Relationships between nitrogen and phosphorus forms and ratios and the development of dinoflagellate blooms in the East China Sea

Abstract

During late spring and early summer of 2005, large-scale (>15 000 km2), mixed dinofla gellate blooms developed along the the coast of the East China Sea. Karenia mikimotoiwas the dom inant harmful algal bloom species in the first stage of the bloom (late May) and was succeeded by Prorocentrum donghaiense approximately 2 wk later. Samples were collected from different stations along both north-south and west-east transects, from the Changjiang River estuary to the south Zhe-jiang coast, during 3 cruises of the Chinese Ecology and Oceanography of Harmful Algal Blooms Pro gram, before and during the bloom progression. Nitrogen isotope tracer techniques were used to measure rates of NO3̃, NH4+, urea, and glycine uptake during the blooms. High inorganic nitrogen (N), but low phosphorus (P) loading from the Changjiang River led to high dissolved inorganic Ndis-solved inorganic P ratios in the sampling area and indicate the development of P limitation. The rates of 15N-uptake experiments enriched with PO43̃ were enhanced compared to unamended samples, suggesting P limitation of the N-uptake rates. The bloom progression was related to the change in availability of both organic and inorganic N and P. Reduced N forms, especially NH4+, were prefer entially taken up during the blooms, but different bloom species had different rates of uptake of organic N substrates. K. mikimotoi had higher rates of urea uptake, while P. donghaiense had higher rates of glycine uptake. Changes in the availability of reduced N and the ratios of N:P in inorganic and organic forms were suggested to be important in the bloom succession. Nutrient ratios and spe cific uptake rates of urea were similar when compared to analogous blooms on the West Florida Shelf. © Inter-Research 2009.