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.

We investigated feeding by Calanus sinicus in the Yellow Sea over four seasons in 2019–2022 using the gut pigment method. Studying the feeding of copepods is important for understanding processes within food webs, but little is known about the feeding ecology of C. sinicus, a major consumer of primary production in the Yellow Sea. The seasonal mean ingestion rate of C. sinicus was 16.6–42.1 for copepodite stage I (CI), 24.1–41.8 for CII, and 32.7–58.5 ng chl ind.⁻¹ d⁻¹ for CIII, with different seasonal fluctuations in each stage. The seasonal mean ingestion rate was 45.8–114.4 for CIV, 50.2–189.3 for CV, 96.9–438.9 for females, and 69.8–132.3 ng chl ind.⁻¹ d⁻¹ for males, with higher values in spring and lower values in summer. The combined grazing impacts of C. sinicus from the CI to adult stages were 1.66%, 1.43%, 2.04%, and 0.65% in spring, summer, autumn, and winter, respectively. The ingestion rate of later developmental stages from CIV to adults was positively related to chlorophyll-a concentration rather than water temperature or salinity, according to a redundancy analysis. These results suggest that C. sinicus population in the Yellow Sea actively feeds on phytoplankton in spring and that most of the grazing impact is attributed to females.

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.