Hydrobiologia 455: 203–212, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 203 Impact of Tapes philippinarum farming on nutrient dynamics and benthic respiration in the Sacca di Goro Marco Bartoli 1,∗ , Daniele Nizzoli 1 , Pierluigi Viaroli 1 , Edoardo Turolla 2 , Giuseppe Castaldelli 2 , Elisa Anna Fano 2 & Remigio Rossi 2 1 Department of Environmental Sciences, Parma University, viale delle Scienze, 43100 Parma, Italy 2 Department of Evolutive Biology, Ferrara University, via Borsari, 44100 Ferrara, Italy Tel: 0521-905976, Fax: 0521-905402; E-mail: bartoli@dsa.unipr.it Received 9 June 2000; in revised form 9 April 2001; accepted 20 April 2001 Key words: clam culture, surficial sediments, benthic fluxes, anoxia, nutrient budget Abstract 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, prob- ably 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 CO 2 production were stimulated by a factor of, respectively, 1.8 and 3.3, whilst nutrient release was 6.5 times higher for NH 4 + and 4.6 times higher for PO 4 3- . 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 μmol NH 4 + m -2 h -1 and 150 μmol PO 4 3- m -2 h -1 ) stimulated by the high biodegradability of clam faeces and pseudofaeces could in turn favour macroalgal growth. Introduction In coastal lagoons, the water column is very shallow and varies from 0 (in the macrotidal environments where the sediments are periodically exposed to the atmosphere) to few meters depth; the chemical com- position of the water and the oxygen availability is strictly linked to surficial sediment features and meta- bolism. At the water–sediment interface, aerobic and anaerobic mineralization processes deplete oxygen, produce reduced compounds and regenerate nutrients (Jørgensen, 1983) . Important transformations such nitrification and denitrification, mediated by special- ised bacteria, or the formation of complexes and the precipitation of orthophosphate also occur at this level (Henriksen & Kemp, 1988; Golterman, 1995). The continuous sedimentation of particulate mat- ter, produced in the water column or transported by the rivers, makes the water–sediment interface a par- ticularly dynamic zone. In particular, in coastal areas, settling particulate matter represents an inexhaustible source of organic carbon for a diversified community of filter feeders and decomposers (Graf et al., 1982; Albertelli et al., 1999). The intensification of molluscs