Award: OCE-1536996

Estuaries, where rivers meet the sea, are highly dynamic and diverse environments that have global importance. Estuaries influence the climate system because they release large amounts of carbon dioxide, an important greenhouse gas, to the atmosphere. Estuaries are also very biologically productive and home to some of the world?s great fisheries. Understanding the cycling of carbon in estuaries is important because it provides the basis for quantifying the high productivity of estuaries and for understanding why estuaries release so much carbon dioxide to the atmosphere. There is an urgency to understand the cycling of carbon in estuaries because they are undergoing rapid change as a result of global and local phenomena. The global phenomena include increases in atmospheric carbon dioxide and the consequences of such increases, such as warming, sea-level rise, and ocean acidification. The local phenomena include the changing characteristics of estuarine watersheds, such as urbanization, which lead to changes in the delivery of riverine materials to estuaries. Despite the importance of estuarine carbon cycling, our knowledge is lacking in numerous ways. Estimates of the amount of carbon dioxide released from estuaries globally have large errors because of a lack of measurements that capture how carbon dioxide levels in estuaries vary in space and time. Furthermore, we do not know the relative importance of global and local phenomena in influencing estuarine carbon cycling. We therefore used the Chesapeake Bay as a model estuary to study estuarine carbon cycling. Prior to our research, carbon cycling in this large and productive estuary was not well characterized. The major activities we undertook to study carbon cycling in the Chesapeake Bay were: (1) high-quality measurements of the carbon dioxide system, (2) analysis of the lower-quality but more spatially and temporally extensive carbon-relevant data, and (3) numerical modeling to isolate the various long-term controls on the carbon cycle of the Chesapeake Bay. The high-quality measurements consisted of a mooring deployment at the mouth of the York River Estuary and four seasonal cruises in the main stem of the Chesapeake Bay. The results of our research have been presented in six peer-reviewed publications and here we provide a brief summary of our major findings. Our high-quality measurements revealed significant seasonal, interannual, and spatial variability of the carbon dioxide system, with important impacts of circulation, temperature, and biological processes. These measurements highlighted the necessity of sustained monitoring in dynamic nearshore environments. The historical data analysis supported the picture of large temporal and spatial variability, as well as the importance of biological processes on the carbon dioxide system, that was found from the high-quality data. The main stem of the Chesapeake Bay, when averaged over a 20-year period, was found to be a weak source of carbon dioxide to the atmosphere. We also found that the cycling of alkalinity, which is the acid-neutralizing capacity of a water body, to be highly dynamic in the tidal tributaries. That is, there were processes that both consumed and produced alkalinity, and we found evidence in support of a large consumption of alkalinity due to an invasive clam in the Potomac River Estuary. Numerical modeling revealed that local phenomena dominated the impacts on long-term changes in the carbon cycle of the Chesapeake Bay but that global phenomena were important as well. Furthermore, the impacts are numerous and often act in opposing ways. For example, the increase in atmospheric carbon dioxide has the direct impact of increasing the uptake of carbon dioxide by the estuary whereas warming has the impact of releasing carbon dioxide from the estuary. Similar counteracting effects were found from changing riverine loads of nitrogen, carbon, and alkalinity. What larger implications does our research have? We think there are three. First, we hypothesized that the dynamic character of estuaries would demand novel approaches for studying them and found that a combination of modern high-quality measurements, historical data analysis, and numerical modeling allowed us to capture this dynamism. This approach could be applied to the many estuarine systems that have long-term historical data to mine and are characterized sufficiently (for example, in terms of riverine inputs) for numerical modeling. Second, 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 carbon dioxide exchange between estuaries and the atmosphere. Third, and finally, we found that the Chesapeake Bay is a highly diverse estuarine system in terms of its carbon dynamics. For example, for a single estuarine system, the Chesapeake Bay has a wide range of alkalinity levels and a wide variety of processes that influence its alkalinity. Therefore, the Chesapeake Bay can serve as a laboratory for studying the carbon dioxide system of many of the world's estuaries. Last Modified: 01/09/2021 Submitted by: Raymond G Najjar