Seasonal and spatial variability of the CO2 system in the Delaware Estuary

Date
2015
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Publisher
University of Delaware
Abstract
Distributions of surface water partial pressure of carbon dioxide ( pCO2), dissolved inorganic carbon (DIC), total alkalinity (TA), and pH were measured along the Delaware Estuary (USA) from June 2013 to April 2015. In addition, DIC, TA, and pH were periodically examined from March to October 2015 in the non-tidal freshwater Delaware, Schuylkill, and Christina Rivers. 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 tidal freshwater Delaware River was a strong CO 2 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 that serve as strong CO2 sources to the atmosphere. Seasonally, pCO 2 changes were greatest at low salinities (0 ≤ S < 5) with pCO2 values in the summer nearly three-fold greater than those observed in the spring and fall. Undersaturated pCO 2 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. Comparably, 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 tidal freshwater Delaware River, it was largely compensated by internal biological processes within the extensive bay system of the lower estuary. DIC and TA exhibited large spatial and seasonal variations throughout the Delaware Estuary (975-2015 and 915-2225 μmol kg-1, respectively). Along the Delaware, Schuylkill, and Christina Rivers, DIC and TA were highest after periods of low discharge and were lowest after periods of high discharge. In the Schuylkill River, during extremely low discharge, DIC and TA values exceeded those at the ocean end-member (> 2100 μmol kg-1). Strong negative relationships between river TA and discharge suggest that changes in HCO-3 concentrations or TA reflect the dilution of the weather derived products in the drainage basin. Furthermore, changes in the DIC to TA ratio at the freshwater end-member may reflect inputs of soil organic matter respiration due to seasonal variations in river discharge. While the reasons remain unclear, mixing of relatively high carbonate freshwater due to eroded limestone and dolomite bedrock minerals from the lower Schuylkill River drainage basin may lead to increased DIC and TA values near the Philadelphia region. In addition to variations in river discharge and mixing from multiple tributaries, internal biological processes within the bay system also led to seasonal shifts in DIC concentrations. Annual DIC input flux to the estuary and export flux to the ocean are estimated to be about 15.4 ± 8.0 × 109 mol C yr-1 and 17.0 ± 10.6 × 109 mol C yr-1, respectively. Based on a CO2 mass balance model, internal estuarine produced CO2 flux is small (5.8 × 109 mol C yr-1) when compared to total riverine flux. Moreover, 36% of the internally produced CO2 is exported to the coastal ocean. In the case of the Delaware Estuary and other coastal systems with long freshwater residence times, much of the DIC produced by net ecosystem production is most likely removed to the atmosphere rather than exported to the sea.
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