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Published: 2011-01-17 00:00:00

Hot dry conditions this summer are contributing to increased algal growth in Waikato’s waterways which could pose a risk to water users.

Environment Waikato water scientist, Bill Vant, said red dinoflagellate algae were found in high levels in two seawater samples after six surfers, all unknown to each other, each developed an itchy rash after returning from surfing at Ruapuke Beach on Waikato’s west coast.

“As well as hearing from the surfers, we have also received reports of two locals suffering diarrhoea after eating shellfish harvested on Ruapuke Beach,” Mr Vant said.

“The hot weather, combined with coastal currents, is believed to be the cause of a concentration of algae in that area,” he said.

Environment Waikato is liaising with the Waikato district health board and territorial authorities. The Waikato District Council has erected signs at the beach advising against swimming and eating shellfish.

Meanwhile, a public health warning issued by the Bay of Plenty district health board remains current after Environment Waikato’s regular testing led to a detection of blue-green algal bloom in Lake Ohakuri, between Taupo and Rotorua.

Blue-green algae are potentially toxic and are also triggered by fine, warm and calm weather. The blooms are common in shallow lakes around Hamilton and Huntly.

Environment Waikato monitoring of the Waikato River has increased to weekly due to the increased likelihood of an occurrence of blue-green algal blooms.

Latest test results show the biovolume of blue-green algae in samples from Lake Ohakuri has dropped from 3.2 to 2.6 cubic millilitres per litre of water, but remains above the recreation health guidelines of 1.8 cubic millilitres per litre.

However, there has been an increase in the amount of blue-green algae in the Maraetai and Karapiro lakes.

Lake Maraetai samples show a rise from 0.3 to 1.3 cubic millilitres per litre of water, and at Lake Karapiro there has been an increase from 0.3 to 1.4 cubic millilitres per litre of water.

Biovolume refers to both the number of cells and their size, which are taken into account as part of new guidelines for measuring and assessing levels of blue-green algae in waterbodies.

Members of the public experiencing any skin irritation or illness after swimming, or who are ill after eating shellfish harvested themselves, are encouraged to see a medical professional.