Studies Of Zooplankton Grazing Impacts On Phytoplankton In Xiamen Harbor

Species composition,abundance variety,community structure of phytoplankton and zooplankton were investigated in Xiamen Harbor in six cruises of November 2004,February,May,August,November 2005 and March 2006.Grazing by mirozooplankton on phytoplankton was quantified by using dilution method,in situ, during March,May,August and November 2005.Growth rates of different size-fractionated phytoplankton,grazing rates and food selectivity of microzooplankton and the secondary productivity of microzooplankton were estimated.The grazing rates of copepods were estimated in Xiamen Harbor in four cruises of May,August,November 2005 and March 2006,in situ,by using of gut fluorescence method.The major results were as follows:1.Species composition,abundance variety,community structure of phytoplankton78 species of phytoplankton in total were identified in Xiamen Harbor.Among these,Bacillariophyta(with 72 species representing 30 genera),Pyrrhophyta(with 5 species representing 2 genera)and Cyanophyta(with 1 species representing 1 genus) were identified.The seasonal peak of cell counts of phytoplankton(6,129×10³ cell·m^-3) was in summer,and in winter cell counts of phytoplankton(23×10³ cell·m^-3)was lowest.The seasonal succession of major dominant genera was remarkable.The first dominant genus in winter was Coscinodiscus,while both in spring and autumn were Chaetoceros.And in summer the first dominant genus was Skeletonema.The trend of annual variety in diversity index of phytoplankton was similar with that of evenness index of phytoplankton which was converse with that of cell counts of phytoplankton.2.Species composition,abundance variety,community structure of zoooplankton106 species(groups)of zooplankton were recorded,including meroplankton larvae,which were divided into five ecological groups.The average biomass of zooplankton was 325.17 mg·m^-3,while the average abundance was 3,555.43 ind·m^-3. Their seasonal dynamics showed bimodal with the first peak in May and the second in November.In the present investigation,copepod was the most dominant genus by both the number of species and abundance in total individual of all cruises.Pelagic larvae was the secondary dominant genus.The annual abundance of pelagic larvae was higher in autumn and winter,while lowest in summer.3.The growth rate of phytoplankton and microzooplankton grazing rates in Xiamen HarborThe growth rates of total phytoplankton in the surface water ranged from 0.46 d^-1 to 1.44 d^-1while microzooplanton grazing rates ranged from 0.12 d^-1to 0.86 d^-1.Both of them reached the maximum value in August and minimum value in March.The carbon flux consumed by microzooplankton per day were 19.25～965.40μg C·L^-1·d^-1 as 18.45～243.18%phytoplankton standing stocks and 31.71～75.80%potential primary productivity.Microzooplankton ingested more primary production of 0.45～20μm phytoplankton than that of 20～200μm phytoplankton.The grazing pressure of microzooplankton on primary production produced by 0.45～20μm and 20～200μm phytoplankton were 82.38～92.95%and 65.35～71.41%,respectively.The secondary production of microzooplankton ranged from 6～162μg C·L^-1·d^-1,which was 16.79～32.04%carbon flux consumed by microzooplankton per day.4.Copepods grazing rates in Xiamen HarborThe dominant species of copepods were Bestiolina amoyensis,Pavocalanus (Paracalanus)crassirostris,Paracalanus parvus,Acartia pacifica,Labidocera euchaeta and Centropages tenuiremis.The grazing rates of 10 species of copepods were determined,in situ,by using of gut fluorescence method.The range of the grazing rates was from 1.68 to 145.36 ng·ind^-1·d^-1.The annual average of daily grazing rates of the copepod population on phytoplankton was 1.69%of phytoplankton standing stock.The daily grazing rate was highest in autumn(3.15%of phytoplankton standing stock)and lowest in summer(0.06%).5.The role of microzooplankton and copepods in the energy flowing and material cycle in Xiamen HarborIn Xiamen Harbor,microzooplankton consumed 18.45～243.18%of phytoplankton standing stocks,while copepods only ingested a small part of phytoplankton standing stocks(0.06～3.15%).And furthermore,the figure of phosphorus cycle in Xiamen Harbor in spring was primitively designed. Microzooplankton was the main part of the secondary productivity in Xiamen Harbor; and the role of copepods in the energy flowing and material cycle in Xiamen Harbor need to be further studied after the investigatin of grazing rates of the copepod population on microzooplankton.