Abstract
Abstract. The Chesapeake Bay is a large coastal-plain estuary that has experienced considerable anthropogenic change over the past century. At the regional scale, land-use change has doubled the nutrient input from rivers and led to an increase in riverine carbon and alkalinity. The bay has also experienced global changes, including the rise of atmospheric temperature and CO2. Here we seek to understand the relative impact of these changes on the inorganic carbon balance of the bay between the early 1900s and the early 2000s. We use a linked land–estuarine–ocean modeling system that includes both inorganic and organic carbon and nitrogen cycling. Sensitivity experiments are performed to isolate the effect of changes in (1) atmospheric CO2, (2) temperature, (3) riverine nitrogen loading and (4) riverine carbon and alkalinity loading. Specifically, we find that over the past century global changes have increased ingassing by roughly the same amount (∼30 Gg-C yr−1) as has the increased riverine loadings. While the former is due primarily to increases in atmospheric CO2, the latter results from increased net ecosystem production that enhances ingassing. Interestingly, these increases in ingassing are partially mitigated by increased temperatures and increased riverine carbon and alkalinity inputs, both of which enhance outgassing. Overall, the bay has evolved over the century to take up more atmospheric CO2 and produce more organic carbon. These results suggest that over the past century, changes in riverine nutrient loads have played an important role in altering coastal carbon budgets, but that ongoing global changes have also substantially affected coastal carbonate chemistry.
Highlights
The well-documented rise in atmospheric CO2 concentrations is one of the most ubiquitous changes in global biogeochemical cycling over the past century (e.g., Keeling et al, 2003)
The lower dissolved inorganic carbon (DIC) and total alkalinity (TA) concentrations are apparent in the northern half of the bay (Fig. 5a, b) where the Susquehanna River delivers ∼ 45 % of the freshwater discharge to the bay
One exception to the low tributary DIC and TA concentrations is the Potomac River where concentrations are higher than all other rivers (Table 2) and approach those of shelf water in the fall season
Summary
The well-documented rise in atmospheric CO2 concentrations is one of the most ubiquitous changes in global biogeochemical cycling over the past century (e.g., Keeling et al, 2003). The ocean’s biological pump maintains atmospheric CO2 significantly lower than it would otherwise be, the uptake of anthropogenic CO2 by the ocean is governed largely by chemical and physical processes. These processes include the diffusion of CO2 across the air–sea interface, the dissolution of CO2 and its dissociation into bicarbonate and carbonate ions, and the transport of anthropogenic dissolved inorganic carbon into the ocean interior by vertical mixing and subduction. Especially estuaries, have unique susceptibility to changes due to their proximity to anthropogenic nutrient and carbon sources and
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