ZOOPLANKTON IN THE BOHAI SEA IN SPRING AND AUTUMN

The zooplankton community was studied in September, October 1998 and in April, May 1999 in the Bohai Sea. The zooplankton samples were collected by vertically towing a medium zooplankton net (mesh size 160μm). Forty seven species were found in the autumn of 1998 while the number was only 27 in the spring of 1999. In the four investigations, the abundance of zooplankton (excluding Noctiluca scintillans ) was 136-31270, 558-6719, 508-13843 and 521-12945 ind/m 3, respectively (the mean abundance among 30 stations was 4076, 3377, 3455 and 5349 ind/m 3, respectively). Fifteen species occupied more than 98% of the zooplankton abundance. The abundance of copepod was 90.11%, 88.86%, 76.94% and 85.29% of that of the total zooplankton in the four investigation, respectively. The Paracalanua parvus and Oithona similis were dominant species in the autumn of the year 1998. The abundance of P. parvus was 6.8-30902 ind/m 3 (on average 2715.0 ind/m 3) and 60-4050 ind/m 3 (on average 1787.9 ind/m 3) in September and October, respectively. That was 67% (range 0%-96%) and 53% (range 11%-77%) of the zooplankton abundance. With abundances of 0-8316 ind/m 3 (on average 645.4 ind/m 3) and 32-3679 ind/m 3 (on average 789.8 ind/m 3), O.similis occupied 16% (0%-65%) and 23% (1%-63%) of the zooplankton abundance in September and October, respectively. The copepods Acartia bifilosa and Centrapages abdominalis were dominant species in the spring of the year 1999. In April, the abundance of A. bifilosa ranged between 0-8389 ind/m 3 (on average 1467.2 ind/m 3), which was 0%-98% (on average 42%) of the total zooplankton abundance. The figures for C. abdominalis were 0-3552 ind/m 3 (on average 761.7 ind/m 3) and 0%-70% (on average 22%). The abundances of the above copepods increased to 0-11941 ind/m 3 (on average 2600.9 ind/m 3) and 3-8238 ind/m 3 (on average 1328.0 ind/m 3), respectively, which was equivalent of 0%-97% (on average 49%) and 0%-70% (on average 25%) of the total zooplankton abundance. Another noteworthy characteristic was that Lamellibranchiata larva and Nauplius larva were abundant in the spring. The abundance of small zooplankton such as Oithona similis, Paracalanus parvus and Acartia bifilosa in the large zooplankton net towing samples were much lower than those in the medium zooplankton net towing samples. Therefore, the analysis of the medium zooplankton net towing samples should be paid more attention.