Microzooplankton And Its Grazing On Phytoplankton In Taiwan Strait And Xiamen Waters

The variations of abundance,biomass,species(or groups)composition of microzooplankton,phytoplankton growth rates as well as microzooplankton grazing rates were studied in Xiamen Waters and Taiwan Strait by means of seasonal,monthly and biweekly investigations from 2003 to 2005,meanwhile,the role of microzooplankton grazing in controlling phytoplankton bloom of the field and mesocosm in the Taiwan Strait was conducted and the impact of mesozooplankton grazing on microzooplankton and phytoplankton was discussed.Results are as following:1.There were remarkable seasonal variations in abundance and biomass of microzooplankton in Xiamen Waters,they were the lowest in February and the highest in May. The average abundances of the whole waters were 466,645,3535 and,2336 ind.1^-1in February, May,September and December,respectively,while mean biomass were 2.00,15.8,11.18 and 3.58μgCl^-1in the same seasons,respectively.It was generally the peak appearing in the western Xiamen Waters,especially in Baozhu Waters.At Stns 18 and 27,the biweekly variations in abundance and biomass of microzooplankton showed that they obviously increased from April with higher values during May to Sep.when water temperature was above 25℃,then decreased,with the lowest in winter.Water temperature was the most important factor for such seasonal variations of mcirozooplankton,especially for tintinnids and copepod nauplii.The total abundance and biomass were in the range of 120～3920 ind.1^-1and 0.61～35.2μgC1^-1 respectively at Baozhu Waters(Stn 18);227～4590 ind.1^-1and 0.95～27.99μg1^-1respectively off Hulishan Waters(Stn 27).There were more higher abundance and more species in tintinnids during July to September,in naked ciliates during May to July,especially in Baozhu Waters. Among tintiunids,Tintinnopsis amoyensis,Tintinnopsis tentaculata,Tintinnopsis minima, Tintinnopsis tocantinensis,Tintinnopsis radix and Leprotintinnus nordquisti were the prominent species in summer and autumn;Tintinnopsis brevicollis and Tintinnopsis gracilis were the prominent species in winter and spring.Among naked ciliates,Tontonia sp.and Laboea sp.were the prominent species in summer,Strombidium spp.was the prominent species in winter and spring.2.The phytoplankton growth rates showed the large fluctuations over an annual cycle,in range of 0.34～2.32d^-1,lowest in February,highest in May,and so did microzooplankton grazing rates,in the range of 0.27～1.67d^-1,lowest in March,highest in June.There were the significantly positive relationship with temperature both growth rates and grazing rates,besides, grazing rates was also positive relationship with chla.Grazing pressure on phytoplankton standing stock and phytoplankton primary productions were 23～81%and 35～118%,respectively over a year.Except January,growth rates were all more than grazing rates, indicating that microzooplanton not completely controlling the phytoplankton growth.3.The clearance rates of Centropages tenuiremis(dominant copepod in winter)for ciliates and phytoplankton were 3.08 and 0.31ml ind^-1h^-1respectively,the ingestion rates were 0.44 and 0.24μgC ind^-1d^-1.The ratio of the ciliates to the diet of Centropages tenuiremis was between 65%and 95%,microzooplankton played the nutritional link between phytoplankton and copepods in Xiamen Waters during winter.Microzooplankton mainly grazed on nano-phytoplankton,not efficiently grazed on micro-phytoplankton,the presence of copepods could not always enhance the algal growth,but may still show grazing on whole size phytoplankton.These results showed there were complicated relationship among phytoplankton,mirozooplankton and copepods and the ultimate effects was determined by various factors.4.The results showed that microzooplankton was dominated by aloricate cilicates, especially for size of＜40μm in July 2004 in the Taiwan Strait.In March 2005,＜40μm aloricate cilicates was still the most abundant component of microzooplankton,the percentage of＞40μm aloricate cilicates decreased obviously,especially in Transect B.Mesodinium rubrum and heterotrophic dinoflagellate(Gyrodinium spirale)were also relatively abundant in March 2005.Tontonia sp.and Strombidium sp.were dominant species of＜40μm aloricate cilicates,while larger Tontonia sp.for＞40μm.The standing crop of microzooplankton varied with depths and seasons,the depth-integrated values ranged from 19.1 to 69.9×10⁶ ind.m^-2and from 6.2 to 48.1×10⁶ ind.m^-2in abundance,from 23.0 to 152.4mgCm^-2and from 15 to 112.9 mgCm^-2in biomass in July-Aug 2004 and March 2005,respectively.The average of the depth-integrated values varied from 104 to 1678 ind 1-^-1and 233 to 1204 ind 1^-1in abundance, from 0.21 to 3.8μgCl^-1and 0.60 to 3.47μgCl^-1in biomass in July 2004 and in March 2005, respectively.For vertical profiles,microzooplankton was dense at surface waters for all stations in March 2005,while at surface waters or the DCM(deeper chla maximum)in July-Aug 2004.The diel variation of total microzooplankton was mainly due to tide,but copepod nauplii and tintinnids had still the behavior of migrating up at night time,naked ciliates always accumulated at near surface.At Stns F1 and F3,abundance of total microzoopankton and Mesodinium rubrum and naked ciliates decreased by the decline of the upwelling day by day,but Mesodinium rubrum and＞40μm naked ciliates always had the maximum at surface,＜40μm naked ciliates was mainly located around DCM layer,copepod nauplii increased a little and mainly located in middle and bottom layer.5.The phytoplankton growth rates and grazing rates were 0.52～0.72d^-1and 0.45～1.33d^-1 in Taiwan Straits in July 2004 respectively,equally to grazing 36%～74%of phytoplankton biomass or 88%～141%of the primary production per day.The phytoplankton growth rates and grazing rates were 0.36～0.50d^-1and 0.12～0.30 d^-1respectively in March 2005,equally to grazing 11～26%of phytoplankton biomass or 30～84%of the primary production per day. The phytoplankton growth rates and grazing rates were 0.43～1.38d^-1and 0.30～1.12d^-1 respectively in July 2005,equally to grazing 23～67%of phytoplankton biomass or 41～90% of the primary production per day.Higher temperature was responsible for higher growth rates in July 2004 than that in March 2005.Moreover,low temperature and high nutrients induced by upwelling and abundant Noctiluca sp.might be responsible for higher growth rates and lower grazing rates in July 2005 when comparing with those in July 2004.6.No general trends were found in the selectivity of grazing on particular phytoplankton groups among the experiments in July 2005.As a whole,microzooplankton avoided dinoflagellates and green algae,mostly preferentially grazed cryptophytes,chrysophyceae and cyanobacteria,no selectively grazed diatom,sometimes avoided grazing.7.Results showed the diatom bloom was in decline during the observation at Stn F1, Pseudo-nitzschia pungens,Asterionellopsis glacialis and Skeletonema costatum were the dominant species,chla concentration decreased from 2.04μg/L on August 1 to 0.37μg/L on August 6,but the abundance and percentage of eyanobacteria and heterotrophic dinoflagellate increased gradually,from 0.04%to 9.59%and from 0.85%to 41.97%respectively. Microzoopankton was mainly composed of aloricate ciliates,loricate ciliates,Mesodinium rubrum and heterotrophic dinoflagellates,the highest abundance was up to 3640 ind/L on August 2,then decreased gradually.There was a significantly positive linear correlation between microzooplamkton grazing rates and chlorophyll a(R²=0.89).Pigment-based data showed significant gazing pressure on biomass and productivity of different pigments.The grazing of micrzooplankton,especially heterotrophic dinoflagellate was one of the important factors of diatom bloom decline.Microzooplankton rapidly responded to phytoplankton bloom in the mesocosm experiment,indicating controlling on phytoplankton boom by increasing grazing rates and abundance at pre-bloom and grazing rates more than growth rates at post-bloom,but microzooplankton might lose controlling on phytoplankton boom.during chla peak of bloom,...