**2. Materials and Methods**

### *2.1. Description of the Study Site*

Gwangyang Bay is located in the south coast of Korean peninsula (Figure 1). In terms of morphological features of the bay, water depth varies from 10 m at the Seomjin River estuary to 50 m at the outer bay. The bay has a semi-diurnal tidal cycle. The bay receives a large discharge (ca. annually 2298 mega MT year<sup>−</sup>1, equivalent to 72.8 m<sup>3</sup> s<sup>−</sup>1) from Seomjin River [26]. It appears that a significant amount of nutrients (19.7 × 10<sup>3</sup> moles N day−1, 0.1 × 10<sup>3</sup> moles P day−1, 18.2 × 10<sup>3</sup> moles Si day−<sup>1</sup> in average) come to the bay from the Seomjin River catchment (ca. 5000 km2) [27]. Since the Seomjin River estuary relative to the Korean river estuaries remains open without barrages, the water mass between river and ocean exchanges more actively. This dynamic condition of the bay tends to shape grea<sup>t</sup> primary productivity and high biological diversity. From both an ecological and economical points of view, Gwangyang Bay (ca. 450 km<sup>2</sup> from the estuary to the outer bay) is the most productive coastal area in Korea. Specifically, Jeonnam Province containing Gwangyang Bay comprised 71% (1,297,815 MT year<sup>−</sup>1) of aqua-cultural resources in a national scale as of 2016 (KOSIS, [28]). In addition, a large industrial area (e.g., oil refineries and steel plants) near the bay can be regarded as a significant pollution source. Thus, the intermittent release of various pollutants might be another factor disturbing water quality and benthic sediments [29].

**Figure 1.** Map of the study sites (black closed circles) in Gwangyang Bay.
