SEASONAL AND INTERANNUAL VARIATION OF BACTERIA IN A SEAGRASS BED OF THE MEDITERRANEAN-SEA - RELATIONSHIP WITH LABILE ORGANIC-COMPOUNDS AND OTHER ENVIRONMENTAL-FACTORS

Seasonal and inter-annual variations in density 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 January 1990 to January 1992. Bacterial parameters 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 photosynthetic pigments. Bacterial density and biomass were high and exhibited marked seasonal variations with highest values in late spring. Bacterial biomass was positively correlated with temperature and negatively correlated with chlorophyll a content. By contrast no correlations were found with the bulk of organic carbon or nitrogen. In the year-to-year comparison the seasonal patterns of bacterial density were similar. However, bacterial parameters showed significant inter-annual variations. Such changes may be related to different organic matter composition and availability or to competition with microphytobenthos for inorganic nutrient uptake. These data evidence that, whereas temperature might be the most important factors controlling bacterial seasonality, the quality of organic matter (sedimentary protein content coupled to C/N values) may be an important factor for explaining the inter-annual variations.