CO2 chemistry in the water column above a Posidonia seagrass bed and related air-sea exchanges

An often sinusoïdal diel pH signal is detectable at -0.5 rn and -8 rn above a Posidonïa oceanica (L.) Delile seagrass bed (Bay of Calvi, Corsica). Total alkalinity, temperature and pH measurements permit description of the CO2 chemistry in the 8 mn water column, and its daily and yearly fluctuations. The homogenous buffer factor and the physicochemical buffer factor are calculated and shown, as expected, to depend essentially on temperature and mean pH. The yearly variation of total alkalinity shows a sudden drop (0.06 meq./1) during summer, which most probably finds its explanation in the sudden increase of activity of Melobesiae epiphytes. However, carbonate )osses and gains in the water column are balanced out over a yearly period. The mean pH is lowest in June-July. 90% of the corresponding maximum acidification is explained by the temperature increase and the total alkalinity diminution. CO2 diel oscillation is largest close to the annual acidity maximum, and drops to zero in winter, accurately reflecting the biological activity of the seagrass bed. The corresponding Tco2 change permits estimation of the net production and respiration rates of the total ecosystem. Net production at its highest is 5.3 g C rn -z d-l in agreement with observations by Bay (1978). Air-sea COz exchanges were measured white monitoring the COz chemistry in the water column. The observed out-fluxes vary considerably from hour to hour. Mean values show a yearly maximum in summer (mean September F0 = 11.7.10-6 mg Coz cm-zmin- 1), but there is no flux during winter and an influx has so far never been measured. The atmospheric partial C02 pressure is 325-330 ppm. It would appear that the reentry of COz into the sea requires β = 10 at pH = 8.29, t = l2°C, PCO2 water = 300 ppm (lowest observed 13 value is 10.45, pH 8.23, t = 11.8°C, PCOz water = 360 ppm). Slight overpressure (25 to 50 ppm) appears to be required to force the surface microlayer. The whole Posidonia belt in the Mediterranean basin may be considered as a CO2 emitter which ceases in winter and is modulated by the ecosystem photosynthesis and respiration, temperature and sea-state.