Nitrogen stable isotope composition as a tracer of fish farming in invertebrates Aplysina aerophoba, Balanus perforatus and Anemonia sulcata in central Adriatic

Information on aquaculture-related environmental impact along the eastern Adriatic coast is very scarce. In this paper, the area affected by fish farm and sewage derived particulate nitrogen in the Murter Sea and semi-enclosed Pirovac Bay (central Adriatic, Croatia) was assessed using the stable nitrogen isotopic composition (δ15N) in particulate organic matter (POM) and benthic sessile invertebrates (Aplysina aerophoba, Balanus perforatus and Anemonia sulcata) in the exposed areas compared to an unpolluted reference site. The results suggest that increased δ15N values in the POM and biota in the impacted areas are predominantly due to nitrogen losses through feed wastage and fish excretion (faeces), but also due to the sewage discharge into the marine coastal ecosystems. The δ15N values of the selected organisms were significantly higher around fish cages, as well as at sites polluted by urban waste, compared to the reference site and some other pristine offshore locations. The 15N enrichments of A. aerophoba, A. sulcata and B. perforatus collected around fish cages compared to the reference location were up to 6.8, 6.5 and 5.2‰, respectively, which exceeds the natural variability. While the δ15N values of A. sulcata and B. perforatus showed an excellent linear correlation with those of the POM, a weaker correlation was found between δ15N of A. aerophoba and the POM, although its δ15N values consistently increased with the increased δ15N values of POM. This is attributed to the presence of different bacterial populations hosting in the sponge. A geochemical map of the δ15N values in the POM and organisms was created, which could be used for tracing the dispersion of 15N loading generated by aquaculture and sewage in the receiving environment, and their transfer into biota in the adjacent coastal ecosystem. Sponges or other organisms, although known as non-selective suspension feeders, but hosting abundant bacterial populations, may have δ15N values lower than the POM, but still reflecting the overall enrichment of the environment in 15N due to the presence of enriched effluents from aquaculture operation or municipal discharges.