ABSTRACT
Chapters 3 and 4 of this volume present evidence that cyanobacterial toxins can and do cause significant adverse health effects. These effects are associ ated with the occurrence of cyanobacterial blooms. As described in Chapter 2, such blooms are a natural occurrence, but occur more frequently in waters which have been subject to certain forms of human interference. The most important types of interference are enrichment of waters with nutrients (eutrophication) from point sources such as municipal wastewater outlets and non-point sources such as agriculture, and the damming of rivers (which increases retention time and exposure to sunlight). Chapter 2 also describes how, although blooms are more frequent and severe in eutrophic waters, they may occur in waters which would be considered in many world regions to be of good or acceptable quality. Of more concern is the fact that the available evidence concerning trends in eutrophication indicates that the situation is severe world-wide (see Table 6.1), although it is improving in some regions whilst deteriorating in others.
