Tracing Different Freshwater Sources for Nutrients and Dissolved Organic Matter in Coastal Waters off Jeju Island Using Radon

Abstract

In order to determine the origins of dissolved inorganic nutrients and dissolved organic matter in coastal waters off Jeju Island, Korea, we measured the concentrations of 222Rn, nutrients, fluorescent dissolved organic matter (FDOM), and dissolved organic carbon in coastal waters during September 2016 and October 2017. The lower salinity waters occurring in the southern coastal waters off Jeju (SCW) were characterized by higher nutrients and humic-like FDOM, while those in the western coastal waters off Jeju (WCW) showed no trend. A significant negative correlation between salinity and 222Rn in the SCW (r2 = 0.62, n = 7 for 2016; r2 = 0.56, n = 14 for 2017) indicates that freshwaters occurring in the SCW originate from fresh groundwater. However, depleted 222Rn activities in the fresh surface waters of the WCW indicate that those in the WCW originate mainly from the remote Changjiang diluted water. In general, the coastal ecosystem in the SCW is characterized by the dominance of brown seaweeds, and the WCW is by scleractinian corals. Our results imply that high nutrient concentrations in the SCW provide favorable conditions for the growth of seaweeds, while relatively low nutrient concentrations and high FDOM concentrations in the WCW provide favorable conditions for survival of corals.