Abstract
Abstract. Distributions of surface water partial pressure of carbon dioxide (pCO2) were measured on nine cruises in the Delaware Estuary (USA). The Delaware River was highly supersaturated in pCO2 with respect to the atmosphere during all seasons, while the Delaware Bay was undersaturated in pCO2 during spring and late summer and moderately supersaturated during mid-summer, fall, and winter. While the smaller upper tidal river was a strong CO2 source (27.1 ± 6.4 mol-C m−2 yr−1), the much larger bay was a weak source (1.2 ± 1.4 mol-C m−2 yr−1), the latter of which had a much greater area than the former. In turn, the Delaware Estuary acted as a relatively weak CO2 source (2.4 ± 4.8 mol-C m−2 yr−1), which is in great contrast to many other estuarine systems. Seasonally, pCO2 changes were greatest at low salinities (0 ≤ S < 5), with pCO2 values in the summer nearly 3-fold greater than those observed in the spring and fall. Undersaturated pCO2 was observed over the widest salinity range (7.5 ≤ S < 30) during spring. Near to supersaturated pCO2 was generally observed in mid- to high-salinity waters (20 ≤ S < 30) except during spring and late summer. Strong seasonal trends in internal estuarine production and consumption of CO2 were observed throughout both the upper tidal river and lower bay. Positive correlations between river-borne and air–water CO2 fluxes in the upper estuary emphasize the significance of river-borne CO2 degassing to overall CO2 fluxes. While river-borne CO2 degassing heavily influenced CO2 dynamics in the upper tidal river, these forces were largely compensated for by internal biological processes within the extensive bay system of the lower estuary.
Highlights
The surface area of estuaries is only about 4 % that of continental shelves, recent studies have concluded that the carbon dioxide (CO2) degassing flux from estuarine waters is as large as the CO2 uptake by the continental shelf (Borges, 2005; Borges et al, 2005; Cai et al, 2006; Chen and Borges, 2009; Cai, 2011)
Global estuarine waters are estimated to emit 0.10–0.45 Pg C yr−1, while continental shelves take up 0.20–0.40 Pg C yr−1 (Borges, 2005; Borges et al, 2005; Cai, 2011; Chen et al, 2013; Regnier et al, 2013; Laruelle et al, 2015)
Estuarine waters are a major source of CO2 to the atmosphere, with partial pressures of CO2 ranging from 350 to 10 000 μatm and air–water CO2 fluxes ranging from −5 to 80 mol C m−2 yr−1 (Raymond et al, 1997; Cai and Wang, 1998; Frankignoulle et al, 1998; Borges, 2005; Borges et al, 2006; Borges and Abril, 2011; Cai, 2011)
Summary
The surface area of estuaries is only about 4 % that of continental shelves, recent studies have concluded that the carbon dioxide (CO2) degassing flux from estuarine waters is as large as the CO2 uptake by the continental shelf (Borges, 2005; Borges et al, 2005; Cai et al, 2006; Chen and Borges, 2009; Cai, 2011). Global estuarine waters are estimated to emit 0.10–0.45 Pg C yr−1, while continental shelves take up 0.20–0.40 Pg C yr−1 (Borges, 2005; Borges et al, 2005; Cai, 2011; Chen et al, 2013; Regnier et al, 2013; Laruelle et al, 2015) Such large estuarine CO2 degassing suggests that much of the terrestrial organic carbon, including that from coastal wetlands, is respired to CO2 during transport through the estuarine zone, though the relative importance of river-supplied CO2 and organic carbon versus those from the coastal wetlands is debatable (Borges and Abril, 2011; Cai, 2011). The majority of past estuarine CO2 studies have been conducted on small estuarine systems, which typically have
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