[A-OS16] Coastal ocean circulation and material cycle

[AOS16-01] Border of temporal variation patterns of pH between nearshore and coastal areas

*Tsuneo Ono¹, Daisuke Muraoka¹, Masahiro Hayashi², Makiko Yorifuji², Kazuaki Tadokoro¹ (1.Fisheries Research and Education Agency, 2.Marine Ecology Research Institute)

Keywords:pH, pCO2, nearshore water, offshore water

There have been marvelous progress in understanding of spacio-temporal variation of surface pCO2 in open oceans, owing to many amount of voluntary ships and research vessels. However, understanding of pCO2 dynamics in coastal areas with bottom depths of less than 50m is still remained unknown, despite that pCO2 is greatly modified in these areas affected by local coastal carbon processes. Although the absence of coastal pCO2 monitoring may cause little difficulties in global carbon flux estimation, it may cause significant problems in ocean acidification research because various calsifiers, including some of important marine resources, inhabits the coastal oceans. To fill out such absence of coastal carbon monitoring data, a series of near-shore stations for pH monitoring had recently been launched arong Japanese coasts. However, we have limited knowledge on the representativeness of nearshore carbon data for general conditions of "coastal carbon cycle." To assess problem, we carried out parallel monitoring of pH in nearshore and offshore water in a coastal area.
We set pH sensors in nearshore (bottom depth 5 m) and offshore ((bottom depth 17 m) stations in Miyako Bay, Iwate prefecture, from Jul 2020 to Oct. 2022. Related hydrographic parameters such as water temperature, salinity, and Dissolved oxygen was also observed. Similar observation was carried out also in two stations (1 m and 25 m of bottom depths) in Kashiwazaki coast, Niigata Prefecture, during the same period. In both areas, pH in nearshore station showed systematically lower values than that in offshore station. Difference of pH between nearshore and offshore stations became up to 0.1 in summer season. We further collected historical shipboard pH monitoring data obtained at the area off Miyako-Bay with bottom depth from 70 m to 100 m, that was operated by R/V Wakataka-Maru in Fisheires Research and education Agensy. Variation of shipboard pH against water temperature showed similer pattern with that observed in offshore Miyako Bay station, while scatterness of pH in narrow water temperature range was significantly larger in offshore station than in shipboard data. In nearshow station, on the other hand, variation of shipboard pH against water temperature showed significantly different patters with that of offshore station and shopboard data. We will further discuss on the hydrograophic cause of pH-water temperature relationship in each three station.