Preliminary Studies On Cascade Grazing Of Mesozooplankton On Phytoplankton And Microzooplankton Community In The Jiaozhou Bay

A preliminary study on grazing speed of different size mesozooplankton on phytoplankton and microzooplankton was carried out with feeding experiments, the Frost's method, and the Landry dilution experiment in 2 stationsin the Jiaozhou Bay, from December 2007 to November 2008. Changes in community structure of phytoplankton and microzooplankton, grazing speed, food composition of mesozooplankton with different size range, and mesozooplankton with different size range and microzooplankton grazing pressures on phytoplankton were studied.During the survey, there were 76 phytoplankton species found in the station A, which mainly were composed of diatom. The eell abundance of phytoplankton changed dramatically in different month and the maximum value was 1301.52×10³ cells L.^-1, which appeared in July, while the minimum was 17.44×10³ cells L^-1, which took place in April. The dominant species were Skeletonema costatum, Thalassiosira nordenski(?)ldii and Eucampia zodiacus. There were 76 phytoplankton species found in the station A, which mainly were composed of diatom as the same as station A. The seasonal variation of cell abundance of phytoplankton was a typical double period's type in station B. The first highest peaks appeared in January,the cell abundance was 1257.48×10³ cells L^-1.Next highest peaks was March, the cell abundance was 1137.67×10³ cells L^-1,lowest peaks appeared in June and November, the cell abundance separately was 11.32×10³ cells L^-1 and 21.77×10³ cells L^-1. The dominant species were Skeletonema costatum, Leptocylindrus danicus and Chaetoceros affinis. The dominant microzooplankton species were Tintinnopsis beroidea, Stromdium sp. and Copepod naupli. The abundance was lowest and averaged 0.68×10⁶ ind m^-3 for microzooplankton in November, while there was a maximum value (6.20×10⁶ ind m^-3) in June. For station B, the highest peaks appeared in March, which averaged value was 4.44×10⁶ ind m^-3, while the lowest peaks appeared in June, which averaged value was 0.49×10⁶ ind m^-3.Through the Landry dilution experiment, the instantaneous growth rate of phytoplankton ranged from 0.38-2.58 d^-1 at 0.99 d^-1 average; ingestion rate of microzooplankton on phytoplankton varied from 0.12-2.23 d^-1 in average of 0.77 d^-1; the percentage of phytoplankton standing crop ingested by microzooplankton ranged from 19.49-1179.88% at 225.22% average; and the percentage of phytoplankton potential production ingested by microzooplankton varied from 25.96-102.06% in average of 72.74%. For station B, the instantaneous growth rate of phytoplankton ranged from 0.45-2.21 d^-1 at 1.12 d^-1 average; ingestion rate of microzooplankton on phytoplankton varied from 0.07-2.12 d^-1 in average of 0.95 d^-1; the percentage of phytoplankton standing crop ingested by microzooplankton ranged from 10.71-771.55%% at 253.21% average;and the percentage of phytoplankton potential production ingested by microzooplankton varied from 19.05-104.86% in average of 78.14%.Compared with the similar studies in other waters around the world,the grazing pressure of microzooplankton in the Jiaozhou Bay is at the middle levels.Through the Frost's method ,for station A, the specific grazing speed of the smaller size mesozooplankton on phytoplankton varied from 15.13-556.67×10³cells ind.^-1d^-1 ind.^-1 d^-1, in average of 158.48×10³cells ind.^-1 d^-1, while the specific grazing speed of the bigger size mesozooplankton on phytoplankton varied from 22.32-637.01×10³cells ind.^-1d^-1, in average of 176.12×10³cells ind.^-1 d^-1. For station B, the specific grazing speed of the smaller size mesozooplankton on phytoplankton varied from 15.28-1360.16×10³cells ind.^-1d^-1, in average of 296.63×10³cells ind.^-1d^-1, while the specific grazing speed of the bigger size mesozooplankton on phytoplankton varied from 12.72-741.93×10³cells ind.^-1d^-1, in average of 195.27×10³cells ind.^-1d^-1. The grazing speed of different size mesozooplankton on phytoplankton was food-density dependent, increasing with phytoplankton cell abundance up to a threshold value,and then decreasing regardless of the cell abundance increase. For station A, the species specific grazing speed of smaller mesozooplankton on microzooplankton varied from 0.37-6.19×10³ ind. ind.^-1 d^-1, in average of 2.21×10³ ind. ind.^-1 d^-1, the species specific grazing speed of bigger mesozooplankton on microzooplankton varied from 0.49-10.07×10³ ind. ind.^-1 d^-1, in average of 3.39×10³ ind. ind.^-1 d^-1, for station B, the species specific grazing speed of smaller mesozooplankton on microzooplankton varied from 0.30-9.12×10³ ind. ind.^-1 d^-1, in average of 1.81×10³ ind. ind.^-1 d^-1, the species specific grazing speed of bigger mesozooplankton on microzooplankton varied from 0.19-10.40×10³ ind. ind.^-1 d^-1, in average of 2.06×10³ ind. ind.^-1 d^-1. Food composition of different size mesozooplankton was mostly phytoplankton, for station A, taking 27.44-97.49% ,in average of 66.96% of the the smaller mesozooplankton total food, and taking 17.79-97.38%, in average of 59.20% of the the bigger mesozooplankton total food;for station B, taking 46.54⁹8.08%,in average of 81.68% of the the smaller mesozooplankton total food, and taking 48.09-98.63% ,in average of 79.02% of the the bigger mesozooplankton total food. Microzooplankton was less taken, but it was a major food source in some month. For station A, the grazing pressure of smaller size mesozooplankton community on phytoplankton community standing crop varied from 0.40-6.16 %, in average of 1.56%, the grazing pressure of microzooplankton community on phytoplankton community standing crop varied from 93.84-99.60 %, in average of 98.54%,and the grazing pressure of bigger size mesozooplankton community on phytoplankton community standing crop varied from 0.17-3.61%, in average of 0.95%, the grazing pressure of microzooplankton community on phytoplankton community standing crop varied from 96.39-99.83%, in average of 99.05%;for station B, the grazing pressure of smaller size mesozooplankton community on phytoplankton community standing crop varied from 1.36-29.85%, in average of 5.72%, the grazing pressure of microzooplankton community on phytoplankton community standing crop varied from 70.15-98.64%, in average of 94.28%,and the grazing pressure of bigger size mesozooplankton community on phytoplankton community standing crop varied from 0.30-17.03%, in average of 3.01%, the grazing pressure of microzooplankton community on phytoplankton community standing crop varied from 82.97-99.70%, in average of 96.99%.Comparison in grazing pressure on phytop lankton by mesozooplankton and microzooplankton,the microzooplankton built much higher pressure on standing phytoplankton crop than that mesozooplankton did in the Jiaozhou Bay.