Are changes in N:P ratios in coastal waters the key to increased red tide blooms?

There is mounting evidence of a global increase in nutrient levels of coastal waters through riverine and sewage inputs, and in both the numbers and frequency (as well as the species composition) of red tides. However, it is still not possible to conclude the extent to which the increase in red tides in coastal waters can be attributed to the increase in nutrient levels, since so many other factors are involved. Undoubtedly, a relationship exists between red tides and the N and P load of coastal waters, and many nutrient enrichment experiments have shown that marine phytoplankton blooms are often nutrient limited. What is now becoming clear, however, is that although in classical Liebigian terms minimum amounts can be limiting, nutrient ratios (such as N:P and Si:P) are far more important regulators. This paper reviews evidence collected by the authors from Tolo Harbour, Hong Kong together with data collected in Japanese and North European coastal waters by various authors, which indicates that both long term and relatively short term changes in the N:P ratio are accompanied by increased blooms of non-siliceous phytoplankton groups and, furthermore, that the growth of most red tide causative organisms in Hong Kong coastal water is optimized at a low N:P (atomic) ratio of between 6 and 15.