Korean Waters Research Articles

Influence of environmental parameters on marine plankton diversity in the southern coastal waters of Korea: Emphasis on thermal stratification

Plankton are fundamental to aquatic ecosystems, forming the base of the marine food web and playing a crucial role in the global carbon cycle. While the impact of environmental factors on marine plankton ecosystems has been widely studied, the seasonal variability in plankton diversity has received comparatively less attention. Here, we investigated the complexity and distribution of plankton species in Korean coastal waters in relation to seasonal environmental changes, particularly those linked to water mass characteristics and the presence of a thermocline, using alpha and beta diversity indices. A thermocline was observed at all stations in July, coinciding with the lowest diversity and richness, but was absent in September, when diversity indices peaked. In the upper layer above the thermocline, diatom species such as Pseudo-nitzschia delicatissima and Skeletonema costatum were dominant. Analysis of the Shannon index suggested that planktonic ecosystems in the South Sea of Korea from May to September 2021 were well-balanced. These findings enhance our understanding of the potential effects of environmental variability and thermal stratification on plankton biodiversity and community dynamics in Korean coastal waters.

Genetic analyses of Anisakis pegreffii (Nematoda: Anisakidae) from the East Asian finless porpoise Neophocaena asiaeorientalis sunameri (Cetacea: Phocoenidae) in Korean waters

The East Asian finless porpoise, Neophocaena asiaeorientalis sunameri, is an endangered species that inhabits the coastal marine environments of East Asia. In the present study, we investigated the overall infection status of anisakid nematodes in East Asian finless porpoises from three sea sectors off the Korean Peninsula. The genetic diversity and population genetic structure of the identified nematode species were evaluated. The prevalence of all stages of anisakid nematodes collected from the stomach was 57.55% (61 among the 106 porpoises examined), and 16 of the hosts were found to have adult worms. The mean number of infected adults was 211 (± 419.54, 5–1455 per host). Only one species of anisakids, Anisakis pegreffii, was identified from randomly selected worms by molecular approaches. Analysis of the mitochondrial (mt) cox2 partial gene in 50 newly generated sequences of A. pegreffii revealed 24 haplotypes, including 14 new haplotypes. We observed below-average levels of nucleotide diversity and haplotype diversity compared to other seas around the world. The mtDNA cox2 haplotypes of the species in the three Korean sea areas showed no genetic structure, suggesting well-connected gene flow within these areas. This study represents the first record of a definitive host of A. pegreffii in Korean waters, providing important information regarding anisakids genetic diversity in the cetacean species inhabiting limited regions.

First report of Coolia palmyrensis in Korea: seasonal and spatial distribution of C. palmyrensis and C. malayensis in Korean coastal waters

Distribution patterns of benthic dinoflagellates that are widely observed in tropical and temperate waters and have toxic potential are changing in response to ocean climate change. Although there have been no outbreaks associated with the genus Coolia affecting humans, it is crucial to understand their changing distribution and clearly identify the species in the study area to prepare for potential toxic events. In this study, five strains of Coolia species were isolated from macroalgae samples collected from Jeju Island and the eastern coastal waters of Korea. Through morphological and molecular analysis of these isolates, one strain was identified as Coolia palmyrensis, marking the first report of this species in Korea, and four strains as C. malayensis. One of the C. malayensis strains was isolated in Pohang on the eastern coast of Korea, where it had not been previously reported. From 2021 to 2023, monitoring of Jeju Island using a quantitative polymerase chain reaction (PCR) assay revealed that C. palmyrensis cells occurred mostly in autumn, with a maximum density of 242 cells g-1, and overwintering populations were observed in 2023. However, C. malayensis cells were not observed in this area. Additionally, C. malayensis was observed in Pohang and Ulsan, located further north than Jeju Island with maximum cell densities of 537 and 201 cells g-1, respectively. These data and our decade of monitoring experience confirmed the introduction and establishment of C. palmyrensis and habitat shift of C. malayensis in Korean coastal waters. This study provides advances for understanding of the relationships between climate-driven alterations and biogeographic distribution of these species.

Revisiting the taxonomy of Korean Ischnochiton species (Polyplacophora, Ischnochitonidae) based on a combined analysis of morphological and molecular data.

Chiton species belonging to the genus Ischnochiton J. E. Gray, 1847 are commonly found in intertidal rocky shores worldwide, with the exception of the northern Atlantic and Arctic oceans. Ischnochiton species are characterised by imbricate girdle scales that are uniform in size, rounded, sculpted with striae or occasionally smooth. However, their species-level taxonomy is complicated due to the high variation in their shell microstructures. Despite more than a hundred species reported worldwide, taxonomic studies of this group remain relatively unexplored in Korean waters, with only a few species recorded to date. In this study, we compared the microstructural characteristics of tegmentum sculpture, girdle scales and radula amongst four Korean Ischnochiton species using high-resolution microscopic images and a scanning electron microscope (SEM). Along with mtDNA cox1 sequence comparison, a comprehensive analysis of their morphology revealed that I.hayamii Owada, 2018 was identified for the first time in Korean waters. This species is morphologically distinguished by its small body size of adults, smooth lateral areas on valves and small perinotum scales sculptured with weak longitudinal ribs. Phylogenetic analysis of the mtDNA cox1 sequence provides distinct resolution at the species level, but interrelationships amongst Ischnochiton species remain unresolved. Results from the morphological and molecular analyses presented in this study offer valuable taxonomic information for accurate species identification amongst closely-related Ischnochiton species.

Nationwide Profiling and Source Identification of Organophosphate Esters in Korean Surface Waters Using Target, Suspect, and Non-Target HRMS Analysis

Organophosphate esters (OPEs) are emerging contaminants that serve as alternatives to regulated substances in aquatic environments. A nationwide large-scale assessment for OPEs, including point sources, remains insufficient. To address this issue, we aimed to investigate OPEs occurrence and novel OPEs via comprehensive target, suspect and non-target analysis. Among the 11 target OPEs, 10 were detected at sampling sites distributed evenly nationwide. The highest mean concentrations were measured for tris-(2-butoxyethyl) phosphate (TBOEP) and tris(2-chloroisopropyl) phosphate (TCIPP). The multivariate statistical analysis revealed that TBOEP and TCIPP are essential components for assessing total OPEs pollution. The systematic risk assessment results evaluated the overall risk contribution of TBOEP and the significant risk impact of 2-ethylhexyl diphenyl phosphate. Promising suspect and non-target analysis enabled frequent detection and identification of 6 antioxidant transformation products (TPs), as well as the tentative identification of 14 OPEs and TPs, including 3 di-OPEs. Based on sampling site classification, we confirmed that major OPEs are significantly discharged near point sources. We believe that this is the first attempt to assess the nationwide risk and potential sources of OPEs in Korean surface waters, providing insights that could support further prioritization and regulation efforts.

Two new species of the family Aoridae (Crustacea, Malacostraca, Amphipoda) from Korean waters.

Two new species of the family Aoridae, one from the genus Aoroides Walker, 1898, and other from the genus Grandidierella Coutière, 1904, are reported from Korean waters. Aoroidesgracilicrus sp. nov. is similar to A.longimerus in having numerous plumose setae on the basis and carpus of gnathopod 1. However, the new species can be distinguished from A.longimerus by the numerous plumose setae on the bases of pereopods 3 and 4 and the slender basis of pereopod 7. Grandidierellanaroensis sp. nov. is morphologically most similar to G.pseudosakaensis. However, the new species can be distinguished by the presence of small distal and proximal processes and a large middle process on the carpus of gnathopod 1, and the subovate propodus of gnathopod 1. Both new species are illustrated and compared to related species. A key to species in the family Aoridae from Korean waters is also provided.

A Three–Year Comparison of Fluctuations in the Occurrence of the Giant Jellyfish (Nemopilema nomurai)

In this study, acoustic, sighting, trawl, and marine environmental surveys were used to determine the vertical distribution and density of giant jellyfish that have been observed in Korean waters over the past 3 years. From 2020 to 2022, annual surveys were conducted in May and July in the East China Sea and waters adjacent to South Korea. The acoustic data were processed by identifying and eliminating all signals considered as noise while excluding those suspected to be jellyfish signals. Subsequently, a single target detection method was employed. Giant jellyfish are distributed mostly in the middle and low layers. In May 2021, the average population density of giant jellyfish was recorded as 11.6 (ind./ha), which was the highest density. In July 2022, this value decreased to 1.7 (ind./ha), marking the lowest density. The sighting survey, which allows for the identification of jellyfish distributed in the surface layer, exhibited a difference of approximately 0.13 times compared to the acoustic survey conducted in the middle and low layers in 2020. In 2021 and 2022, this difference was approximately 0.11 times and 0.24 times, respectively. The average of this difference was 0.16 times or greater.

Multi-Criteria Model for Identifying and Ranking Risky Types of Maritime Accidents Using Integrated Ordinal Priority Approach and Grey Relational Analysis Approach

Accidents in marine operations are occurring consistently despite government safety initiatives and efforts to lower the number of accidents and the ensuing human casualties. Since each accident type has a different frequency and casualty rate, identifying risky accident types is important to determine the priority for taking necessary risk reduction actions. Usually, a risk is calculated using two criteria, i.e., the frequency and fatality of an accident. However, the accident statistics show that for the last 5 years from 2018 to 2022, the injury rate is more than three times the death rate in maritime accidents in Korean waters. Considering the importance of injury, unlike other previous studies, we perform a risk analysis with three criteria, i.e., frequency, death, and injury to complement the conventional risk calculation methods, which can help decision-makers allocate the limited resources to the riskiest types of accidents in order of priority. In doing so, we employed an integrated ordinal priority approach (OPA) and grey relational analysis (GRA) method to assign proper weight to each criterion and rank eight accident types. We categorized the accidents types into three different groups where safety accidents and collisions were ranked as the most dangerous types. The combined OPA and GRA technique has been effectively applied to other risky industries, as well as the maritime industry. Additionally, the proposed method is suitable for multi-criteria models when each criterion has a different importance. Finally, the method can be integrated into the framework of the risk ranking process to enhance the analysis results.

First record of Mytilisepta virgata (Bivalvia: Mytilidae) in Russian waters

Mytilisepta virgata was first collected as empty shells attached to plastic debris in south-western part of Peter the Great Bay (Sea of Japan), near Russian-Korean border. This species may be considered as a potentially non-indigenous species in Russian waters due to coastal warming, influence of warm-water currents and its presence in North Korean waters. A description, illustrations, data on distribution and ecology and taxonomic comments are provided.

Seasonal Phytoplankton Characteristics Related with Region-Specific Coastal Environments in the Korean Peninsula

The seasonal dynamics of phytoplankton communities in Korean coastal waters (KCWs) are influenced by complex interactions between ocean currents and nearshore human activities. Despite these influences, the understanding of seasonal phytoplankton changes and their environmental relationships in KCWs remains limited. We investigate the influence of the distinct characteristics of the three seas surrounding the KCWs (the Yellow Sea, the South Sea, and the East Sea) on seasonal phytoplankton communities based on field surveys conducted at 23 stations between 2020 and 2021. The East Sea exhibited higher winter temperatures due to the Jeju and Tsushima warm currents, while summer temperatures were lower compared to the other regions, highlighting the role of currents and deeper oceanic waters. The Yellow Sea showed significant freshwater influence with low salinity levels from major rivers, contrasting with the higher salinity in the East Sea. These differences led to a disparity in the productivity of the two regions: the highest value of Chl. a was observed to be 6.05 µg L–1 in the Yellow Sea in summer. Diatoms dominated in nutrient-rich conditions, particularly in the Yellow Sea, where they comprised up to 80–100% of the phytoplankton community in summer, winter, and spring. PCA analysis revealed positive correlations between diatoms and Chl. a, while cryptophytes, which thrive in the absence of diatom proliferation, showed no such correlation, indicating their opportunistic growth in nutrient-limited conditions. This study highlights the significant impact of region-specific hydrographic factors on phytoplankton communities in KCWs, with diatoms dominating in summer and cryptophytes and dinoflagellates showing seasonal and regional variations. Understanding these dynamics is crucial for predicting phytoplankton bloom dynamics and their ecological implications in coastal ecosystems.

Epiphytic Bacterial Community Analysis of Ulva prolifera in Garorim and Muan Bays, Republic of Korea.

The bacterial communities related to seaweed can vary considerably across different locations, and these variations influence the seaweed's nutrition, growth, and development. To study this further, we evaluated the bacteria found on the green marine seaweed Ulva prolifera from Garorim Bay and Muan Bay, two key locations on Republic of Korea's west coast. Our analysis found notable differences in the bacterial communities between the two locations. Garorim Bay hosted a more diverse bacterial population, with the highest number of ASVs (871) compared to Muan Bay's 156 ASVs. In Muan Bay, more than 50% of the bacterial community was dominated by Pseudomonadota. On the other hand, Garorim Bay had a more balanced distribution between Bacteroidota and Pseudomonadota (37% and 35.5%, respectively). Additionally, Cyanobacteria, particularly Cyanothece aeruginosa, were found in significant numbers in Garorim Bay, making up 8% of the community. Mineral analysis indicated that Garorim Bay had higher levels of S, Na, Mg, Ca, and Fe. Function-wise, both locations exhibited bacterial enrichment in amino acid production, nucleosides, and nucleotide pathways. In conclusion, this study broadens our understanding of the bacterial communities associated with Ulva prolifera in Korean waters and provides a foundation for future research on the relationships between U. prolifera and its bacteria.

A new species of Janiralata Menzies, 1951 (Isopoda: Asellota, Janiridae) from Korea

A new isopod species, Janiralata kwangsooi sp. nov., from Dokdo Island located in the East Sea off the Korean Peninsula is described here. This species is distinguished from its congeners by the number of antennular articles, setation of mouthparts and pereopods, length and width ratio of appendages, and shape of male pleopods I and II. To aid species identification, a taxonomic key to species of Janiralata Menzies, 1951 in the Japanese and Korean waters is provided. A partial sequence of mitochondrial cytochrome-c-oxidase (COI) of this new species is also provided for DNA barcoding and compared with those of congeners publicly available in GenBank.

Insights and machine learning predictions of harmful algal bloom in the East China Sea and Yellow Sea

The increase in harmful algal blooms (HABs) globally has been linked to climate change and anthropogenic activities such as agricultural runoff and urbanization. This study focused on analyzing the impact of these factors on HAB occurrences in the East China Sea and the Yellow Sea, identifying influential factors, and predicting future HAB events. For this study, random forest and numerical modeling were employed, with datasets encompassing physical and chemical properties of river water, seawater, and precipitation to assess the impact of discharge on HABs. Additionally, climate change scenarios derived from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) were employed to predict future HAB occurrences, supported by a sensitivity analysis to identify influential factors affecting HAB occurrence. This study demonstrated that the growth rate and occurrence of HABs in the East China Sea (ECS) and Korean coastal waters (KCW) distinctively increased in July and November after the operation of the Three Gorges Dam (TGD). It is likely affected by the decreasing discharge from the Yangtze River (YR) owing to the operation of the TGD. Using the Random Forest model, future HAB events were predicted in good agreement with observations. The sensitivity results revealed that environmental properties, such as precipitation, water temperature, and salinity are major features affecting the HAB trends in both the KCW and YR basins. Moreover, based on the random forest model and climate change scenarios, HAB events were predicted to increase in frequency in July, September, and October. Therefore, the findings can contribute to preventing biological pollution of the ocean system in the ECS and KCW by supporting efficient environmental management.

Improving a ship's energy efficiency in Korean coastal waters using tidal current information

This study investigates the impact of tidal currents on a ship’s speed, fuel consumption, and energy efficiency for Greenhouse Gas (GHG) reduction. The research was conducted in Korean coastal waters from Busan Port to Incheon Port during spring and neap tides. The results show that if a tanker sails with the most favorable tidal current, it can improve fuel efficiency by up to 2.46 % and reduce sailing hours by up to 2.17 hr. This change can also improve the Carbon Intensity Indicator (CII) Rating from "B" to "A". The study concludes that utilizing tidal currents can enhance energy efficiency and contribute to GHG reduction without any financial investment.

First report of Perkinsus marinus occurrence associated with wild Pacific oysters Crassostrea gigas from the west coast of Korea

This study reports the occurrence of Perkinsus marinus associated with wild Pacific oyster (Crassostrea gigas) specimens collected along the west coast of Korea. Confirmation of P. marinus presence was achieved by conventional PCR using World Organization of Animal Health (WOAH)-recommended primers that specifically targeted regions of the rDNA locus (ITS1, 5.8S, and ITS2). Sequencing of 10 samples revealed two distinct sequences differing by a single base pair, indicating potential haplotype variability. One sequence closely resembled the P. marinus strain found in Maryland, USA, whereas the other exhibited divergence, indicative of species diversity in the Korean strain, as was evident from the haplotype network analysis. Further validation involved the Ray’s Fluid Thioglycollate Medium (RFTM) assay, which initially yielded inconclusive results, possibly due to low infection intensity. Subsequently, RFTM and 2 M NaOH assays conducted on the isolates in the present study, cultured P. marinus cells in standard DMEM/F12 medium, and a positive P. marinus strain (ATCC 50509), revealed characteristic hypnospores of P. marinus upon Lugol’s iodine staining. These comprehensive investigations underscore the conclusive confirmation of P. marinus in Korean waters and mark a significant milestone in our understanding of the distribution and characteristics of this parasite in previously unreported regions.

Threats to a Temperate Kelp Forest Species, Ecklonia cava, through Tropical Fish Herbivory Associated with Sea Surface Warming in the East China Sea

Kelp forests in Korean waters, mainly consisting of Ecklonia cava, provide ecologically and economically important ecosystems. However, they are severely threatened by increasing sea surface temperature (SST). In 2023, an unusually high SST was observed in the northern East China Sea, where the average SST from August to November 2023 was found to be 1.1 °C higher than the average SST during the same period over the last two decades. Our photo images and videos reveal increasing feeding on E. cava populations by tropical herbivore rabbitfish (Siganus canaliculatus, Siganus fuscescens) associated with the impact of increasing SST. Given the fall reproductive peak of E. cava population, increased herbivory by tropical rabbitfish could have a significant adverse impact on the composition of temperate kelp forests.

The Long-Term Dynamics of Walleye Pollock Stocks in Relation to Oceanographic Changes in the East Sea

The decline in walleye pollock (Gadus chalcogrammus) stocks in Korean waters is a major concern for fishery conservation and management. However, the causes and mechanisms of this collapse remain unclear. This study investigated the complex dynamics influencing the abundance of walleye pollocks in the East Sea of Korea over several decades, by analyzing data from long-term changes in biological factors including composition of length and sex, catch, and oceanographic condition. Prior to the mid-1980s, the catch ratio of juveniles was higher than that of adults, with a higher proportion of females in both juvenile and adult catches compared to males. Especially, high fishing pressure on female individuals can be an important factor contributing to declining reproduction. Consequently, after the mid-1980s, there was a sudden decline in juvenile pollock catches. In the late 1980s, there was a rapid increase in sea surface temperature (SST) in the spawning grounds, resulting in a decrease in both the duration of suitable temperature for spawning and the regional proportion for suitable spawning conditions. Consequently, the decline in pollock stocks after the late 1980s due to overfishing of pollock in the mid-1980s was further exacerbated by the effects of SST warming after the late 1980s. These findings highlight the impact of overfishing and environmental factors on pollock stocks and indicate the need for appropriate fishery management practices to ensure the sustainable use of fishery resources.

New Species of Eupelte (Crustacea, Copepoda, Harpacticoida) from the East Sea, with Notes on the Zoogeography of the Genus

A new species of the genus Eupelte Claus, 1860, was obtained from the algal bed bottom in Dokdo Island, East Sea of Korea, using a light trap. Specimens of both sexes of the Korean new species are described in detail and compared with its congeners. The present species, Eupelte dokdoensis sp. nov., is closely similar to E. aurulenta Wells and Rao, 1987, found in algal sands from the Andaman and Nicobar Islands, and E. setacauda Monk, 1941, collected from seaweeds in the Californian coast of the USA. These species share the body length (0.5–07 mm), the nine-segmented A1, the setation of the first and second endopodal segments of P1 (one and four setae), P4 exp-3 with three inner setae, and enp-3 of P2–P4 with five setae in both sexes. The new species can be readily discernable from the previous two species by the short rostrum with a serrate distal margin, the shape of the maxillipedal basis, the setation and surface ornamentation of P1 enp-2 (covered with long setules), the exp-3 of P3 and P4 with thicker middle inner seta, and the setation of female and male P5 exopod and baseoendopod (six and six, four and two setae, respectively). In addition, the zoogeography of the valid species of the genus Eupelte is briefly discussed, and a diagnostic key to the females of the species is provided. Eupelte dokdoensis sp. nov. described herein is the first report of the genus from Korean waters and is the fourteenth member of the genus.

Environmental Factors Influencing Annual Changes in Bycatch per Unit Effort of Delphinus delphis around Their Main Hotspot in Korean Waters

In this study, the characteristics of temporal changes in bycatch per unit effort (BPUE) as an index of the relative density of the common dolphin around their widest hotspot in the East Sea/Sea of Japan were examined from 2011 to 2021. BPUE rapidly increased from 2017 to 2019. The annual changes in BPUE were due to changes during March–April, which is the most abundant season for common dolphins. The annual relationship between BPUE and four variables (Pacific herring, common squid catches, chlorophyll-a concentration, and sea surface temperature) around their main hotspot for March–April was investigated using generalized linear models (GLMs) for gamma distribution. A stepwise Bayesian information criterion for the gamma GLM yielded significant retention of two variables, Pacific herring catch and chlorophyll-a concentration, over the study period, indicating that the rapid increase of the relative density of the common dolphin in the early spring during 2017–2019 could be due to the increase of their prey abundance caused by higher primary productivity. Therefore, ecosystem productivity altered by nutrient conditions could be a key biological process that enhances habitat use of cetaceans in highly productive seasons and regions.

Estimation of bioluminescence intensity of the dinoflagellates Noctiluca scintillans, Polykrikos kofoidii, and Alexandrium mediterraneum populations in Korean waters using cell abundance and water temperature

Many dinoflagellates produce bioluminescence. To estimate the intensity of bioluminescence produced by populations of the heterotrophic dinoflagellates Noctiluca scintillans and Polykrikos kofoidii and autotrophic dinoflagellate Alexandrium mediterraneum in Korean waters, we measured cellular bioluminescence intensity as a function of water temperature and calculated population bioluminescence intensity with cell abundances and water temperature. The mean 200-second-integrated bioluminescence intensity per cell (BLcell) of N. scintillans satiated with the chlorophyte Dunaliella salina decreased continuously with increasing water temperature from 5 to 25°C. However, the BLcell of P. kofoidii satiated with the mixotrophic dinoflagellate Alexandrium minutum continuously increased from 5 to 15°C but decreased at temperatures exceeding this (to 30°C). Similarly, the BLcell of A. mediterraneum continuously increased from 10 to 20°C but decreased between 20 and 30°C. The difference between highest and lowest BLcell of N. scintillans, P. kofoidii, and A. mediterraneum at the tested water temperatures was 3.5, 11.8, and 21.0 times, respectively, indicating that water temperature clearly affected BLcell. The highest estimated population bioluminescence intensity (BLpopul) of N. scintillans in Korean waters in 1998–2022 was 4.22 × 1013 relative light unit per liter (RLU L-1), which was 1,850 and 554,000 times greater than that of P. kofoidii and A. mediterraneum, respectively. This indicates that N. scintillans populations produced much brighter bioluminescence in Korean waters than the populations of P. kofoidii or A. mediterraneum.