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

Tracking morphological variability of phytoplanktonis 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 phytoplanktonmorphology. However, to fully understand the effects of mixing and trophic state on phytoplanktonfunctional group (FG) distribution, the entire water columnmust be considered since integrated or sub-superficially collected samples, may not adequately represent phytoplanktonstructure. This idea was tested by analyzing vertical profiles of phytoplanktonFGs, 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, mesotrophicIkizcetepeler reservoir, northwestern part of Turkey. Thermal stratification occurred in the reservoir from April to September, and z mix / z eu values increased after the breaking down of the thermocline. Nine FGs characterized the seasonal cycle and C- strategistorganisms were typical representatives of the assemblages. Nutrient availability and temperature were found to influence phytoplanktondynamics, whereas water withdrawal played a secondary role. Groups Y , P , B , J , F , Lo and Tc showed heterogeneous distribution along the water columnand, temperature heterogeneity, high DIN values in surface waters, z mix / z eu values and sedimentation loss were the most important factors regulating their distribution. The results show that phytoplanktonmorphological spectrum throughout the entire water column, contributes useful information to assess the influence of mixing and trophic state on phytoplanktondynamics in mesotrophicreservoirs.