Introduction

'Ecological catastrophe' is defined as any disastrous event upon natural populations, communities, or ecosystems. Ecological catastrophes are caused by various stressors such as exposure to toxic substances, desertification, fire, and flood. Among various stressors this article presents the link between catastrophic events related to the release of toxic substances and ecological impacts. Environmental pollution and chemical accidents involving the massive amount of release of toxic chemicals have the potential to cause devastating ecological impacts (ecological catastrophe). This article focuses especially on the historical environmental pollutions or chemical accidents involving the release of massive amount of toxic substances that have the potential to cause ecological catastrophes and presents an overview of the ecological consequences actually reported in such events.

In the light of history, several events in areas such as Minamata (Japan), Bhopal (India), Seveso (Italy), Chernobyl (Ukraine), Prince William Sound (Alaska), Sandoz (Switzerland), Aznalcollar (Spain), and lately Jinlin (China) have highlighted such potential. The ecological impact observed in these areas was unique and differed greatly in the chemicals involved, the area covered, and the time periods affected. For example, in Minamata, there was chronic pollution of a rich fishery bay by the release of mercury from a chemical plant; in Bhopal and Seveso there was extensive contamination of areas with highly toxic chemicals due to an accident in chemical plants; in Chernobyl there was the most significant unintentional release of radiation into the environment due to the explosions at the nuclear power plant; in Prince William Sound, there were the significant marine oil spills due to the oil tanker Exxon Valdez accident; in Sandoz, there was pollution of the Lower Rhine River as far downstream as the Netherlands with a pesticides mixture occurring as a result of a fire at a chemical manufacturing plant; at Aznalcollar, a part of Dofiana National Park, 60 km downstream, was polluted with acidic water and a metal-rich sludge; and inJilin, the Songhua River was polluted by the release of 100 of benzene and other compounds by an explosion at a petrochemical plant. Regarding the major chemical pollutions, in many cases, the effects on human health have been a primary concern and the related literature has focused primarily on human health issues. This article examines the various aspects of ecological impacts caused by the release of toxic substances in certain major environmental catastrophic events involving Minamata, Bhopal, Seveso, Chernobyl, Prince William Sound, Sandoz, and Aznalcollar.

A part of the inorganic mercury used as a catalyst had changed into methylmercury by a side reaction in the plant. Wastewater containing both inorganic mercury and methylmercury had been discharged into the bay, and methylmercury had accumulated in the fish. Minamata disease was caused by the consumption of fish or shellfish contaminated by methylmercury from the bay. The signs and symptoms of Minamata disease, a disorder of the central nervous system, in humans include sensory disturbances of the extremities, loss of coordination, and bilateral concentric contraction of the visual field. As of March 1997, more than 2900 Minamata disease patients were certified, of whom about 2300 victims were located in the Yatsushiro Sea area. The ecological impacts in the Minamata area caused by massive mercury pollution are described below.

Minamata Bay received 70-1501 or more of inorganic mercury. Methylation takes place in sediment and in fish. Inorganic mercury in the environment can migrate into fish and accumulate as methylmercury. Almost all of the mercury found in the edible part of fish is methylmercury. In addition, it is estimated that approximately 600 kg of methylmercury had been discharged into the bay from an acetaldehyde process at the plant between 1938 and 1965. The transition of methylmercury emissions from the acet-aldehyde plant in Minamata Bay as well as the production of acetaldehyde are shown in Figure 1. The release of methylmercury into the bay has markedly continued to increase after 1950, along with an increase in acetaldehyde production, but then began to decrease after 1960. These findings suggest that the sharp increase in methylmercury, probably along with an increase in inorganic mercury emissions into the bay, resulted in ecological damage to the bay as well as methylmercury poisoning to humans during the 1950s. The trend of methylmercury release correlated well with the mercury concentrations in bivalves and some fish in the bay (described later), indicating that the reduction in

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