There are three main treatment methods to limit algae growth in ponds and lakes.
1. Starve the algae!
This method works best in pond and lakes that have relatively small nutrient inputs. Nutrient inputs can come from lawn and garden fertilizer, agricultural activity, nutrient laden groundwater, or nutrient laden surface water. The primary nutrients required for algae growth are Nitrogen and Phosphorus. When these two nutrients are found in even relatively small concentrations algae blooms occur. In a pond or lake with low nutrient concentrations, an algae bloom will often use up the available nutrients and resulting in limited additional algae growth. For those ponds with higher nutrient concentrations or consistent nutrient inputs the algae blooms often persist and take over the pond or lake.
Beneficial bacteria break down available nutrients. When combined with phosphorus binders and proper aeration these beneficial bacteria provide a very natural and effective method for limiting algae growth. This method provides healthier function for the entire pond ecosystem. Because bacteria populations plummet as they digest nutrients it is necessary to apply beneficial bacteria at regular intervals while water temperatures are above 50 degrees Fahrenheit.
2. Algaecide treatments.
For ponds and lakes with high nutrient concentrations combined with high alkalinity and high pH it can be nearly impossible to remove enough nutrients from the pond to impact algae growth. In these situations, there are several highly effective algaecides that can kill algae and help to prevent future growth for a period of time. For planktonic algae (very small floating algae) liquid algaecides are most effective because they mix throughout the water column. For filamentous (string) algae that grow on the pond or lake bottom before floating to the surface at the end of their life cycle, granular algaecides are very effective. Most granular algaecides also limit addition growth for 4-6 weeks. In most situations algaecides must be applied at regular intervals to prevent regrowth.
3. Block the Sun!
Pond dye is an economical and effective approach to preventing sunlight from reaching the pond bottom. No sunlight = no plant growth. To be effective pond dye must be applied before growth starts and maintained throughout the growing season. Depending on inflow/outflow conditions, dye is typically applied on a 2 to 4 week interval to constantly shade the pond bottom. There are several available dyes that also contain beneficial bacteria to digest and reduce available algae causing nutrients.