What factors control the distribution of aquatic palynomorphs in surface sediments of Osaka Bay, central Japan? With special reference to the cysts of <i>Alexandrium catenella/pacificum</i> (Dinophyceae)

Distribution contour maps of aquatic palynomorphs in Osaka Bay were made to determine their horizontal distribution using 21 surface sediment samples. The aquatic palynomorphs consisted of planktonic autotrophic dinoflagellate cysts, raphidophyte cysts, heterotrophic dinoflagellate cysts, heterotrophic ciliate cysts, heterotrophic crustacean resting eggs, benthic foraminiferal linings, turbellarian egg capsules, as well as planktonic chlorophycean resistant cells. Most of these resistant cells are thought to have been transported via the Yodo River (Yodogawa), since most of them originated from freshwater areas, mainly Lake Biwa. Most of these palynomorphs were distributed mainly off the mouth of the Yodo River, while benthic foraminiferal linings were distributed mainly off Kobe Airport. A canonical correspondence analysis between each palynomorph and environmental factors including distance from the mouth of the Yodo River (Distance A), water depth, dissolved oxygen (DO), surface water temperature, salinity, transparency, grain size composition (Mdφ), chemical oxygen demand (COD), nitrogen, phosphorus, and chlorophyll was performed. The results showed that the autotrophic dinoflagellate Spiniferites bulloideus and foraminiferal linings were strongly positively correlated with Distance A and Mdφ but negatively correlated with DO, whereas cysts of Alexandrium catenella/pacificum, which often form red tides with paralytic shellfish poisoning in Osaka Bay, were positively correlated with water depth and negatively correlated with COD. Turbellarian egg capsules, heterotrophic dinoflagellate cysts, Echinidinium spp., and foraminiferal linings were positively correlated with salinity and transparency. However, these results do not logically explain the relationship between the distribution of these palynomorphs and environmental factors, because the distribution of aquatic palynomorphs in a narrow sea area with complex marine environments such as Osaka Bay is caused by a combination of biological factors and physical factors that act after the formation of palynomorphs.