Ecosystem modeling analysis of size-structured phytoplankton dynamics in the York River estuary, Virginia (USA). I. Development of a plankton ecosystem model with explicit feedback controls and hydrodynamics

An ecosystem simulation model was developed to investigate potential mechanisms controlling the size-structured phytoplankton and nutrient dynamics in the mesohaline zone of the York River estuary. The model included 12 state variables in a unit volume (m3) describing the distribution of carbon and nutrients (nitrogen, phosphorus) in the surface mixed layer. General size-scale relationships and density-dependent feedback control terms were used in the ecosystem model. Forcing functions included incident solar radiation, water temperature, wind stress, river flow and tide, which include advective transport and turbulent mixing. Advective transport and turbulent mixing were incorporated into the model explicitly without coupling to other hydrodynamic models. The ecosystem model was developed in Fortran90 using differential equations that were solved numerically using the fourth-order Runge-Kutta (explicit ) technique. After calibrating the ecosystem model, forcing and state variables in the model were validated using pre-existing data and field data collected over an annual cycle. Model predictions for forcing and state variables generally followed the pattern of field observations and were within the range of field data. Model sensitivity analysis was also performed to examine how sensitive model output was to specified changes in parameter values. Model output was not sensitive to changes in most parameters, suggesting that the model is relatively robust. These results suggested that the model including explicit feedback controls and hydrodynamic processes captures plankton/nutrient dynamics and can be used for additional modeling analyses of phytoplankton and nutrient dynamics in the York River estuary, Virginia.