A study on grazing speed of mesozooplankton on phytoplankton and microzooplankton was carried out with in situ feeding experiment, the Frost′s method, and the Landry dilution experiment in 6 stations in area of frequent red tide occurrence in the East China Sea, from April 27 to June 5, 2005. Changes in community structure of phytoplankton and microzooplankton, grazing speed, food composition of Calanus sinicus, and mesozooplankton and microzooplankton grazing pressures on phytoplankton were studied.
In the survey area, the specific growth rate of phytoplankton species ranged from 0.09-5.70 at (2.01±1.33) d^-1 in average; and that of microzooplankton ranged from 0-3.89 in average of (0.46±0.79) d^-1. The species specific grazing rate of C. sinicus on phytoplankton varied from 0.01-8.43 in average of (2.72±2.14) d^-1 in speed of 0.05-838.23 averaged at (52.72±154.21) cells ind.^-1d^-1. C. sinicus took mostly some harmful algal blooms species, such as Karenia mikimotoi, Pseudo-nitzschia delicatissima, Pseudo-nitzschia pungens, Chaetoceros curvisetus, Scrippsiella trichoidea, and Prorocentrum dentatum. The grazing speed of C. sinicus on phytoplankton was food-density dependent, increasing with phytoplankton abundance up to a threshold value, and then decreasing regardless of the abundance increase. Gazing speed of C. sinicus on phytoplankton community carbon ranged from 26.94-1495.98, averaged at (326.15±575.28) ngC ind.^-1d^-1, whereas that of C. sinicus population did on the carbon ranged from 0.42-3.25 in average of (1.54±1.14) ngC L^-1 d^-1.
Species specific grazing rate of C. sinicus on microzooplankton ranged from 0.01-8.52 in average of (3.13±2.16)d^-1, in speed of 0-21.18 in average of (3.83±5.98) ind.^-1d^-1. C. sinicus often grazed fast on dominant microzooplankton species, such as Codonellopsis mobilis, Favella campanula, Tintinnopsis beroidea and Strombidium sulcatum at a speed positively related to microzooplankton abundance. Gazing speed of C. sinicus on microzooplankton community carbon ranged from 7.12^-110.85 averaged at (35.37±40.06) ngC ind.^-1d^-1, whereas that by C. sinicus population were from 0.01^-1.93 in average at (0.57±0.72) ngC L^-1 d^-1.
Food composition of C. sinicus was mostly phytoplankton, taking 45.17%-98.83 % in average of 77.28%±1920% of the total food. Microzooplankton was less taken but it was a major food source in some places. Gazing speed of mesozooplankton on phytoplankton community carbon ranged from 0.53-4.97 in average of (2.16±1.63) ngC L^-1 d^-1. Mean species gazing speed of microzooplankton on phytoplankton community carbon ranged from 0.04^-13.20 averaged at (2.91±5.22) ngC ind.^-1d^-1 in speed of 61.07-8632.85 averaged at (2801.01±4198.46) ngC L^-1 d^-1. Comparison in grazing pressure on phytoplankton by mesozooplankton and microzooplankton, the microzooplankton built much higher pressure on standing phytoplankton crop than that mesozooplankton did, taking nearly 98% of total grazing pressure by herbivorous zooplankton in the study area. Our results showed that the mesozooplankton abundance in the area controlled phytoplankton growth by cascade grazing on microzooplankton.