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For the first time the annual carbon budget on the West Antarctic Peninsula shelf was studied with continuously measured CO₂system parameters (pH andpCO₂) from a subsurface mooring. The temporal evolution of the mixed layer dissolved inorganic carbon (DIC) is investigated via a mass balance. The annual mixed layer DIC inventory change was 1.1 ± 0.4 mol m⁻² yr⁻¹, which was mainly regulated by biological drawdown (−2.8 ± 2.4 mol m⁻² yr⁻¹), diapycnal eddy diffusion (2.6 ± 1.3 mol m⁻² yr⁻¹), entrainment/detrainment (0.9 ± 0.4 mol m⁻² yr⁻¹), and air‐water gas exchange (0.4 ± 2.1 mol m⁻² yr⁻¹). Significant carbon drawdown was observed in the spring and summer, which was replenished by the physical processes mentioned above. These observations suggest this area is an annual atmosphere CO₂sink with a mixed layer net community production of 2.8 ± 2.4 mol m⁻² yr⁻¹. These results highlight the significant seasonality in the DIC mass balance and the necessity of year‐round continuous observations for robust assessments of biogeochemical cycling in this region.

Understanding decadal changes in the coastal carbonate system is essential for predicting how the health of these waters responds to anthropogenic drivers, such as changing atmospheric conditions and riverine inputs. However, studies that quantify the relative impacts of these drivers are lacking. In this study, the primary drivers of decadal trends in the surface carbonate system, and the spatiotemporal variability in these trends, are identified for a large coastal plain estuary: the Chesapeake Bay. Experiments using a coupled three‐dimensional hydrodynamic‐biogeochemical model highlight that, over the past three decades, the changes in the surface carbonate system of Chesapeake Bay have strong seasonal and spatial variability. The greatest surface pH and aragonite saturation state (Ω_AR) reductions have occurred in the summer in the middle (mesohaline) Bay: −0.24 and −0.9 per 30 years, respectively, with increases in atmospheric CO₂and reductions in nitrate loading both being primary drivers. Reductions in nitrate loading have a strong seasonal influence on the carbonate system, with the most pronounced decadal decreases in pH and Ω_ARoccurring during the summer when primary production is strongly dependent on nutrient availability. Increases in riverine total alkalinity and dissolved inorganic carbon have raised surface pH in the upper oligohaline Bay, while other drivers such as atmospheric warming and input of acidified ocean water through the Bay mouth have had comparatively minor impacts on the estuarine carbonate system. This work has significant implications for estuarine ecosystem services, which are typically most sensitive to surface acidification in the spring and summer seasons.

The Arctic Ocean is more susceptible to ocean acidification than other marine environments due to its weaker buffering capacity, while its cold surface water with relatively low salinity promotes atmospheric CO₂uptake. We studied how sea‐ice microbial communities in the central Arctic Ocean may be affected by changes in the carbonate system expected as a consequence of ocean acidification. In a series of four experiments during late summer 2018 aboard the icebreakerOden, we addressed microbial growth, production of dissolved organic carbon (DOC) and extracellular polymeric substances (EPS), photosynthetic activity, and bacterial assemblage structure as sea‐ice microbial communities were exposed to elevated partial pressures of CO₂(pCO₂). We incubated intact, bottom ice‐core sections and dislodged, under‐ice algal aggregates (dominated byMelosira arctica) in separate experiments under approximately 400, 650, 1000, and 2000 μatm pCO₂for 10 d under different nutrient regimes. The results indicate that the growth of sea‐ice algae and bacteria was unaffected by these higher pCO₂levels, and concentrations of DOC and EPS were unaffected by a shifted inorganic C/N balance, resulting from the CO₂enrichment. These central Arctic sea‐ice microbial communities thus appear to be largely insensitive to short‐term pCO₂perturbations. Given the natural, seasonally driven fluctuations in the carbonate system of sea ice, its resident microorganisms may be sufficiently tolerant of large variations in pCO₂and thus less vulnerable than pelagic communities to the impacts of ocean acidification, increasing the ecological importance of sea‐ice microorganisms even as the loss of Arctic sea ice continues.

Understanding the vulnerability of estuarine ecosystems to anthropogenic impacts requires a quantitative assessment of the dynamic drivers of change to the carbonate (CO₂) system. Here we present new high‐frequency pH data from a moored sensor. These data are combined with discrete observations to create continuous time series of total dissolved inorganic carbon (TCO₂), CO₂partial pressure (pCO₂), and carbonate saturation state. We present two deployments over the winter‐to‐spring transition in the lower York River (where it meets the Chesapeake Bay mainstem) in 2016/2017 and 2017/2018. TCO₂budgets with daily resolution are constructed, and contributions from circulation, air‐sea CO₂exchange, and biology are quantified. We find that TCO₂is most strongly influenced by circulation and biological processes; pCO₂and pH also respond strongly to changes in temperature. The system transitions from autotrophic to heterotrophic conditions multiple times during both deployments; the conventional view of a spring bloom and subsequent summer production followed by autumn and winter respiration may not apply to this region. Despite the dominance of respiration in winter and early spring, surface waters were undersaturated with respect to atmospheric CO₂for the majority of both deployments with mean fluxes ranging from −9 to −5 mmol C·m⁻²·day⁻¹. Deployments a year apart indicate that the seasonal transition in the CO₂system differs significantly from one year to the next and highlights the necessity of sustained monitoring in dynamic nearshore environments.

Estuaries play an uncertain but potentially important role in the global carbon cycle via CO₂outgassing. The uncertainty mainly stems from the paucity of studies that document the full spatial and temporal variability of estuarine surface water partial pressure of carbon dioxide ( pCO₂). Here, we explore the potential of utilizing the abundance of pH data from historical water quality monitoring programs to fill the data void via a case study of the mainstem Chesapeake Bay (eastern United States). We calculatepCO₂and the air‐water CO₂flux at monthly resolution from 1998 to 2018 from tidal fresh to polyhaline waters, paying special attention to the error estimation. The biggest error is due to the pH measurement error, and errors due to the gas transfer velocity, temporal sampling, the alkalinity mixing model, and the organic alkalinity estimation are 72%, 27%, 15%, and 5%, respectively, of the error due to pH. Seasonal, interannual, and spatial variability in the air‐water flux and surfacepCO₂is high, and a correlation analysis with oxygen reveals that this variability is driven largely by biological processes. Averaged over 1998–2018, the mainstem bay is a weak net source of CO₂to the atmosphere of 1.2 (1.1, 1.4) mol m⁻² yr⁻¹(best estimate and 95% confidence interval). Our findings suggest that the abundance of historical pH measurements in estuaries around the globe should be mined in order to constrain the large spatial and temporal variability of the CO₂exchange between estuaries and the atmosphere.

Despite the important role of alkalinity in estuarine carbon cycling, the seasonal and decadal variability of alkalinity, particularly within multiple tidal tributaries of the same estuary, is poorly understood. Here we analyze more than 25,000 alkalinity measurements, mostly from the 1980s and 1990s, in the major tidal tributaries of the Chesapeake Bay, a large, coastal‐plain estuary of eastern North America. The long‐term means of alkalinity in tidal‐fresh waters vary by a factor of 6 among seven tidal tributaries, reflecting the alkalinity of nontidal rivers draining to these estuaries. At 25 stations, mostly in the Potomac River Estuary, we find significant long‐term increasing trends that exceed the trends in the nontidal rivers upstream of those stations. Box model calculations in the Potomac River Estuary indicate that the main cause of the estuarine trends is a declining alkalinity sink. The magnitude of this sink is consistent with a simple model of calcification by the invasive bivalveCorbicula fluminea. More generally, in tidal tributaries fed by high‐alkalinity nontidal rivers, alkalinity is consumed, with sinks ranging from 8% to 27% of the upstream input. In contrast, tidal tributaries that are fed by low‐alkalinity nontidal rivers have sources of alkalinity amounting to 34% to 171% of the upstream input. For a single estuarine system, the Chesapeake Bay has diverse alkalinity dynamics and can thus serve as a laboratory for studying the numerous processes influencing alkalinity among the world's estuaries.