Award: OCE-1537013

Final Report for NSF OCE1537013: Collaborative Research: Multiple Stressors in the Estuarine Environment: What drives changes in the Carbon Dioxide system? Invoking a combination of fieldwork, historical data analysis and physical-biogeochemical modeling, this project has greatly enhanced our understanding of carbonate system cycling in the Chesapeake Bay. The resulting eight publications focus on these various aspects of the project, with the final publication (Da et al., 2021) identifying the primary drivers of decadal trends in this estuarine carbonate system, as well as their spatiotemporal variability. Combining historical data, new climate quality carbonate system data and our coupled physical-biogeochemical model of the Chesapeake Bay, we found that the greatest surface pH and aragonite saturation state reductions occurred in the summer in the middle (mesohaline) Bay: −0.24 and −0.9 per 30 years, respectively, with increases in atmospheric CO2 and 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 aragonite saturation state occurring 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. Another aspect of this project (St-Laurent et al., 2020) focused on longer term impacts of global vs. regional watershed changes on the inorganic carbon balance of the Chesapeake Bay. Specifically, our objective in this case was to better understand the relative impact of these changes on the inorganic carbon balance of the bay between the early 1900s and the early 2000s.We again used our linked physical-biogeochemical and conducted sensitivity experiments 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 found 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. Broader impacts from this work have been achieved through our Chesapeake Bay Environmental Forecasting System (www.vims.edu/cbefs), which now provides forecasts not only of salinity, temperature and oxygen, but also multiple acidification metrics such as pH, alkalinity and aragonite saturation state (Bever et al., 2021). These metrics are particularly important for hatchery operators, who need to make decisions regarding the use or treatment of Bay intake water for their daily operations. The success of this forecast system is largely attributable to the stakeholder focus group meetings which were held throughout the project duration, ensuring strong communication between project PIs and our end-users. Bever, A.J., M.A.M. Friedrichs, P. St-Laurent, 2021. Real-time environmental forecasts of the Chesapeake Bay: Model setup, improvements, and online visualization. Environmental Modelling and Software, 105036, https://doi.org/10.1016/j.envsoft.2021.105036 Da, F., M.A.M. Friedrichs, P. St-Laurent, E.H. Shadwick, R.G. Najjar, K. Hinson, 2021. Mechanisms driving decadal changes in the carbonate system of a coastal plain estuary. J. Geophys. Res. Oceans, 126(6), e2021JC017239, https://doi.org/10.1029/2021JC017239 St-Laurent, P., M.A.M. Friedrichs, R.G. Najjar, E.H. Shadwick, H. Tian, Y. Yao, 2020. Relative impacts of global changes and regional watershed changes on the inorganic carbon balance of the Chesapeake Bay, 2020, Biogeosciences, 17, 3779-3796, https://doi.org/10.5194/bg-17-3779-2020 Last Modified: 05/20/2022 Submitted by: Marjorie A Friedrichs