The importance of small phytoplankton in warming and stratified water conditions has been emphasized, but little information on the small phytoplankton contribution is currently available in the Southern Ocean. In this study, we analyzed temporal variations in the carbon uptake rates of large and small phytoplankton and the small phytoplankton contributions to the total primary production. The data were collected in the Amundsen Sea polynya region using three different field observations (early January, January-February, and February-March) from 2012 to 2016. The total primary production in the polynya region decreased from 0.84 ± 0.18 g C·m −2 ·day −1 in early January to 0.19 ± 0.04 g C·m −2 ·day −1 in February-March, which is typical of the seasonal variation in the primary production of phytoplankton in the Amundsen Sea. In contrast, the small phytoplankton contributions to the total carbon uptake rates increased considerably from early January (14.9 ± 8.4%) to February-March (50.8 ± 18.4%) in the polynya region. This substantial increase of small phytoplankton contribution was not caused by increasing biomass and the specific uptake rates of small phytoplankton, but rather by seasonally declining biomass and the specific uptake rates of large phytoplankton during the study period. The overall contribution (25.5 ± 15.7%) of small phytoplankton to the primary production in the polynya region based on the three different time observations is significantly higher than the previous result (14.9 ± 8.4%) based on one time observation. Therefore, a seasonal variation in the small phytoplankton contribution in the Amundsen Sea should be considered in further studies.

The East China Sea (ECS) is the largest marginal sea in the northern western Pacific Ocean. In comparison to various physical studies, little information on the seasonal patterns in community structure of phytoplankton is currently available. Based on high performance liquid chromatography (HPLC) pigment analysis, spatiotemporal variations in phytoplankton community compositions were investigated in the northern ECS. Water temperature and salinity generally decreased toward the western part of the study area but warmer conditions in August led to strong vertical stratification of the water column. In general, major inorganic nutrient concentrations were considerably higher in the western part with a shallow water depth, and consistent with previous results, had no discernable vertical pattern during our observation period except in August. This study also revealed PO4-limited environmental conditions in May and August. The monthly averaged integral chlorophyll-a concentration varied seasonally, highest (35.2 ± 20.22 mg m−2) in May and lowest (5.2 ± 2.54 mg m−2) in February. No distinct vertical differences in phytoplankton community compositions were observed for all the sampling seasons except in August when cyanobacteria predominated in the nutrient-deficient surface layer and diatoms prevailed at deep layer. Canonical correlation analysis results revealed that nutrient distribution and the water temperature were the major drivers of the vertical distribution of phytoplankton communities in August. Spatially, a noticeable difference in phytoplankton community structure between the eastern and western parts was observed in November with diatom domination in the western part and cyanobacteria domination in the eastern part, which were significantly (p < 0.01) correlated with water temperature, salinity, light conditions, and nutrient concentrations. Overall, the two major phytoplankton groups were diatoms (32.0%) and cyanobacteria (20.6%) in the northern ECS and the two groups were negatively correlated, which holds a significant ecological meaning under expected warming ocean conditions.

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The current phytoplankton community structure is expected to change, with small phytoplankton becoming dominant under ongoing warming conditions. To understand and evaluate the ecological roles of small phytoplankton in terms of food quantity and quality, the carbon uptake rates and intracellular biochemical compositions (i.e., carbohydrates, CHO; proteins, PRT; and lipids, LIP) of phytoplankton of different sizes were analyzed and compared in two different regions of the western East/Japan Sea (EJS): the Ulleung Basin (UB) and northwestern East/Japan Sea (NES). The average carbon uptake rate by the whole phytoplankton community in the UB (79.0 ± 12.2 mg C m–2 h–1) was approximately two times higher than that in the NES (40.7 ± 2.2 mg C m–2 h–1), although the average chlorophyll a (chl a) concentration was similar between the UB (31.0 ± 8.4 mg chl a m–2) and NES (28.4 ± 7.9 mg chl a m–2). The main reasons for the large difference in the carbon uptake rates are believed to be water temperature, which affects metabolic activity and growth rate, and the difference in euphotic depths. The contributions of small phytoplankton to the total carbon uptake rate were not significantly different between the regions studied. However, the rate of decrease in the total carbon uptake with increasing contributions from small phytoplankton was substantially higher in the UB than in the NES. This result suggests that compared to other regions in the EJS, the primary production in the UB could decrease rapidly under ongoing climate change. The calorific contents calculated based on biochemical compositions were similar between the small (1.01 ± 0.33 Kcal m–3) and large (1.14 ± 0.36 Kcal m–3) phytoplankton in the UB, whereas the biochemical contents were higher in the large phytoplankton (1.88 ± 0.54 Kcal m–3) than in the small phytoplankton (1.06 ± 0.18 Kcal m–3) in the NES. The calorific values per unit of chl a were higher for the large phytoplankton than for the small phytoplankton in both regions, which suggests that large phytoplankton could provide a more energy efficient food source to organisms in higher trophic levels in the western EJS.

Transparent exopolymer particles (TEPs) are defined as acidic polysaccharide particles and they are influenced by various biotic and abiotic processes that play significant roles in marine biogeochemical cycles. However, little information on their monthly variation, relationship and contribution to particulate organic carbon (POC) is currently available particularly in coastal regions. In this study, the water samples were collected monthly to determine TEP concentrations and POC concentrations in a southern coastal region of Korea, Jaran Bay from April 2016 to March 2017. The TEP concentrations varied from 26.5 to 1695.4 μg Xeq L−1 (mean ± standard deviation (S.D.) = 215.9 ± 172.2 μg Xeq L−1) and POC concentrations ranged from 109.9 to 1201.9 μg L−1 (mean ± S.D. = 399.1 ± 186.5 μg L−1) during our observation period. Based on the 13C stable isotope tracer technique, monthly carbon uptake rates of phytoplankton ranged from 3.0 to 274.1 mg C m−2 h−1 (mean ± S.D. = 34.5 ± 45.2 mg C m−2 h−1). The cross-correlation analysis showed a lag-time of 2 months between chlorophyll a and TEP concentrations (r = 0.86, p < 0.01; Pearson’s correlation coefficient). In addition, we observed a 2 month lag-phased correlation between TEP concentrations and primary production (r = 0.73, p < 0.05; Pearson’s correlation coefficient). In Jaran Bay, the TEP contribution was as high as 78.0% of the POC when the TEP-C content was high and declined to 2.4% of the POC when it was low. These results showed that TEP-C could be a significant contributor to the POC pool in Jaran Bay.