Abstract
Few estuaries have inorganic carbon datasets with sufficient spatial and temporal coverage for identifying acidification baselines, seasonal cycles and trends. The Chesapeake Bay, though one of the most well-studied estuarine systems in the world, is no exception. To date, there have only been observational studies of inorganic carbon distribution and flux in lower bay sub-estuaries. Here, we address this knowledge gap with results from the first complete observational study of inorganic carbon along the main stem. Dissolved inorganic carbon (DIC) and total alkalinity (TA) increased from surface to bottom and north to south over the course of 2016, mainly driven by seasonal changes in river discharge, mixing, and biological carbon dioxide (CO2) removal at the surface and release in the subsurface. Upper, mid- and lower bay DIC and TA ranged from 1000-1300, 1300-1800, and 1700-1900 µmol kg-1. The pH range was large, with maximum values of 8.5 at the surface and minimums as low as 7.1 in bottom water in the upper and mid-bay. Seasonally, the upper bay was the most variable for DIC and TA, but pH was more variable in the mid-bay. Our results reveal that low pH is a continuing concern, despite reductions in nutrient inputs. There was active internal recycling of DIC and TA, with a large inorganic carbon removal in the upper bay and at salinities <5 most months, and a large addition in the mid-salinities. In spring and summer, waters with salinities between 10 and 15 were a large source of DIC, likely due to remineralization of organic matter and dissolution of CaCO3. We estimate that the estuarine export flux of DIC and TA in 2016 was 40.3 + 8.2 × 109 mol yr-1 and 47.1 + 8.6 × 109 mol yr-1. The estuary was likely a large sink of DIC, and possibly a weak source of TA. These results support the argument that the Chesapeake Bay may be an exception to the long-standing assumption that estuaries are heterotrophic. Furthermore, they underline the importance of large estuarine systems for mitigating acidification in coastal ecosystems, since riverine chemistry is substantially modified within the estuary.
Highlights
Since industrialization, the oceans have absorbed at least 25% of anthropogenic carbon dioxide release due to fossil fuel combustion and land use change ( Sabine et al, 2004; Le Quéré et al, 2017)
At the beginning of the freshet’s influence on the bay, in March, Dissolved inorganic carbon (DIC) and total alkalinity (TA) increased down the bay at the surface with mixing but was largely uniform below 10 m (Figure 4). pH values in the upper bay were among the lowest all year (∼7.2), even at the surface
DIC and TA increased from surface to bottom and north to south, bay-wide, and pH decreased from surface to bottom and increased from north to south
Summary
The oceans have absorbed at least 25% of anthropogenic carbon dioxide release due to fossil fuel combustion and land use change ( Sabine et al, 2004; Le Quéré et al, 2017). There is global evidence of ocean acidification in open ocean waters (Bates et al, 2014), pH declines in estuaries have largely been attributed to eutrophication (Abril et al, 2004; Sarma et al, 2011). The combination of acidification and eutrophication can result in an additional pH decline in estuaries, due to changes in buffering capacity ( Cai et al, 2011; Breitburg et al, 2015). Inorganic carbon data for most estuarine systems are insufficient to define spatial and seasonal heterogeneity, which is necessary for establishing baseline pH information from which trends can be evaluated. This information gap is a serious obstacle for resource management in these economically and socially valuable systems
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