Occurrence of mucous aggregates and their impact on Posidonia oceanica beds.

Mucous macro-aggregates of both pelagic and benthic origin are recurrently observed in the meadows formed by the seagrass Posidonia oceanica around the island of Ischia (Gulf of Naples, Italy). In the past two decades, major events occurred in 1991, 1993 and 2000, when a thick layer of mucilage covered vast areas of the meadows. To investigate the environmental triggers for mucilage formation and the effects of macro-aggregates on the functional and structural status of P. oceanica, a number of abiotic and biotic parameters were monitored over three years within the frame of a research project on Adriatic and Tyrrhenian mucilages (MAT). Basic environmental parameters (salinity, temperature, irradiance, dissolved oxygen concentration) in the water column and inorganic nutrient concentrations above and within P. oceanica meadows were measured. As descriptors of the status of the seagrass, shoot density and the nitrogen content of the leaves were monitored. Moreover, a reconstructive technique (lepidochronology) was employed to track back annual plant production. During the three-year study, mucous aggregates produced by benthic algae were observed in spring-summer and a major event of macro-aggregates of pelagic origin affecting P. oceanica canopy was observed in autumn 2000. Each of these episodes was observed for a few weeks to one month. We hypothesize nutrient limitation to explain the short duration of the benthic algal blooms that were responsible of macroaggregate formation; a highly dynamic circulation resulted in the dispersion of the pelagic aggregates deposited on the meadows. None of the descriptors of the structure of the meadow and of plant performance showed any obvious alterations in relation to the occurrence of mucilage coverage. Presumably, the short duration of the mucilage episodes recorded were not enough to induce such alterations, at least at the temporal and spatial scales considered.