Specific responses to nitrogen and phosphorus enrichment in cyanobacteria: factors influencing changes in species dominance along eutrophic gradients.

Anthropogenic eutrophication is a worldwide problem, causing proliferation of cyanobacterial masses, some of which may be toxic. However, little is known about whether the response to nutrient enrichment differs among cyanobacterial species. To address this issue, distinct patterns in growth and competitive response of benthic cyanobacteria under N and P nutrient regimes were studied. Nine cyanobacterial species, collected from Guadarrama river biofilms at several locations with different nutrient concentrations, were isolated and used for a series of N and P enrichment bioassays. In competition experiments with a mixture of all nine species, a great predominance of certain cyanobacteria over others was noted at high nutrient conditions, while under low nutrient conditions some others dominated. On the basis of these results four selected strains were subjected to a gradient of different concentrations of phosphate, nitrate and ammonium, in independent bioassays, both in monocultures and mixed cultures. Depending on the concentration of N and P, stimulation or inhibition of growth was observed. Some species grew better, dominating at high nutrient concentrations, while higher yields were recorded for others under low nutrient regimes, dominating in these conditions. Results from this study clarify previously published field observations, whereby a group of species occurred mostly in downstream nutrient-rich locations, while other was typical of upstream oligotrophic conditions. Our findings concerning differential growth in relation to nutrient concentrations may be useful for environmental management, because they help us predict which cyanobacteria may be expected to occur under certain conditions.