A significant net sink for CO2 in Tokyo Bay

Most estuaries and inland waters are significant source for atmospheric CO2 because of input of terrestrial inorganic carbon and mineralization of terrestrially supplied organic carbon. In contrast to most coastal waters, some estuaries with small freshwater discharge are weak source or sometimes sink for CO2. Extensive surveys of pCO2 in Tokyo Bay showed that the overall bay acts as a strong net sink for atmospheric CO2. Although small area was a consistent source for CO2, active photosynthesis driven by nutrient loading from the land overwhelmed the CO2 budget in the bay. Here we show a comprehensive scheme with a border where air-sea CO2 flux was ±0 between nearshore waters emitting CO2 and offshore waters absorbing CO2. The border in Tokyo Bay was extremely shifted toward the land-side. The shift is characteristic of highly urbanized coastal waters with an extensive sewage treatment system in the catchment area. Because highly urbanized coastal areas worldwide are expected to quadruple by 2050, coastal waters such as Tokyo Bay are expected to increase as well. Through extrapolation of Tokyo Bay data, CO2 emission from global estuaries would be expected to decrease roughly from the current 0.074 PgC year-1 to 0.014 PgC year-1 in 2050.