| Abstract: |
The Thalia Creek (TC) system, a tidal tributary connecting to the Western Branch of the Lynnhaven River in the City of Virginia Beach, VA, has consistently failed State dissolved oxygen criteria. The TC system commonly exhibits high concentrations of chlorophyll a (>100 ï?g/l) and chronic severe hypoxia (< 2 mg/L) during the summer. It is anticipated that additional development pressures within the region will further exacerbate the degradation of water quality in TB-TC and the downstream Western Branch of the Lynnhaven River ecosystem. Because of its failure to meet State dissolved oxygen criteria, the City of Virginia Beach and US Army Corps of Engineers (Norfolk District) are currently assessing eutrophication related processes and seeking an ecosystem-based approach to improve the water quality conditions within the TB-TC system.
An intensive field survey was conducted in summer 2009 using high-frequency observations of DO, Chl fluorescence, and turbidity in conjunction with multiple-site in-stream synoptic grab sample surveys. In situ sediment oxygen demand (SOD) studies were also conducted to determine the benthic impacts on the oxygen budget. Observations showed large diurnal DO oscillations resulting in severe hypoxia at night during fair weather, whereas phytoplankton exhibited semi-diurnal as well as diurnal fluctuations during the study period. Estimates of net ecosystem metabolism suggest the TC system is heterotrophic and characterized by elevated primary production and community respiration. High respiration rates often results in a hypoxic conditions that can increase in severity and duration following rainfall events. The DO variation is predominated by benthic community with diurnal swings but it is strongly influenced by phytoplankton through competitions of light and nutrients. The influence of physical processes on associated biological process is visible and varies along the salinity gradient. Biological processes are more dominant in the upstream regions whereas the influence of the physical processes increases downstream.
A diagnostic model has been developed to study the coupling effects between phytoplankton and benthic community and between biological and physical processes in the TC system. The model results show that a strong coupling and competition between the benthic community and the water column exist in the system resulting in phase shifting between DO and phytoplankton. The findings of data analysis and diagnostic model study are being used to guide the development of 3D numerical model to simulate eutrophication processes and will be used for designing a management plan to improve the water quality conditions in the TC system.
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