Harmful algal blooms (HABs) are expanding worldwide. The harmful dinoflagellate Karlodinium veneficum is of concern because its toxigenic properties cause fish kills. Despite considerable study on nutrient-HAB relationships, there is a lack of data on HAB nutrient physiology because of the complexity of HAB nutrition. Many bloom-forming harmful algae consume particulate prey when nutrients are not available in the dissolved form. The goal of this dissertation was to apply statistical time series analysis, together with a series of laboratory experiments, and multi-nutrient quota models to improve our understanding and predictive capability of this important HAB species. Statistical time series analysis of K. veneficum abundance in Chesapeake Bay showed the predictive power of multiplicative factors (i.e., physical factors, nutrients, and prey) and the importance of temporal lags in some of these factors in bloom promotion.

In laboratory experiments, feeding rates were determined for K. veneficum on prey when both were in varying nutritional conditions. Highest feeding rates were found for K. veneficum initially under low nitrogen:phosphorus condition and fed nitrogen-rich prey. Based on these data, a conceptual model was developed of mid-Bay summer K. veneficum blooms that incorporates the role of prey with a high nitrogen:phosphorus ratio originating from river inputs and a source inocula of K. veneficum from southern Bay waters with a lower nitrogen:phosphorus content. Further laboratory experiments were conducted using multi-wavelength fluorometry to measure growth, grazing and photo-physiology of K. veneficum with single and multiple prey species. Growth of K. veneficum increased with increasing prey concentrations of the cryptophyte Rhodomonas salina, but declined with Synechococcus as the prey.

Subsequent multi-nutrient mechanistic modeling was undertaken, simulating the growth of dinoflagellate K. veneficum and its common prey, Rhodomonas. The model was run varying nutrient ratios (molar nitrogen:phosphorus of 4, 16 and 32) and temperatures. The modeled biomass of K. veneficum was highest when they consumed prey under high nitrogen:phosphorus conditions. When nutrients were in balanced proportions, lower biomass of the dinoflagellate was attained at all temperatures in the model. This study underscores the importance of considering prey and their nutritional quality, as well as dissolved nutrients, in modeling HAB dynamics.