Abstract
Abstract. Carbonate geochemistry research in large estuarine systems is limited. More work is needed to understand how changes in land-use activity influence watershed export of organic and inorganic carbon, acids, and nutrients to the coastal ocean. To investigate the seasonal variation of the inorganic carbon system in the Delaware Estuary, one of the largest estuaries along the US east coast, dissolved inorganic carbon (DIC), total alkalinity (TA), and pH were measured along the estuary from June 2013 to April 2015. In addition, DIC, TA, and pH were periodically measured from March to October 2015 in the nontidal freshwater Delaware, Schuylkill, and Christina rivers over a range of discharge conditions. There were strong negative relationships between river TA and discharge, suggesting that changes in HCO3− concentrations reflect dilution of weathering products in the drainage basin. The ratio of DIC to TA, an understudied but important property, was high (1.11) during high discharge and low (0.94) during low discharge, reflecting additional DIC input in the form of carbon dioxide (CO2), most likely from terrestrial organic matter decomposition, rather than bicarbonate (HCO3−) inputs due to drainage basin weathering processes. This is also a result of CO2 loss to the atmosphere due to rapid water transit during the wet season. Our data further show that elevated DIC in the Schuylkill River is substantially different than that in the Delaware River. Thus, tributary contributions must be considered when attributing estuarine DIC sources to the internal carbon cycle versus external processes such as drainage basin mineralogy, weathering intensity, and discharge patterns. Long-term records in the Delaware and Schuylkill rivers indicate shifts toward higher alkalinity in estuarine waters over time, as has been found in other estuaries worldwide. Annual DIC input flux to the estuary and export flux to the coastal ocean are estimated to be 15.7 ± 8.2 × 109 mol C yr−1 and 16.5 ± 10.6 × 109 mol C yr−1, respectively, while net DIC production within the estuary including inputs from intertidal marshes is estimated to be 5.1 × 109 mol C yr−1. The small difference between riverine input and export flux suggests that, in the case of the Delaware Estuary and perhaps other large coastal systems with long freshwater residence times, the majority of the DIC produced in the estuary by biological processes is exchanged with the atmosphere rather than exported to the sea.
Highlights
The global carbon cycle involves dynamical processes of carbon exchange among the earth’s atmosphere, land, vegetation, coastal zones, and oceans
At the bay mouth (S > 30), dissolved inorganic carbon (DIC) and total alkalinity (TA) concentrations remained fairly constant throughout all seasons (1920–1990 and 2095–2180 μmol kg−1, respectively)
In the fall and winter, DIC and TA generally varied linearly in relation to salinity, the change in pH was small across the salinity gradient (Fig. 3)
Summary
The global carbon cycle involves dynamical processes of carbon exchange among the earth’s atmosphere, land, vegetation, coastal zones, and oceans. Most carbon fluxes in inland waters involve inputs from soil-derived carbon, chemical weathering of carbonate and silicate minerals, wetlands, dissolved carbon in sewage waste, and organic carbon produced by phytoplankton in surface waters (Battin et al, 2009; Tranvik et al, 2009; Regnier et al, 2013; Abril et al, 2014). To balance the influx of carbon, a large fraction is returned to the atmosphere by organic carbon decomposition within inland waters, transported to adjacent waters, buried in freshwater. Joesoef et al.: Seasonal variability of the inorganic carbon system in a large coastal plain estuary sediments, and in some cases released as methane (CH4) gas (Downing et al, 2008; Bastviken et al, 2011)
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