Do Strong Winds Impact Water Mass, Nutrient, and Phytoplankton Distributions in the Ice‐Free Canada Basin in the Fall?. Issue 1 (4th January 2020)

• Record Type: Journal Article
• Authors: Nishino, Shigeto; Kawaguchi, Yusuke; Inoue, Jun; Yamamoto‐Kawai, Michiyo; Aoyama, Michio; Harada, Naomi; Kikuchi, Takashi
• Is Part Of: Journal of geophysical research. Volume 125:Issue 1(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-01-04
• Subjects: Arctic Ocean -- Canada Basin -- nutrients -- phytoplankton -- wind forcing -- warm‐core eddies; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC015428
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: In general, strong wind events can enhance ocean turbulent mixing, followed by episodic nutrient supply to the euphotic zone and phytoplankton blooms. However, it is unclear whether such responses to strong winds occur in the ice‐free Canada Basin, where the seasonal pycnocline is strong and the nutricline is deep. In the present study, we monitored a fixed‐point observation (FPO) station in the Canada Basin for about 3 weeks in the fall of 2014 to examine the oceanic and biological responses to strong winds. At the FPO site, oceanic microstructure measurements, hydrographic surveys, and water sampling were performed with high temporal resolution, recording internal wave propagation, eddy passage, and water mass changes. Strong winds and internal wave propagation significantly enhanced the mixing above and at the seasonal pycnocline, but their effects were diminished at the nutricline, which was much deeper than the seasonal pycnocline. Therefore, wind‐induced mixing did not increase the upward nutrient supply from the nutricline and did not impact phytoplankton (chlorophyll a ) distribution in the surface layer of the FPO site. The temporal evolution of the chlorophyll a concentration was most closely related to water mass changes. We also observed prominent subsurface chlorophyll a maxima with abundant large‐sized phytoplankton that were likely carried by warm‐core eddies to the FPO site. Phytoplankton biomass may have been sustained by the high concentration of ammonium within the eddy and ammonium regeneration at the seasonal pycnocline, where particulate organic matter likely accumulated. Plain Language Summary: The Arctic basins, once covered with multiyear ice, are gaining ice‐free areas in summer and fall as an effect of global warming. An ice‐free upper ocean is more susceptible to wind forcing than ice‐covered waters. Here, we studied the oceanic and biological responses to strong winds in the ice‐free Canada Basin, where a density gap (pycnocline) was seasonally formed at a depth of about 20 m between the sea surface layer containing sea ice meltwater and the layer below. Although significant wind‐induced mixing was observed above and at this seasonal pycnocline, mixing was attenuated at nutricline depths (~60 m), where nutrient concentrations increased strongly with depth. Thus, nutrients were not supplied from the nutricline to the sea surface layer through wind‐induced mixing, and phytoplankton biomass did not increase in response to strong winds. However, phytoplankton biomass sometimes increased around the seasonal pycnocline, where ammonium was available for use in photosynthesis. This ammonium was likely supplied by warm‐core eddies transporting shelf water into the Canada Basin. Another conceivable source of ammonium was particulate organic matter suspended at the seasonal pycnocline that decomposed, producing ammonium. Key Points: A nutricline deeper than the seasonal pycnocline in the Canada Basin makes nutrient and phytoplankton distributions unresponsive to wind The increase in phytoplankton in the Canada Basin is partly explained by the effects of warm‐core eddies with high ammonium concentrations Particulate organic matter suspended at the seasonal pycnocline depth in the Canada Basin may be an ammonium source for photosynthesis