Certain species of microscopic phytoplankton produce toxins that can be accumulated by moving up through the food chain to poison animals, such as clams, fish, seals, walruses, whales, and ultimately humans. Recent observations in the Bering and Chukchi seas have identified large populations of Alexandrium catenella, a saxitoxin-producing, harmful species of phytoplankton. A. catenella germinates from cysts on the seabed as water warms during spring and summer and enters the vegetative stage where they grow and reproduce in the water column, forming a harmful algal bloom (HAB). When the water cools and daylight wanes in autumn the vegetative cells then form dormant cysts, which sink to the seabed. Recent warming of ocean temperatures in the Pacific Arctic, and along the coast of Alaska in particular, presents a distinct threat since A. catenella germination and growth rates increase with increasing temperature. This project aims to help in understanding the evolving biophysical environment of the Bering and Chukchi seas for decision making related to food security for humans that consume seafood from the region. The project’s key objectives are to identify the major pathways and ‘hotspots’ of A. catenella in the Pacific Arctic. This project will use the Regional Arctic System Model (RASM), which includes ocean, sea ice, atmosphere, and biogeochemistry models, and expand it with a life cycle model of A. catenella. The expanded model (RASM-HAB) will be used to identify the