Limnetica 32

Estuaries have been the focus of several studies of the carbon biogeochemical cycle. Despite their small area compared with other coastal regions, estuaries may play a significant role in the carbon biogeochemical cycle due to their intense biogeochemical activity resulting from river and anthropogenic inputs of organic and inorganic matter. Most of these studies suggest that estuaries are sources of carbon inputs to the atmosphere. This study describes a study conducted in the Ave estuary (northern Portugal) in summer 2011 and winter 2012 to quantify the air-water CO₂ fluxes. Surface waters were always oversaturated in CO₂ relative to the atmosphere. CO₂ partial pressure ranged from ≈ 689 to ≈ 1111 µatm in summer during the flood and the ebb, respectively, whereas winter values ranged from ≈ 767 to ≈ 1021 µatm during the ebb and the flood, respectively. These results suggest that the Ave estuary releases CO₂ to the atmosphere at rates ranging from 6 to 34 mmol C m^–2d^–1, in summer and from 13 to 40 mmol C m^–2d^–1 in winter. These values are lower than estimates for other Portuguese estuaries, such as the Tagus, Douro or Sado.