Chesapeake Bay Carbonate Cycle: Past, Present, And Future

Multiple natural and anthropogenic drivers are expanding the variability of the estuarine carbonate system (CO2 system). These changes in the CO2 system are threatening the health of ecologically and economically important bivalve species. This dissertation investigates the Chesapeake Bay CO2 system by using numerical models and historical water quality data, focusing on the past three decades, the contemporary period, and the late 2060s. In Chapter 2, sensitivity experiments are conducted with a 3-D Chesapeake Bay hydrodynamic-biogeochemical model and reveal that the magnitude of decadal trends in the CO2 system over the past 30 years is much greater than that observed in the open ocean. This is due to a combination of impacts from the land, atmosphere, and ocean. The greatest surface pH and aragonite saturation state (Ω_AR) reductions have occurred in the summer in the mesohaline Bay (–0.24 units and –0.9 over the past 30 years, respectively), with nearly equal influences from increased atmospheric CO2 and reduced nutrient loading. Increases in riverine total alkalinity (TA) and dissolved inorganic carbon (DIC) have raised surface pH in the upper oligohaline Bay. On top of these long-term trends, short-term CO2 system variability is particularly pronounced in smaller tributaries of the Chesapeake Bay. These relatively shallow regions are important sites for the shellfish aquaculture industry as well as oyster restoration, and could be more susceptible to coastal acidification due to terrestrial runoff. Chapter 3 examines the primary controls of the CO2 system in a tidal estuary of the Chesapeake Bay: the York River Estuary (YRE). Model results show that on average, wetland inputs account for 27% and 20% of the total DIC and TA inputs to the estuary, respectively. In addition, wetlands increase estuarine CO2 outgassing by a factor of two relative to a simulation without wetlands. Strong quasi-monthly variability in DIC and TA is driven by tidal cycles, which cause fluctuations between net heterotrophy and net autotrophy. Model results in a wetter year compared to those in a drier year show that greater net heterotrophy is largely responsible for a tripled net biological DIC production and the increased CO2 outgassing in the wetter year. Chapter 4 investigates the impact of extreme river discharge and climate change on calcium carbonate saturation state (Ω_Ca) in the YRE. Model results show that year-to-year differences in river discharge produce differences in Ω_Ca that are comparable in magnitude to the long-term reductions in Ω_Ca projected to occur over the next 50 years. Although a similar high discharge event in the future will have 20–40% less of an impact on Ω_Ca, increasing atmospheric CO2 will decrease baseline Ω_Ca. Shallow regions in the lower YRE, where most oyster reefs are located, typically recover faster after a high discharge event compared to regions farther upstream. This dissertation complements existing knowledge on Chesapeake Bay CO2 system and provides useful information to regional stakeholders. Local managers need to know the causes of acidification over different spatiotemporal scales, and specifically what portion is due to drivers they can control (e.g., nutrient reductions) versus drivers they have less control (e.g., climate change). These analyses of the CO2 system variability at spatiotemporal scales that are relevant to the Bay’s shellfish industry provide information regarding environmental challenges that the industry faces today and will likely face more often in the future