Benthic bacterial response to variable estuarine water inputs.

Estuarine waters are known to enhance productivity in coastal environments, but little is known about the impact that nutrient-rich waters can have on benthic microbial processes. After intensive surface sediment sampling in a wide coastal area impacted by river plume waters, we performed on-site microcosm experiments in which we added estuarine waters, rich in both organic and inorganic N and P, to pristine offshore sediments. This experimental approach has been conducted in different coastal areas for two consecutive years, in which the river-plume waters differed in both inorganic and organic N and P content. Benthic bacterial response (total bacterial abundance and biomass, aminopeptidase, beta-d-glucosidase and bacterial C production) was investigated in treated and non-treated sediments. All structural and functional microbial variables increased significantly after plume-water supply (within 12-24 h bacterial abundance in the sediment doubled and enzymatic activities increased up to >50%). Results indicate that inorganic N supply from plume waters was sufficient to induce a significant response in benthic bacterial abundance, independently from the presence of high and/or variable N:P ratios. However, bacterial carbon production and exo-enzymatic activities increased significantly when the supply of organic P from plume waters was associated with a decrease of organic N:P ratios. We conclude that plume waters have important effects on benthic bacterial dynamics, but the extent of their biogeochemical implications is largely dependent upon their organic P availability and on stoichiometric ratios of organic nutrients supplied by plume waters.