Air–water fluxes and sources of carbon dioxide in the Delaware Estuary: spatial and seasonal variability

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.