PRESERVATION OF DINOFLAGELLATE CYSTS IN SURFACE SEDIMENTS OF INNER BAYS

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For clarifying a change such as nutrients in aquatic environments, such phytoplankton communities as diatoms and dinoflagellates are very useful. In dinoflagellates the vegetative cell community is more useful than the cyst community, especially the eutrophication. Unfortunately all modern dinoflagellates can not produce a resting cyst which is preserved in sediments. Therefore, when we employ the cyst community, we have to understand a bias of dinoflagellate cyst preserved in sediments. Unfortunately there are only a few study on this subject for the moment.In this study, we collected plankton and cyst samples from Matoya Bay in Mie the 0mura Bay in Nagasaki Prefecture, where historical record of dinoflagellate vegetative cells has been kept throughout over past 30 years. In 2 sites of Matoya Bay and 1 site 0mura Bay, three core sediments were collected with a handy piston core with out serious disturbance of sediments. After measurement of 210Pb, the average sedimentation rates of these cores are estimated as 0.55cm/year (Matoya Bay St.1), 0.41cm/year (Matoya Bay St. 2) and 0.31cm/year (0mura Bay St.5). According to these sedimentation rates, the record in sediments may be back to 120, 125 and 145 years ago respectively.Dinoflagellate cyst community of St.1 in Matoya Bay consists of at least 18 genera and 34 species including several undeterminable species. In gonyaulacoid group, Lingulodinium machaerophorum (= L. polyedrum), Spiniferites bulloideus (= Gonyaulax scrippsae), Spiniferites belerius were dominated and Brigantedinium spp. (= Protoperidinium spp.) were abundant in protoperidinioid group. 32 genera and 134 species of plankton form were identified. Among them, only 9 genera and 17 species in cyst forms were corresponding with plankton species. In particular, no cysts of prorocentlales and dinophysales were recovered from the sediments. This strongly suggests that the information from cyst community is very limited in comparison with plankton community.