SEASONAL-CHANGES OF BENTHIC BACTERIA IN A SEAGRASS BED (POSIDONIA-OCEANICA) OF THE LIGURIAN SEA IN RELATION TO ORIGIN, COMPOSITION AND FATE OF THE SEDIMENT ORGANIC-MATTER

Variations in number and biomass of benthic bacteria were examined in the surface sediments of a Mediterranean seagrass bed [Posidonia oceanica (L.) Delile] in the Gulf of Marconi (northwestern Mediterranean Sea) from 1990 to 1991. The annual dynamics of benthic bacterial density and biomass were compared to changes in elemental (organic C and total N) and biochemical (lipids, proteins, carbohydrates) composition of sediment organic matter, as well as to microphytobenthic biomass, dissolved inorganic nutrients and ATP. Bacterial densities exhibited marked seasonal variations (5.12 to 322.7x10(8) cells g(-1) sediment dry wt) with highest values in late spring. Bacterial standing stocks (15.8 to 882.33 mu g C g(-1) of sediment dry wt) were high. Bacterial biomass did not correlate with organic C, total N or to specific biochemical components, but correlated significantly with chlorophyll a, ATP and porewater phosphate concentrations. There is evidence that benthic bacteria were responding to variations of algal biomass. Bacterial biomass accounted, on average, for 30% of total living carbon (calculated on the basis of the ATP concentrations) and 8.4% of total organic carbon.