Abstract:The compositions of particulate organic carbon (POC), fatty acids (FA) and chlorophyll-a (Chl-a) and their correlations with phytoplankton groups were studied during a major diatom (Skeletonema costatum (Grev.) Cleve) bloom in July, August and September of 2005 in Jiaozhou Bay. The concentrations of Chl-a and POC peaked in the bloom stage (August) and the percentage of Skeletonema costatum (Grev.) Cleve reached to 86% in Station A5 and 59% in Station C3, dominated the phytoplankton species. Total particulate FA, followed the patterns of POC and Chl-a, also showed the highest concentrations in the bloom stage and that were 2-3 times and 20 times higher at the two sampling stations C3 and A5, respectively, compared with the values measured in the pre- and post-bloom months (July and September). The fatty acids were mainly composed of 16∶0, 14∶0, 18∶0; 16∶1ω7, 16∶1ω5+ω9, 18∶1ω9, 18∶1ω7; 20∶5ω3 (DHA), 22∶6ω3 (EPA) and i/ai-15∶0. Monounsaturated FA (MUFA) and polyunsaturated FA (PUFA) fractions were higher in the bloom stage than that in the pre-bloom stage at both stations. In the post-bloom stage, however, the percentages of straight chain saturated FA (SSFA) and branched chain saturated FA (BSFA) all increased while the MUFA and PUFA decreased significantly at Station A5, but increased conspicuously at Station C3, especially the ω3 components. The fate and composition changes of particulate FA indicate that phytoplankton (diatom) was the major contribution during the bloom month at both stations, while detritus and bacteria contributed significantly in the post-bloom stage at Station A5. It is suggested that the death of phytoplankton cells after the bloom largely reduced algal abundance in the diatom bloom at Station A5, but predation of zooplankton appeared to be the controlling factor for disappearance of the bloom at Station C3.