The Yellow Sea Bottom Cold Water (YSBCW) refers to seawater with a water temperature of 10 °C or less found at the bottom of the center of the Yellow Sea. The spatiotemporal variability of the YSBCW directly affects the distribution of organisms in the marine ecosystem. In this study, hydroacoustic and net surveys were conducted in April (spring) to understand the spatial distribution of the sound scattering layer (SSL) and estimate the density of Euphausia pacifica (E. pacifica) in the YSBCW. Despite the shallow water in the YSBCW region, E. pacifica formed an SSL, which was distributed near the bottom during the daytime; it showed a diel vertical migration (DVM) pattern of movement toward the surface during the nighttime. The mean upward and downward swimming speeds around sunset and sunrise were approximately 0.6 and 0.3–0.4 m/min, respectively. The E. pacifica density was estimated in the central, western, and eastern regions; the results were approximately 15.8, 1.3, and 10.3 g/m², respectively, indicating significant differences according to region. The results revealed high-density distributions in the central and eastern regions related to the water temperature structure, which differs regionally in the YSBCW area. Additional studies are needed regarding the spatial distribution of E. pacifica in the YSBCW and its relationship with various ocean environmental parameters according to season. The results of this study contribute to a greater understanding of the structure of the marine ecosystem in the YSBCW.

We measured the monthly egg production rate (EPR) of Paracalanus parvus s. l. (P. parvus) at a fixed station in Busan Harbor from January 2020 to December 2021 to elucidate the seasonal variation in the EPR and the factors involved. Over the 2 years, the monthly EPR ranged from 0.5 to 47.0 eggs female⁻¹ d⁻¹ (average 13.3 eggs female⁻¹ d⁻¹). The seasonal mean EPR was high in spring (April to June) and summer (July to September) and low in autumn (October to December) in both years. Egg hatching success was high (average 94%). Monthly nauplii production in the P. parvus population ranged from 9 to 37,123 ind. m⁻³ d⁻¹ (average 4605 ind. m⁻³ d⁻¹), with a peak in March and May in 2020 and in April in 2021. The monthly EPR of P. parvus was more dependent on chlorophyll-a concentration than on water temperature. Nauplii production was dependent on the abundance of adult females, while the chlorophyll-a concentration had a positive effect on nauplii production. In this study, while there was no clear seasonal variation in the abundance of adult females, the EPR showed a clear pattern of seasonal variation. These results imply that for monitoring potential environmental changes in Busan Harbor, measuring the EPR of copepods may be a better indicator than copepod abundance.

This study investigated variation in mesozooplankton community structure and indicator species in the Yellow Sea and northern East China Sea, based on four surveys conducted at different times of year. Total mesozooplankton abundance ranged from 1063 to 4515 ind. m⁻³, and dry weight ranged from 23.3 to 44.6 mg m⁻³, with higher values observed in May and August compared to October and January. Redundancy analysis explained a modest fraction of the variation in community structure (17.6% in total; Axis 1: 10.5%, Axis 2: 4.6%), with temperature, salinity, and chlorophyll a jointly contributing to the observed gradients. Cluster and indicator species analyses delineated three ecologically distinct regions: (1) a neritic coastal area dominated by coastal copepods and meroplankton; (2) a cold-water region associated with the Yellow Sea Bottom Cold Water (YSBCW); and (3) a warm offshore region influenced by the Jeju and Yellow Sea Warm Currents. Oithona atlantica was consistently linked to the YSBCW, suggesting its potential as a biological indicator of cold-water mass, whereas Clausocalanus minor was confined to warm offshore waters and reflected the seasonal northward expansion of warm currents. These findings demonstrate a clear coupling between mesozooplankton community dynamics and hydrographic conditions during the survey periods.

We estimated the total production of Calanus sinicus over four seasons by measuring somatic and egg production rates in the Yellow Sea. In the process of measuring somatic production, the length–weight relationship and growth rates of C. sinicus in this region were also derived. The population abundance and biomass of C. sinicus were especially high in spring. All copepodite stages had similar proportions in spring, whereas the late stages and adults were most common in summer. The mean total production of the C. sinicus population was 107.6–259.7 μg C m⁻³ day⁻¹, ranging from higher values in spring to lower values in summer. The annual total production of C. sinicus in the Yellow Sea was 64.3 mg C m⁻³ year⁻¹ (4032 mg C m⁻² year⁻¹). The production of C. sinicus was positively correlated with chlorophyll a concentration, rather than water temperature or salinity. This study is the first to quantify the secondary production of C. sinicus in the Yellow Sea. These findings will broaden our understanding of the ecology of C. sinicus and its contribution to the Yellow Sea food web.

We examined the possible influence of the anticyclonic Ulleung Warm Eddy (UWE) on copepod community production, fecal pellet production (FPP), and suspended fecal pellet (SFP) biomass in the Ulleung Basin. Copepod biomass and production appeared to show spatial variability across the UWE, with relatively higher values observed at the eddy ring and outside the eddy compared to the eddy core. Large copepods, particularly Neocalanus, contributed substantially at the eddy ring and outside the eddy, whereas medium-sized copepods were more prevalent in the eddy core. FPP of the copepod community and SFP biomass also appeared to show a similar elevated trend at the eddy ring. Furthermore, SFP exhibited distinct diel variability at the eddy ring, reaching maxima at 100 m during the day and at 10–30 m at night, whereas little diel variation occurred at the eddy core. These results suggest that the UWE influences copepod size structure, production, and fecal pellet dynamics, highlighting potential implications for upper-ocean biogeochemical cycling and carbon export efficiency. Rather than relying on a statistically replicated regional survey, this study provides a process-oriented observational assessment of copepod community responses across the UWE.