Quantification of groundwater versus fish-farm sources of nutrients in the coastal water off Jeju Island, Korea, using fluorescent dissolved organic matter as a tracer

Abstract

The concentrations of nutrients and dissolved organic matter (DOM) in the coastal waters off Jeju Island, Korea, are known to be influenced mainly by submarine groundwater discharge (SGD) and fish-farm effluents, in addition to the background open-ocean seawater from the Tsushima Current. However, the differentiation of the effect of fish-farm effluents from SGD is very challenging since most of the farms in this island utilize groundwater as a rearing water. Thus, in this study, we measured the concentrations of nutrients and fluorescent DOM (FDOM) in seawater, fresh groundwater (FGW), and farm water in two coastal areas off Jeju Island, in May and August 2021, in order to determine the effect of SGD versus fish-farm effluents. The results showed that humic-like FDOM (FDOMH) and NH4+ concentrations were greatly enriched owing to organic matter decomposition in the farm waters. We used FDOMH, as a tracer for the farm effluent. The end-member mixing model for FDOMH and salinity showed that FGW contributed about 73% of NO3− (avg. 17 ± 20 μM), whereas the farm effluent contributed about 95% of NH4+ (avg. 5.6 ± 4.1 μM) and 71% of FDOMH (avg. 0.86 ± 0.49 R.U.) in the coastal waters. Thus, our result shows that fish-farm effluents can be an important source of anthropogenic DOM and nutrients in coastal waters, where farms are being operated intensively.