Nutrient and dissolved inorganic carbon variability in the North Pacific
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SPECIAL SECTION: ORIGINAL ARTICLE Ocean Mixing Processes (OMIX): Impact on Biogeochemistry, Climate an Ecosystem
Nutrient and dissolved inorganic carbon variability in the North Pacific Sayaka Yasunaka1 · Humio Mitsudera2 · Frank Whitney3 · Shin‑ichiro Nakaoka4 Received: 16 April 2020 / Revised: 30 July 2020 / Accepted: 19 August 2020 © The Author(s) 2020
Abstract A compilation of surface water nutrient (phosphate, nitrate, and silicate) and partial pressure of C O2 (pCO2) observations from 1961 to 2016 reveals seasonal and interannual variability in the North Pacific. Nutrients and calculated dissolved inorganic carbon (DIC) reach maximum concentrations in March and minimum in August. Nutrient and DIC variability is in-phase (anti-phase) with changes in the mixed layer depth (sea surface temperature) north of 30 °N, and it is anti-phase (in-phase) with changes in Chl-a north of 40 °N (in 30 °N–40 °N). Seasonal drawdown of nutrients and DIC is larger toward the northwest and shows a local maximum in the boundary region between the subarctic and subtropics. Stoichiometric ratios of seasonal drawdown show that, compared to nitrate, silicate drawdown is large in the northwestern subarctic including the Bering and Okhotsk seas, and drawdown of carbon is larger toward the south. Net community production in mixed layer from March to July is estimated to be more than 6 gC/m2/mo in the boundary region between the subarctic and subtropics, the western subarctic, the Gulf of Alaska, and the Bering Sea. Nutrient and DIC concentrations vary with the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation which cause changes in horizontal advection and vertical mixing. The DIC trend is positive in all analysis area and large in the western subtropics (> 1.0 μmol/l/yr). Averaged over the analysis area, it is increasing by 0.77 ± 0.03 μmol/l/yr (0.75 ± 0.02 μmol/kg/yr). Keywords Nitrate · Phosphate · Silicate · DIC · PDO · NPGO
1 Introduction The North Pacific is characterized by areas of high-nutrient but moderate-chlorophyll because of iron limitation in the subarctic (Martine and Fitzwater 1988), and of low-nutrient and low-chlorophyll due to strong stratification in the
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10872-020-00561-7) contains supplementary material, which is available to authorized users. * Sayaka Yasunaka [email protected] 1
Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan
2
Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan
3
Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, Canada
4
Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Japan
sun-rich subtropics (Karl and Church 2019). The boundary region between the subarctic and the subtropics is an area of strong CO2 uptake (Takahashi et al. 2009). A long-term decrease and ENSO-related variation of surface nutrients (phos
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