Bottom‐associated phytoplankton bloom and its expansion in the Arctic Ocean

• Published in: Global change biology 2022-12, Vol.28 (24), p.7286-7295
• Main Authors: Shiozaki, Takuhei, Fujiwara, Amane, Sugie, Koji, Nishino, Shigeto, Makabe, Akiko, Harada, Naomi
• Format: Article
• Language: English
• Subjects: Ablation; Arctic Ocean; Blooms; Bottom water; Chlorophyll; Chlorophylls; Climate change; Diatoms; Global warming; Ice cover; Irradiance; Marine microorganisms; Ocean floor; Phytoplankton; phytoplankton bloom; Plankton; Polar environments; Primary production; Satellite observation; Satellites; Sea ice; sea ice reduction; Sediments; Shelving; subsurface chlorophyll maximum; Surface water; Transparency (optical)
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.16421

Phytoplankton production in the Arctic Ocean is increasing due to global warming‐induced sea ice loss, which is generally assessed through satellite observations of surface chlorophyll. Here we show that a diatom bloom can occur near the seafloor rather than at the surface in the open Arctic Ocean. Light can reach the seafloor underlying nutrient‐rich bottom water after the spring bloom because the surface water becomes oligotrophic and increases transparency in the region of shallow Arctic shelf. Our microcosm experiment demonstrated that diatoms formed a bloom when sediments on the shelf region, which contained abundant viable diatom cells, were exposed to even weak light reaching the seafloor (~1% of the surface irradiance). Repeated shipboard observations in the shelf region suggested that such bottom‐associated blooms occurred occasionally and the primary production was significantly underestimated by satellite observations. The average bottom irradiance (2003–2017) in the Arctic Ocean is particularly promoted in summer in the eastern East Siberian Sea and the Foxe Basin, which were ice‐covered throughout the year until the 1990s. Our results imply that hidden bottom‐associated blooms are now widespread across the shallow Arctic shelf region. We found a novel type of phytoplankton bloom occurring in the bottom water of open Arctic shelf region. This bloom is triggered by light reaching the shallow seafloor underlying nutrient‐rich bottom water. Since this bloom cannot be detected by surface observation, satellite data analysis significantly underestimates the regional primary production. This discovery challenges the conventional notion that phytoplankton blooms always occur at the surface. Furthermore, our analyses suggest that the hidden bottom‐associated blooms are now widespread across the Arctic Ocean with the recent sea ice loss.