The interplay between nutrient load, N partitioning and turbulence: three key factors in the dynamics of coastal plankton

Abstract

Two intensive surveys were conducted in the coastal waters of Barcelona (NW Mediterranean) to assess short-term variations of biological parameters in relation to environmental conditions. We focused on two attributes: wave height, as a proxy for mechanical energy entering the system, and nutrient inputs, whose variability in total load and relative composition is a central characteristic of coastal areas. The interplay between nutrients and turbulence was also studied by means of two experimental simulations with natural planktonic communities. Experimental nutrient additions varied both in terms of total nutrient load and in the balance of elements, with a particular focus on N sources. In the experiments, shifts in community composition appeared to be mostly related to the interaction between turbulence and N partitioning: under still conditions, ammoniumrich waters favored small organisms and reinforced the microbial loop, whereas nitrate mostly favored diatom growth. Turbulence added complexity to the final outcome, because mixing tended to favor large over small osmotrophs. The results suggested that the rapid growth of small autotrophs and heterotrophic bacteria in ammonium-rich waters could be partly counteracted by diatom increases if nutrient pulses were coupled with turbulent mixing, and diatom bursts after nitrate enrichments could be largely enhanced by concomitant turbulence. This was consistent with the observations in the field, where a fine balance between moderate mixing and nutrient availability pushed our system toward diverse trophic pathways: sudden nutrient fluxes uncoupled from water motion tended to favour bacteria and heterotrophic nanoflagellates, while their concurrence with some water-column mixing shaped a favorable scenario for large autotrophs. Ultimately, these two distinct biological responses pointed toward two main disturbance scenarios: Episodes of nutrient enrichment uncoupled from mixing, mostly related to episodic water spills from the nearby city; episodes of increased wind caused by passing weather fronts that promoted some water column mixing and the entrainment of nutrients from bottom sediments or from adjacent water masses