The Development and Proliferation of Summer Algal Blooms in the Oligo/Poly Haline Portion of the Chesapeake Bay Observational and Numerical Modeling Studies
Back River is located along the western shoreline of the upper Chesapeake Bay and is about 8 miles in length and 2 3 meters in average depth. Each summer, algal blooms with chlorophyll a concentration exceeding 100 mg/L are regularly observed in Back River. This excessive primary production, exhibited by the high chlorophyll a concentrations, is among the highest in the Chesapeake Bay and its tributaries. In order to explain the unusual phenomenon, a field survey was conducted in the low salinity water of Back River by Boynton and Ceballos (2014) which suggested that pH values exceeding 8.5 on average can trigger a release of phosphorus from the sediment. Based on the observation and related studies, a hypothesis was formulated that a pH mediated feedback mechanism may exist in this system by which elevated phytoplankton growth can enhance pH and the increased pH, in turn, can trigger a phosphorus release from the sediment in Back River where phosphorus is normally limited. A numerical modeling approach was used to verify the hypothesis. In this study, we developed a dynamic pH model with four state variables and coupled it into "integrated compartment model" (ICM) water quality model with 19 state variables. The coupled model is then set up in the upper Chesapeake Bay including Back River. The calibrated model gives reasonable results for hydrodynamics and water quality variables. For testing our hypothesis, we applied the pH model kinetics in Back River and compared with the scenario "with" and "without" it. The results show a positive feedback loop interacting among phytoplankton, total inorganic carbon, pH and phosphate, which verifies that the effect of pH on sediment phosphorus release is important to sustain the high phytoplankton biomass during summer.