What is red tide?

Red tides are natural phenomena in which the rapid multiplication of microscopic, unicellular algae discolour the water by the pigments contained in the algal cells. Depending on the pigments, the massive growth of algal cells may turn the water into pink, red, brown, reddish-brown, deep green or other colours.

	Red tides in Hong Kong waters

Most species of algae do not cause threats to the environment and human, rather, they serve as the energy producers at the base of the food web.

	Skeletonema costatum	Eucampia zodiacus

The term "red tide" has been used widely to describe the phenomena in which the water is discoloured by high algal biomass. However, most red tides are harmless. On the other hand, a few algal species may cause adverse impacts such as fish kill, seafood contamination or harmful effect on humans even at low cell densities and the blooms are so-called harmful algal blooms (HABs).

Ceratium furca	Thalassiosira nordenskioeldii	Alexandrium catenella	Heterosigma akashiwo
Harmless algal species		Harmful algal species

Although red tides occur quite frequently in Hong Kong averaging about 20-30 incidents in a year, the blooms associated with harmful effects are not common and seldom caused large scale impact.