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Stratigraphic cluster analysis using aquatic palynomorphs preserved in the core sediments revealed a rapid eutrophication due to anthropogenic activities from the mid 1960s in Beppu Bay, East Kyushu, Japan. These assemblages were divided into three major units: BP-I, BP-II and BP-III, and also only dinoflagellate cyst assemblages were divided into the following four units in Beppu Bay: BP-A, BP-B, BP-C, and BP-D. Unit boundaries based on aquatic palynomorphs and dinoflagellate cysts were different except in the upper part, BP-III and BP-D, both of which clearly indicated anthropogenic eutrophication in both sea water and bottom sediments. On the other hand, in dinoflagellate cyst assemblages, Unit BP-A was characterized by stable occurrence of  Spiniferites bulloideus  and  Spiniferites hyperacanthus, Lingulodinium machaerophorum  of Gonyaulacales, and reduction of heterotrophic Peridinioid  Brigantedinium  spp. In Unit BP-C there was a clear decrease of  L. machaerophorum.  Unit BP-B was characterized by decreases of  S. bulloideus ,  S. hyperacanthus , and  L. machaerophorum , and little increase of  Spniferites bentori.  Unit BP-C was characterized by an increase in  S. bulloideus  and heterotrophic Peridinioid  Echinidinium  spp. Unit BP-D was subdivided into Subunit BP-D1 where dinoflagellate cysts showed a marked increase in  S. bulloideus  accompanied by the appearance of  L. machaerophorum  and  Tuberculodinium vancampoae , and Subunit PB-D2 where there was a decrease of total dinoflagellate cysts. From the dinoflagellate cyst assemblages, the marine environment of the period of BP-A Unit was suggested to be warm and stable. However,  L. machaerophorum  started to decrease in BP-B. The clear decrease of  L. machaerophorum  suggest that the marine environment became cooler than that of Unit BP-A. Significant increases of  S. bulloideus, S. hypearcanthus, L. machaerophorum, T. vancampoae, Brigantedinium  spp., and  Polykrikos kofoidii  were characteristic of Unit BP-D. The increase in total dinoflagellate cyst density and the increase of the ratio of heterotrophic dinoflagellate cysts in Subunit BP-D1 are manifestations of the Oslo fjord Signal and Heterotroph Signal, respectively. In addition, the decrease in microforaminiferal lining that continued from Unit BP-C to Unit BP-D might indicate deterioration of the bottom sediment environment.

frontiers in marine science

Marine Environmental Change Induced by Anthropogenic Activities – From a Viewpoint of Aquatic Palynomorph Assemblages Preserved in Sediment Cores of Beppu Bay, West Japan