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.