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The East/Japan Sea (EJS) is a highly productive marginal sea in the northwest Pacific, consisting of three basins (Ulleung Basin: UB, Yamato Basin: YB, and Japan Basin: JB). To find causes of the reportedly high primary productivity in summer in the EJS, especially in the UB, we measured primary productivity, phytoplankton composition, and other environmental variables. The water column was strongly stratified in the EJS compared with the Western Subarctic Pacific (WSP). Integrated primary productivity was two times higher in the EJS (612 mg C m 22 d 21 ) than in the WSP (291 mg C m 22 d 21 ). The vertical distributions of physicochemical and biological factors confirmed that production in the subsurface chlorophyll maximum layer in the study regions was an important factor regulating primary productivity within the water column. While picoplankton (<2.7 mm) dominated in the WSP, JB, and YB, micro/nanoplankton (2.7 mm) dominated in the UB. Contribution by picoplankton to total biomass and primary productivity in the UB was significantly lower than in the other regions. CHEMTAX analysis using marker pigments showed that diverse phytoplankton groups inhabited the study regions. Cluster and canonical correspondence analyses showed high correlation between the spatial variation in phytoplankton assemblages with the water mass properties mainly represented by water temperature and nitrate concentration. Overall, our results suggest that the hydrographic structure of water column in the study region is an important controlling factor of the biomass and productivity of phytoplankton as well as their diversity in size and taxonomic groups.

We examined stable carbon and nitrogen isotope ratios for a large variety of consumers in intertidal and subtidal habitats, and their potential primary food sources [i.e., microphytobenthos (MPB), phytoplankton, and Phragmites australis] in a coastal bay system, Yeoja Bay of Korea, to test the hypothesis that the transfer of intertidal MPB-derived organic carbon to the subtidal food web can be mediated by motile consumers. Compared to a narrow δ13C range (−18 to −16‰) of offshore consumers, a broad δ13C range (−18 to −12‰) of both intertidal and subtidal consumers indicated that 13C-enriched sources of organic matter are an important trophic source to coastal consumers. In the intertidal areas, δ13C of most consumers overlapped with or was 13C-enriched relative to MPB. Despite the scarcity of MPB in the subtidal, highly motile consumers in subtidal habitat had nearly identical δ13C range with many intertidal foragers (including crustaceans and fish), overlapping with the range of MPB. In contrast, δ13C values of many sedentary benthic invertebrates in the subtidal areas were similar to those of offshore consumers and more 13C-depleted than motile foragers, indicating high dependence on phytoplankton-derived carbon. The isotopic mixing model calculation confirms that the majority of motile consumers and also some of subtidal sedentary ones depend on intertidal MPB for more than a half of their tissue carbon. Finally, although further quantitative estimates are needed, these results suggest that direct foraging by motile consumers on intertidal areas, and thereby biological transport of MPB-derived organic carbon to the subtidal areas, may provide important trophic connection between intertidal production and the nearshore shallow subtidal food webs.

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