兵庫県播磨灘北部沿岸域における過去数百年間の珪藻群集と海洋環境の変化

摘要

兵庫県赤穂市坂越湾と相生市沖で採取した柱状試料を用いて，過去約数百年間の珪藻群集および堆積物の全有機炭素（TOC）・全窒素（TN）濃度の時空間的変化を明らかにした.さらに，珪藻群集と最近の人為汚染・改変との関連性，および1980年代以降の環境保全対策が珪藻群集に与えた影響に関して検討した.各種分析を行った結果によると，播磨灘北部沿岸域では1930年代以降に水域の富栄養化が始まり，その初期段階には珪藻群集が増加しはじめ，1960年代の高度経済成長期において水域の富栄養化がさらに進行した.1980年代以降，坂越湾と相生市沖では，異なる環境変遷をたどったことが示唆され，1990年代以降に坂越湾ではマガキ養殖の影響，相生市沖では播磨灘北部の水質改善の影響を受けたと考えられる.%~(210)Pb and ~(137)Cs 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 ~(210)Pb and ~(137)Cs 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 oc- curred 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.