Frequent and large scale harmful algal blooms have developed in the highly eutrophic water in the Changjiang River estuary and East China Sea coast in the past decade. Sampling and experiments were conducted during late spring and early summer of 2005 to study the dinoflagellate blooms in this region when large scale (> 15,000 km2) mixed dinoflagellate blooms developed. Karenia mikimotoi was the dominant harmful algal bloom (HAB) species in the first stage of the bloom and was succeeded by Prorocentrum donghaiense approximately 2 weeks later. Samples were collected from different stations along both north-south and west-east transects during 3 cruises. Nitrogen (N) uptake experiments were conducted during the time period that covered pre-bloom, bloom development and bloom decline. Kinetics of N uptake by phytoplankton assemblages were measured as a function of substrate concentration (4 N sources, NO3−, NH4+, urea, glycine) with 15N isotope techniques. The bloom progression was related to the change in available N and phosphorus (P) composition. Reduced N, especially NH4+ and urea, were preferentially taken up during the blooms. Both the biological availability of reduced N and the N:P ratio were suggested to be important factors for the bloom development. High inorganic nitrogen loading from the river and phosphorus from coastal water supported the bloom development. Relatively high organic nitrogen levels, and low DIN:DIP led to a succession of dinoflagellates. The temporal and spatial variability in kinetics parameters were reported as N-specific maximum uptake rates (Vmax, h-1), absolute maximum N uptake rates (ρmax, µM h-1) and half-saturation constant (Ks, µmol L-1) during the bloom progression. The results of comparison these indices with ambient nutrients and bloom progression confirmed the preferentially taken up of N by different species during the blooms. Remarkable correspondence was found about the nutrient ratios and specific uptake rates of urea between blooms in the East China Sea and in analogous blooms on the West Florida Shelf. Similar N uptake rates were also shown between the East China Sea and the Chesapeake Bay. These findings suggest that there may be general relationships between these blooms species and the availability of different nutrients and the N:P ratio. To more firmly estimate these relationships, additional laboratory experiments and more comparison among different ecosystem will be required. Such relationships will contribute to our understanding of, and ability to model, these bloom dynamics.