The Prominent Spring Bloom and Its Relation to Sea‐Ice Melt in the Sea of Okhotsk, Revealed by Profiling Floats. Issue 6 (26th March 2021)

• Record Type: Journal Article
• Authors: Kishi, S.; Ohshima, K. I.; Nishioka, J.; Isshiki, N.; Nihashi, S.; Riser, S. C.
• Is Part Of: Geophysical research letters. Volume 48:Issue 6(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-03-26
• Subjects: dissolved oxygen -- net community production -- phytoplankton bloom -- profiling float -- sea ice melt -- seasonal ice zone; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL091394
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Seven profiling floats equipped with oxygen sensors deployed in the Sea of Okhotsk provide time series data for 33 cases of spring phytoplankton bloom period, including 9 cases in which sea ice existed just before the bloom (prior‐ice case). As an index of biological productivity, we calculated net community production (NCP) based on the increasing oxygen rate using the Redfield ratio. The total NCP in the euphotic layer averaged for prior‐ice cases is 31.3 mmolC m −2 day −1 ), ∼3 times higher than that of non‐ice cases. In addition to intensification of surface stratification, other factors of sea‐ice melt likely enhance the bloom. The influence of sea‐ice melt is particularly large in the southwestern region, where the iron availability likely limits phytoplankton growth. A suggested scenario is that when the sea ice containing sediment/iron is transported from the northern shelves, a prominent bloom is induced via the iron supply by sea‐ice melt. Plain Language Summary: The seasonal ice zone is a high biological productivity area with a large spring phytoplankton bloom. The enhanced biological production results in significant CO2 uptake, which could play an important role in carbon budget. However, understanding of high biological productivity in seasonal ice zones is poor due to limited observations. This study examines the spring bloom in and around the Sea of Okhotsk, a typical seasonal ice zone, using profiling floats equipped with oxygen sensors. Based on the rate of oxygen increase for 30 cases of the spring bloom period, we estimated NCP as a quantitative indicator of biological production and CO2 uptake. It is statistically shown that a large spring bloom, corresponding to high NCP, is strongly related to sea‐ice melt. The cause is suggested to be sediments/iron released from melting sea ice as well as enhancement of stratification and light availability. This study will also provide the first attempt to evaluate the impact of significant sea ice decline in the Sea of Okhotsk on the carbon budget in the present and future. Key Points: Profiling floats equipped with oxygen sensors observed 30 cases of spring phytoplankton bloom in and around the Sea of Okhotsk, seasonal ice zone Total net community production estimated in the euphotic layer in cases where sea ice existed is much higher than that of no ice cases In addition to the intensification of surface stratification, sediment/iron released from melting sea ice likely enhances the spring bloom