Annual changes in Clytia hemisphaerica abundance was analyzed, according to the monthly zooplankton samples collected using 50cm diameter net （mesh size 500μm） in 2009--2010. The grazing pressure on zooplankton popu- lation in the Jiaozhou Bay was also calculated via the feeding rates of C. hemisphaerica. As far as seasonal variation of abundance and the grazing pressure on zooplankton were concerned, C. hemisphaerica became one of the dominant species that peaked once a year, i.e., in May and June, 2009 and June, 2010. Its abundance values were 34.5, 41.3, 45.3ind/m3 during these three periods, accounting for 74.0%, 95.2%, 48.9% of the total medusa population, respectively. In May and June, 2009, the daily grazing pressure values on zooplankton were 0--28.08%, 0--74.99%, respectively; while the grazing pressure was 0--105.96% in June, 2010. In May and June, 2009, C. hemisphaerica was mainly distributed near the Bay mouth, outside and center. The daily grazing pressure on zooplankton in D6 station was up to 74.99% in June, 2009. In June, 2010, C. hemisphaerica was mainly distributed in western of Jiaozhou Bay, in which the daily grazing pressure on zooplankton of C 1 station reached 105.96%. The outbreak of C. hemisphaerica seriously impacts the zooplankton commu- nity structure. However, in other months of 2009 and 2010, C. hemisphaerica significantly diminished, exerting minimal impact on the zooplankon population.%利用2009、2010年胶州湾浅水I型浮游生物网的逐月调查资料，分析了半球美螅水母数量周年变动；并结合实验测定的摄食率，估算了其对胶州湾浮游动物的摄食压力。结果表明，2009、2010年半球美螅水母丰度，以及该种对浮游动物生物量摄食压力具有明显的季节性。2009年5、6月．2010年6月分别为该种的数量高峰期，成为胶州湾浮游动物优势种之一，丰度分别为34．5、41．3、45．3ind／m2，分别占当月水母总丰度的74．0％、95．2％、48．8％。其对浮游动物生物量日摄食压力分别为0—28．08％、0—74．99％、0—105．96％。2009年5、6月该种在湾口、湾外附近以及湾中心均出现了密集区；其中，6月份，D6站位半球美螅水母对浮游动物生物量的日摄食压力高达74．99％；2010年6月，该种在湾西附近出现了密集区，其中C1站位该种对浮游动物生物量日摄食压力高达105．96％；严重影响了胶州湾浮游动物的群落结构。2009、2010年其它月份该种丰度显著降低，对浮游动物的影响很小。
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