Résumé
We experimentally simulated the temporality of the interactions between intertidal seagrasses and phytoplankton in the context of nutrient enrichment from continental origin. Phytoplankton development was retarded after 1-day exposure to intertidal Zostera noltii Hornemann vegetation samples, with respect to control treatments. This was not explained by resource competition and we hypothesized a direct interference between seagrass leaves and phytoplankton. After separation from the vegetation in 5-day incubations, the final yield of large phytoplankton, mainly diatoms and haptophytes, was determined by nutrient loading. In contrast, Synechococcus-like cells (SYN), phototrophic picoeukaryotes and nanophytoplankton decreased after day 3, most likely due to grazing control. A second experiment was designed to test the pelagos-benthos coupling in more detail. Therefore, we compared the effect of aboveground biomass of Z. noltii alone with the effect of the entire vegetation including the sediment. This experiment did not provide unequivocal support for our initial hypothesis on direct interference competition between Z. noltii leaves and phytoplankton. Surprisingly, we found a spectacular decrease of SYN, phototrophic picoeukaryotes and nanophytoplankton in treatments with Z. noltii vegetation including the sediment, which could be attributed to benthic grazing by suspension feeders.