Red tides and phytoplankton

Phytoplankton are tiny plants floating in the sea that are highly responsive to changes in nutrients such as nitrogen and phosphorus in the water. When large amounts of nutrients are present, a process called 'eutrophication' can take place in which some phytoplankton species rapidly reproduce, causing widespread algal blooms some of which take the form of 'red tides'. These red tides can be harmful to fish culture, as well as affecting marine recreation and other water uses.

Prorocentrum sigmoides -an important diatom species in Hong Kong waters

Overall, the aim of the phytoplankton monitoring programme is threefold: first, to identify changes in the phytoplankton community (e.g. changes in dominant phytoplankton species or in the abundance of their populations); second, to pinpoint, where possible, any links that may exist between these changes and nutrient levels in marine water; and thirdly, to detect the presence of any toxic species of phytoplankton.

The phytoplankton monitoring programme was set up in response to a significant increase in red tides during the 1980s, and was originally tasked to study phytoplankton populations in specific places where red tides frequently occurred, such as Tolo Harbour. It now covers all Hong Kong's territorial waters, and forms an integrated part of the Marine Monitoring Programme.

Today, the EPD monitors phytoplankton populations at 25 stations across nine WCZs, taking monthly samples from one metre below the surface of the water. In the laboratory, scientists identify and count the phytoplankton species in each sample, and compare the results geographically between stations and over time.

The EPD's monitoring of phytoplankton has thrown considerable light on the complex relationships between water, temperature, nutrients, and phytoplankton growth. It has helped the EPD build up a major database of information about the more than 200 phytoplankton species found in Hong Kong waters, of which around 81 are known to cause red tides. In particular, the programme has helped assess the effectiveness of the Government's efforts to reduce nutrients in marine water, especially in sensitive areas such as Tolo Harbour. Following the implementation of the Tolo Harbour Action Plan in 1988, for example, the number of red tide incidents in the harbour declined from a peak of 43 in 1988 to just 12 in 2005. Overall in Hong Kong waters, 1988 represented a peak (88 red tide incidents were reported), since which time the number has slowly come down to around 30 a year in recent years. Fish kills relating to red tides are rare these days compared with the 80s and 90s.

Red tide incidents in Tolo Harbour (1986-2005)