Spatio-temporal changes of diatom assemblages and marine environments over the past few hundread years in coastal areas of northern Harima-Nada, Hyougo Prefecture, southwestern Japan

Abstract

²¹⁰Pb and ¹³⁷Cs age dating, total organic carbon (TOC) and total nitrogen (TN) contents analysis, and diatom analysis were conducted in sediment samples from two short cores collected from Sakoshi Bay (core HNA) and off Aioi City (core HNB) , northern Harima-Nada, southwest Japan. The results of ²¹⁰Pb and ¹³⁷Cs dating shows that ages at the bottom of core HNA and that of core HNB were estimated at 1883AD and 1786AD, respectively. Spatio-temporal changes of diatom assemblages and TOC and TN contents were as follows : In coastal areas of northern Harima-Nada, eutrophication occurred earlier than the initiation of the high economic growth period, and diatoms flourished during the 1940s-1960s. Deterioration of marine environments peaked in the 1960s-1970s. In the 1980s, Sakoshi Bay and a site off Aioi City underwent different paleoenvironmental changes. Since the 1990s, water and bottom environments in Sakoshi Bay were deteriorated by oyster farming, while water environments off Aioi City were influenced by the improvement of water quality in northern Harima-Nada.