Spatial heterogeneity and seasonal succession of phytoplankton functional groups along the vertical gradient in a mesotrophic reservoir

Tracking morphological variability of phytoplankton is a useful tool to better understand environmental changes. Highly dynamic systems such as reservoirs are convenient environments to investigate the effects of environmental variables on phytoplankton morphology. However, to fully understand the effects of mixing and trophic state on phytoplankton functional group (FG) distribution, the entire water column must be considered since integrated or sub-superficially collected samples, may not adequately represent phytoplankton structure. This idea was tested by analyzing vertical profiles of phytoplankton FGs, their relative biomass, physical and chemical parameters, through monthly samplings over a 2 years period, at three stations located along the longitudinal axis in the monomictic, mesotrophic I kizcetepeler reservoir, northwestern part of Turkey. Thermal stratification occurred in the reservoir from April to September, and zmix/zeu values increased after the breaking down of the thermocline. Nine FGs characterized the seasonal cycle and C-strategist organisms were typical representatives of the assemblages. Nutrient availability and temperature were found to influence phytoplankton dynamics, whereas water withdrawal played a secondary role. Groups Y, P, B, J, F, Lo and Tc showed heterogeneous distribution along the water column and, temperature heterogeneity, high DIN values in surface waters, zmix/zeu values and sedimentation loss were the most important factors regulating their distribution. The results show that phytoplankton morphological spectrum throughout the entire water column, contributes useful information to assess the influence of mixing and trophic state on phytoplankton dynamics in mesotrophic reservoirs.