The in situ primary production rates and various environmental variables were investigated in the Chukchi Sea during the RUSALCA expedition, which was conducted in 2012, to identify the current status of primary production. A ¹³C–¹⁵N dual-tracer technique was used to measure the daily primary production rates, which ranged from 0.02 to 1.61 g C m⁻² d⁻¹ (mean ±SD = 0.42 ± 0.52 g C m⁻² d⁻¹). The primary production rates showed large regional differences, with the southern region (0.66 ± 0.62 g C m⁻² d⁻¹) producing approximately 5 times as much as the northern region (0.14 ± 0.10 g C m⁻² d⁻¹), which was primarily due to the differences in phytoplankton biomasses induced by regional nutrient conditions. The primary production rates in the Chukchi Sea were averaged using data acquired during the three different RUSALCA expeditions (2004, 2009, and 2012) as 0.33 g C m⁻² d⁻¹ (SD = 0.40 g C m⁻² d⁻¹), which was significantly lower than previously reported rates. In addition to strong seasonal and interannual variations in primary production, recent decreases in the concentrations of major inorganic nutrients and chlorophyll a could be among the reasons for the recent low primary production in the Chukchi Sea because the primary production is mainly affected by nutrient concentration and phytoplankton biomass. The nutrient inventory and primary production appear to be largely influenced by the freshwater content (FWC) variability in the region due to the significant relationships between FWC, nitrate inventory (r = 0.54, p < 0.05), and primary production rates (r = 0.56, p < 0.05). Moreover, we found highly significant relationships between the nutrient inventory and the primary production rates (r = 0.75, p < 0.001). In conclusion, the primary production in the Chukchi Sea is primarily controlled by nutrient availability, which is strongly related to the FWC variability. Our results imply that the predicted increase in freshwater accumulation might cause a decrease in primary production by lowering the nutrient inventory in the euphotic zone of the Chukchi Sea.

In recent years, the change of marine environment due to climate change and declining primary productivity have been big concerns in the East/Japan Sea, Korea. However, the main causes for the recent changes are still not revealed clearly. The particulate organic carbon (POC) to chlorophyll-a (chl-a) ratio (POC:chl-a) could be a useful indicator for ecological and physiological conditions of phytoplankton communities and thus help us to understand the recent reduction of primary productivity in the East/Japan Sea. To derive the POC in the East/Japan Sea from a satellite dataset, the new regional POC algorithm was empirically derived with in-situ measured POC concentrations. A strong positive linear relationship (R² = 0.6579) was observed between the estimated and in-situ measured POC concentrations. Our new POC algorithm proved a better performance in the East/Japan Sea compared to the previous one for the global ocean. Based on the new algorithm, long-term POC:chl-a ratios were obtained in the entire East/Japan Sea from 2003 to 2018. The POC:chl-a showed a strong seasonal variability in the East/Japan Sea. The spring and fall blooms of phytoplankton mainly driven by the growth of large diatoms seem to be a major factor for the seasonal variability in the POC:chl-a. Our new regional POC algorithm modified for the East/Japan Sea could potentially contribute to long-term monitoring for the climate-associated ecosystem changes in the East/Japan Sea. Although the new regional POC algorithm shows a good correspondence with in-situ observed POC concentrations, the algorithm should be further improved with continuous field surveys.

In the Philippine Sea, mesoscale eddies have been frequently observed, but little is known about their contributions to the biological productivity and community structure of phytoplankton in the region. In situ carbon and nitrogen uptake rates and phytoplankton community structure were investigated in the Philippine Sea based on the ¹³C‐¹⁵N dual‐tracer technique and photosynthetic pigment analysis from late August to September 2018. Based on the comparison of the phytoplankton community structure among the cold eddy, warm eddy, and outer area, we found that the phytoplankton composition was different in the warm eddy with relatively lower contributions of large phytoplankton. However, a distinct enhancement in phytoplankton biomass over the euphotic zone was not detected even in eddy occurrence. In terms of community, the picoplankton Prochlorococcus was persistently dominant, which was attributed to a low supply and chemical composition of nutrients. The average integrated primary productions (IPP) were 80.4, 75.9, and 76.3 mg C m⁻²d⁻¹ for the cold eddy, warm eddy, and outer area, respectively. The IPP under the eddy condition was not largely different from that in reference sites, even though the vertical distribution of productivity maximum was found to be different depending on eddy condition. These weak biological responses to mesoscale eddies might be attributed to the sustaining picoplankton‐dominant community structure and no substantial upward inputs of nutrients to the euphotic zone. Our findings represent an important contribution to understanding the biological response of mesoscale eddies in the oligotrophic Philippine Sea.

Picocyanobacteria (Prochlorococcus and Synechococcus) play an important role in primary production and biogeochemical cycles in the subtropical and tropical Pacific Ocean, but little biological information on them is currently available in the North Pacific Ocean (NPO). The present study aimed to determine the picocyanobacterial contributions to the total primary production in the regions in the NPO using a combination of a dual stable isotope method and metabolic inhibitor. In terms of cell abundance, Prochlorococcus were mostly dominant (95.7 ± 1.4%) in the tropical Pacific region (hereafter, TP), whereas Synechococcus accounted for 50.8%–93.5% in the subtropical and temperate Pacific region (hereafter, SP). Regionally, the averages of primary production and picocyanobacterial contributions were 11.66 mg C m⁻²·h⁻¹ and 45.2% (±4.8%) in the TP and 22.83 mg C m⁻²·h⁻¹ and 70.2% in the SP, respectively. In comparison to the carbon, the average total nitrogen uptake rates and picocyanobacterial contributions were 10.11 mg N m⁻²·h⁻¹ and 90.2% (±5.3%) in the TP and 4.12 mg N m⁻²·h⁻¹ and 63.5%, respectively. These results indicate that picocyanobacteria is responsible for a large portion of the total primary production in the region, with higher contribution to nitrogen uptake rate than carbon. A long-term monitoring on the picocyanobacterial variability and contributions to primary production should be implemented under the global warming scenario with increasing ecological roles of picocyanobacteria.

The macromolecular concentrations and compositions of phytoplankton are crucial for the growth or nutritional structure of higher trophic levels through the food web in the ecosystem. To understand variations in macromolecular contents of phytoplankton, we investigated the macromolecular components of phytoplankton and analyzed their spatial pattern on the Chukchi Shelf and the Canada Basin. The carbohydrate (CHO) concentrations on the Chukchi Shelf and the Canada Basin were 50.4–480.8 μg L⁻¹ and 35.2–90.1 μg L⁻¹, whereas the lipids (LIP) concentrations were 23.7–330.5 μg L⁻¹ and 11.7–65.6 μg L⁻¹, respectively. The protein (PRT) concentrations were 25.3–258.5 μg L⁻¹ on the Chukchi Shelf and 2.4–35.1 μg L⁻¹ in the Canada Basin. CHO were the predominant macromolecules, accounting for 42.6% on the Chukchi Shelf and 60.5% in the Canada Basin. LIP and PRT contributed to 29.7% and 27.7% of total macromolecular composition on the Chukchi Shelf and 30.8% and 8.7% in the Canada Basin, respectively. Low PRT concentration and composition in the Canada Basin might be a result from the severe nutrient-deficient conditions during phytoplankton growth. The calculated food material concentrations were 307.8 and 98.9 μg L⁻¹, and the average calorie contents of phytoplankton were 1.9 and 0.6 kcal m⁻³ for the Chukchi Shelf and the Canada Basin, respectively, which indicates the phytoplankton on the Chukchi Shelf could provide the large quantity of food material and high calories to the higher trophic levels. Overall, our results highlight that the biochemical compositions of phytoplankton are considerably different in the regions of the Arctic Ocean. More studies on the changes in the biochemical compositions of phytoplankton are still required under future environmental changes.