No abstract

In recent decades, Antarctic ice sheets have rapidly retreated, thus contributing to rising sea levels. An estimated 2720 billion tonnes of ice was lost from Antarctica between 1992 and 2017, corresponding to a global sea-level rise of about 7.6 mm (Shepherd et al., 2018). In particular, grounded ice reduction in West Antarctica accounted for ∼86% of the total Antarctic ice loss. The rapid ice reduction in West Antarctica caused by the increase in glacial flow is believed to be driven by the thinning of the buttressing ice shelves, in turn associated with increasing ocean melt. Notably, the fastest rate of decline in ice volume was observed in the Amundsen Sea sector during the late 2000s (Turner et al., 2017), with some potential anthropogenic origins (Holland et al., 2019).The Dotson Ice Shelf (DIS) is about 70 km long and 50 km wide, and is situated between the Martin Peninsula (MP) and the Bear Peninsula (BP) on the Marie Byrd Land coast, in the Amundsen Sea embayment, West Antarctica (Figure 1). It buttresses the flow of the Kohler and Smith glaciers. A rapid thinning of the DIS has been

Phytoplankton in the Arctic Ocean are subject to nitrogen limitation in the summer, however, how severely the nitrogen stress affects phytoplankton physiology remains largely unknown. In the summers of 2015-2018, we examined the distribution of phytoplankton photophysiological properties across two contrasting regions of the Arctic Ocean with distinctly different levels of nitrogen availability in the upper water column. We quantified the extent of nitrogen stress using a highly sensitive fluorescence induction and relaxation system to obtain continuous underway measurements and via discrete sample analyses of phytoplankton physiology, as well as nutrient enrichment incubations. The results revealed vast regions in the Chukchi Sea where phytoplankton photosynthesis was severely nitrogen-stressed. Thereby, the maximum quantum yield of photochemistry in photosystem II showed only a small decrease (12 ± 9%) relative to its nutrient-replete values, while the maximum photosynthetic electron transport rates under saturating irradiance were impaired to a greater extent (40 ± 17%). This phytoplankton photosynthesis response is indicative of a severe nitrogen limitation, which results in dramatic reduction in growth and net primary production rates. Nutrient enrichment incubations also revealed a marked increase in large-size phytoplankton growth (>10 μm) after the nitrogen stress was alleviated, suggesting that the larger cells were more susceptible to nitrogen stress. These results are important for understanding how regional nitrogen fluxes control variability in the primary production and phytoplankton community structure and how these processes might change with rapid climate changes in the Arctic Ocean. Plain Language Summary Nutrient availability is the main bottom-up controls of phytoplankton physiology and growth in the upper ocean. The distribution of nutrient limitation in the global ocean varies greatly in space and time, so phytoplankton responses to this factor are essential for understanding the marine ecosystem. Although nitrogen limitation was previously shown in the Arctic Ocean in the summer, how nitrogen stress affects phytoplankton physiology remains largely unknown. This study investigates, with high spatial resolution, the distribution of phytoplankton physiological status and quantifies the effects of nitrogen stress in the western Arctic Ocean. Our results revealed severe nitrogen limitation in the summer that results in dramatic reduction in growth and net primary production in this region of the ocean. Therefore, alterations in nitrogen fluxes along with climate change in the Arctic Ocean would be important for controlling phytoplankton growth and primary production in this region.

Satellite observations and modeling data have suggested a significant increase in net primary production in the Arctic Ocean over the last decade due to retreating sea ice and the development of light availability caused by Arctic warming. Subsequently, under‐ice blooms (UIBs) are being recognized as an important phenomenon from the traditional perspective. However, the role of sea‐ice algae in UIBs is still unknown due to the limited availability of continuous observations. We analyzed data on primary producer‐derived lipid biomarkers from sinking particles collected over 1 year using time‐series sediment traps on the East Siberian Sea and Chukchi Sea slopes. Based on the seasonal changes in sympagic organic carbon derived from the data of the ice proxy (IP25) flux and pelagic biomarkers, such as highly branched isoprenoid trienes, epi‐brassicasterol and dinosterol, a UIB was identified in summer 2018 on the East Siberian Sea slope. Compared to the nutrient distribution on the Chukchi Sea slope, the UIB on the East Siberian Sea slope might have been triggered by the nutrient supply. The estimated flux‐weighted mean sympagic organic carbon value measured during the UIB period (May−August) was 1.04 mg m−2 d−1 on the East Siberian Sea slope, approximately five times greater than recorded that on the Chukchi Sea slope (0.23 mg m−2 d−1) during the same period. Our findings suggest that the importance of sea‐ice algae as primary producers has increased as the UIB phenomenon has become more important in the Arctic Ocean and that sea‐ice environments face changes due to Arctic warming.

The disposal of mine tailings into the marine environment is considered an essential option to secure the economic efficiency of deep-sea mining, but it might adversely affects the ecosystem. To examine the potential impacts of tailing disposal from polymetallic nodules and polymetallic sulfide mines on phytoplankton communities, addition experiments of crushed fine particles into surface seawater were conducted in the open Indian Ocean and changes in chlorophyll a fluorescence and community composition were analyzed. The addition of tailings had serious adverse effects on phytoplankton fluorescence and photosynthetic activity, regardless of mine type. The adverse effects seemed to mainly be due to the physical properties of the mine tailings. These also showed discriminatory effects on phytoplankton, resulting in great changes in community composition. The results suggest that mine tailings could have significant adverse impacts on phytoplankton assemblages, but the degree of impact greatly varies depending on the phytoplankton groups. The discriminatory impacts would cause changes in biomass, community structure, and thus ecological function.