Nutrient loading from lawn fertilizer runoff is a major factor in promoting algae growth in urban ponds. It is ironic that the commercial products that make your lawn so lush and green are the same products that make your pond green and smelly. Fertilizer (phosphorus and nitrogen) fuels the out-of-control growth of algae and aquatic plants. Without nutrients, these pests are unable to grow and flourish. Phosphorus is the limiting nutrient in aquatic environments and is the fundamental base of the entire ecosystem. Due to large scale watershed runoff, it is impossible to completely prevent nutrient influx in urban ponds, however, there are some things you can do to reduce it. While algaecides/herbicides address the symptoms of the problem, taking some of the steps below can help address the cause of the problem.
We have seen it a million times…..someone running a fertilizer spreader directly adjacent to a community pond. If mechanical spreaders are used, it is important to keep fertilizer pellets away from the water’s edge. We recommend keeping a “buffer zone” of at least 25 feet between the pond’s edge and any fertilizer applications. Even a light rainfall can wash a significant amount of nutrients into the pond causing severe algae blooms overnight. It is also important to bag your grass clippings and fallen leaves rather than blow them into the pond.
Nutrient loading by waterfowl is another major concern. We recommend that the number of ducks and swans be kept to a minimum. This is especially true for very small ponds. Not only do waterfowl add excess nutrients to the water, they also muddy up water and litter walk paths and fountains with feces.
There are a few commercial products available that are used for nutrient deprivation. Phoslock and aluminum sulfate (alum) are the two most common products. Phoslock is used in highly fertile waters to restore water quality and reduce algae growth. Alum is a chemical product that is used for both clearing muddy water and locking up available nutrients thereby restricting noxious aquatic growth. Alum should only be applied by a qualified lake management professional.
When used under the right conditions, supplementation with beneficial pond bacteria can also be effective at nutrient reduction. Microbes work by consuming the excess nutrients, thereby reducing the available nutrients that algae need to grow. Therefore, future algae blooms should decline depending on the amount of future nutrient influx. Through experience, we have found that bacteria treatments work best in small, highly oxygenated ponds. Recirculating systems with little or no water exchange respond most effectively to bacteria treatments. Besides aeration, water quality variables (most notably pH and alkalinity) also determine microbe effectiveness. Since chemicals “work” right away, the expectations are often that microbes will also work quickly. Bacteria work naturally and we suggest not to expect improvements in water quality for at least 3 to 4 weeks. It should be noted that organic, microbial treatments will have no effect on rooted aquatic weeds.