were quantified and classified into three functional traits (size class, floating, life form) used to estimate functional beta diversity. We showed that water transparency was higher in the dam, and dry periods presented higher total phosphorus. Our results also demonstrated that the dam promotes species turnover between zones and seasonal periods and that the functional morphological groups respond to local limnological conditions. However, the dam led to a reduction in functional beta diversity—functional homogenization—of phytoplankton species, mainly downstream. We demonstrated that the dam is promoting unfavorable conditions and reducing the environmental heterogeneity, leading to a significant loss of functional traits variation between phytoplankton communities, which compromises the ecosystem processes provided by phytoplankton.