The Research Of Nutrient Structure And Its Function On The Succession Of Phytoplankton Predominant Species In The High Frequent Harmful Algae Blooms Occurrence Areas In East China Sea

With the fast development of economy, human factors increasingly do harm to the environment resulting to the water quality deterioration and the frequent red tide which has been increasing in frequency. Changjiang estuary and the adjacent area are the worst endangered area which is called the high frequent harmful algae blooms occurrence areas in East China Sea. Content of N,P,Si and their structure are closely related to the phytoplankton among many environment factors causing the red tide, which are the important factors influencing the plankton community and its succession.Beginning with the nutrient structure and distribution in the high frequent harmful algae blooms occurrence areas in East China Sea, we analyzed yearly inter-month variation of nutrient structure in this area, then analysis was done to study the distribution during the year, and made further research on nutrient structure level and distribution by using nutrient structure data from large-scale red tide in spring & summer in 2005, meaning to gather information of nutrient structure temporal and spatial variation in this area; and then preliminarily studied the function of nutrient structure on succession of phytoplankton predominant species, which was confirmed by large-scale Red Tide Generation, Succession and Vanishment in East China Sea in 2005, and this analysis was verified in more precise time scale; finally, prelimimary discussion was made on nutrient replenishment mechanism according to the variation of nutrient and nutrient structure in typical transects during the dinoflagellate red tide. It is reasonable to provide Red Tide occurrence mechanism in East China Sea with scientific foundation. Main research work and conclusions are listed:1. Through analyzing the nutrient structure inter-month variation of 13 cruises from 2002 to 2007, the inter-month variation of DIN/P shows a distinct tendency of"three peaks", however, DIN/Si tends to rise, then drop, and then rise again, but Si/P has only one peak in its inter-month variation. On the whole, in early April, DIN/P and DIN/Si achieve their peak value, and Si/P is low; then in the middle and late May, both DIN/P and Si/P appear in high level, and then drop deeply, while DIN/Si continue dropping from early April to middle and late June; In the early October, DIN/P and DIN/Si show their high value again, while Si/P is still low at that time.2,According to distribution of nutrient structure data in 4 cruises in spring, summer, autumn and winter respectively between 2002–2003, Isoline of DIN/P at the surface, middle, and bottom indicate a tendency of decline from coast to offshore during the year, and the distribution of isolines parallels with coast line; Isoline of DIN/Si shows a complexity in tendency: it was decreased from coast to ocean in spring which was opposite in summer, DIN/Si is higher in south area than it in north in autumn, which is opposite in winter.3,According to analysis on nutrient structure during large-scale red tide in 2005, in the early period of Diatom Red Tide, the averages of DIN/P, DIN/Si and Si/P are approximate in the surface and middle layers, but much higher than that of in the bottom; In the early stage of Dinoflagellate Red Tide and Vanishing period of Diatom Red Tide, these three references are approach in the surface and middle layers, the divergence between the two layers and bottom is increasing largely compared with early stage of Diatom Red Tide, and the fluctuation range of these rates is larger in surface and middle water than that in the early Diatom Red Tide; In vanishing stage of Dinoflagellate Red Tide, the difference of DIN/P between surface and middle & bottom layers becomes larger and larger, so does the difference of Si/P between surface & middle and bottom, while DIN/Si is dropping on the whole, and the differences between layers is not significant.4,By principle of nutrient relative limiting, statistic analysis of nutrient structure in 2002-2003 suggests that, besides spring, DIN wouldn't be the potential limiting factor generally; and phosphate is the potential limiting factor mainly deviating from the middle of research zone to coast, so is Silicate in outsea. Associated with Threshold Values that limiting the growth of phytoplankton, the analysis shows that nutrient level in surface water does not play a negative role in the growth of phytoplankton in spring & winter; and it is Si that becoming the limiting factor after Diatom Red Tide in some sites. In summer, Si limiting zone is in outsea, whereas it only happens in southern part of outsea in autumn.5,Through statistic analysis of the nutrient structure and Chl-a inter-monthly variations of 13 cruises from 2002 to 2007, DIN/P and DIN/Si increased with the growth of diatom and decreased with the growth of dinoflagellate,Si/P decreased with the growth of diatom, on the contrary, it increased with the growth of dinoflagellate.Low DIN/P and high phosphate would do good to the growth of the dinoflagellate,moreover,high DIN/P and high DIN are favor to the growth of dinoflagellate.Low DIN/Si and high silicate would be propitious to the growth of diatom for that silicate is important to diatom. The conclusion listed was confirmed with the day variation of nutrient structure in repeat investigation area in 2005 during the red tide, which proved validity of conclusion listed in a certain extent.6,According to the comparison of salinity, nutrient and nutrient structure in typical sections around onset of Dinoflagellates Red Tide in 2005, the surface water is mainly suffered by replenishment of land-based fresh water, such as Changjiang diluted water, and land-based replenishment is stronger in the north than in the south, the diluted water has more replenishment of Silicate and Nitrate, then Phosphate. However, in the bottom high-salty water from Taiwan warm current takes mainly effect, and the south part of research area is suffering more replenishment from Taiwan warm current, which bringing far more Phosphate than Nitrate and Silicate, and sediment release might be one of important sources of Silicate.