Why Ponds Turn Green
September 18, 2015
By Mark Tancig, Leon County Public Works
Leon County Government staff ensures our local lakes, ponds, and streams are thriving and healthy by monitoring and testing our water resources. However, during hot summer months in Leon County, it is not uncommon to notice a pond, lake, or stormwater pond turn pea-soup green. Some may wonder how such a change can happen so quickly and many folks, especially those living along the body of water, want to know what can be done to get it back to a “good,” “nice,” or “pretty” color again. The how and the why are not so hard to explain. But the “What can be done?” answer is more complex.
“Why do ponds turn green?”
Ponds and other bodies of water turn green due to an abundance of algae. There are several thousand different species of algae belonging to several different taxonomic groups, but most freshwater algae belong to the green algae group. Some of our local waterbodies may also contain blue-green algae (technically bacteria from the cyanobacteria group). Green algae, like plants, use sunlight and available nutrients to grow. In the presence of abundant sun and nutrients, they can grow at an amazing rate and turn a clear water system green in a matter of days to weeks. This is what biologists call an algal bloom. During an algal bloom, there can be millions of algae cells in one cup of water. See, I told you the how and the why weren’t hard to explain.
“What can we do?”
Algae are able to grow to such numbers only when they have enough food. Again, algae use sunlight and nutrients to grow, so reducing either the amount of light or nutrients is the key. Since the sun comes up every day, we are limited in ways to reduce sunlight. There are dyes that can be safely applied to ponds that reduce how much sunlight penetrates the water and can reduce the amount of algae. However, this is a band-aid approach that will not provide a long-term solution. For a long term solution to reduce algae, the nutrients in the pond need to be addressed.
Nutrients enter the pond from the surrounding land surface that drains to it, called the pond’s drainage basin. Several steps can be taken to reduce the amount of nutrients within the drainage basin, such as reducing or eliminating the amount of fertilizer applied to landscaped areas, following all label directions when applying fertilizers, minimizing any bare or eroded areas, properly maintaining septic systems, and properly disposing of pet waste. Practices that slow the flow of runoff and increase the amount of water absorbed into the ground also help reduce the total nutrient load. These include reducing paved surfaces, incorporating mulch and mulch beds into the landscape, and amending the soil with compost. Maintaining a vegetated buffer around the waterbody can also help to filter and slow down runoff entering the pond and allows the nutrients to be taken up before they enter the water. Ponds surrounded by a thickly vegetated buffer also seem to attract fewer Canada geese and the mess (and additional nutrients) that follows them. Another way to affect nutrient levels is to plant, or simply allow to grow, desirable aquatic plants in the pond. When desirable aquatic plants are growing in a pond, they compete with the algae for the nutrients in the water and can help to reduce the algae population. In addition, they provide habitat for small, sometimes microscopic, critters that feed on algae, further reducing their population. These plants and small critters also provide habitat and food for larger animals, like fish, and you begin to create a food pyramid with algae at its base.
Leon County Government monitors and tests local waterbodies and reports the findings each year, which includes measures of algae growth in local lakes, streams, and rivers. These reports, along with other information can be found at www.LeonCountyFL.gov/Water. Leon County Public Works staff can answer questions regarding algal blooms and other water resource issues at 850.606.1500.
Mark Tancig is a Water Resource Specialist with the Leon County Public Works Department. For gardening questions, email UF/Leon County Cooperative Extension Service at Ask-A-Mastergardener@leoncountyfl.gov