Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Analyses of EPA long-term datasets (1985--1994) combined with field studies and ecosystem model development were used to investigate phytoplankton and nutrient dynamics in the York River estuary. Analysis of the EPA dataset showed that algal blooms occurred during winter-spring followed by smaller summer blooms. Peak phytoplankton biomass during the winter-spring blooms occurred in the mid reach of the mesohaline zone whereas during the summer bloom it occurred in the tidal fresh-mesolialine transition zone. River discharge appears to be the major factor controlling the location and timing of the winter-spring blooms and the relative degree of potential nitrogen (N) and phosphorus (P) limitation. Phytoplankton biomass in tidal fresh water regions was limited by high flushing rates. Water residence time was less than cell doubling rate during seasons of high river flow. Positive correlations between PAR at 1m depth and chlorophyll a suggested light limitation of phytoplankton in the tidal fresh-mesohaline transition zone. A significant relationship between the delta of salinity between surface and bottom water and chlorophyll a distribution suggested the importance of tidal mixing for phytoplankton dynamics in the mesohaline zone. Accumulation of phytoplankton biomass in the mesohaline zone was generally controlled by N with the nutrient supply provided by benthic or bottom water remineralization. In general, phytoplankton dynamics appear controlled to a large extent by resource limitation (bottom-up control) rather than zooplankton grazing (top-down control). The dynamics of phytoplankton size structure were investigated in the freshwater, transitional and estuarine reaches of the York River over an annual cycle. The contribution of large cells (micro-plankton, >20 mum) to total biomass increased downstream during winter whereas that of small cells (nano-, 3--20 mum) pico-plankton, <3 mum) increased downstream during summer. I conclude from these studies that spatial and seasonal variations in size structure of phytoplankton observed on the estuarine scale are determined both by the different preferences of micro-, nano-, and picoplankton for nutrients and by their different light requirements. Analyses of phytoplankton size structure are, thus, necessary to better understand phytoplankton dynamics and to better manage water quality in estuarine systems. An ecosystem model was developed to integrate these data and to investigate mechanisms controlling the size-structured phytoplankton dynamics in the mesohaline zone of the York River estuary. The model developed in Fortran90 included 12 state variables describing the distribution of carbon and nutrients (nitrogen, phosphorus) in the surface mixed layer. Forcing functions included incident radiation, temperature, wind stress, mean flow and tide including advective transport and turbulent mixing. Model results supported the general view that phytoplankton dynamics are controlled by abiotic mechanisms (i.e. bottom-up control) rather than biotic, trophic interactions in the York River estuary. Model sensitivity tests showed that small cells (pico-, nano-sized) are more likely regulated by temperature and light whereas large cells (micro-sized) are regulated by physical processes such as advection, and tidal mixing. Microphytoplankton blooms during winter- pring resulted from a combination of longitudinal advection and vertical diffusion of phytoplankton cells rather than in-situ production.



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