Bacterial activity in NW Mediterranean seagrass (Posidonia oceanica) sediments

We examine here the hypothesis that benthic bacterial activity in seagrass [Posidonia oceanica (L.) Delile] meadows is dependent on seagrass growth and availability of inorganic nutrients in the sediments. This was achieved by measuring bacterial activity (ammonification rates, and exoproteolytic and exoglucosidase activities) during an annual cycle in five P. oceanica meadows in the NW Mediterranean. Benthic bacterial activity was high, and tended to increase with increasing seagrass production. This trend is likely to involve a direct effect derived from the greater supply of organic carbon in productive meadows, and an indirect effect derived from the fact that productive meadows develop over nutrient-rich sediments and yield nutrient-rich detritus. Phosphorus availability to bacteria was low, for plant detritus was deficient in phosphorus relative to bacterial requirements, and bacterial activity was reduced after seagrasses depleted inorganic phosphorus from the sediments at the onset of exponential plant growth. These results indicate that, on local and annual time scales, benthic bacterial activity is directly related to seagrass production in the NW Mediterranean, because of enhanced inputs of organic matter by the seagrasses, while on seasonal scales, bacteria and seagrass metabolism are inversely related, apparently because of competition for inorganic nutrients.