Spatial heterogeneity and seasonal succession of phytoplankton functional groups along the vertical gradient in a mesotrophic reservoir
This study based on analysis from vertical profiles of phytoplankton FGs (both on a morphology-based approach and according to the functional groups), their relative biomass, physical and chemical parameters, through monthly samplings over a two years period, at three stations located along the longitudinal axis in the monomictic, mesotrophic Mediterranean reservoir. Nutrient dynamics, temperature and light availability were mainly driven by the water circulation patterns, governing the phytoplankton morphological variability (C→S→R) in İkizcetepeler reservoir. Nutrient availability and temperature constituted the main environmental constraints influencing phytoplankton dynamics, while zmix/zeu ratio played a secondary role. A similar conclusion for mesotrophic lakes was achieved by Naselli-Flores (2000) by studying 21 Sicilian man-made lakes. Despite the variations in zmix/zeu ratio as a result of stratification vs. mixing events and water withdrawal, C-strategists were typical representatives of phytoplankton assemblage. Groups Y, P, B, J, F, Lo and Tc showed heterogeneous distribution along the water column and vertical heterogeneity of phytoplankton was affected by nutrients (especially DIN values), temperature and zmix/zeu values which were mainly driven by the water circulation pattern. 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.
Similar studies about phytoplankton structure along the vertical gradient at seasonal time scales are not so detailed, and, to our knowledge, vertical distribution of phytoplankton functional groups was not investigated in Mediterranean reservoirs and only a short term study (Becker et al., 2009b) and a long term one (Crossetti and Bicudo, 2008) were performed in tropical Brazilian reservoirs. Therefore, our results provide evidence on vertical heterogeneous distribution of phytoplankton functional groups in mesotrophic Mediterranean reservoir, allowing for a deeper understanding of the role of different groups in such systems.