Hong Kong's algal blooms - red alerts from nature
Environmental degradation is the most likely cause of a flood of colourful tides that washed up on Hong Kong's shores last month
The end of November saw areas of inshore waters across Hong Kong and east Shenzhen turn improbably colourful due to extensive red tides. Nighttime scenes were even more arresting, as seawater stirred by ferries flashed blue-green with luminescence.
The main phytoplankton in the blooms even turned out to be a bizarre microorganism.
It's scientifically fascinating but the red tides might be a sign that intense development in and around Hong Kong is increasingly unsustainable.
Red tides are formed by blooms of microscopic algae. They can occur naturally but they are often linked to pollution, such as phosphates from sewage and fertilisers. In the first six months, red tides were recorded on 35 days, reportedly the most since 1988.
A few red tide algae contain highly toxic chemicals. In 1998, one such species caused massive fish kills at fish farms. Otherwise, red tides are mostly harmless.
The Agriculture, Fisheries and Conservation Department said the main species causing November's red tides was Noctiluca scintillans. It's a dinoflagellate - a single-celled organism with two tiny whip-like structures, making it a feeble swimmer. Though unable to photosynthesise, N. scintillans can sometimes have a green algae cell inside, and form green blooms.