July 1 2006 12:00 AM

We’ve all seen those slimy green scums on ponds, lakes, or pools. They’re called algae. And they stubbornly occur in water – the one place in a landscape that shouldn’t look green. But what are algae? Where did they come from? What kind of damage can they do? And are they really that bad?

The problem of algae is hard to escape – algae are everywhere. More than 30,000 varieties have been discovered, and scientists have found that algae have been around for at least two billion years. They occur in virtually every habitat on earth, as long as water and sunlight are found there, even if the water is present for a very short time. They can survive severe environments, from icy mountain glaciers to boiling hot springs to excessively salty water. However, for the purposes of this article, we will limit ourselves to algae that grow in ponds, lakes, and streams.

So what are algae? You’ll notice that we’ve been using the plural form to talk about them – one alga is microscopic; masses of hundreds of millions of alga are called algae. It’s these masses that we see when we look at a lake, and it’s these masses that are the real enemy.

Algae are traditionally considered to be simple, primitive plants, some made of only one or two cells. Most make their own food materials through photosynthesis using sunlight, water, and carbon dioxide – just like any other plant. While they also contain chlorophyll and produce oxygen, all lack the leaves, roots, and flowers associated with the more familiar higher plant forms.

Algae can float freely in water, coloring it green, or they can coat the sides of a pond with a green or brownish slime. In the ocean, they provide the food base for most marine food chains. Without algae, our waters would not sustain life and mankind would not benefit from its countless qualities and boundless beauty. However, in very high densities (called algal blooms) algae not only discolors water, but can also outcompete or poison other life forms in it.

But how did it get to your pond or pool? That can be answered with another question: where did you get your water? All water – even purified drinking water – can have algae in it. Algae can form spores, which are special, microscopic, and very tough cells that can survive just about anything – even the local water purification system. Local water companies will kill nearly all of the algae in water, but some is bound to survive, and it only takes a single spore to birth a huge, visible, mucky colony. Algae can also enter a pond with fish, fish food, or just about anything else you put inside.

The most common kind of algae is called planktonic algae. These are the single-celled culprits that create most algal blooms, although they also bedevil pond owners worldwide. They reproduce rapidly, and can be green, brown, or red in color. They can be toxic to animals and can give water an unpleasant taste or odor.

Spirogyra are another common type of algae. These algae look like strings or filaments, and are familiar as being the green “hair” on the rocks, sides, or bottom of a pond. There are more than four hundred species of Spirogyra worldwide, adapted to a variety of environments, but are not as likely to “bloom” as planktonic algae.

Planktonic algae will bloom in nutrient-rich water. Nutrients can be produced by a few fish, heavy feeding of fish, or even bird traffic. Any of these circumstances can throw off the balance of a pond's ecosystem, and algae will quickly take advantage of that imbalance, growing rapidly and dying back when the nutrients are depleted. Even non-toxic chemicals such as those running off of a farmer’s field have been found to cause algal blooms. If non-toxic runoff from a landscape is reaching a pond, or even the rocks surrounding a pond, it might be inviting an algae explosion.

This can be a problem even after the algae die back. They can sink to the bottom of a pool or pond and form sludge. A lot of sludge – a couple acre feet of water can easily sustain growth of several tons of algae per season. This will decrease your water volume over time and possibly necessitate dredging.

As the dead algae decompose, the decomposition may cause oxygen depletion in the deeper waters. This can result in fish kills, or even chemical changes in the mud on the bottom, which could release chemicals or toxic gases. Some species of algae even produce neurotoxins, which, if present in a high enough concentration of water, can cause serious health problems in humans if that water is ingested.

Naturally, those are extreme instances. But smaller amounts of algae are not without their own hazards. For one, algae aren’t aesthetically pleasing. They can make a pristine pond look like a cloudy, stagnant bowl of pea soup. They can also make rocks or hard surfaces extremely slippery, especially a problem around pools. They can clog screens, filters, or pipelines, plug irrigation or pumping equipment, and stain and rot wood. They can also lessen water flow and trap unsightly debris.

In small amounts in natural environments, algae are an important part of the ecosystem, providing an essential link in the food chain. However, in recreational or aesthetic water features, they can be a serious problem that needs to be dealt with aggressively.