A comparative study of carbon dioxide degassing in river- and marine-dominated estuaries

The partial pressure of carbon dioxide (pCO2), concentration of total dissolved inorganic carbon, and total alkalinity were measured at both high tide and low tide in the surface water of three Georgia estuaries from September 2002 to May 2004. Of the three estuaries, Sapelo and Doboy Sounds are marine-dominated estuaries, while Altamaha Sound is a river-dominated estuary. During all sampling months, the three estuaries were supersaturated in CO2 with respect to the atmosphere (39.5–342.5 Pa, or 390–3380 matm) because of CO2 inputs from within the estuarine zone (mainly intertidal marshes) and the river. Overall, pCO2 in the river-dominated estuary is much higher than that in the marine-dominated estuaries. The calculated annual air–water CO2 flux in Altamaha Sound (69.3 mmol m22 d21) is 2.4 times those of Sapelo and Doboy Sounds (28.7–29.4 mmol m22 d21). The higher CO2 degassing in the river-dominated estuary is fueled largely by CO2 loading from the river. Because of the substantial differences between riverand marine-dominated estuaries, current estimates of air–water CO2 fluxes in global estuaries (which are based almost entirely on river-dominated estuaries) could be overestimated. Recent studies have shown that estuaries are significant sources of carbon dioxide (CO2) to the atmosphere, with partial pressure of carbon dioxide (pCO2) varying from about 40 to 960 Pa (or ,400–9500 matm) (Frankignoulle et al. 1998; Borges 2005; Borges et al. 2005). Even though the surface area of global estuaries is only about a 20th that of continental shelves (Woodwell et al. 1973), it is argued that CO2 degassing by estuaries (Borges 2005; Borges et al. 2005) could nearly counterbalance the continental shelf CO2 sink (Tsunogai et al. 1999; Borges et al. 2005; Cai et al. 2006), which is about 30–70% of the atmospheric CO2 sink of the open ocean (1.2–1.6 Pg C yr21) (Takahashi et al. in press). However, most estuarine CO2 studies have focused on estuaries that receive substantial freshwater discharge; much less attention has been given to estuaries that receive little freshwater discharge besides precipitation and groundwater (Frankignoulle et al. 1998; Borges 2005; Borges et al. 2005). Definitions of estuaries vary widely. Most definitions restrict an estuary to the mouth of a river or a body of seawater reaching inland, while others argue that an estuary extends to the continental shelf (Perillo 1995). One of the most frequently cited definitions of an estuary is that of Cameron and Pritchard (1963): ‘‘a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which seawater is measurably diluted with freshwater derived from land drainage.’’ According to this definition, all river mouths and coastal brackish lagoons qualify as estuaries, although the former have been the focus for most estuarine studies. Following Elliott and McLusky (2002), we have adopted the most widely held point of view that considers both river mouths and coastal brackish lagoons to be estuaries. The inclusion of coastal brackish lagoons as estuaries is also consistent with the fact that the most cited surface area of global estuaries was estimated ‘‘without differentiating mouths of rivers and coastal brackish lagoons’’ (Woodwell et al. 1973). For this study, we refer to mouths of rivers that receive significant amounts of upland river inflow as riverdominated estuaries and coastal brackish lagoons that receive little freshwater besides precipitation and groundwater as marine-dominated estuaries. The salt marsh–surrounded estuaries of the southeastern United States cover approximately 3 3 109 m2. Riverand marine-dominated estuaries are typical features of this region, with marine-dominated estuaries covering approximately 50% of the total estuarine area in this region (National Ocean Service 1985). In this paper we present a comparative study of CO2 in riverand marinedominated estuaries around Sapelo Island, Georgia (Fig. 1). The proximity of these two types of estuaries and their similarities in physical conditions provide a unique opportunity to examine the CO2 differences between these two types of estuaries. We also discuss this study’s global implications on air–water CO2 fluxes of estuaries.