Influence of terrestrial runoff on phytoplankton species richness-biomass relationships: A double stress hypothesis

The relationship between biomass and species richness (SR) was studied along productivity gradients in coastal phytoplankton assemblages under the influence of terrestrial runoff. In particular, the investigation focused on (a) whether phytoplankton blooms affect the shape of the relationship, (b) whether the relationship is taxon specific (between dinoflagellates and diatoms), and (c) the potential mechanisms regulating the shape of the relationship. Analysis was based on phytoplankton and physicochemical data from six coastal areas in the Aegean Sea, Eastern Mediterranean, covering the whole range from oligotrophy to eutrophication. A SR-biomass relationship was not observed for the more oligotrophic areas, whereas hump-shaped curves were found in areas yielding higher maximum biomass values, corresponding to bloom events occurring mainly during winter. Furthermore, the observed hump-shaped relationships were taxon specific since they were more pronounced in the case of diatoms than dinoflagellates. It is hypothesized that SR at the leftmost part of the hump-shape is nutrient regulated, whereas at the rightmost part is limited by extreme abiotic stress occurring during the formation of blooms in coastal ecosystems. The results of this study suggest that a double stress mechanism related to abiotic factors may be responsible for the hump-shaped SR-biomass relationships occurring in coastal phytoplankton communities at various levels of productivity.