Abstract
Estuaries play a crucial role in regional carbon cycling. Until now, accurate estimations of the impact of environmental variables on estuarine air–water CO2 fluxes have been mostly characterized by a low spatial-temporal sampling resolution. This study reports on the variations of CO2 partial pressure (pCO2) and related environmental parameters, at both tidal and seasonal temporal scales, in the surface seawater of a station located in the lower section of the Tagus estuary, Portugal. The study was carried out from February to December 2007. Air–water CO2 fluxes suggest that the lower estuary acted as a relatively weak source of CO2 to the atmosphere, with an average rate of 7.2 mol∙m−2∙year−1, with highest fluxes occurring in winter. Over a tidal cycle, pCO2 was mainly influenced by tidal-induced mixing. Results suggest an influence of upper and central estuary inputs with higher pCO2 values. pCO2 varied seasonally, with values decreasing from ~890 µatm in winter to ~400 µatm in summer and increasing again to ~990 µatm in autumn. The generalized linear model (GLM) applied to the data set explained 69.3% of the pCO2 variability, pointing to the thermodynamic effect of temperature and biological activity as the most relevant processes in CO2 dynamics. Tidal variation of pCO2 corresponded to ~35% of its seasonal variability, denoting the importance of tide conditions on the dynamics of inorganic carbon. Results showed distinct patterns in the dynamics of CO2 at the tidal scale. This outcome suggests that disregarding tidal variability in the use of seasonal data sets may lead to significant errors in annual carbon budget estimations.
Highlights
Inner estuaries act as sources of CO2 to the atmosphere due to their heterotrophic ecosystem metabolic status [1,2,3,4]
This paper focuses on the study of CO2 variability at different temporal scales of a mid-latitude estuarine system, the Tagus estuary (SW Portugal), contributing to characterize the dynamics of air–water CO2 fluxes with higher temporal resolution
The Tagus river inputs reflect the common pattern of a mid-latitude system, with higher discharges in winter and lower ones in spring/summer (Table 1), despite the high degree of hydrological regime regularization imposed by a significant number of dams upstream
Summary
Inner estuaries act as sources of CO2 to the atmosphere due to their heterotrophic ecosystem metabolic status [1,2,3,4]. A compilation of available air–water CO2 fluxes in inner estuaries indicates that these near-shore ecosystems emit CO2 to the atmosphere at an average of 20.8 mol·C·m−2 ·year−1 [5,6,7]. Scaling up this value yields CO2 emissions of 0.10 Pg·C·year−1 [8], which is much lower than published values by Chen and Borges [9], for example, mainly because of the contribution of a considerable amount of unpublished or new data from Asia and the Arctic [10].
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