A three-dimensional mechanistic model of Prorocentrum minimum blooms in eutrophic Chesapeake Bay.

Planktonic Prorocentrum, common harmful dinoflagellate, are increasing in frequency, duration, and magnitude globally, as exemplified by the number of blooms of P. minimum in Chesapeake Bay that have nearly doubled over the past 3 decades. Although the dynamics of transport and seasonal occurrence of this species have been previously described, it has been challenging to predict the timing and location of P. minimum blooms in Chesapeake Bay. We developed a new three-dimensional mechanistic model of this species that integrates physics, nutrient cycling and plankton physiology and embedded it within a coupled hydrodynamic-biogeochemical model originally developed for simulating water quality in eutrophic estuarine and coastal waters. Hindcast simulations reproduced the observed time series and spatial distribution of cell density, in particular capturing well its peak in May in the mid-to-upper part of the estuary. Timing and duration of the blooms were mostly determined by the temperature-dependent growth function, while mortality due to grazing and respiration played a minor role. The model also reproduced the pattern of overwintering populations, which are located in bottom waters of the lower Bay, and are transported upstream in spring by estuarine flow. Blooms develop in the mid-upper parts of the estuary when these transported cells encounter high nutrient concentrations from the Susquehanna River and favorable light conditions. Diagnostic analysis and model-sensitivity experiments of nutrient conditions showed that high nitrogen:phosphorus conditions favor bloom development. The model also captured the observed interannual variations in the magnitude and spatial distribution of P. minimum blooms.