Spatial and temporal physiochemical characteristics of the Seomjin River and Estuary, Gwangyang Bay, and Yeosu Strait on the southern coast of South Korea

Abstract

Coastal area has been compromised through eutrophication, seawater intrusion, heavy metal contamination, and sediment accumulation. We investigated the physicochemical properties of seawater and heavy metal concen- trations in marine sediments along the southern coast of South Korea extending from the Seomjin River and Estuary and the adjoining Gwangyang Bay into the Yeosu Strait for four months (March, May, August, and October 2019). The results indicated a relatively high salinity downstream of the Seomjin River in August and October, indicating seawater intrusion into the river from the estuary. High nutrient concentrations were observed at the mouth of Yeosu Strait and Gwangyang Bay owing to freshwater inflow, whereas a high chlo- rophyll-a (chl-a) concentration was observed in the farthest from the Seomjin Estuary in August, indicating a discrepancy between primary productivity and nutrient concentrations. This was likely due to favorable con- ditions, such as high salinity and less grazing effect (a low zooplankton population) in the Yeosu Strait, which could increase the diatom population and their primary productivity. The concentrations of heavy metals (Cd, As, and Hg) in the surface sediments were below the legal standard values stated in the Marine Sediment Quality Guidelines of Korea. However, some fish, including Scyliorhinus torazame, Sepia esculenta, and Pseudopleuronectes yokohamae, contained Hg and As above the legal standard values, requiring adequate management of Hg and As in fisheries.