Algae are primitive plants that uses photosynthesis to combine water, carbon dioxide and other nutrients to form sugars for energy and growth. During day time, algae produce oxygen, a beneficial by product. But when sunlight is not available at night, algae respire and absorb oxygen. The respiration uses the stored sugars and oxygen to form carbon dioxide, which depletes the oxygen in the pond.

Algae is commonly referred to as “pond scum” or “pond moss” and typically forms greenish mats upon the water’s surface. Algae usually begins its growth along the edges or bottom of the pond and “mushrooms” to the surface buoyed by the oxygen it has produced. Algae can also form fur-like growths on logs, rocks, and even the backs of turtles. The texture of these growths may be slimy, cottony, or coarse.

Algae differ from most pond plants or lake plants in that they lack roots, leaves and other structures. Algae can essentially be divided into two different physical categories: Planktonic Algae, Filamentous Algae and Chara.

Planktonic algae are single-celled organisms. They are so small that planktonic algaes species can be indentified under a microscope. They pass through even the finest filter. Their abundant growth make it easy to identify visually. If conditions are right, with plenty of nutrients (ammonia and phosphorus) and sunlight, concentration of five million algae cells per milliliter of pond water can be measured. A Planktonic Algae bloom often appear as a paint-like scum on top of the water’s surface. The water column turns green throughout and is often described as “pea soup.”

Planktonic Algae can sometimes be mistaken for other growth, such as Duckweed or Watermeal (two common plants found in ponds and still lake waters). Duckweed is much larger in size than planktonic algae. You may easily determine if your waterbody has Duckweed or Watermeal by placing the bottom of a glass jar or drinking glass into the water. Remove the jar or glass. Duckweed will look like little flat leaves with small root hairs. Watermeal will look like small green grains.

Not only does planktonic algae ruin the aesthetic look of your pond, it can cause a great danger to fish when trying to treat it. What happens is when the planktonic algae dies off rapidly, from either weather change or treating the water with an algaecide, it will deplete oxygen from the water, possibly causing a fish kill if the oxygen depletion is great enough. Although it is not a 100% guarantee to prevent a fish kill, having an aeration system will reduce the possible risk of one.

This filamentous species grows in long strands. They often resemble the appearance of long stringy hairs, cotton-like in appearance. Individual filaments are a series of cells joined end to end which give the thread-like appearance. Filamentous algae can form thick, greenish looking mats on the water’s surface, and many times it is attached to a substrate such as rocks, logs and other plants. Therefore, they are commonly founds on rocks and waterfalls. They eventually tangle together, forming thick, unsightly mats that can double their weight within 24 hours.

Some filamentous algae may be bright green and slimy, while some may look more like “horse-hair” with a course texture, like that of steel wool. Filamentous Algae will often form mats that float on the surface of the pond resembling wet wool. Most pond owners commonly refer to this type of algae as “pond scum”, but can often be called string algae, or floating algae. This is a very common type of pond algae, but if left untreated can cover the entire surface of your pond.

Blue-green algae isn’t technically algae. The term actually refers to cyanobacteria, a phylum of bacteria. Despite being completely different organisms, cyanobacteria very much resemble common algae. The name blue-green algae comes from the color of the bacteria blue-green color. They photosynthesize like algae, but they are actually bacteria. Cyanobacteria, commonly referred as “pond scum”, can often smell like sewage or manure. But it can also contain a harmful bacteria and be dangerous to pets and humans.  Therefor. it has much more potential to cause harm to an aquatic ecosystem (such as a pond, water garden, golf course waterway, etc.) and even the surrounding people and animals. Warning signs of blue-green algae toxicity are: dead fish and/or waterfowl, unexplained sickness/death of a cat or dog, unpleasantly scented water, and skin rashes following human contact with water. Furthermore, one should be cautious in observing pet behavior if worried about the toxicity of an algae bloom.

Not all blue-green algae is toxic.  Cyanobacteria forms a blanket of slime on the surface of a body of water. It could be described as a thin layer a green viscous liquid.  This bacteria can also appear reddish-purple or even brown.  It is also known in the marine environment as “red tide.” If this layer of bacteria grows enough, it can completely block out sunlight from the depths of the water.  Excessive development can prevent growth of competing algae, and completely dominate a body of water.

Harmful algae blooms can decrease the water quality, produce an awful odor or taste, and cause the production of algal toxins.  These toxins could cause serious illness or death.  Toxic alga coincides with a lot of sunlight, warm temperatures and slow moving water.  It can be problematic if the water is left untreated. Common side effects of cyanobacteria poisoning in pets are: vomiting, diarrhea, jaundice, seizures, disorientation, coma, shock, excessive salivation, shortness of breath, and death.  With these consequences looming, it is important to be educated and proactive in the fight against blue-green algae.

Chara is a gray-green branched algae that is often confused with submerged rooted plants. The common names for these are Chara and Stonewort. Chara is very common on the bottom of lakes and ponds. Chara often looks like a matted tangle of plants, forming a carpet on the lake or pond bottom. Chara is often called muskgrass or skunkweed because of its foul, musky, almost garlic-like odor when crushed in your hands. Chara has no flower, will not extend above the water’s surface, and often has a grainy or crunchy texture due to calcium deposits on its surface. Chara has cylindrical, stem like branches with forked leaves. Each leaf-like structures has 6 – 16 branchlets around each node. For final identification, look for little bumps or spots on the leaflets.

Chara can only be chemically treated early in its growth cycle. Its course texture later in its growth cycle is caused by the absorbtion of mineral deposits. Once chara has absorbed these materials, it begins to have an innate resistance to algaecide. As a result, the only working treatment at that stage is a floating cover, which will completely block photosynthesis.

Stonewort is similar, but has smooth branches and the stems are more translucent. The leaves also appear to be covered in a green gelatinous substance. Stonewort also lacks the musky odor found with Chara.