Since natural swimming pools use the same biological processes of filtration and sanitation as lakes and streams, there is a theoretical risk of harmful algae blooms as seen above.
These algae blooms have been known to close beaches, and are generally a result of excess phytoplankton growth. Blue-Green algae (Cyanobacteria) is the most common phytoplankton species involved in them, and can cause sickness in people and animals.
Luckily, scientists know the cause of these harmful algae blooms, and they can be prevented with proper planning within natural swimming pools.
The known cause of eutrophication (harmful algae blooms), is as simple as an excess of nutrients within the water. The particular nutrients of concern are nitrogen and phosphorous, which are also the core nutrients found within fertilizers.
There is a natural ratio of nitrogen to phosphorous that is present in water, which allows plant growth to occur. However, when runoff occurs bringing additional nutrient into the water in the form of fertilizer, this ratio is thrown off and phytoplankton growth is no longer limited.
With an abundance of nutrients present, phytoplankton will continue to grow while reducing oxygen present in the water, resulting in conditions that can kill plant life, and harm animal and human swimmers.
In order to prevent algae blooms from occurring in a natural swimming pool, the main focus is to prevent any surface water runoff (nutrients) from entering the pool. This is accomplished in a number of ways, and can vary depending on the topography of the yard.
The first key is to use a water proofing material on the interior of the pool, so no nutrients can enter through the soil, this is important for all installations.
Another important step for all installations is lab testing any substrate that will come in contact with the pool water (gravel in the filter), as well as the pool water itself prior to use. As expected, the main things we are looking for within the test results, are low levels of phosphates and nitrates (phosphorous and nitrogen).
Lastly, the yard surrounding the natural swimming pool must be graded to guide any runoff away from the pool. With severe slopes in yards, this sometimes requires retaining walls and/or French drains.