Study On Nutrients And Algae Growth Dynamics In Estuarine Brackish Lake

Yangtze estuary and its adjacent sea areas in China are typical regions where red tide frequently occur, Chongming island which locates at Yangtze estuary is one of the most promising development space of Shanghai, and North lake is a typical brackish water lake at the north of Chongming. This dissertation investigated water quality of North lake and impacts of red tide on its environment, discussed impact mechanism of red tide , which benefits effectively exploring resource of brackish water lake in the future. Series of related research were developed and obtained preliminary results.The results of seasonal observation and analysis indicated that several parameters had evident relationships with seasonal variation. Spatial distribution differences of pH and water temperature were not distinct; spatial distribution differences of transparence, dissolved oxygen and salinity were enormous in spring but relatively small in summer and autumn. Strong correlation existed between values of DO, pH and concentrations of CDOM and Fluorescence Chlorophyll-a content, which demonstrated that their distribution were influenced by degradation of organic matters and photosynthesis of algae. It was showed that transparence had relationship with meteorology and hydrology parameters such as water depth, precipitation and wind. Water temperature is affected by temperature and prevailing wind.Seasonal variation and spatial distribution of main nutrients were investigated monthly. The results showed that particulate nitrogen, phosphorus were dominant, which followed by organic dissolved and inorganic form. Variation of N/P value indicated that North Lake had experienced stages from N-limit to P-limit, then to N/P balance. Enormous variation of N/P has relationship with rapid bloom of dominant algae. The average values of NO₃-N, NO₂-N, DOP, DIP, and surface COD_Mn was relatively high in the eastern lake. Eutrophication assessment using with modified TSIM method showed that North Lake was under the condition of oligotrophic in November, eutrophic in April and May, and mesotrophic in other months. Water quality was basically good, while red tide occurred in spring was its main environmental problem. Large-area red tide caused by Prorocentrum donghaiense Lu occurred in most parts of this lake in April, and disappeared in May, during which Chlorophyll-a content was much higher in eastern than western part of the lake. This red tide was strongly affected by many parameters such as meteorology , hydrology, nutrients and biological factors, and happened under the condition of suitable weather, good illumination, long residence time and rich nutrients. Bloom of Chaetoceros spp happened in October , which soon became the dominant speices. High nitrogen and silicate was necessary elements for Chaetoceros spp bloom, effluent of Yangtze rive and suitable temperature and salinity were influenced parameters.Fluorescence method using Pam and acetone solution extraction method were used to analyzed the concentration of Chlorophyll-a. It was found that strong correlation existed between them. However, modification is necessary when different algae employed for tests.Influence of growth and photosynthesis on Prorocentrum minimum under the condition of different nutrients level were investigated primarily. It was indicated that Prorocentrum minimum could increase rapidly under high nutrients level. Reproduce of Prorocentrum minimum required a period time to adjust the surroundings after the addition of nutrients; luxury absorbtion existed. It was estimated that the optimal N/P value was about 16, the initial concentrations of N and P were very important to the average absorb speed. Certain relationship exists between them, though nonlinear relationship was observed. The decrease of nutrients and the growth of algae fit to Monod equation basically. The peak value of Fluorescence Chlorophyll-a content of every group was achieved later than Fv/Fm. Maximum photosynthesis capability of algae was more sensitive to the environment than Fluorescence Chlorophyll-a content, the average value of Fv/Fm,qN,ETR_max was low for the low nitrogen group than that of other groups. Low nutrients can inhibit photosynthesis capability and rate of Prorocentrum minimum in some degree.