Increasing frequency of harmful algal blooms (HABs) have been observed in eutrophic coastal ecosystems. The relationship between environmental factors (nutrients and light) and bloom-forming dinoflagellates were explored in this dissertation by both historical data analysis and laboratory experiments. The growth and nitrogen (N) acquisition of the HAB dinoflagellates Prorocentrum minimum, P. donghaiense, Karlodinium veneficum and Karenia brevis, were studied. It is hypothesized that estuarine species Prorocentrum spp. develops blooms in relative high N:P ratio water, while K. veneficum blooms near or lower than the Redfield ratio; these species will grow faster in the N:P ratio in which they develop blooms, even when these nutrients are not at limiting levels; Prorocentrum spp. preferentially take up more DIN in high DIN:DIP ratio water, while Karlodinium can better use other source of N in the low DIN water; low-light-adapted nitrogen acquisition by Prorocentrum spp. serves as an adaptive advantage to grow in low light waters.

Historical data analysis showed that P. minimum generally develops blooms in high DIN, high N:P ratio, but turbid water in Chesapeake Bay, while K. veneficum blooms near or lower than the Redfield ratio, when DIN was depleted, but organic N sources were still available. Following these results, the effects of ambient N:P ratio and light on the growth and N acquisition of P. minimum and P. donghaiense were studied in both batch and continuous culture (turbidistat). Prorocentrum spp. were grown in a wide range of N:P ratios, and across a wide range of light intensities in turbidistat. Experiments to determine rates of N acquisition of different N sources were conducted using 15N tracer techniques at each N:P ratio and light treatment.

However, in culture, the growth of the Prorocentrum species was not regulated by the ambient N:P ratio. When nutrients were sufficient, light, instead of ambient N:P ratio, regulated the algal ability to acquire N. The adaptive strategies of the two types of dinoflagellates, Prorocentrum spp. and Karlodinium/Karenia spp., are different. Prorocentrum minimum was shown to take up N in the dark. This light independent N uptake allows it to be more competitive in the relative low light near-shore water. Karlodinium/Karenia spp. apparently only takes up N in the light phase, but it can be mixotrophic and directly use organic sources, and thus may be more competitive after DIN was depleted.

The Droop model, which describes the growth rate regulated by the cell quota, was used to interpret the relationship between N acquisition and the growth rate over the diel cycle of growth. Prorocentrum spp. continuously take up nitrogen at night to supplement the cell quota, and reaches the maximum cell quota at the beginning of light phase, when they reach the higher growth rate in a diel cycle.

In eutrophic coastal systems (e.g., Chesapeake Bay), the ambient N:P ratio, as well as light may be critical factors for HAB growth. The dinoflagellates studied here have different adaptive strategies to grow in low light and to take advantage of high nutrients in the eutrophic waters. Prorocentrum spp. may dominant the high DIN water, while Karlodinium/Karenia spp. prefers organic nutrients.