The combined effects of anthropogenic and biological CO
inputs may lead to more rapid acidification in coastal waters compared to the open ocean. It is less clear, however, how redox reactions would ...contribute to acidification. Here we report estuarine acidification dynamics based on oxygen, hydrogen sulfide (H
S), pH, dissolved inorganic carbon and total alkalinity data from the Chesapeake Bay, where anthropogenic nutrient inputs have led to eutrophication, hypoxia and anoxia, and low pH. We show that a pH minimum occurs in mid-depths where acids are generated as a result of H
S oxidation in waters mixed upward from the anoxic depths. Our analyses also suggest a large synergistic effect from river-ocean mixing, global and local atmospheric CO
uptake, and CO
and acid production from respiration and other redox reactions. Together they lead to a poor acid buffering capacity, severe acidification and increased carbonate mineral dissolution in the USA's largest estuary.The potential contribution of redox reactions to acidification in coastal waters is unclear. Here, using measurements from the Chesapeake Bay, the authors show that pH minimum occurs at mid-depths where acids are produced via hydrogen sulfide oxidation in waters mixed upward from anoxic depths.
Syntheses of carbonate chemistry spatial patterns are important for predicting ocean acidification impacts, but are lacking in coastal oceans. Here, we show that along the North American Atlantic and ...Gulf coasts the meridional distributions of dissolved inorganic carbon (DIC) and carbonate mineral saturation state (Ω) are controlled by partial equilibrium with the atmosphere resulting in relatively low DIC and high Ω in warm southern waters and the opposite in cold northern waters. However, pH and the partial pressure of CO
(pCO
) do not exhibit a simple spatial pattern and are controlled by local physical and net biological processes which impede equilibrium with the atmosphere. Along the Pacific coast, upwelling brings subsurface waters with low Ω and pH to the surface where net biological production works to raise their values. Different temperature sensitivities of carbonate properties and different timescales of influencing processes lead to contrasting property distributions within and among margins.
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.
Interactions between riverine inputs, internal cycling, and oceanic exchange result in dynamic variations in the partial pressure of carbon dioxide (pCO₂) in large estuaries. Here, we report the ...first bay-wide, annual-scale observations of surface pCO₂ and air–water CO₂ flux along the main stem of the Chesapeake Bay, revealing large annual variations in pCO₂ (43–3408 μatm) and a spatial-dependence of pCO₂ on internal and external drivers. The low salinity upper bay was a net source of CO₂ to the atmosphere (31.2 mmol m−2 d−1) supported by inputs of CO₂-rich Susquehanna River water and the respiration of allochthonous organic matter, but part of this region was also characterized by low pCO₂ during spring and fall phytoplankton blooms. pCO₂ decreased downstream due to CO₂ ventilation supported by long water residence times, stratification, mixing with low pCO₂ water masses, and carbon removal by biological uptake. The mesohaline middle bay was a net CO₂ sink (−5.8 mmol m−2 d−1) and the polyhaline lower bay was nearly in equilibrium with the atmosphere (1.0 mmol m−2 d−1). Although the main stem of the bay was a weak CO₂ source (3.7 ± 3.3 × 10⁹ mol C) during the dry hydrologic (calendar) year 2016, our observations showed higher river discharge could decrease CO₂ efflux. In contrast to many other estuaries worldwide that are strong sources of CO₂ to the atmosphere, the Chesapeake Bay and potentially other large estuaries are very weak CO₂ sources in dry years, and could even turn into a CO₂ sink in wet years.
Dissolved inorganic carbon (DIC) and its stable isotope (δ13C-DIC) are powerful tools for exploring aquatic biogeochemistry and the carbon cycle. Traditionally, they are determined separately with a ...DIC analyzer and an isotope ratio mass spectrometer. We present an approach that uses a whole-water CO2 extraction device coupled to a Cavity Ring-Down Spectroscopy (CRDS) CO2 and isotopic analyzer to measure DIC and δ13C-DIC simultaneously in a 3–4 mL sample over an ~11 min interval, with an average precision of 1.5 ± 0.6 μmol kg−1 for DIC and 0.09 ± 0.05‰ for δ13C-DIC. The system was tested on samples collected from a Chesapeake Bay cruise in May 2016, achieving a precision of 0.7 ± 0.5 μmol kg−1 for DIC and 0.05 ± 0.02‰ for δ13C-DIC. Using the simultaneously measured DIC and δ13C-DIC data, the biogeochemical controls on DIC and its isotope composition in the bay during spring are discussed. In the northern upper bay, the main controlling processes were CO2 outgassing and carbonate precipitation, whereas primary production (surface) and degradation of organic carbon (subsurface) dominated in the southern upper bay and middle bay. By improving the mode of sample introduction, the system could be automated to measure multiple samples. This would give the system the potential to provide continuous shipboard measurements during field surveys, making this method more powerful for exploring the complicated carbonate system across a wide range of aquatic settings.
•A CRDS-based approach was developed to simultaneously determine DIC and δ13C-DIC.•High accuracy and precision comparable to the traditional methods of NDIR and IRMS.•Main biogeochemical controls on DIC and δ13C-DIC in Chesapeake Bay in early May.
Abstract
Interactions between riverine inputs, internal cycling, and oceanic exchange result in dynamic variations in the partial pressure of carbon dioxide (
p
CO
2
) in large estuaries. Here, we ...report the first bay‐wide, annual‐scale observations of surface
p
CO
2
and air–water CO
2
flux along the main stem of the Chesapeake Bay, revealing large annual variations in
p
CO
2
(43–3408
μ
atm) and a spatial‐dependence of
p
CO
2
on internal and external drivers. The low salinity upper bay was a net source of CO
2
to the atmosphere (31.2 mmol m
−2
d
−1
) supported by inputs of CO
2
‐rich Susquehanna River water and the respiration of allochthonous organic matter, but part of this region was also characterized by low
p
CO
2
during spring and fall phytoplankton blooms.
p
CO
2
decreased downstream due to CO
2
ventilation supported by long water residence times, stratification, mixing with low
p
CO
2
water masses, and carbon removal by biological uptake. The mesohaline middle bay was a net CO
2
sink (−5.8 mmol m
−2
d
−1
) and the polyhaline lower bay was nearly in equilibrium with the atmosphere (1.0 mmol m
−2
d
−1
). Although the main stem of the bay was a weak CO
2
source (3.7 ± 3.3 × 10
9
mol C) during the dry hydrologic (calendar) year 2016, our observations showed higher river discharge could decrease CO
2
efflux. In contrast to many other estuaries worldwide that are strong sources of CO
2
to the atmosphere, the Chesapeake Bay and potentially other large estuaries are very weak CO
2
sources in dry years, and could even turn into a CO
2
sink in wet years.
We surveyed the carbonate system along the main channel of the Chesapeake Bay in June 2016 to elucidate carbonate dynamics and the associated sources of oxygen-consuming organic matter. Using a two ...endmember mixing calculation, chemical proxies, and stoichiometry, we demonstrated that in early summer, dissolved inorganic carbon (DIC) dynamics were controlled by aerobic respiration in the water column (43%), sulfate reduction in the sediment (39%), atmospheric CO₂ invasion (13%), and CaCO₃ dissolution (5%). A mass balance of the DIC concentration and its stable isotope suggested that the apparent δ13C of oxygen-consuming organic matter was −19.4 0.3‰. The bulk composition of particulate organic matter also reflected a dominance of algal material (C/N = ~ 6, δ13C > −25‰). Therefore, we concluded that the decomposition of autochthonous organic matter (i.e., eutrophication-stimulated primary production) was the dominant process consuming oxygen, while allochthonous organic matter (terrestrially derived) made minor contributions to oxygen consumption in the hypoxic zone in June 2016. These findings in the Chesapeake Bay contrast with another hypoxic estuarine ecosystem, the Pearl River Estuary in China where allochthonous organic matter contributed significantly to oxygen consumption. The differences between these two systems in terms of hydrology, quantity and quality of organic matter, and physical characteristics are discussed to yield new insights on the formation and maintenance of hypoxia. In both systems, autochthonous organic matter dominates oxygen depletion, indicating that nutrient management and reduction are useful actions to control and mitigate the occurrence of hypoxia for the restoration of ecosystem.
Acidification can present a stress on organisms and habitats in estuaries in addition to hypoxia. Although oxygen and pH decreases are generally coupled due to aerobic respiration, pH dynamics may be ...more complex given the multiple modes of buffering in the carbonate system. We studied the seasonal cycle of dissolved oxygen (DO), pH, dissolved inorganic carbon, total alkalinity, and calcium ion (Ca2+) along the main channel of Chesapeake Bay from May to October in 2016. Contrary to the expected co‐occurrence of seasonal DO and pH declines in subsurface water, we found that the pH decline ended in June while the DO decline continued until August in mid‐Chesapeake Bay. We discovered that aerobic respiration was strong from May to August, but carbonate dissolution was minor in May and June and became substantial in August, which buffered further pH declines and caused the seasonal DO and pH minima mismatch. The rate of calcium carbonate (CaCO3) dissolution was not primarily controlled by the saturation state in bottom water, but was instead likely controlled by the supply of CaCO3 particles. The seasonal variability of Ca2+ addition in the mid‐bay was connected to Ca2+ removal in the upper bay, and the timing of high carbonate dissolution coincided with peak seasonal biomass of upper Bay submerged aquatic vegetation. This study suggests a mechanism for a novel decoupling of DO and pH in estuarine waters associated with CaCO3, but future studies are needed to fully investigate the seasonality of physical transport and cycling of CaCO3.
In coastal regions and marginal bodies of water, the increase in partial pressure of carbon dioxide (
p
CO
2
) in many instances is greater than that of the open ocean due to terrestrial (river, ...estuarine, and wetland) influences, decreasing buffering capacity and/or increasing water temperatures. Coastal oceans receive freshwater from rivers and groundwater as well as terrestrial-derived organic matter, both of which have a direct influence on coastal carbonate chemistry. The objective of this research is to determine if coastal marshes in Georgia, USA, may be “hot-spots” for acidification due to enhanced inorganic carbon sources and if there is terrestrial influence on offshore acidification in the South Atlantic Bight (SAB). The results of this study show that dissolved inorganic carbon (DIC) and total alkalinity (TA) are elevated in the marshes compared to predictions from conservative mixing of the freshwater and oceanic end-members, with accompanying pH around 7.2 to 7.6 within the marshes and aragonite saturation states (Ω
Ar
) <1. In the marshes, there is a strong relationship between the terrestrial/estuarine-derived organic and inorganic carbon and acidification. Comparisons of pH, TA, and DIC to terrestrial organic material markers, however, show that there is little influence of terrestrial-derived organic matter on shelf acidification during this period in 2014. In addition, Ω
Ar
increases rapidly offshore, especially in drier months (July). River stream flow during 2014 was anomalously low compared to climatological means; therefore, offshore influences from terrestrial carbon could also be decreased. The SAB shelf may not be strongly influenced by terrestrial inputs to acidification during drier than normal periods; conversely, shelf waters that are well-buffered against acidification may not play a significant role in mitigating acidification within the Georgia marshes.
Here, we explicitly define a half-cell reaction approach for pH calculation using the electrode couple comprised of the solid-state chloride ion-selective electrode (Cl-ISE) as the reference ...electrode and the hydrogen ion-selective ion-sensitive field effect transistor (ISFET) of the Honeywell Durafet as the hydrogen ion H+-sensitive measuring or working electrode. This new approach splits and isolates the independent responses of the Cl-ISE to the chloride ion Cl− (and salinity) and the ISFET to H+ (and pH), and calculates pH directly on the total scale pHtotalEXT in molinity (mol (kg-soln)−1) concentration units. We further apply and compare pHtotalEXT calculated using the half-cell and the existing complete cell reaction (defined by Martz et al. (2010)) approaches using measurements from two SeapHOx sensors deployed in a test tank. Salinity (on the Practical Salinity Scale) and pH oscillated between 1 and 31 and 6.9 and 8.1, respectively, over a six-day period.
In contrast to established Sensor Best Practices, we employ a new calibration method where the calibration of raw pH sensor timeseries are split out as needed according to salinity. When doing this, pHtotalEXT had root-mean squared errors ranging between ±0.0026 and ±0.0168 pH calculated using both reaction approaches relative to pHtotal of the discrete bottle samples pHtotaldisc. Our results further demonstrate the rapid response of the Cl-ISE reference to variable salinity with changes up to ±12 (30 min)−1. Final calculated pHtotalEXT were ≤±0.012 pH when compared to pHtotaldisc following salinity dilution or concentration. These results are notably in contrast to those of the few in situ field deployments over similar environmental conditions that demonstrated pHtotalEXT calculated using the Cl-ISE as the reference electrode had larger uncertainty in nearshore waters. Therefore, additional work beyond the correction of variable temperature and salinity conditions in pH calculation using the Cl-ISE is needed to examine the effects of other external stimuli on in situ electrode response. Furthermore, whereas past work has focused on in situ reference electrode response, greater scrutiny of the ISFET as the H+-sensitive measuring electrode for pH measurement in natural waters is also needed.
•Electrode responses are split out and isolated using half reactions.•A new approach for pH calculation using half reactions is explicitly defined.•pH calculation uses single ion activity coefficients for the chloride and hydrogen ions.•This new pH calculation approach is optimized for salinities between 0.105 and 35.•pH can now be calculated directly on the total scale in molinity (mol (kg soln)−1).
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