This is Siglap Canal, which runs between Bedok Reservoir Road and the sea off East Coast beach. Yesterday, greenish seawater below the foot bridge located between Marine Terrace housing estate and Victoria Junior College was seen during high tide.

The colour could be due to the presence of phytoplanktons, a marine algae, which contains the green pigment chlorophyll.

Phytoplankton is a flora of free-floating, often minute organisms that drift with water currents. Like land vegetation, phytoplankton uses carbon dioxide, releases oxygen, and converts minerals to a form animals can use. In fresh water, large numbers of green algae often colour lakes and ponds, and cyanobacteria may affect the taste of drinking water.

Oceanic phytoplankton is the primary food source, directly or indirectly, of nearly all sea organisms. Composed of groups with siliceous skeletons, phytoplankton varies seasonally in amount, increasing in spring and autumn with favourable light, temperature and minerals.

Phytoplankton populations in the oceans have been shown to rise and fall according to cycles lasting several years to decades.

However, scientists examining records of phytoplankton kept from 1899 to 2008 noted that phytoplankton biomass fell by 1 per cent a year in eight of earth's 10 ocean basins, resulting in a cumulative loss of roughly 40 per cent.

Rising sea surface temperatures over the same period are thought to be the primary cause of this decline.