Abstract
Phytoplankton communities in seasonally ice-covered areas are largely affected by ice algae. The Okhotsk Sea is the southernmost sea ice zone in the northern hemisphere with a sizeable seasonal ice cover, thus ice algae of the Okhotsk sea ice have a large potential to seed the early spring bloom. Little is known about the Okhotsk ice algal communities and their seeding effects. We investigated the dynamics of the composition and the photophysiological performances of an Okhotsk ice algal community in a 6-day laboratory incubation experiment that simulated the natural ice melt conditions. Centric diatoms, especially Thalassiosira spp., overwhelmingly dominated the ice algal community throughout incubation, whereas pennate diatoms, mostly Navicula and Nitzschia, showed little growth with much higher mortality. The maximum photochemical efficiency of Photosystem II (Fv/Fm) was the lowest at the beginning of the ice melt, suggesting a suppressed photosynthetic functioning by changes in salinity. The cellular pigment contents decreased by 30% due to physical damage of the cellular membrane, evidenced by deformed plastids under a microscope. The transcript level of the rbcL gene that encodes the large subunit of RubisCO was significantly higher during ice melt and decreased in the no-ice period, suggesting an urgent need for osmoprotectants under the melt condition. Full recovery of photosynthesis and growth was also made after complete ice melt. Our results indicated high seeding potential of Thalassiosira to spring blooms owing to their photophysiological plasticity to dynamic salinity changes.
Highlights
Sea ice algae can form a large fraction of sea-ice biomass (Tedesco et al, 2019)
To simplify the calculation of the algal growth during the ice melt period, we assumed that the algal cells remaining in sea ice and
Results showed that despite the large difference in μw between centric and pennate diatoms, the calculated μi for the two groups were similar, around 0.05 day−1, and very close to that of the μw of pennate diatoms in the no-ice period. Those released into the seawater had the same growth rate, μi. By doing this we can imagine that all the algal cells were staying in sea ice until the moment ice had melted out (t31) when all cells were released into the seawater
Summary
Sea ice algae can form a large fraction of sea-ice biomass (Tedesco et al, 2019). There is a substantial standing stock of ice algae in the Okhotsk Sea ice. Dense. Sea Ice Melt Affects Bloom ice algal communities have commonly been observed at the bottom of Okhotsk sea ice, especially in spring (Leonov et al, 2007). Chlorophyll a (Chl a) concentrations in the Okhotsk sea ice ranged between 0.2 to 3.5 mg m−2, and was up to an order of magnitude higher than the under-ice seawater (Granskog et al, 2015). Granskog et al (2015) found considerable excess proteinlike compounds in the Okhotsk Sea ice relative to the under-ice seawater, suggesting active biological production within the ice Chlorophyll a (Chl a) concentrations in the Okhotsk sea ice ranged between 0.2 to 3.5 mg m−2, and was up to an order of magnitude higher than the under-ice seawater (Granskog et al, 2015). McMinn et al (2008) reported an extremely high Chl a concentration of 1.6 × 103 mg Chl a m−3 in the pack ice near Shiretoko with an average thickness of around 70 cm. Granskog et al (2015) found considerable excess proteinlike compounds in the Okhotsk Sea ice relative to the under-ice seawater, suggesting active biological production within the ice
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