Phytoplankton strategies and diversity under different nutrient levels and planktivorous fish densities in a shallow Mediterranean lake

Two mesocosm experiments were carried out to investigate the dynamic effects of nutrients (nitrogen and phosphorus) and planktivorous fish additions on phytoplankton strategies and diversity. The phylogenetic and functional approaches were used to understand phytoplankton ecology in shallow Mediterranean lakes. The experimental approach is new for the study of algal functional groups. Nutrient loading and fish stocks enhanced biomass of small algae, but decreased phytoplankton diversity and species richness. Faster species replacement and fluctuations in diversity occurred above loadings of 1 μM P and 21 μM N. Mesotrophic conditions favoured a diverse pool of species, including nostocales and unicellular flagellate algae (functional groups Sn, S1, L0, Y, Reynolds et al., 2002). C-strategist chlorophytes (small algae from functional group X1) dominated mid-successional assemblages with good light and accessible nutrients. High nutrient concentrations, dim light, presence of organic matter and of larger zooplankton favoured to functional groups S1 of oligophotic filamentous cyanobacteria and J of mixotrophic Scenedemus species. Intermediate nutrient levels wit h total phosphorus ≤10 μM, water quiescence, transparency and smaller zooplankton prompted dominance of chroococcal cyanobacteria (functional groups X1 and K). Resulting patterns agree and reinforce the validity of plankton functional groups associated with warm, shallow enriched systems, although some changes in the groups are suggested in relation to the structuring role of nutrients and grazing on the functional scheme for phytoplankton.