A mechanistic model to predict seasonal timing and spatial distribution of Prorocentrum minimum blooms in Chesapeake Bay

Planktonic Prorocentrum species are among the most commonly recognized harmful algae that are increasing in frequency, duration, and magnitude globally. In Chesapeake Bay, the number of blooms of this species has increased from ~13 per year in the 1990s to >20 per year in the early 2000s. Although the dynamics of its transport and seasonal occurrence were clearly described in early field surveys, it has proven challenging to predict the precise timing and locations of P. minimum blooms in the estuary. We developed a new, spatially explicit, mechanistic model ROMS-RCA-HABs for P. minimum that integrates physics, nutrient cycling, food web interactions, physical factors and nutrient physiology. Hindcast simulations showed that the model accurately reproduced the observed cell density at the monitoring stations in Chesapeake Bay, with the peak bloom occurring in May and in the mid-to-upper part of the estuary. The mechanistic model also revealed a life strategy as suggested in the earlier field studies: Prorocentrum seed populations overwinter in the southern most of the estuary, are transported up-estuary by the estuarine return flow in the bottom layer, and develop into a bloom when encountering optimal nutrient conditions in the upper-mid Bay during the spring while residual populations are advected downstream during the summer and fall. The model also showed that Prorocentrum minimum growth was limited by dissolved inorganic phosphate during blooms.