Ecosystem modeling analysis of size-structured phytoplankton dynamics in the York River estuary, Virginia (USA). II. Use of a plankton ecosystem model for investigating controlling factors on phytoplankton and nutrient dynamics

An ecosystem simulation model was used to investigate potential mechanisms controlling the size-structured phytoplankton and nutrient dynamics in the mesohaline zone of the York River estuary, The York River ecosystem model (Sin & Wetzel 2001, Mar Ecol Prog Ser 228:75-90)was calibrated and validated based on field observations and laboratory measurements prior to the exercises reported here. Analyses of model sensitivity to state variable changes and parameter variations were performed to examine hypotheses proposed from previous studies regarding controls on phytoplankton and nutrient dynamics in the York River estuary. The model results supported the general view that phytoplankton dynamics may be controlled by abiotic mechanisms (i.e. bottom-up control) rather than biotic, trophic interactions in the estuary. Larger mesozooplankton appear to be controlled by top-down mechanisms. Model sensitivity studies showed that small phytoplankton cells (pico-, nano-) are more likely to be regulated by temperature and light, whereas large cells (micro-) are more likely to be regulated by physical processes such as advection and tidal mixing, Micro-phytoplankton blooms during winter-spring resulted from a combination of vertical advection and diffusion of phytoplankton cells rather than in situ production