Daily, biweekly, and seasonal temporal scales of pCO2 variability in two stratified Mediterranean reservoirs

Temporal scales of variability for the partial pressure of CO2 (pCO(2)) in the surface waters of two stratified Mediterranean reservoirs were examined through the temporal decomposition of 5month time series with hourly sampling frequency. pCO(2) time series included similar patterns of variability at daily, biweekly, and seasonal scales regardless of the difference in amplitude of the pCO(2) variation in the two reservoirs studied. Daily variability was strongly related to the day-night cycles of metabolic activity, accounting for about one third of the total amplitude in pCO(2) variation. At a biweekly scale, wind forcing led to higher rates of air-water CO2 exchange and subsequently temporary partial mixing events associated to relevant increase of CO2 concentration in surface waters. Seasonal variability accounted for one third of the amplitude of the pCO(2) variability and was coupled to the seasonal dynamics of water temperature and thermal stratification of the water column. Our results provide evidence that CO2 emission from stratified water bodies shows significant variability at daily, biweekly, and seasonal scales; all of which should be taken into consideration in the analyses of the carbon fluxes. The wind-induced mixing events, operating at temporal scales between daily and seasonal cycles, may become a major factor controlling the pCO(2) dynamics. Hence, some of the most common models for computing CO2 fluxes from pCO(2) were not able to reproduce the biweekly response patterns of CO2 emissions to wind forcing.