Carbon 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.