Nutrient Limitation Of Phytoplankton In The Changjiang Estuary

In this paper, nutrient status of phytoplankton was studied in the Changjiang Estuary in each of the four seasons, especially in spring and autumn. Field investigation and lab experiment answered the following questions: Which nutrient limit the growth of phytoplankton, when and where does limitation happen? 1. The relationships between phytoplankton and nutrients were completely reviewed. The influences of nutrients on the physiology and biochemistry of phytoplankton cells, population growth, phytoplankton production, community structure was listed. And four methods dealing with the relationships were compared. 2. During November 16-23, 1998, community structure of phytoplankton, Chl-a, primary production and the physical and chemical environmental factors were investigated in the Changjiang Estuary. The nutrients limiting algal growth II ~fliI,l potential (AGP) were estimated by dissolved inorganic N:P ratios as well as by nutrient enrichment bioassays using Skeletonema costatum as the test algae. P was severely limiting at both stations by the method of N:P ratios, but by bioassays P was the most important nutrient regulate phytoplankton growth at station 26(1230 E, 310 N) and N at station 6(122 40?E, 310 40?N). The Changjiang Estuary may be divided into three parts: light-limited area, light- and phosphorus-limited area and nitrogen-limited area. 3.During May 11 to May 15, 1999, we took on a cruise in the Changjiang estuary and adjacent areas to determine which nutrient is the limiting factor of the growth of phytoplankton. From the analyses on the density of phytoplankton, chlorophyll a, nutrients, temperature and salinity, as well as the result of field and lab experiments, we know that P is the limiting factor in the diluted areas and is more limiting than Nov. 1998. This is due to the volume and direction of the diluted water and the vertical steady state of water. This area is divided into four parts according to the nutrient status of phytoplankton. 4. Phytoplankton density and dominant species in each season was compared. Skeletonema cosfatum is the dominant species in spring and autumn, and in the inner part of the estuary in summer, Chaetoceros lorenzianus is the dominant species in outer part of the estuary in summer; Coscinodiscus spp. dominants phytoplankton in winter. 5. The biogeochemical character of the main nutrients N and P in the Changjiang River and its estuary was analyzed. P tends to exit in the suspended matter and easy to deposit in the sediments. This is the main reason why P does not rise as pollution increases.