Effect of fish farming wastes on sedimentary and particulate organic matter origin in a Southern Mediterranean area (Gulf of Castellammare, Sicily): a multiple stable isotope study (delta13C and delta15N)

Abstract

Carbon and nitrogen stable isotope analysis was used to investigate the dispersion area of waste material coming from fish farming activities in the western Mediterranean. Tests were conducted to see if uneaten feed and faecal material isotopic signals, originating from fish farms, could be detected in particulate organic matter (POM) and sedimentary organic matter (SOM). The detectable dispersion distance (from under cages as far as 1000 m) of cage-derived organic material was also examined. To do this, carbon (d13C) and nitrogen (d15N) composition in POM and SOM collected around the cages, in some control areas and in the waste material, was measured. Mean POM d13C was delta22.9F0.2x, while SOM d13C was 22.1F0.1x, and did not show significant differences along a distance gradient. Mean POM d15N was 3.9F1.0x, while SOM d15N was 3.4F1.3x, showing significant differences between 15N-enriched sites positioned near the cages and 15N-depleted sites positioned at about 1000 m from the cages. The mixing model applied to each reservoir (POM and SOM) as targets showed an incidence of autochthonous carbon (mostly phytoplankton in the particulate and sand microflora in the sediments) of about 24% in POM and of about 19% in SOM. Terrigenous carbon, which represented 37% in POM and 33% in SOM, increased in moving from sites nearby cages to more distant sites. Farming waste carbon represented 39% in POM and 48% in SOM. The inputs of autochthonous N represented about 24% in POM and about 18% in SOM, and with terrigenous N (representing 62% and 70%, respectively, in POM and SOM) showed higher contribution than cage-derived nitrogen. On average, farming waste nitrogen in POM was 15%, while it was 11% in SOM. d15NPOM showed a significant difference between cage sites and sites positioned at about 300 m. The latter was similar to sites located at 1000 m from the cages. Results indicate that in a Mediterranean oligotrophic area, with a bottom about 25 m deep and a mean current speed of 10–12 cm s 1, the influence of carbon and nitrogen from farming waste can be isotopically detected both in the particulate matter and the sediments in a wide area around fish farming cages. Sediments around the cages have been observed to be organic-enriched at about 1,000 m from cages. Dispersion of cage waste by hydrodynamic advection, consumption and defecation by wild fish, and resuspension from the bottom currents were invoked as three combined factors to explain the greater impact area found in this study than has been previously reported in the literature.