Studies On The Response Of Phytoplankton To Nutrient Input By Mesocosm Experiments In East China Sea

In recent years, influenced by the activities of human beings, eutrophication of the marine environment become more and more serious and enlarged in level and scale as well, in the East China Sea, especially in the Yangtse River estuary and its adjacent sea area. Thus serous problems in marine environment were induced, such as the worsen of sea water, unbalance of marine ecosystem and the high frequency of HABs. All of those do harm to the development of marine economy, even to the health of human beings. In order to find the reason of HABs'occurring and how the two common HABs species succession in algae bloom process by interspecific competition, the data from four mesocosm experiments conducted outside of Yangtze River Estuary in the East China Sea were analyzed. So we can give more information for HABs controlling in the East China Sea. The main research work and conclusions are listed as follows:1. The effects of nutrients input on the phytoplankton growthAccording form both chlorophyll a concentration and cell abundance, increasing the contents of nitrate and phosphate in the sea water has visible effects on the growth of phytoplankton. When the concentration of phosphate or nitrate was increased only, the growth of phytoplankton was limited in an appropriate level, because the phytoplankton absorbs the N and P with the ratio nearly 16:1, so N or P may become the limiting factors firstly.The the phytoplankton growth will be more and more remarkable with the increasing of the concentration of phosphate input, but it will no longer increase when the concentration of phosphate rise to some degree. There is no obvious difference on the max biomass of phytoplankton between Paroxysmic input or time-by-time input with phosphate. But the growth rate will increased when the phostrate was input paroxysmally. Gradually increases with phostrate can maintain a longer growth period and cause more final biomass.2. Interspecific competition of two important HABs species and the responses to the nutrient inputSkeletonema costatum has more sensitivity response to nutrient input than Prorocentrum dentatum (also called Prorocentrum donghaiense), in the mesocosm with rich nutrient it is superiority in community because of its rapid growth. In hte middling enrichment of nutrient, Skeletonema costatum becomes the winer of the competition firstly, and then with the nutrient consumpted by the phytoplankton Prorocentrum dentatum will become more and more in both ratio and cell abundance.The dominant species in the algal communities were variational in different nutrient concentrations. In low-phosphate condition, the dominant species was Prorocentrum dentatum, while Skeletonema costatum became dominant in phosphate-enriched condition. Prorocentrum dentatum grows better in the condition with lower nutrients concentrations because of its strong endurance for"starvation". On the contratry the growth of Skeletonema costatum is more fit to higher nutrients concentrations. In other words, it has stronger competition than Prorocentrum dentatum in eutrophication of the marine environment.When the sea water in the mesocosms was obtained during the red tide caused by prorocentrum dentatum, the initial cell density of prorocentrum dentatum was Absolutely superiority. Increasing the nitrate and phosphate at the same time will do more favorite to Skeletonema costatum, its cell abundance increased and it became one of the dominant species rapidly, although its initial cell density was very small. But its cell abundance was inferior to prorocentrum dentatum all the time. So we think that the interspecific competition between prorocentrum dentatum and Skeletonema costatum is influenced by the ratio of initial cell density.In the mesocosm experiments conducted in the March 2003, during the red tide caused by prorocentrum dentatum, the growth of Skeletonema costatum was limited. Considering the temprature and salinity data, we thought the main reason of the above result may be lower temperature.During the red tide time, the concentration of phosphate always much lower and it easily becomes the limiting factor to the growth of phytoplankton, sometimes Nitrate. P has the key effects on the breakout of the red tide, without the complementarity of N and P, red tide will clear away rapidly.