If you have noticed a green substance in Hamilton Harbour, don’t panic — aliens have not landed. On parts of the Island, Bermuda’s usual pristine aquamarine water has turned into a murky pea soup, but this is simply due to the presence of algae.

A Government spokesman said: “We think this is a plankton bloom. The green is the chlorophyll (photosynthetic pigments) of plankton cells in the water, i.e. algae. It is normal for plankton to ‘bloom’, to multiply quickly, in spring and early summer as the water warms up and nutrients are available for them to grow and divide.

“Why it is so pronounced this year is unknown, but could be related to water temperatures being between 1.5 to 2 degrees Centigrade cooler than normal this year. This may have suppressed the bloom a bit so that when it did occur it was more pronounced.”

Tests by the Bermuda Institute of Ocean Sciences have concluded the algae poses “no significant health risk”, however, some children may experience an allergic reaction.

The Government is therefore advising children to swim beyond bloom areas, under supervision.

BIOS sea water samples collected on Friday did not show “any significant levels of sewage contamination”.

The Government spokesman said: “I understand that there is some concern that it could be caused by sewage but this is unlikely as there are no known sewage outfalls that discharge directly into Hamilton Harbour. Sewage from Hamilton and the surrounding areas is either treated and pumped deep underground or discharges via a submarine outfall onto the South Shore 900m from the shoreline, at Seabright Point near Hungry Bay.

“Although blooms such as this are normal, it is probably wise to advise not swimming in them until the species causing the bloom is identified or at least until the bloom has ‘decayed’ or diluted down.”

The green algae has also been spotted at White’s Island Beach. However, it is most common at Mills Creek, because the water is generally warmer and has the highest nutrient concentrations of the inshore bays. It also has the lowest flushing rate — the weakest flow of water in and out of a bay or harbour.