Inter-annual variability of CO2 exchanges between an emersed tidal flat and the atmosphere

Carbon dioxide (CO2) exchanges between a tidal flat (Wadden Sea, Netherlands) and the atmosphere were measured during a three-year survey. CO2 exchanges were monitored during 1–2 days each month between September 2006 and September 2009 using a flux chamber. The flux of CO2 was separated into two fluxes: the dark flux and the gross light flux, with the dark flux representing the flux during darkness and the gross light flux the difference between the net CO2 flux (measured under light) and the dark flux. It was argued that dark and light fluxes may deviate from respiration and photosynthesis, as the fluxes between wet tidal sediment and the atmosphere are affected by the partial pressure of CO2 in pore water, which is only gradually changed by sources and sinks of CO2 in the sediment. Light and dark fluxes were empirically related to environmental parameters in order to interpolate between succeeding measurements. The dark flux appeared to increase with temperature and the light flux became more intense with increasing irradiance with signs of saturation at high light levels on many but not all measurement days. These relations with environmental parameters showed seasonal and inter-annual variability. Fluxes were negligible just after the site was emersed and it took up to 3 h of emersion until fluxes were adapted to environmental conditions. Dark and light fluxes both showed strong seasonality with high values in summer and low values in winter. The tidal flat appeared as a source of atmospheric CO2 in the first year of measurement (+0.35 mol CO2 m−2 yr−1) and a sink in the following two years (−1.59 and −0.72 mol CO2 m−2 yr−1). Since the source of CO2 was observed during an extremely warm year, we suggest that climate warming might influence the carbon budget of tidal flats.