Largehead hairtail (Trichiurus japonicus) is one of the most commercially important fishes in the western North Pacific. To identify the dietary changes across ontogeny and seasons, we investigated its stomach contents and stable isotope signatures (δ¹³C and δ¹⁵N) in the South Sea of Korea. The index of relative importance revealed that the dietary composition of largehead hairtail consisted mainly of Pisces (1.6%–84.8%) and Euphausiacea (3.0%–93.0%). However, the proportions of dietary items differed by season and anal length (ALs), suggesting a trophic adaptability response to seasonal food availability and size-related trophic variation, respectively. Our isotopic results showed that the largehead hairtail is placed intermediately between pelagic and benthic fishes and was more¹³C and¹⁵N-depleted than other carnivorous fishes, suggesting a high consumption of pelagic species. Similarly, seasonal variations were observed in the δ¹³C and δ¹⁵N values of the largehead hairtails (PERMANOVA,p=0.001). Interestingly, their δ¹³C values in spring decreased significantly with ALs (r²= 0.594,p< 0.001), suggesting an ontogenetic change due to the increased proportion of Euphausiacea to the nutrition of largehead individuals during this specific season. Based on a combination of stomach contents and stable isotope signatures, overall results provide new insights into the feeding characteristics of the largehead hairtail, which improves our understanding of its trophic ecology depending on season and body size.

In estuaries, freshwater discharge functions as a regulator of physicochemical and biological properties. We assessed the seasonal variability of the phytoplankton community in response to hydrological features and nutrient dynamics in two contrasting estuaries in the presence and absence of a dam on the western coast of Korea. Diversity and evenness were also evaluated through chemotaxonomic analysis. Seasonal and inter-estuarine comparisons revealed the overall predominance of diatoms in all seasons and the differentiated composition of small phytoplankton populations in each estuary, which was mainly characterized by significant contribution of cryptophytes in the continuously flushed estuary in contrast to the seasonal occurrence of pelagophytes and cyanobacteria in the intermittently perturbed estuary. Our one-way analysis of similarity and similarity percentage analysis showed obvious inter-estuarine discrepancy of the phytoplankton community in winter in relation to the high dissolved inorganic nitrogen concentration in the natural estuary, implying that the impact of freshwater inflow on the phenological response of phytoplankton can be maximized during active seawater intrusion in dry seasons in the prevalence of freshwater and seawater in the estuary. The contribution swing of diatoms and cryptophytes during the study period reflects the seasonal variability in nutrient dynamics, including absolute concentrations and stoichiometric ratios, which is especially associated with P-limitation on the western coast of Korea. The occurrence of cyanobacteria with a summer peak of phytoplankton biomass in the dammed estuary during the study period indicates the role of an estuarine dam as a conduit transporting phytoplankton and dissolved inorganic nutrients. These findings further suggest that the phytoplankton community structure, differentiated by the presence and absence of dams, influences bottom-up regulation and thus the food web structure in estuaries.

The aim of this study was to assess seasonal variation in the food-web structure of fish assemblages in the East (two sites) and the South (one site) Seas of Korea, and to compare the isotopic niche areas between the regions. To do this, we analyzed the community structures and the δ¹³C and δ¹⁵N values for fish assemblages, and their potential food sources collected during May and October 2020. There were spatial differences in the diversity and dominant species of fish assemblages between the two seas. The fish assemblages in the South Sea had relatively wide ranges of δ¹³C and δ¹⁵N (−22.4‰ to −15.3‰ and 7.4‰ to 13.8‰, respectively) compared to those (−22.1‰ to −18.0‰ and 9.8‰ to 13.6‰, respectively) in the East Sea. The δ¹³C and δ¹⁵N values of suspended particulate organic matter, zooplankton, and fish assemblages differed significantly among sites and between seasons (PERMANOVA, p < 0.05, in all cases). Moreover, isotopic niche indices were relatively higher in the South Sea compared to those in the East Sea. Such differences in food-web characteristics among sites are likely due to the specific environmental effects (especially, major currents) on the differences in the species compositions and, therefore, their trophic relationships. Overall, these results allow for a deeper understanding of the changing trophic diversity and community structure of fish assemblages resulting from climate variability.

The common squid Todarodes pacificus is a dominant species within epipelagic communities and an important commercial species in the South and East seas of Korea and in the East China Sea. In this study, to examine the dietary composition, trophic position, and ontogenetic dietary shifts of the common squid, we analyzed stomach contents and stable isotope values (δ¹³C and δ¹⁵N) in muscle tissue across different body sizes (mantle length, ML) and seasons (winter, spring, and summer 2021) in the South Sea of Korea. Regardless of the season, the diet of the common squid predominantly comprised Pisces (37.9–94.0%) and Cephalopoda (6.0–61.0%). However, in the smallest individuals (ML < 10 cm), the stomach contents in August primarily comprised Crustacea (95.5%), thereby indicating a seasonal difference in dietary composition and ontogenetic dietary shifts only during summer. Similarly, our isotope results revealed seasonal isotopic variation among sampling periods and significant positive correlations between ML and both δ¹³C and δ¹⁵N values during the summer. These results reveal a seasonal variability in major diet components among the common squid of different size classes and that their feeding strategies are probably adjusted in response to the temporal availability of prey. Collectively, the findings of this study enhance our understanding of the feeding ecology of T. pacificus and thereby provide valuable information that will contribute to the ecological fishery management of this commercially important species.

The geomorphology of coastal lagoons has an important effect on the hydrological balance between fresh water and seawater, which exhibit a wide range of salinity from nearly fresh water to hypersaline water. The present study aimed to characterize seasonal biogeochemical properties of suspended particulate organic matter (POM) in response to environmental variability driven by hydrological dynamics in contrasting coastal lagoon systems. We selected two lagoons distinguished by their geomorphological features, a permanently open artificial lagoon (Gyeongpoho) and an intermittently open natural lagoon (Hyangho), analyzed environmental characteristics, and compared the seasonal differences in the biochemical compositions of POM. Variations in temperature and precipitation in both lagoons showed the characteristics of a typical monsoon climate but variation in salinity differed 6.5−35.0 in Gyeongpoho and 1.1−10.8 in Hyangho. A self-organizing map analysis using the environmental data revealed that the spatiotemporal variations in salinity and nutrient concentrations differed significantly between the two lagoon systems, indicating a difference in hydrologic connectivity between lagoons and marine systems. Furthermore, a canonical correlation analysis highlighted that the POM properties differed according to physicochemical factors. The differing environmental conditions may affect the spatial and temporal variations in organic matter composition, resulting in significant differences in the biochemical composition of POM between two lagoon systems. Overall, this information provides insight into the origin and transport process of POM under changing coastal lagoon environments caused by climatic and human activities.