Local Variability of CO2 Partial Pressure in a Mid-Latitude Mesotidal Estuarine System (Tagus Estuary, Portugal)

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.

Cardinal Buoys: An Opportunity for the Study of Air-Sea CO2 Fluxes in Coastal Ecosystems

“…The phase inversion observed in November and April (Figure 5), which occurred every year, was the consequence of the opposite SST seasonality between the CWM and the OWM (lower SST in summer in OWM and lower SST in winter in CWM). The influence of tides on pCO 2 dynamics is prominent in estuarine ecosystems (De la Paz et al, 2007;Bozec et al, 2012;Oliveira et al, 2018), but has also been observed in various continental shelves of the world ocean (DeGrandpre et al, 1998;Hofmann et al, 2011;Horwitz et al, 2019). Several studies have reported the impact of the tidal cycle on pCO 2 over various European continental shelf provinces, for example in the nWEC (Litt et al, 2010), in the Bay of Brest (Bozec et al, 2011) or in the Cadiz Bay (Ribas-Ribas et al, 2011.…”

“…The shallower depth in the CMW favors light penetration, which can result in higher pelagic production (when nutrients are not depleted) compared to the deeper OWM. The role of benthic production processes on CO 2 variations is also important in proximal shallow areas (Hammond et al, 1999;Cai et al, 2000;Forja et al, 2004;Waldbusser and Salisbury, 2014;Oliveira et al, 2018). The low δpCO 2 non−therm associated to CWM during the productive period might include both higher pelagic and benthic production, with a predominance of the latter.…”

Decadal Dynamics of the CO2 System and Associated Ocean Acidification in Coastal Ecosystems of the North East Atlantic Ocean

“…For the latter, the relatively deep water column and the limited amount of light reaching the seafloor might limit the contribution of the benthic compartment to the pelagic signal observed at the surface. Conversely, in the two shallow nearshore ecosystems, the benthic compartment may exert a strong control on the carbonate signal with high OM production/remineralization and/or CaCO 3 dissolution/precipitation characteristic of benthic communities in various coastal environments (Hammond et al, 1999;Cai et al, 2000;Forja et al, 2004;Martin et al, 2007;Waldbusser and Salisbury, 2014;Oliveira et al, 2018). These two nearshore stations were characterized by different benthic communities with maerl (coralline red algae) beds in the bay of Brest (e.g., Martin et al, 2007;Longphuirt et al, 2007;Lejart and Hily, 2011) and seagrass and macroalgae beds at SOMLIT-pier (Ouisse et al, 2011;Bordeyne et al, 2017).…”