Potential of diatoms in sediments as seeds for autumn blooms in the Pacific Arctic shelf

The Pacific gateway to the Arctic has a vast continental shelf spanning the northern Bering and the Chukchi Seas. Within this shelf region, diatoms are crucial in sustaining high primary production and facilitating the sinking particulate organic carbon flux from spring to summer. Consequently, the bottom sediments have abundant viable diatoms, including resting stages. Despite the importance of diatoms, our understanding of the dynamics of this organism in sediments and their capacity to initiate primary production in the Pacific Arctic shelf remains limited.

In this study, we delved into the photophysiological capabilities of diatoms in the surface sediments collected from the Chukchi Sea in autumn through a laboratory incubation experiment at 3°C under the light conditions of 300 or 30 µmol photons m^-2 s^-1 for seven days. This experiment revealed that diatoms, mainly Chaetoceros, quickly resumed photosynthesis after light exposure and reached the maximum photosynthetic carbon fixation rates within only several days. These results suggest that diatoms in sediments have a significant potential to function as “seeds” for bloom formation in the sunlit water column. We further examined diatom communities, including resting spores, in the water column of the Chukchi Sea during autumn using scanning electron microscopy (SEM) and DNA metabarcoding techniques, as well as environmental parameters. Consequently, intense winds and subsequent water turbulence in the shallow Chukchi caused the predominance of Chaetoceros resting spores, probably derived from the sediments, in the diatom assemblages. As speculated from the incubation experiment mentioned above, diatom resting spores from the sediments can germinate immediately in the water column. Thus, settled diatoms could work as seeds for subsequent autumn blooms by being supplied from the seafloor along with nutrient-rich water.

The recent delayed sea ice formation in the autumn Arctic leads to increased storm occurrence over open water and enhanced vertical mixing, resulting in more frequent autumn blooms. Therefore, diatoms in sediments could be one of the critical contributors to autumn blooms in the shallow Pacific Arctic.