Dynamic forcing of coastal plankton by nutrient imbalances and match-mismatch between nutrients and turbulence

We carried out 2 experimental simulations in which natural planktonic communities were subjected to a combination of turbulence and variable nutrient forcing. Nutrient addition was varied both in terms of total nutrient load and in the balance of elements. Unbalanced nutrient inputs (with regard to Redfield values) and varying nitrogen sources are frequent in coastal areas heavily exposed to human perturbations. The relative abundance of nitrate and ammonium has been related to shifts in the size distribution of the planktonic community, so we explored whether uneven inputs of ammonium and nitrate could be major factors driving plankton dynamics in coastal environments. The occasional uncoupling between turbulence and nutrient pulses is also a distinctive feature of nearshore waters compared with open ocean environments. Thus, we further tested the match–mismatch between nutrient enrichments and turbulent mixing. Both turbulence and nutrient inputs had a significant positive effect on the growth of planktonic organisms, with some synergistic effects. Shifts in community composition appeared to be mostly related to the interplay between turbulence and N partitioning. Under still conditions, ammonium-rich waters favoured small organisms and reinforced the microbial loop, whereas nitrate mostly favoured diatom growth. Turbulence added complexity to the final outcome, because mixing tended to favour large over small osmotrophs. Accordingly, the rapid growth of small autotrophs and heterotrophic bacteria in ammonium-rich waters may be partly counteracted by diatom increases if nutrient pulses are coupled with turbulent mixing; diatom bursts after nitrate enrichments may also be enhanced by concomitant turbulence.