This paper reports the phenomena of eutrophication and red tides correlated with human activities,especially economic development. The fertilizer production in China has been accelerated since the 1970s and has approximately doubled the rate of nitrogen input into the terrestrial N cycle. While the rate of the total population growth of China maintains relatively stable the rate of population growth in cities and towns in the 1980s and 1990s was 10 times higher than ever before. As a result, the urban sewage discharge increased by 400%, which carries 1 million tons of nitrogen into the environmental waters per year. The mining of P minerals and application of P-containing fertilizers greatly enhanced the input of P into the waters. Harmful algal blooms are consequence of the eutrophication. The high frequency of algal bloom is related to the quick economic development since the 1970s and the eutrophication of environmental waters. Analysis indicates that the number of the red tide events in the China Seas roughly follows the curve of the growth rate of GDP with a time lag of about 5-6 years. The time lag is explained as the period of the cycle of accumulation, denitrification and release of the nutrients with transportation and resuspension of the sediment.