Abstract
The effects of sediment elutriate, prepared from a sediment resuspension simulation process, on the growth of bacterioplankton and of different sized phytoplankton as well as on their grazing by protozooplankton were investigated in Mediterranean coastal ecosystems during spring phytoplankton bloom. In parallel, the response of bacterioplankton, phytoplankton and protozooplankton to sediment elutriate enrichment was assessed in microcosm experiments. Elutriate addition provoked a high enrichment in nutriments of the overlying water, particularly in ammonium, dissolved organic matter but also in trace metal elements and herbicides. Overall, elutriate addition significantly increased the biomass of bacterioplankton and of total phytoplankton, but the phytoplankton size fractions responded differently to elutriate enrichment. The picophytoplankton showed a significant strong enhancement in both growth and production rates, relatively to control, while these rates significantly decreased for nano- and microphytoplankton. Accordingly, the phytoplankton size structure shifted in elutriate microcosm toward a dominance of the picophytoplankton concomitant with a significant reduction in larger size fractions. The elutriate addition caused also a clear shift in the taxonomic composition of protozooplankton, associated with a significant modification of its functional diversity, with the dominance of pico-sized cell consumers (such as aloricate ciliates, heterotrophic nanoflagellates and mixotrophic dinoflagellates) in the detriment of grazers of nano- and microphytoplankton (such as heterotrophic dinoflagellate). The protozooplankton grazing on small prey (bacterioplankton and picophytoplankton) increased, while feeding on nano- and micro-sized phytoplankton decreased after elutriate addition. These results have implication for the carbon transfer pathway, which could change from an herbivorous food web to a microbial food web, influencing so the exportation of biogenic carbon through the other trophic compartments.