The effect of mariculture facilities on water column trophodynamics as revealed by biochemical features of suspended organic matter (South Tyrrhenian, MED).

A comparison of a Mediterranean aquaculture impacted area and control areas was made to assess the effect of fish farm waste discharge on the biochemical features of the water column. Trophic variables commonly used in marine ecology such as total suspended matter, suspended chlorophyll-a, biochemical features of particulate organic matter (proteins, lipids and carbohydrates) and biopolymeric carbon were chosen as the best descriptors of trophic conditions. An initial analysis of data from the impact area was carried out in order to test the effect of farm waste using a gradient of distances downstream from the fish farm cages (50 m, 300 m, 1000 m). The results were then compared with a control site 750 m upstream. Subsequently, the cage data set was asymmetrically compared with data from five controls collected some years before, when no aquaculture activity was present in the Gulf. The analysis revealed differences in chlorophyll-a, carbohydrates and some trophic ratios between the farm impact area and the controls taken upstream. A clear pattern of trophic enrichment of the water column around the fish farm was evidenced since concentrations in the sites increased along with their distance from the cages. The downstream sites overall were significantly different, trophically speaking, from the five control areas, while the trophic variables of the upstream control were not different from the external controls. Results showed that fish farm facilities provided an organic enrichment of the water column up to at least 1000 m downstream from the cages, producing a deviation of trophodynamics from normal ambient conditions.