Manila clam introduction in the Sacca di Goro Lagoon (Northern Italy): ecological implications

The manila clam was introduced in 1986 in the Sacca di Goro lagoon (Po River Delta, northern Italy) and in a few years became one of the backbone of the local economy. Few years later, the lagoon was affected by macroalgal blooms, followed by the biomass breakdown which impacted the clam farming, with mass mortality in summer. From late 1987 to 1994, an intensive, monthly-based monitoring plan of water column was thus performed, aiming at analyzing salinity, dissolved oxygen, chlorophyll a and nutrient patterns in a grid of sampling stations, with respect to hydrodynamics and freshwater inputs. Later on, monitoring of spatial distribution, biomass evolution and ecophysiology of the dominant macroalgal community was performed (1990-1998), together with experiments focusing on macroalgal production and factors controlling macroalgal growth and decay. Furthermore, the role of clams in coupling benthic and pelagic processes was addressed as a possible feedback loop in the lagoonal ecosystem. Regular monitoring was afterwards flanked by an experimental approach aiming at more detailed studies of how clams affect benthic processes, in particular aerobic and anaerobic respiration, nutrient recycling, the regulation of denitrification and the analysis of nitrogen cycling within farmed areas (1993-2005). More recently, different European projects addressed the relationships between environmental and socio-economic issues in the clam farming management. In this review we summarize the main outcomes of these research activities with the overall aim of quantifying, at the lagoon scale, the role of the farmed species as regulators of some of the multiple processes involved in the ecosystem metabolism and biogeochemistry. Special emphasis is given to how clams affect uptake, dissipation and recycling pathways of nutrients, especially nitrogen. Overall, the clam farming system is analyzed for its implications in the whole lagoon metabolism and for possible ecological feedbacks which in turn can impact the local socio-economic system.