Impact of Tapes philippinarum farming on nutrient dynamics and benthic respiration in the Sacca di Goro

The introduction of the short-necked clam Tapes philippinarum into the Sacca di Goro has over a short period made this coastal environment one of the top European clam production sites. In recent years, this activity has been seriously impacted due to the appearance in the lagoon of large macroalgal beds and the occurrence of dystrophic events causing anoxia and massive deaths of molluscs in the cultivated areas. Tapes cultivation sites now cover more than one third of the lagoon surface at densities sometimes attaining 2000-2500 adult individuals m(-2); such densities and the harvesting methods, based on sediment dredging, probably have a strong impact on the benthic system. Whilst a number of studies have reported water-sediment interface induced modifications due to oyster or mussel farming there have been few attempts to quantify how clam farming affects biogeochemical cycles of oxygen and nutrients, in particular in the Sacca di Goro. Two areas, a farmed and a control one, were compared for benthic fluxes and results were correlated with clam biomass. Oxygen, carbon dioxide, ammonium, reactive silica and phosphorus fluxes were stimulated several fold by the presence of Tapes due to the clams, respiration and excretion activities, but also to the reducing conditions in the surface sediments. On average, the whole lagoon dark sediment O-2 demand and CO2 production were stimulated by a factor of, respectively, 1.8 and 3.3, whilst nutrient release was 6.5 times higher for NH4+ and 4.6 times higher for PO43-. Our results indicate that clam farmers should carefully consider sustainable densities of Tapes in order to prevent the risk of sediment and water anoxia. Rapid nutrient recycling (up to 4000 mumol NH4+ m(-2) h(-1) and 150 mumol PO43- m(-2) h(-1)) stimulated by the high biodegradability of clam faeces and pseudofaeces could in turn favour macroalgal growth.