Effect of N:P ratios on response of Mediterranean picophytoplankton to experimental nutrient inputs

The effect of variations in N:P ratios on Mediterranean picoplankton Synechococcus sp. was tested with a nutrient enrichment experiment in large-scale mesocosms in a coastal Mediterranean bay community during summer 1998. By adding either N or P in excess of 42 mmol m^-2 d^-1 N or 2.1 mmol m^-2 d^-1, the mesocosm units (16 m ³) received nutrient ratios varying between 2.5 to 160, i.e. 8-fold lower to 8-fold higher than background levels (N:P 20). The total phytoplankton had increased significantly after 1 wk of the experiment on mesocosms receiving high N:P ratios (excess N), with diatoms being mainly responsible for this increase. In contrast, Synechococcus sp., which along with small flagellates initially dominated the water column, rapidly increased in biomass, abundance and primary production, with higher abundance in mesocosms receiving excess N and P, and lower abundance in mesocosms receiving the background N:P load of 20 that is normal for the Bay of Blanes. Our study indicated that this complex pattern resulted from the response of the grazing community, which was highest at the 'background' ratio. Indeed, the potential grazers of Synechococcus sp. (heterotrophic nanoflagellates and phagotrophic ciliates) were highly abundant at the background ratio. Our results also showed that primary production of Synechococcus sp. was significantly inversely correlated (r = -0.98, p < 0.05) with specific grazing rates, suggesting that picophytoplankton responses to N:P loads arose from top-down effects related to the responses of the grazing community to the various N:P levels. The top-down effects were, however, obscured during the early part of the experiment, when there were transient imbalances between growth and loss rates of picophytoplankton owing to time lags between the growth of picophytoplankton and the growth of their grazers (heterotrophic nanoflagellates and phagotrophic ciliates). This resulted in a general net increase in Synechococcus sp. abundance in all mesocosms.