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Estuaries play a crucial role in regional carbon cycling. Until now, accurate estimations of the impact of environmental variables on estuarine air&ndash, 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&ndash, 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&minus, 2∙year&minus, 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 &micro, atm in winter to ~400 &micro, atm in summer and increasing again to ~990 &micro, atm 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.

Accurate knowledge of Henry's law constants (Kaw) is very important for fate and transport studies of organic chemicals. Here field-derived Kaw values for polychlorinated biphenyl (PCB) congeners (PCBs 28, 49, 52, 118, 138, 149, and 153) were determined in the open Atlantic Ocean, at locations where air and seawater were assumed to be at equilibrium. Field-based Kaw values were derived from air and seawater samples simultaneously collected at these locations, and their relationship with temperature was determined. The average field-derived Kaw for PCBs 28, 49, 52, 118, and 138 was generally 2–3 times higher than laboratory-derived values reported in the literature. Possible causes of differences between the field and the laboratory data may be attributed to differences between pure water and seawater and the use of much higher persistent organic pollutant concentrations in laboratory studies compared to that of the remote ocean. Field-derived Kaw values for PCB 149 and PCB 153 were up to 10 times higher than literature values. The comparison of field- and laboratory-derived Kaw shows that their temperature dependence is not significantly different (95% confidence) from those reported in the literature., Department of the Environment, Food and Rural Affairs (DEFRA) and the Natural Environment Research Council (NERC) on persistent organic pollutants at Lancaster University and the Spanish Ministry of Science and innovation through the MIMOSA and ATOS projects.