Master of Science (M.Sc.)
Virginia Institute of Marine Science
Mark J Brush
Iris C anderson
Walter R Boynton
While deeper estuaries typically demonstrate predictable responses to increased nutrient loads, responses in shallow systems are more varied, due in part to the presence of multiple benthic autotrophs. Shallow systems are particularly vulnerable to increases in watershed nutrient loads due to their position at the interface between land and open water. The prevailing conceptual model of eutrophication for shallow systems currently describes a succession in the dominant autotroph from seagrass to macroalgae to phytoplankton, but this model does not include benthic microalgae, which can sequester nutrients in photic systems. The Virginia Eastern Shore is characterized by shallow lagoons connected to upland watersheds through small tidal creeks, where the main source of fresh water and nutrients is groundwater. While some studies have characterized the response of the lagoons to nutrient loads, little is known about the tidal creeks and whether they act as filters, transformers, or conduits for land-based nutrients. We examined the role tidal creeks play in modulating watershed nutrient inputs in the Great Machipongo River (GMR) system, the largest tidal creek complex on the seaside of the Virginia Eastern Shore. We developed a field monitoring program that provided data to calibrate a reduced complexity Estuarine Ecosystem Model (EEM). Production, respiration, and net ecosystem metabolism were quantified, using both the open water and component methods, seasonally at three sites within this system. These rates together with monthly concentrations of standing stock nutrients and water column chlorophyll, monthly DataFlow surveys of physiochemical parameters, seasonally and spatially-intensive benthic chlorophyll surveys, and a bathymetric survey were used to develop and calibrate the EEM. The model was used to assess the degree to which tidal creeks export (via flushing), remove (via denitrification), or transform (via autotrophic uptake) land-based nutrient loads to the adjacent lagoons during baseflow and storm conditions. Component metabolism studies showed the system was overall net autotrophic, with increasing dominance of benthic processes towards the head of the estuary. Open water metabolism studies suggested the system was overall net heterotrophic, but we believe this conclusion is biased by the surrounding marshes and violations of the constant water mass assumption. The creek system exported 61,476 kg N y-1 as phytoplankton biomass, an amount approximately equal to inputs from the watershed and atmosphere, and imported 172,830 kg N y-1 in dissolved inorganic forms for a net import of 111,354 kg N y-1 from Hog Island Bay. Phytoplankton uptake, benthic microalgal uptake, and denitrification accounted for 216%, 343%, and 38% of the annual input of watershed and atmospheric N to the system, indicative of rapid cycling and advection of nutrients from Hog Island Bay. The storm simulation showed that almost all of the additional 28,635 kg N y-1 added from the watershed was flushed to Hog Island Bay and a small portion was denitrified. This study indicates that GMR system function is dominated by benthic processes, and the system acts as a transformer and filter of land-based nutrients during normal conditions and a conduit of nutrients during storm conditions.
© The Author
Dean, Britt Leighanne, "Modulation of Watershed Nutrient Loads By Tidal Creek Ecosystems on The Virginia Eastern Shore" (2016). Dissertations, Theses, and Masters Projects. Paper 1477068164.