Stratification is a fundamental process influencing nutrient availability and biological productivity in coastal marine ecosystems. In this study, we examine multi-decadal variability in winter stratification and nutrient distribution within Western margin of the East Sea (WES), using observational data collected between 1990 and 2023. Stratification characteristics were quantified using the Brunt-Väisälä frequency (N²), with particular focus on changes in intensity and the depth of maximum stratification. These variables were then analyzed in relation to environmental factors such as upper-layer temperature, surface wind speed, and East Korea Warm Current (EKWC) transport. Over the study period, stratification intensity increased at an average rate of 1.1% per year, while the maximum stratification depth shoaled by approximately 0.6% annual trends that became more pronounced after 2015. These shifts were closely linked to enhanced thermal stratification and increased EKWC volume transport. Enhanced stability of the water column was accompanied by reduced vertical mixing, which in turn intensified the nitrate gradient between surface and subsurface waters. The most marked nitrate accumulation was observed near 100 m, where stratification was strongest. This study highlights how physical oceanographic changes during winter can regulate nutrient supply prior to the spring bloom, potentially influencing seasonal productivity in marginal seas. The results emphasize the importance of long-term monitoring of vertical structure in assessing the ecological impacts of ocean variability.

In seasonally stratified marine environments, the dynamics of benthic–pelagic coupling plays a crucial role in shaping food web structures and fisheries production. We examined fish food web structures across three distinct shelf areas in the Southern Sea of Korea (SSK) during both stratified (summer) and mixed (spring) water conditions using stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N). In spring, fish communities exhibited a broader range of δ¹³C values compared with summer, indicating more diverse feeding strategies. Seasonal variations in the proportion of benthic and pelagic prey in consumer diets highlighted shifts in benthic–pelagic coupling, illustrating how consumers adjust their reliance on benthic or pelagic resources. The relative importance of the benthic pathway varied among species groups throughout the year. During stratified conditions, reduced benthic–pelagic coupling led to increased reliance on benthic prey, particularly in the oligotrophic region influenced by the Tsushima Warm Current (TWC). The food web spanned five trophic levels, with a median of 3.6. Several species, notably benthic ones, declined in their trophic positions during the summer stratification. These results suggest that fish food webs in the SSK are shaped by temperature-driven seasonal bottom-up control. Our findings further offer insights into how increased water-column stratification could impact the trophic niches of shelf-food webs in the TWC region.

Located within the subpolar front (SPF) zone, the eastern coast of Korea is subject to frequent environmental changes on both spatial and temporal scales. In this study, we analyzed seasonal changes in the algal community structure and related environmental factors at two sites, Chodo (CD) and Sageunjin (SG), which were influenced by cold and warm-water masses, respectively, in the SPF zone. Sea surface temperature (SST) exhibited greater seasonal change in CD, whereas significant wave height was higher in SG. The salinity and nutrient concentrations decreased during summer and increased in winter at both sites. Seasonal shifts in the dominant species were distinct, with turf species positively correlated with SST dominating the intertidal zone of SG during winter, and canopy species were associated with nutrient concentrations dominating the intertidal zone of CD. Bryopsidales, which was positively correlated with the N:P ratio, dominated the SG subtidal zone in summer, whereas Fucales, which was positively correlated with the SST, dominated the CD subtidal zone. These results enhance our understanding of seaweed responses to environmental changes in SPF zones and provide a basis for predicting future changes in coastal ecosystems.

The brackish water clam, Corbicula japonica, acts as an ecosystem engineer in estuaries. To identify its resource-use patterns in the low-turbidity temperate Seomjin River estuary of Korea, we analyzed stable isotope and fatty acid (FA) biomarkers to differentiate allochthonous and autochthonous dietary sources, and examined the effects of clam size, salinity gradient, and season. The δ¹³C and δ¹⁵N values were consistent across the three factors. The δ¹³C values of the clams were similar to those of both riverine- and estuarine-suspended particulate organic matter (R- and E-SPOM), while their δ¹⁵N values were 2–4‰ higher, indicating an equal contribution of both sources to the clam diet. Biogeochemical proxies and FA compositions of SPOM indicate that estuarine phytoplankton significantly contribute to the E-SPOM pool. Moreover, the similarity in FA profiles between Corbicula and E-SPOM indicates that phytoplankton-derived organic matter is the primary source of nutrition for the clam, with minimal impact from growth, salinity gradient, or seasonal changes. Our study suggests that in low-turbidity estuaries with high phytoplankton production, allochthonous organic matter makes a negligible contribution to clam nutrition, compared to high-turbidity estuaries. This finding could provide insights into the variations in the trophic structure of estuarine food webs across diverse regions.

The eastern coasts of Korea (ECK) and Japan (ECJ) are located at the southernmost limit of walleye pollock distribution in the Northwest Pacific. Following the climate regime shift (CRS) in the late 1980s, pollock catches in these regions have declined sharply, with different trends emerging. This study examined the relationship between environmental factors, such as sea surface temperature (SST) and habitat suitability, and changes in pollock catches from the late 1980s to 2022. From the late 1980s to the late 1990s, El Niño and positive Pacific decadal oscillation (PDO) phases dominated, increasing SST in the ECK and ECJ habitats and rapidly decreasing catches. Although spawning grounds (SGs) have maintained high habitat suitability, nursery ground (NG) suitability has declined. From the late 1990s to 2022, La Niña and negative PDO phases prevailed, with SST continuing to rise along the ECK, further reducing catches. SG suitability remained high, but NG suitability declined. Along the ECJ, SST decreased after the late-1990s CRS, stabilizing catches. After the mid-2010s, the SST increased along the ECJ, reducing pollock catches, although SG suitability remained high. This study elucidates SST changes during early life stages and their effects on pollock catch, habitat, and resources in future marine environments.

Kelp transplantation is a nature-based strategy aimed at restoring coastal habitat integrity and marine biodiversity. However, its functional consequences for trophic integration within benthic food webs remain poorly understood. Using δ¹³C and δ¹⁵N stable isotope analyses, we evaluated how Undaria pinnatifida transplantation alters consumer trophic structures and isotopic niche characteristics in Oeyeondo, South Korea. While basal source remained isotopically uniform across sites, the introduction of U. pinnatifida triggered significant isotopic shifts in consumers, reflecting a reorganization of carbon assimilation pathways. At the transplanted site, herbivores exhibited significantly enriched δ¹³C values (−14.7 ± 2.0‰ to −13.2 ± 0.3‰) compared to the control site (−19.3 ± 1.2‰), indicating direct assimilation of kelp-derived carbon. Conversely, grazers showed depleted δ¹³C values (−20.6 ± 0.6‰) reflecting a shift toward alternative benthic resources. Isotopic niche metrics revealed a broader community-level niche width at the transplanted site, driven by increased resource diversity and niche partitioning. These findings demonstrate that kelp transplantation effectively restructures benthic food web dynamics by providing new energy pathways, offering a robust functional framework for evaluating marine forest restoration success.