LAUSR

Abstract Intensive phytoplankton blooms, which mainly consist of diatoms, take place in the northern Bering and Chukchi Seas from spring to summer. Little is known, however, about the diatoms contributing to new production in these waters during summer when the water column is often stratified. In this study, using a 13C, 15N dual isotope tracer technique plus scanning electron microscopy, we assessed the diatom genera or species contributing to new production in surface waters of the northern Bering and Chukchi Seas in July 2013. Relatively high concentrations of nitrate, nitrite, phosphate, and silicate were observed at Bering Strait and Chukchi Shelf stations, whereas at the other stations surface nitrate was generally depleted. Surface ammonium levels were relatively high (0.05–1.52 µM), suggesting the high activity of heterotrophic organisms. In surface waters, hourly nitrate uptake rates ranged from 0.03 to 3.73 mg N m−3h−1, while the ammonium uptake rates varied between 0.04 and 0.43 mg N m−3h−1. As a result, the mean f-ratio was computed as 0.62 ± 0.23 during observation. We found that the nitrate uptake rates were positively correlated with ambient nitrate levels, indicating that the activity was mainly regulated with the physical process of nitrate supply. The diatom Chaetoceros (mainly C. socialis/gelidus) numerically dominated the surface diatom assemblages. Contributions of C. socialis/gelidus (9 ± 4 µm in size) to the total armored phytoplankton abundance increased with an increase in the water column stratification, whereas Thalassiosila spp. (26 ± 8 µm) showed the opposite trend. These results were consistent with the survival strategy of small diatoms for nutrient uptake in oligotrophic waters through their higher cell surface area to volume ratios. On the other hand, the carbon biomass of Thalassiosira became relatively high in surface waters, and significantly correlated with f-ratio. We conclude that the centric diatom Thalassiosira plays a key role in determining new production in surface waters of the study area during summer.