Impact of mussel farming on sedimentary geochemical properties of a Northern Adriatic area influenced by freshwater inflows

• Higher deposition of organic matter and anoxic condition under the mussel farm.
• Phosphorus represent a good tracer of the mussel biodeposition.
• Pelite normalization enforce the impact signal of the mussel farm in sediments.
• Grid sampling represent a good approach to evaluate the impact of mussel farms.

The interactions of mussel aquaculture with physico-chemical properties and the biogenic elemental composition of the surface sediment were studied at a long-line farm, located in a 12 km2 area, which included the farm (4 km2) characterized by relevant freshwater inputs along the North-Western Adriatic coastline. Negative redox potentials were detected at two stations directly affected by the farm. Total phosphorus content reached the highest concentration at stations located beneath the farm. OC/TP and TN/TP showed similar patterns, which was characterized by average values underneath the farm (respectively 45.5 and 4.5) that were significantly lower than those measured at the control stations. Our hypothesis, that these patterns could be related to organic matter enrichment of the surface sediment underneath the farm, due to intense mussel biodeposition, was confirmed by the mass accumulation rates, estimated to be respectively 23, 2.3, 0.94 and 0.26 g m−2 y-1 for organic carbon, total nitrogen, total and organic phosphorus. Data concerning the sediment chemistry were normalized with respect to the pelite fraction thus increasing the capability of detecting the environmental impact of the farm under variable background sedimentary fluxes.