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Development of Mechanistic Model that Simulates the Light Dynamics and Phytoplankton Growth Kinetics in Little Vermilion Bay
Little Vermilion Bay (TV-12) coastal restoration project is located in the Northwestern corner of Little Vermilion Bay at its intersection with the Gulf Intracoastal Waterway (GIWW) in Vermilion Parish, Louisiana. The project area encompasses 964 acres of shallow (0.3m-0.9m) brackish bay with a series of vegetated terraces constructed for sediment trapping. The focus of this paper is to investigate and estimate parameters of light dynamics and growth kinetics for the eventual development of a mechanistic model of the phytoplankton production in this system. Chlorophyll a represent phytoplankton concentration and phycoerythrin is the blue green algae pigment. Both of which effect the light dynamics in water column. Solar Irradiance was measured by a bulb quantum sensor at various times, depths and in various locations of Little Vermilion Bay. Light Irradiance is represented in terms of photosynthetically active scalar radiation (PAR) and was measured as Photon Flux Density (PDF).Scalar irradiance at any point in the water column is influenced by surface solar irradiance, biomass, turbidity and water depth. Lambert- Beer Law was used to determine light attenuation coefficient and integrate over the depth to estimate average irradiance. Specific parameters necessary for mechanistically modeling phytoplankton production were estimated. They include light attenuation coefficient Ko(PAR), maximum specific growth rate (µmax) and optimum average PFD (Iopt). These parameters varied with species of the phytoplankton community and temporally and specially. The project area was divided into 8 representative sampling locations. For mechanistic modeling purposes the light dynamics and the effects of phytoplankton biomass on average scalar irradiance Ian(PAR) at 8 sampling locations were investigated .For each location Scalar Irradiance were taken at 0.1m interval. Only location 1 and location 8 were 0.9m deep. The remaining locations were very shallow and depths for these locations varied from 0.2m-0.4m. At each location an exponential decrease of scalar irradiance was observed with increase of depth. The R2 value for 8 locations indicating a very good exponential relationship between depth and scalar irradiance. The analysis between chlorophyll and light attenuation coefficients indicated that Ko decreases exponentially with increase of biomass. A linear partitioning of Ko resulted in Kb + Kw where Kb = the light attenuation due to biomass and Kw = the light attenuation due to water. The specific growth rate for the biomass for each location is dependent on hydrology and dilution rate, maximum specific growth rate, periodicity, self-shading, average scalar irradiance ,nitrogen and phosphorous, concentration of Phycoerythrin, temperature and turbidity. As depth increases effect of depth on average scalar irradiance (Ian) became more prominent. Dark zone increased with increase of depth which reduces productivity. Steel’s equation is an exponential peak shaped function used to model Specific Growth Rate (µn) where growth is limited by low irradiance and photoinhibited by high light levels. Surface Irradiance Io changes with the duration of the day and year. Maximum surface irradiance was 2396 µmolsec-1m-2. The turbidity value varied from 18.6 to 961.9 NTU during the studies.
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