Modeling the Circulation, Coastal Connectivity, and Harmful Algal Blooms in the Gulf of Maine

Ruoying He
North Carolina State University


The Gulf of Maine (GOM) is a continental shelf sea that supports productive shellfisheries that are frequently impacted by Alexandrium fundyense blooms and outbreaks of paralytic shellfish poisoning (PSP). To monitor and predict HAB abundance and distribution in the GOM, we have coupled a population dynamics model for Alexandrium into a GOM ROMS. In the wake of the historic bloom of 2005 in the western GOM, this coupled model was used to diagnose the underlying causes, including 1) high abundance of resting cysts in fall 2004 that provided a large inoculum; 2) storms with strong northeast winds that carried toxic cells towards, and along the coast; and 3) abundant fresh water runoff, providing macro- and micro-nutrients, a stratified water column, and an alongshore transport mechanism. Sensitivity experiments show that simulations initiated from A. fundyense cyst distributions capture large-scale seasonal patterns in the distribution and abundance of A. fundyense cells. We proposed that cyst abundance is a first-order predictor of regional bloom magnitude in the following year, and that cyst abundance may hold the key to interannual forecasts of PSP severity. The coastal ocean transport and connectivity in the GOM and Georges Bank (GB) are further quantified using the Lagrangian probability density functions (PDFs) based on ROMS circulation simulations and numerical particle trajectory calculations. We focus on the spring and summer seasons when toxic A. fundyense blooms often occur. The seasonal-averaged PDFs highlight the mean circulation patterns in the GOM. On the interannual time scale, Lagrangian PDFs do show significant variability.