Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 40

suffocate. Some harmful algae, like dinoflagellate, also produce toxic material and may transfer the toxic
to humans through the food web. The 1998 red tide in the Bohai Sea of China spread over a 50 thousand
km 2 area, killing many fish and causing $80 million in losses.
Fig. 1.34 Pollution causes the bad odor the Weihe River water and kills all fish (See color figure at the end of this topic)
Algal blooms rarely occurred in seas near China before the 1980s because the nutrients supply was not
sufficient. Eutrophication due to intensified human activities in recent decades has released the nutrient
constraint and red tide has occurred quite often. Figure 1.35 shows the number of red tide events per
decade from the 1950s to 1990s. Almost no red tides occurred in the 1950s and 1960s but the number of
red tide events increased in an explosive way to more than 240 events in the 1990s. Comparing Fig. 1.35
with Fig. 1.33 it can be seen that the red tide events follow almost the same growth curves of urban
population and sewage discharge. The growth curves exhibit the same law with a turning point about 1980,
when the economy of China took off. There is no doubt that the high frequency of red tide events is a
result of extremely high rates of fertilizer usage and wastewater discharge, which are in turn the results of
the high speed of economic development and industrialization of china. If the trend continues, most of
sea life in the seas near China will be extinct and the seas will become seas of death.
Fig. 1.35 The number of red tide events per decade occurring in the seas near China in the period of the 1950s-1990s