CLASSIFICATION OF SEA AREAS BASED ON THE CHARACTERISTICS OF LONG-TERM VARIATION OF SEAWATER pCO₂ IN OSAKA BAY

Abstract

 We calculated the partial pressure of carbon dioxide (pCO₂) of seawater in Osaka Bay using water quality data observed since the 1970's, and analyzed the spatial and temporal variability of pCO₂ in the urban inner bay. As a result of comparing the spatial distribution of pCO₂ averaged by season every decade, a spatial gradient of pCO₂ was formed in both surface layer and bottom layer during the high temperature period. However, the surface layer in the deep part of the bay tended to absorb atmospheric CO₂, while the bottom layer tended to store high concentrations of CO₂. Furthermore, a cluster analysis was conducted based on the time series of pCO₂ data from all the survey sites to classify the sea area of Osaka Bay into four areas which are the inner part of the bay, the center part of the bay, the southern part of the bay, and the mouth of the bay. The increase trend of pCO₂ of surface seawater in the whole Osaka Bay was 2.3 μatm/year, which was slightly larger than that of the surface seawater observed in the northwest Pacific Ocean. Furthermore, the pCO₂ in the inner part of the bay which is affected by urban drainage showed a strong seasonal variation but the trend of increase was different between in the surface layer (1.3 μatm/year) and in the bottom layer (5.8 μatm/year).