A Numerical Modeling Study of Mesodinium Bloom Formation and Retention in a River-Dominated Mesotidal Estuary

The Columbia River estuary experiences extensive seasonal red-colored blooms caused by a mixotrophic ciliate of the genus Mesodinium. Although the blooms are non-toxic, they have a significant influence on the levels of nutrients, light and oxygen in the estuary. Mesodinium spp. displays very particular physiology that makes it one of few planktonic species able to thrive in a highly flushed system: a high growth rate due to its ability to photosynthesize using the photosynthetic organelles of its preys, and complex vertical migration patterns. Knowledge of the migration pattern is based on limited observations of Mesodinium behavior in culture and recent in-situ measurements collected in the Columbia River estuary. A more comprehensive understanding is needed of the mechanisms allowing Mesodinium spp. to be retained and experience rapid growth. To this end, we extended the finite element circulation model SELFE to include a 5-component behavioral model that simulates the relationships between nutrients, detritus, Mesodinium spp. and its cryptophyte prey. We then used the model to investigate various migration patterns and growth scenarios to determine their role in the formation and retention of the Mesodinium spp. bloom in the brackish water of the estuary.