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Arctic phytoplankton are highly sensitive to seawater physical and chemical conditions, especially in the context of rapid climate change and sea ice loss. We studied the spatial and seasonal distributions of diatoms, dinoflagellates and coccolithophores, and clarified their associations with light, temperature and nutrients in the western Barents Sea in late summer 2017, and winter, spring and early summer 2018. Diatoms, composed mainly of Chaetoceros, Fragilariopsis and Thalassiosira, bloomed in spring at the southern border of the marginal ice zone with mean abundance of 1.1 × 106 cells L−1 and biomass of 119.5 µg C L−1, and were observed to follow the retreat of sea ice in the Arctic water to the north at the shelf break near Nansen Basin, contributing to the progression of the summer situation. Dinoflagellates flourished in surface waters south of Svalbard in summer, with maxima of 2.2 × 105 cells L−1 and 78.2 µg C L−1. High abundances and calcite mass of coccolithophores were detected in the southern Barents Sea and southwest of Svalbard in summer, with maxima of 3.3 × 105 cells L−1 and 4.7 µg C L−1. Two distinct phytoplankton assemblages, closely linked with Atlantic water and Arctic water, were geographically separated by the Polar Front in two summers, with a percent similarity below 11.9%, suggesting great influence of the two water masses on large-scale distributions of phytoplankton. Redundancy analysis revealed that temperature was one of the most important factors in shaping the seasonal distributions of diatoms, while irradiance showed positive correlation with dominant dinoflagellates of each season. From the perspectives of phytoplankton composition and carbon biomass, our findings highlight the governing effect of physical seawater conditions on driving seasonal patterns of phytoplankton biogeography, as well as the pivotal role of nutrients in supporting the phytoplankton growing seasons in the western Barents Sea. [ABSTRACT FROM AUTHOR]

Living coccolithophore assemblages collected at discrete depths (0–490 m) from eleven stations across the shelf and slope regions of the Yellow and East China Seas (YECS) during fall 2013 were analyzed using a scanning electron microscope. A total of 32 taxa were recorded, and the predominant taxa were Emiliania huxleyi , Gephyrocapsa spp., Syracosphaera spp. and Algirosphaera robusta . The body coccoliths of A. robusta exhibited an unusual morphology with incomplete hoods, which were recorded in nearly half of the samples and may represent a new variety. In addition, an agglutination relationship was observed between Gephyrocapsa coccoliths and the tintinnid Dictyocysta lepida in oligotrophic waters. Total coccolithophores reached a maximum cell abundance of 252 × 10 3 cells/l (on average 27.8 × 10 3 cells/l), with the contribution of Gephyrocapsa ericsonii , E. huxleyi and Gephyrocapsa oceanica accounting for 36.4%, 29.6% and 15.3%, respectively, of the total abundance. Coccolithophore assemblages in the YECS were a mixture of coastal, shelf and subtropical taxa, with the diversity decreasing in a radial direction from the Okinawa Trough to the inner shelves. Distinct deep-water flora existed in the Kuroshio slope waters (100–490 m), which was precisely reflected by the cluster analysis, illustrating very low percent similarity (48.1%) with the other groups. The occurrence of subtropical taxa in the coccolithophore assemblages can be used as benign warmer water signals. In the bottom turbid layers, the coccolithophore assemblages were largely composed of free coccoliths (84.3%), implying complex processes, such as cell death, resuspension and lateral advection, in the sandy and detrital waters. To clarify the relationship between the species distribution and ambient environments, a redundancy analysis (RDA) was applied to distinguish how much the variation in the taxon composition could be attributed to changes in the environmental conditions. Conclusively, salinity and temperature, which to some extent could reflect the physical properties of water stability and stratification, were key factors in driving the distribution patterns of living coccolithophores in the study areas. [ABSTRACT FROM AUTHOR]

Spring and winter distributions of living coccolithophores in the Yellow Sea were studied using a polarizing microscope based on two surveys in April 2010 and January 2011. Nine species were recorded, including Emiliania huxleyi and Gephyrocapsa oceanica as the predominant forms. The cell abundance of all coccolithophores in the spring was significantly higher than in the winter ( p<0.01), with means of 15 387 and 2470 cells l-1, respectively. Notably, we observed a habitat selection of coccolithophores in the southeastern Yellow Sea (32-35°N, 123-125°E). This habitat was characterized by sharp temperature gradients during both seasons caused by water exchange between the shelf waters and the Yellow Sea Warm Current. Moreover, the cell abundance, standing crop and estimated fluxes of coccolithophores were abnormally high compared to other regions. The habitat preference of coccolithophores in the Yellow Sea was directly related to the hydrodynamic conditions. The unfavorable light conditions caused by the intense convective mixing were responsible for the low biomass of coccolithophores during the winter. However, the increasing temperature influenced by the warm current, in combination with water column stability, triggered the spring bloom of coccolithophores in the southern Yellow Sea. [ABSTRACT FROM AUTHOR]

A total of twenty-seven species of marine-living coccolithophores were recorded from seawater samples that were collected during two spring cruises along the shelf regions of the Yellow Sea (YS) and the East China Sea (ECS). They were classified into ten families and four orders, with some additional species incertae sedis. Most of these species were heterococcolithophores, and no holococcolithophores were examined from the YS waters. Six species were recorded for the first time from the coastal waters of the China Seas and their morphological characteristics are described and photographically illustrated in this paper. They were Cyrtosphaera lecaliae, Syracosphaera histrica, Syracosphaera marginaporata, Pappomonas cf. sp. type 3, Calyptrolithophora papillifera and Corisphaera strigilis. Three types of Emiliania huxleyi, type A, type B/C and type C, were examined. Species of the genus Syracosphaera, in addition to E. huxleyi and Gephyrocapsa oceanica, frequently occurred at the surveyed sites. The coccolithophore assemblages in the offshore waters of the ECS were characterized by high species diversity—fourteen species in one sample. This finding indicated that the shelf waters adjacent to the Kuroshio path were ideal habitats for living coccolithophores. The variation in taxonomic composition of these algae could be associated with differences in their preferred habitats. [ABSTRACT FROM AUTHOR]

Studies on the diversity and distribution of bacterial populations will improve the overall understanding of the global patterns of marine bacteria and help to comprehend local biochemical processes and environments. We evaluated the composition and the dynamics of bacterial communities in the sediment of Jiaozhou Bay (China) using PCR-denaturing gradient gel electrophoresis (DGGE). Sediment samples were collected from 10 different sites in May, August, and November 2008 and in February 2009. There was significant temporal variation in bacterial community composition at all sites. However, the spatial variation was very small. The DGGE analyses of bacterial communities were used to divide the 10 stations into three types. Canonical correspondence analysis (CCA) revealed that the changes in bacterial communities were driven by sediment properties. Sequence analysis of DGGE band-derived 16S rRNA gene fragments revealed that the dominant bacterial groups in the sediment were of the classes γ-proteobacteria and δ-proteobacteria and phyla Bacteroidetes and Nitrospirae. Our results provide considerable insight into the bacterial community structure in Jiaozhou Bay, China. [ABSTRACT FROM AUTHOR]

• Inter-annual and seasonal variations in zooplankton community structure were analyzed. • Warmer conditions in spring of 2017–2018 benefited copepod community. • A regime shift might have occurred in 2016/2017 in the Yellow Sea. Inter-annual and seasonal variations in zooplankton community structure in the Yellow Sea, China were studied in relation to climatic events and local environment from 2014 to 2018. Three zooplankton assemblages were identified based on cluster analysis, which were associated with seasonal temperature changes. Group 1 composed spring stations with moderate water column temperatures. The spring zooplankton assemblage had the highest mean total abundance with crustaceans, including euphausiids, amphipods and copepods dominating samples. Stations in the basin area during summer–autumn and all stations in winter composed Group 2, with low bottom water temperatures being a prominent feature. Zooplankton assemblage of Group 2 mainly consisted of a mixture of low-temperature species and some tropical species that were transported into the region by the Yellow Sea Warm Current. Coastal stations with high water temperature during summer–autumn made up Group 3. Zooplankton was characterized as summer–autumn neritic assemblage with high jellyfish abundance. Some meroplankton and brackish water species, i.e. Labidocera euchaeta , were indicator species confirming the influence of the river discharge. Both PDO (from positive to negative) and ENSO (from negative to positive) reversed their signs in 2016/2017. The abundances of total zooplankton, Noctiluca scintillans , Calanus sinicus and small copepods were much higher in spring of 2017–2018 compared to spring 2015 due to the warmer conditions. In addition, zooplankton community of summer 2017 was characterized by high abundances of copepods and gelatinous zooplankton, Diphyes chamissonis and Doliolum denticulatum. This was influenced by a strong river runoff and warm current transport, and differed from 2014 and 2016 summers. Our results suggest that a regime shift occurred in 2016/2017, which was supported by sharp changes in physical settings as well as zooplankton community structure in the Yellow Sea. However, a longer time series is required to uncover detailed processes and response mechanisms of environmental conditions of the Yellow Sea to climate change. [ABSTRACT FROM AUTHOR]