Have you ever noticed a pea-soup color or blobs of green, slimy material, floating in many areas on your pond? This green scum is algae and is often a sign of an unbalanced pond and problems with water quality. There are many ways to treat it but how does algae form in the first place?
First let’s look at exactly what algae are. Algae are primitive, primarily aquatic, one-celled or multicellular plant-like organisms that lack true stems, roots, leaves, and other structures but usually contain chlorophyll. Most pond scum is algae, the green slime on rocks in a stream is also algae, as is the pea soup green seen in some nutrient-enriched lakes. In ponds, algae are referred to as pond scum or pond moss and typically form greenish mats upon the water’s surface. It usually starts growing along the edges or bottom of the pond and “mushrooms” to the surface sustained by the oxygen it has produced. Algae blooms may discolor water, poison, or asphyxiate aquatic life. Algae can cause displeasing odors that smell like sewage or manure. The most common types of algae are Planktonic Algae that cause the pea soup color and can appear as a paint-like scum on top of the water’s surface. Then there is Filamentous Algae that may look like long stringy hairs, cotton-like in appearance. This type of algae can form thick, greenish looking mats on the water’s surface and may attach to rocks, logs, and other plants. Blue-Green Algae, also called cyanobacteria, can be toxic as it can contain harmful bacteria and be dangerous to humans and animals. One form of algae that resembles submerged plants is called Chara or Stonewort. Chara looks like a matted tangle of plants that forms a carpet on the bottom of your pond. It has a crusty, course texture and a musky odor when crushed in your hands. It is recommended that Chara be treated early in its growth cycle before it has a chance to absorb mineral deposits that cause it to grow.
The most common reason why ponds have excessive algae blooms is nutrient pollution like excessive nitrogen, phosphorous, carbon, and potassium. Generally, the amount of phosphorous controls the amount of algae found in your pond. These are the chemicals pond plants use, combined with carbon dioxide and water, to grow and make new leaves. In a pond, these nutrients are invisible, dissolved in the water. These can originate from fertilizer, runoff, septic systems, agricultural land, and biodegradable materials (i.e. decomposing lawn clippings, dead leaves, fecal matter from fish, frogs, birds, and other aquatic life). Oxygen deficient water columns are also a cause of algae growth. Ponds with low oxygen levels and high bottom sludge accumulations have a tendency to go anaerobic. Under this condition, oxygen levels can be further depleted due to the amount of oxygen required to degrade the bottom sludge and, as a result, ponds become stagnant and odorous and algae blooms get worse over the summer months. These conditions combined with bright sunlight on the pond surface and warm temperatures, will allow algae to grow out of control.
Now that you know how algae are formed, learn how to keep your pond scum under control.