Preprints
https://doi.org/10.5194/bg-2021-116
https://doi.org/10.5194/bg-2021-116

  19 May 2021

19 May 2021

Review status: this preprint is currently under review for the journal BG.

Geophysical and biogeochemical observations using BGC Argo floats in the western North Pacific during late winter and early spring, Part 2: Biological processes during restratification periods in the euphotic and twilight layers

Chiho Sukigara1, Ryuichiro Inoue2, Kanako Sato2, Yoshihisa Mino3, Takeyoshi Nagai1, Andrea J. Fassbender4, Yuichiro Takeshita5, and Eitarou Oka6 Chiho Sukigara et al.
  • 1Tokyo University of Marine Science and Technology, Tokyo, 1088477, Japan
  • 2Japan Agency for Marine-Earth Science and Technology, Yokosuka, 2370061, Japan
  • 3Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, 4648601, Japan
  • 4NOAA/OAR Pacific Marine Environmental Laboratory, Washington, 98115, USA
  • 5Monterey Bay Aquarium Research Institute, California, 95039, USA
  • 6Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, 2778564, Japan

Abstract. Two Argo floats equipped with oxygen, chlorophyll (Chl), backscatter, and nitrate sensors conducted daily vertical profiles of the water column from a depth of 2000 m to the sea surface in the western North Pacific from January to April of 2018. Data for calibrating each sensor were obtained via shipboard sampling that occurred when the floats were deployed and recovered. Float backscatter observations were converted to particulate organic carbon (POC) concentrations using an empirical relationship derived from contemporaneous float profiles of backscatter and shipboard observations of suspended organic carbon particles. During the float deployment periods, repeated meteorological disturbances (storms) passed over the study area and caused the mixed layer to deepen. During these events, nitrate was entrained from deeper layers into the surface mixed layer, while Chl and POC in the surface mixed layer were redistributed into deeper layers. After the storms, the upper layer gradually restratified, nitrate concentrations in the surface layer decreased, and Chl and POC concentrations increased. When the floats observed the same water mass, the net community production within the euphotic layer (0–70 m), determined from the increases in POC, was 126–664 mg C m−2 d−1 (10.5–55.3 mmol C m−2 d−1) close to the values reported from a nearby area. The C/N ratio of the increase in POC and the decrease in nitrate was closed to the Redfield ratio, which indicates that the sensors were able to observe the net biochemical processes in this area despite the relatively low concentrations of nitrate and POC. To determine the fate of particles transported from the surface ocean to the twilight layer, the ratio of oxygen consumption and nitrate regeneration rates were compared. This O2/N ratio approached the Redfield ratio when the floats followed the same water mass continuously, but the consumption rate of POC was significantly lower than what would be expected based on the oxygen consumption and nitrate release rates. This suggests that dissolved organic carbon was the main substrate for the respiration in the twilight layer.

Chiho Sukigara et al.

Status: open (until 07 Jul 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-116', Anonymous Referee #1, 28 May 2021 reply

Chiho Sukigara et al.

Chiho Sukigara et al.

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Short summary
We combined ship-borne water sampling with the use of two Argo floats equipped with biogeochemical sensors to determine the changes in primary productivity associated with the passage of storms and resultant disturbance in the subtropical western North Pacific. We found that the episodic influx of carbon to the surface facilitated by storms played a key role in promoting primary production. Particulate carbon transported to the twilight layer were not the major substrate for the respiration.
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