The Effects Of Nutrients And Allelopathy On The Phytoplankton In The East China Sea

Effects of nutrients and allelopathy on the phytoplankton growth and communitystructure were studied by field experiment during MC2005-3 and MC2005-5 of HAB"973" project in the East China Sea and algae culturing in laboratory. In addition theeffect of the different granule, isolated by cross-flow ultrafiltration from Prorocentrumdonghaiense on Skeletonema costatum was studied and the characteristic of theallelopathic matters of Prorocentrum donghaiense were also discussed.After algae bloom, growth of phytoplankton was inhibited by lack of nutrients inthe seawater, especially by N and P which showed different inhibiting mechanism indifferent areas and the phytoplankton grew faster after adding nutrients. The fieldexperiment after the diatom blooms indicated that high nutrients conditions benefitedthe competition of diatom against dinoflagellate while the dinoflagellate dominatedunder low nutrients conditions. The field experiment after the dinoflagellage bloomsproved this result again, and it also indicated that under high N and P conditions, theconcentration of Si in the water was an important factor in regulating the ratio of diatomand dinoflagellage in the community and the growth of diatom were obviously inhibitedby the low Si concentration.The growth stimulative effect of diatom bloom on P. donghaiense may be the mainreason for the appearance of dinoflagellate bloom after diatom bloom in spring of 2005.The result that the promotion effect of seawater after of K. mikimotoi bloom on P.donghaiense growth corresponded with the highest K. mikimotoi cell density maybeprovided experimental evidence for the P. donghaiense bloom appearing after K.mikimotoi in field. The East China Sea was considered as the P limited area. UnderP-limited condition, the growth of P. donghaiense was significantly promoted by the S.costatum cell-free filtrate, which maybe indicated the allelopathy of S. costatum on P.donghaiense was one of the reasons that the dinoflagellate (pyrrophyta) bloom appearedafter diatom bloom in East China Sea. In addition, the promotion of P. donghaiensecell-free filtrate on A. tamarensen growth under P-limited condition made A.tamarensen as the sub-superior species possible. However, the cell-free filtrate of P.donghaiense under this condition also promoted the growth of itself, so A. tamarensencouldn't become the first superior species in the bloom. In conclusion, expect for thenutrients and other physical and chemical factors, allelopathy may be an importantfactor influencing the succession of species in the bloom.The promotion effect of P. donghaiense cell-free filtrate on the growth of S.costatum increased along with the growth of P. donghaiense. In different growth phases,the effect of cell-free filtrate between 100KDa and 0.45μm were most intensive. Thecell-free filtrate of P. donghaiense had obviously protein-like fluorescence andhumic-like fluorescence, the fluorescence intensities got stronger along with algaegrowth. In addition, the fluorescence style of protein-like and humic-like are vary indifferent growth phases. A great deal of carbohydrate existed in the culture medium of P.donghaiense, and the concentration of carbohydrate also increased with the increasingculturing time and were highest in the cell-free filtrate between 100KDa and 0.45μm asthe promotion effect, which indicated that the promotion effect of P. donghaiensecell-free filtrate on the growth of S. costatum was probably caused by the exudation ofdissolved carbohydrate.