BGD

Biogeosciences Discussions

BGD

Biogeosciences Discuss.

1810-6285

Göttingen, Germany

10.5194/bg-2021-116

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

Sukigara

Chiho

¹ Inoue

Ryuichiro

² Sato

Kanako

² Mino

Yoshihisa

³ Nagai

Takeyoshi

¹ Fassbender

Andrea J.

https://orcid.org/0000-0002-5898-1185

⁴ Takeshita

Yuichiro

⁵ Oka

Eitarou

⁶

Tokyo University of Marine Science and Technology, Tokyo, 1088477, Japan

Japan Agency for Marine-Earth Science and Technology, Yokosuka, 2370061, Japan

Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, 4648601, Japan

NOAA/OAR Pacific Marine Environmental Laboratory, Washington, 98115, USA

Monterey Bay Aquarium Research Institute, California, 95039, USA

Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, 2778564, Japan

19 05 2021

2021 1 26

2021

This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/

This article is available from https://bg.copernicus.org/preprints/bg-2021-116/

The full text article is available as a PDF file from https://bg.copernicus.org/preprints/bg-2021-116/bg-2021-116.pdf

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.