Effect of bacteria on the phytoplankton response to P-replete and P-deplete riverine water inputs

Human population growth and intensified anthropogenic activities are altering the Earth environment, especially in coastal areas. The increase in N and C compared to P nutrient inputs via continental runoff that is, the increase in C:P and N:P ratios, might cause P limitation in microbial communities. We used a mesocosm approach to elucidate the impact of riverine water inputs with different P availability (P-deplete vs P-replete) on bacterial and phytoplankton communities from the Ría de Vigo (NW Iberian Peninsula) in winter, spring, and autumn. Generally, phytoplankton growth was stimulated (up to 1.3-fold) while bacterial abundance was reduced (up to 0.79-fold) after riverine water inputs. Intriguingly, P-replete riverine inputs did not significantly stimulate bacterial growth causing even a similar reduction (up to 0.87-fold) than the P-deplete riverine water inputs in spring, suggesting that P-limitation would not be the only factor explaining the overall negative impact of riverine water discharge on bacteria. Overall, inputs of P-depleted (high DIN:DIP ratio) riverine water caused a relaxation in the competition between phytoplankton and bacteria and, on some occasions, led to a beneficial impact of bacteria on phytoplankton response. Our findings strongly support that the expected future increase in P limitation in this coastal system could reduce bacterial abundance due to their high P demand compared to that of phytoplankton, which could alleviate competition with phytoplankton and promote positive interactions but would also lead to reduced remineralization rates in the long term.