Mixotrophic species influencing phytoplankton community structuring during the filling phase of a subtropical reservoir

Aiming to evaluate temporal alterations in the dynamics of the phytoplankton community in response to environmental changes occurring during the filling phase of Santa Clara reservoir, samples were taken weekly from April to June 2005 in the region close to the dam, following vertical gradients of light and temperature. Sampling depths comprised the subsurface (sup), upper limit of the euphotic zone (Z(eu)), lower limit of the epilimnion (Z(mix)) and near the bottom (Z(max)). Low light availability hindered phytoplankton growth throughout the period. Descriptor species (biovolume > 5%) were classified in 17 functional groups (FGs): X2, Y, D, C, P, Lo, E, B, Na, MP, A, J, K, S1, Ws, F and M. Phytoplankton biovolume was low during the filling phase, and characterized a meso-oligotrophic environment. Mixotrophic species belonging to functional groups X2, Y, Lo and E provided the greatest contribution to total biovolume and dominated throughout the period. Water column thermal structure and a tendency of a decrease in nutrient concentrations influenced the change in the dominance of FGs. Our results suggest that mixotrophic strategy was determinant for phytoplankton production and biomass, representing a competitive advantage over strictly autotrophic phytoplankton, suggesting that during the filling phase of Santa Clara reservoir, the microbial loop played an important role in the flow of energy and carbon to higher trophic levels of the food web. Therefore, our initial hypothesis that the disruption of longitudinal connectivity in the river and alterations in the mixing regime and light availability would change phytoplankton community structure causing a replacement in the dominance of FGs containing turbulence-tolerant species by FGs containing species with a higher demand for water column stability and light availability was not corroborated. Although a species replacement was observed, this was not due to enhanced resource availability, but rather due to the trophic abilities of the FGs.