Source/sink of CO2 in the southwestern part of Peter the Great Bay (sea of Japan)

Abstract

In this study, the carbon dioxide (CO₂) system on the continental shelf of the southwestern part of Peter the Great Bay (PGB), Sea of Japan was investigated during the autumn of 2015. During this season, the PGB represents an ocean-dominated shelf interaction system as the source of nutrients on the shelf is the subsurface part of the Sea of Japan. Weak seasonal upwelling occurred during the study period (October 20–23, 2015), forming a two-layer water structure on the shelf with a sharp pycnocline. The upper warm layer (12–15 °C) is characterized by nutrient depletion and supersaturation of dissolved oxygen concentrations (apparent oxygen utilization (AOU) < 0), and it simultaneously acts as a sink to atmospheric carbon dioxide (pCO₂<pCO_2atm). The layer beneath the pycnocline is characterized by low temperatures (2–5 °C) and a high concentration of nutrients; it exhibits an oxygen deficit (AOU>0) and an excess of carbon dioxide (pCO₂>pCO_2atm). Nitrate concentrations in the near-bottom layer of the waters suggest that these waters upwelled to the PGB shelf at depths of 200–300 m in the open Japan-East Sea. We observed a high intensity of photosynthesis corresponding to the pycnocline zone, with chlorophyll a concentration exceeding 10 mg/m³, which was attributed to the phytoplankton bloom. The estimated primary production in the euphotic layer of the southwestern part of PGB is up to 4.98 gC m⁻² day⁻¹. Based on the dissolved inorganic carbon (DIC)/nutrient ratio in the subsurface waters of the northwestern Sea of Japan, the shelf becomes a sink for atmospheric carbon dioxide after upwelling. Owing to the heating of surface waters during summer, the PGB acts as a source of CO₂ to the atmosphere.