Chesapeake Bay Inorganic Carbon: Spatial Distribution and Seasonal Variability

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 (CO 2 ) 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 , respectively. 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 CaCO 3 . We estimate that the estuarine export flux of DIC and TA in 2016 was 40.3 ± 8.2 × 10 9 mol yr −1 and 47.1 ± 8.6 × 10 9 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.

Alkalinity in Tidal Tributaries of the Chesapeake Bay

“…Early studies suggested conservative behavior of alkalinity in the Patuxent (Brust & Newcombe, ) and Susquehanna (Carpenter et al, ) River Estuaries and the possibility of a sink in the James River Estuary (Wong, ). More recent studies suggest alkalinity sinks in the tidal fresh and oligohaline portions of the Susquehanna (Brodeur et al, ; Cai et al, ) and Potomac (Cerco et al, ) River Estuaries and sources (due to sulfate reduction) in the York River Estuary (Raymond et al, ), which are in broad agreement with our findings. Brodeur et al () suggested that the alkalinity sink in the upper mainstem bay was due to SAV in the Susquehanna Flats, although these authors found the mainstem bay as a whole to be a weak source on average, with significant seasonal fluctuations.…”

Seasonal Variability of the CO2 System in a Large Coastal Plain Estuary

“…A recent study in the CB by Brodeur et al () observed latitudinal gradients in DIC and TA throughout the mainstem that are similar to our findings. We find good agreement with respect to the seasonal ranges in DIC and TA in the midbay (corresponding most closely R2 in this analysis) and lower bay (corresponding to R3W and R3E) with the observations of Brodeur et al () in both the mixed layer and below. The pH comparison is complicated by inconsistency in units, but we note that our observations are in good agreement with respect to overall seasonality, with decreased pH with depth and distance from the Bay mouth.…”

“…Here we present new observations spanning four seasons (Autumn 2016 to Summer 2017) to diagnose the spatiotemporal variability of the CO 2 system in the main stem of the CB, the largest estuary in the continental United States. This study builds on a growing body of work focused on the seasonality of the CO 2 system in the region (e.g., Brodeur et al, ; Shadwick, Friedrichs, et al, ; Shen, Testa, Li, et al, ). New shipboard observations are used to partition the seasonality of DIC into physical and biological drivers.…”

High‐Frequency CO2 System Variability Over the Winter‐to‐Spring Transition in a Coastal Plain Estuary

“…These observations suggest that the broad patterns in CO 2 system changes from one season to the next are captured by sampling with monthly or sparser resolution (see also Brodeur et al, ); however, attribution of the drivers of those changes to particular physical and/or biological processes is more robust with higher‐frequency observations. Furthermore, the discrete seasonal sampling does not allow the air‐sea CO 2 flux to be captured appropriately.…”

“…Investigations of the role of continental shelves in the uptake of atmospheric CO 2 have resulted in a broad classification of estuaries as net sources of CO 2 to the atmosphere and sources of inorganic and organic carbon to the continental shelves and coastal oceans (Borges et al, ; Cai et al, ; Chen & Borges, ; Flecha et al, ; Frankignoulle et al, ; Jiang et al, ). Sustained measurements of the CO 2 system and quantification of carbon fluxes in nearshore environments have increased in recent years (Brodeur et al, ; Joesoef et al, ; Shadwick et al, ; Signorini et al, ; Vandemark et al, ), and regional budgets have been developed (Herrmann et al, ; Laruelle et al, ; Najjar et al, ).…”