Modeled Karenia brevis accumulation in the vicinity of a coastal nutrient front

Abstract

The alongshore independent distribution of Karenia brevis, a dominant harmful algal bloom dinoflagellate in the Gulf of Mexico, was investigated in a shelf environment using the Expanded Eulerian physical-biological modeling approach. The physical model included an ocean of variable depth with a frontal region at the 25 m isobath, and moderate upwelling-favorable winds. Nutrients were available from a surface source conceptually associated with outwelling from Florida bays, and from a near bottom offshore source conceptually associated with upwelling or sediment flux. The biological model included physiological rate processes, biochemical quotas, and behavioral responses based on cellular optimization and environmental conditions. The population distribution at the end of a 37 d simulation was examined. It was found that cells accumulated in the vicinity of the nearshore front. This accumulation began as soon as cells arrived near the front from the offshore boundary. Approximately 70 % of the population was concentrated in the vicinity of the front by the end of the simulation. The trapping mechanism was interpreted to be a combination of fluid advection and swimming behavior. Four additional 37 d simulations were performed, where: (1) offshore bottom nutrient source was eliminated, (2) wind stress was doubled, (3) inhibition control on swimming behavior was imposed, and (4) chemotaxis control on swimming behavior was eliminated. Comparison of results from the simulations indicated that chemotaxis can play an important role in frontal accumulation.