Impact of Commercial Clam Harvesting on Water Column and Sediment Physicochemical Characteristics and Macrobenthic Community Structure in a Lagoon (Sacca Di Goro) of the Po River Delta

Abstract

The Sacca di Goro is a hypertrophic lagoon of the Po river Delta, which is heavily exploited for rearing ofthe Manila clam, Tapes philippinarum. Harvesting of the clams could add to the general disturbance regime in the lagoon through damage to the benthic community, the release of porewater nutrients which could fuel macroalgal growth and of reduced compounds from the sediment leading to depletion of water column oxygen. We tested these hypotheses during an in situ harvesting experiment. No significant differences were found in population or biomass densities, or community indices between samples collected before and after harvesting. Similarly, no differences were recorded in trophic (food source) availability. Whilst, there was a slight reduction in water column oxygen, pH and Eh values in the plume of suspended sediment, these changes were small. Similarly, increases in water column concentrations of ammonium and reactive phosphorus during harvesting were not significant. Calculations based on profiles of sediment ammonium content, before and after harvesting, demonstrated a release of ammonium equivalent to 13 mmol m−2. In terms of the oxygen and nitrogen budgets of the lagoon the effects of harvesting seem to be of little importance and of the same order of magnitude of sediment-water fluxes driven by clam activity, both terms being subjected to high energy hydrodynarnism.