Stable isotopic signatures (δ13C and δ15N) of suspended particulate organic matter as indicators for fish cage culture pollution in Sansha Bay, China

The stable isotope (delta C-13 and delta N-15) signature of suspended particulate organic matter (SPOM) is widely used to assess the environmental status of water bodies in different ecosystems. However, the influence of intensive fish cage culture on SPOM remains unclear. The environmental parameters and stable isotopic values (delta C-13 and delta N-15) of SPOM were investigated in Sansha Bay (China) in 2019 from winter to spring, with fish feed varying significantly between seasons. Dissolved inorganic nitrogen (DIN) concentrations showed no significant differences between winter and spring (t = -1.28, df = 19, P > .05), with averages of 0.65 mg L-1 and 0.57 mg L-1 , respectively. The large quantity of fish feed was applied in the area found to have less impact on inorganic nutrients concentrations, due to intensive macroalgae cultures in offshore water, forming a nutrient buffer zone. The values of delta C-13 and delta N-15 of SPOM were significantly higher in spring than in winter (P < .01). Due to organic particulates from unconsumed fish feed, delta N-15 of SPOM was much higher in cage culture water areas, decreasing towards the bay mouth. Significantly positive N-15 values were observed in SPOM from NO3 dominant water, with the highest delta N-15 in SPOM of 19.31%o, which significantly positively correlated with Chl a during spring (P < .05). The delta C-13 of SPOM was significantly higher in spring than in winter (P < .01), with the delta C-13 distribution of SPOM also more homogenized in spring than in winter, due to hydrodynamics and intensive fish cage culture. These findings suggest that stable isotope values of SPOM can be used as effective indicators for the evaluation of water pollution level in cage aquaculture systems.