Carbon Dioxide Uptake by a Temperate Tidal Sea

Carbon dioxide (CO₂) exchange between the atmosphere and the Wadden Sea, a shallow coastal region along the northern Netherlands, has been measured from April 2006 onwards on a tidal flat and over open water. Tidal flat measurements were done using a flux chamber, and ship borne measurements using an equilibrator to determine the partial pressure of CO₂ in the water. A strong response of CO₂ exchange rate on irradiance changes was observed at the tidal flat surface, with generally strong uptake of CO₂ during daylight conditions and release, presumably due to respiration, during dark conditions. The rate of respiration appeared to increase with temperature. Daytime CO₂ uptake tended to be higher than nighttime release all year round. Apparently, tidal flats in temperate regions of the globe may be a sink for CO₂ on an annual basis. In contrast to terrestrial CO₂ exchange, uptake by processes active on and in tidal flats responds only slowly to changing irradiance. This slow reaction is attributed to CO₂ storage in wet soil. Measurements in the water between tidal flats suggest that the estuarine, open water part of the Wadden Sea is a source of CO₂ in spring.