Abstract
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.
Highlights
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
Coastal waters have been regarded as CO2 sources to the atmosphere because of their input of terrestrial organic carbon and subsequent mineralization[2,3]
Kuwae et al.[4] have recently discussed the possibility of net carbon fixation in certain types of coastal systems including those affected by intense anthropogenic activities
Summary
These upwelling events cause conspicuous milky turquoise waters[14] due to the oxidation of hydrogen sulfide in anoxic bottom waters We observed these bottom waters on 16 and 23 September, 2010, in which pCO2 values ranged from 765 to 1,161 μatm with an average of 967 μatm and the average CO2 flux was estimated to be 54.0 mmolC m−2 day−1. On the basis of this average duration and area, the observed CO2 flux would yield an annual flux of 5.9 × 108 gC year−1 This value accounts for only 1.2% of the air-sea CO2 exchange in the entire bay. Sewage treatment plants along Tokyo Bay remove organic carbon from freshwater at a rate of 7.7 × 1010 gC year−1 28 This value is comparable to the uptake of CO2 observed in this study (5.2 × 1010 gC year−1). The CO2 budget of the global coastal waters would be expected to be a sink rather than a source
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