In recent years, significant changes in environmental conditions and marine ecosystems have been observed in the East Sea/Japan Sea. This study investigates the long‐term environmental dynamics and phytoplankton responses in the Ulleung Basin, situated in the southwestern East Sea/Japan Sea, utilizing satellite and in situ data from 2002 to 2021. Over this period, there was a noticeable increase in sea surface temperature (SST) (r = 0.5739, p < 0.01), accompanied by decreasing mixed layer depth (MLD) and chlorophyll‐a (Chl‐a) concentration (r = −0.6193 and −0.6721, respectively; p < 0.01). Nutrient concentrations within the upper 50 m significantly declined for nitrate and phosphate. A reduction in the N:P ratio indicated a shift from phosphorus‐limited to nitrogen‐limited environment. Moreover, primary production (PP) demonstrated a decreasing trend (r = −0.5840, p < 0.01), coinciding with an increase in small phytoplankton contribution (r = 0.6399, p < 0.01). Rising SST potentially altered the water column's vertical structure, hindering nutrient entrainment from the deep ocean. Consequently, this nutrient limitation may increase small phytoplankton contribution, resulting in a decline in total PP. Under the IPCC's SSP5‐8.5 scenario, small phytoplankton contribution in the Ulleung Basin is projected to rise by over 10%, resulting in a 29% average PP decrease by 2100. This suggests a diminishing energy supply to the food web in a warming ocean, impacting higher trophic levels and major fishery resources. These findings emphasize the critical need for understanding and monitoring these environmental shifts for effective fisheries management and marine ecosystem conservation.

The vertical distribution patterns of daily primary production and dark carbon fixation were investigated at three stations in the East/Japan Sea (hereafter East Sea), a semi-enclosed marginal sea in the northwest Pacific Ocean. Our results displayed consistent vertical patterns of daily primary production at two of the stations, while the third station exhibited a markedly different distribution pattern, highlighting localized variations in production dynamics. In contrast, dark carbon fixations displaying varying vertical patterns among the stations are not specific enough to have much meaning. Water column-integral values showed differences in the contribution of dark carbon fixation to total primary production, ranging from 4.5% to 27.1%. These variations may reflect environmental parameters such as nutrient concentrations. However, our study is limited by the lack of direct data on the microbial community structure and chemoautotrophic activities, which are crucial for a more comprehensive understanding of these patterns. Understanding the environmental drivers of dark carbon fixation is crucial for elucidating carbon cycling dynamics in the East Sea. Notably, dark carbon fixation could contribute up to one-third of primary production in the region as an additional source of newly produced organic matter, highlighting the need for further investigation into this previously overlooked process.

Over the past two decades, the environmental characteristics of the northern East China Sea (NECS) that make it a crucial spawning ground for commercially significant species have faced substantial impacts due to climate change. Protein (PRT) within phytoplankton, serving as a nitrogen-rich food for organisms of higher trophic levels, is a sensitive indicator to environmental shifts. This study aims to develop a regional PRT algorithm to characterize spatial and temporal variations in the NECS from 2012 to 2022. Employing switching chlorophyll-a and particulate organic nitrogen algorithms, the developed regional PRT algorithm demonstrates enhanced accuracy. Satellite-estimated PRT concentrations, utilizing data from the Visible Infrared Imaging Radiometer Suite (VIIRS), generally align with the 1:1 line when compared to in situ data. Seasonal patterns and spatial distributions of PRT in both the western and eastern parts of the NECS from 2012 to 2022 were discerned, revealing notable differences in the spatial distribution and major controlling factors between these two areas. In conclusion, the regional PRT algorithm significantly improves estimation precision, advancing our understanding of PRT dynamics in the NECS concerning PRT concentration and environmental changes. This research underscores the importance of tailored algorithms in elucidating the intricate relationships between environmental variables and PRT variations in the NECS.

The Ross Sea, one of the most productive regions of the Southern Ocean, plays a key role in biological carbon sequestration that contributes significantly to the global carbon cycle. In this study, we investigated the vertical distribution of particulate organic carbon (POC) and the biochemical composition of particulate organic matter (POM) in relation to phytoplankton assemblage structure during the austral summer of 2023. Stations were classified as either Phaeocystis antarctica ( P. antarctica )‐dominated or mixed assemblages composed of diatoms and P. antarctica . P. antarctica ‐dominated stations were characterized by carbohydrate (CHO)‐ and transparent exopolymer particle (TEP)‐enriched POM in the euphotic zone, whereas mixed assemblages contained relatively protein (PRT)‐rich material. Concentration‐based vertical attenuation patterns, assessed using power‐law fits to suspended POM profiles, indicated a weaker decline in Non‐biopolymeric carbon (Non‐BPC) with depth under P. antarctica dominance, whereas mixed stations exhibited steeper declines in suspended POC concentrations. These compositional differences in POM influenced the persistence of suspended POM remaining below the euphotic zone. Our findings suggest that phytoplankton assemblage structure, driven by environmental factors such as hydrographic structure, plays an important role in regulating the lability and potential preservation of organic carbon in Antarctic waters. The results highlight how future climate‐driven shifts in assemblage structure could alter the composition and persistence of organic carbon transferred into deeper waters of the Ross Sea.