Competitors had been due to kayak over three miles on Lochnindorb, north of Grantown-on-Spey, as part of this year's Wicked Wolf Triathlon on July 21, which also involves an 18.6-mile mountain bike ride and a run stretching more than six miles.
However, the kayaking has been dropped because of the presence of the algae blooms in the loch.
If swallowed, blue green algae can cause stomach upsets or more serious health problems and can also cause skin irritations.
Organiser Gemma Powell said: "We have been monitoring the situation and have taken advice from environmental health officers, hoping that the algae would disperse before our event, but unfortunately it is still present.
"The Wicked Wolf is still set to be a really enjoyable event and we hope that participants will not be put off. We are replacing the kayak section with a fun 3k run (1.86 miles) along the loch side."
The event is named after the Wolf of Badenoch, Alexander Stewart, who won a fearsome reputation in the 14th century as Lord of Badenoch for terrorising the northern counties and sacking Elgin Cathedral.
Algae in Scotland's waterways is not new, but it can be treated. A recent study showed that Loch Leven, Scotland's largest lowland loch is now cleaner than at any time in the last 20 years when it was blighted by algae.
According to the Scottish Environment Protection Agency, algal blooms can result from a combination of natural factors, including availability of nutrients and light, water temperature and wind conditions. Typical nutrient sources from human activity in Scotland include discharges from sewage works, industry, and agriculture.