Keller, David PeterCarbon and nitrogen are involved in many important biological and environmental processes and can even influence the global climate (i.e. CO2 as a greenhouse gas). In this dissertation the role of dissolved organic matter (DOM) in marine C and N cycling is studied. Research is also presented that looks at phytoplankton as potential sources of C and N in the upper Chesapeake Bay estuary food web. To better understand DOM cycling a model was constructed to simulate dissolved organic carbon (DOC) and nitrogen (DON) cycling in marine surface waters. Using the model DOM cycling was simulated in the context of: (1) a steady- state comparison of idealized oceanic, coastal, and estuarine ecosystems, (2) the seasonal cycle in eutrophic waters, and (3) a focus on the roles of viruses and microzooplankton. The results suggest that DOM cycling is intricately tied to the biomass concentration, ratio, and productivity of phytoplankton, zooplankton, viruses, and bacteria. The first set of simulations highlights the importance of certain processes in each ecosystem. The second set of simulations shows how DOM cycling, particularly the sources of DOM, changes seasonally. The third set of simulations highlights differences in the top-down and bottom-up roles of viruses and microzooplankton and their subsequent effect on DOM cycling and trophic interactions. To better understand C and N cycling in the upper Chesapeake Bay the biomass distribution and floral composition of the phytoplankton community was studied during the winter and spring to determine if phytoplankton could play an important role in the estuarine turbidity maximum (ETM) food web. This research suggests that the general distribution of phytoplankton in the upper Bay is somewhat like a classic estuarine "salt wedge" diagram with two distinct phytoplankton communities separated by a zone of increased mortality due to salinity stress and ETM entrapment. High concentrations of phytoplankton pigment degradation products were often observed in the ETM suggesting that this is an area of high phytoplankton mortality and/or an area where phytoplankton derived particulate organic matter was being concentrated. These results suggest that phytoplankton have the potential to play an important role in C and N cycling and the ETM food web.DissertationBiology, OceanographyBiogeochemistryEnvironmental SciencesChesapeake BayDOMmarine biogeochemical modelmarine ecosystem modelPhytoplankton