Identification of autochthonous, allochthonous, and reworked (derived fossil) taxa in diatom analyses is of fundamental importance for understanding how taphonomic processes affect diatom assemblages and for estimating paleoenvironments in coastal areas. To characterize modern and Neogene diatom assemblages, we surveyed the distributions of modern diatom assemblages and of Neogene fossil diatom assemblages in the Toberi River area, eastern Hokkaido, Japan. Then, on the basis of these results, we analyzed two cores collected near the Toberi River mouth and examined changes in the fossil diatom assemblages during the late Holocene. The results showed that not only allochthonous and extinct diatom taxa, but also extant taxa that have persisted over an extended geological time period, should be excluded when diatom assemblages are analyzed to reconstruct Holocene paleoenvironments in this area.

在硅藻分析中鉴定自生，异源和返工（衍生的化石）分类群对于理解分子过程如何影响硅藻组合以及评估沿海地区的古环境具有至关重要的意义。为了表征现代和新近纪的硅藻组合，我们调查了日本北海道东部Toberi河地区的现代硅藻组合和新近纪化石硅藻组合的分布。然后，根据这些结果，我们分析了在Toberi河口附近收集的两个岩心，并研究了全新世晚期硅藻化石组合的变化。结果表明，不仅异源和灭绝的硅藻类群，而且在延长的地质时期内仍存在的现存类群，