Open water swimming at Alderford Lake won't be permitted for at least a week after a routine test was carried out on Saturday. It discovered the algae bloom, which means a build up of photosynthetic particles which make the water unsafe to swim in.
The Tilstock Road attraction sees hundreds of people normally swimming there at a weekend, as well as walking around it and visiting the farm shop and restaurants, though everything else remains open.
A statement on the website read: "As a natural lake, it is not uncommon for the lake to develop algae blooms, most of which are completely unharmful.
"Luckily, we have never had a blue green algae bloom before but of course we are vigilant with our routine testing to ensure that these blooms are caught as early as possible so we have taken the decision to close the lake until we retest next week.
"The Lakeside Kitchen, walk, escape islands and play area remain open. We ask dogs are kept on leads and away from the water.
"If you have swam or used the lake over the last few days and you begin to feel unwell, please seek medical advice.
"The team are busy contacting all bookings for this weekend and please be patient, we will get to you as soon as possible. Please wait to be contacted rather than calling or emailing us."
A spokesman said a further update would be provided after another of the weekly tests, towards the end of next week.
Meanwhile tickets are on sale for Festival on the Lake which takes place on Saturday July 16 with music from Toploader, Scouting for Girls and The Feeling.
For details visit alderford.com.