Aquaculture impact on benthic microbes and organic matter cycling in coastal Mediterranean sediments: a synthesis

Microbial assemblages and organic matter composition as well as their response to the disturbance induced by mussel and fish-farm biodeposition were compared in several areas of the Mediterranean: La Spezia (Ligurian Sea), Gaeta (Tyrrhenian Sea), Cattolica (Adriatic Sea) and Cyprus (Levantine Sea), on both unvegetated and Posidonia bed sediments. In all systems investigated, organic matter (as biopolymeric carbon) accumulated in aquaculture impacted sediments. Among the main biochemical classes, lipids appeared to be a good tracer of aquaculture impact, especially in fish-farm sediments. Exoenzymatic activities displayed higher values in sediment beneath the cages, indicating faster organic matter cycling. A significant accumulation of chloroplastic pigments was observed under mussel farms (related to the deposition of faeces and pseudo faeces), but also on fish-farm sediments. The presence of newly deployed cages induced rapid changes in the benthic compartment, and sediments reached rapidly (6 weeks) reducing conditions. Bacterial and total microbial density increased significantly in impacted sediments. Although the contribution of prokaryotic auto-fluorescent cells was very low, this component displayed a clear response to organic biodeposition, increasing significantly beneath the cages. Conversely, eukaryotic autofluorescent cells decreased. The ratio of benthic auto-fluorescent cells to total bacterial number proved to be a new, sensitive descriptor of biodeposition impact. Heterotrophic bacteria (CFU, and particularly the number of gramnegative bacteria: Cytophaga=Flexibacter-like bacteria) increased significantly in sediments beneath the farm. The similarities and differences observed among sites, latitudes, microbial components and typology of impact (mussel vs. fish farm) are analysed and discussed.