Llanelli’s North Dock has had a suspected positive sampling of blue green algae.
Visitors have been advised against swimming there.
Signs were posted immediately the algae was suspected there on Friday and announcements made on radio.
Carmarthenshire County Council public protection department was informed of the possible presence of the algae by Natural Resources Wales.
Council head of public protection Philip Davies said: “Visitors, and people that normally use the area for recreation, are asked to look out for the warning signs and not to come into contact with water where the algae might be present. We advise people and pets against swimming there until further notice.
“It is important to note that people and animals can become ill after coming into contact with the algae.”
Mr Davies said: “We will continue to monitor conditions at the dock and other council owned water courses closely and will be taking regular samples to test for levels of the algae in the water.
“We are working closely with all the relevant authorities to advise them on how to minimise the risk to both the environment and local people.”
The blue green algae is a naturally forming bloom caused by sunlight on retained water areas like lakes and impounded docks.
The natural color of these algae can give bodies of water a dark-green appearance. Temperature and sun exposure dramatically influences the types and mix of blue-green algae in the water.
Llanelli North Dock has had the algal blooms every year with the exception of the last two since 2000 and Sandy Water Park had it last year. Its occurrence is unlikely in poorer weather.
Carmarthenshire council parks manager Rory Dickinson said: “There is nothing we can do but let nature take its course. The bloom will disappear as quickly as it appeared.
“Rangers noticed a greenish tinge on the water on Friday and called for testing. The tests were inconclusive and further tests will be taken this week. In the meantime signs warning of the algae have been erected as a precaution.
“We have been asked by triathlon athletes who regularly use the dock for training to flush the dock out but it is important to note the dock waters are healthier than waters in the estuary. The dock is ordinarily replenished on spring tides.”
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