Ultraviolet Light vs. Algae - Water Garden Facts

by: Douglas Hoover

What is the best way to control algae and impurities in backyard water gardens? It is not as simple as you think! Ultraviolet light was discovered to have adverse effects on certain organisms commonly found in ponds, such as specific types of bacteria and algae. For this reason, UV light treatment of ponds has become very popular and effective in controlling certain forms of bacteria such as pathogens and planktonic algae.

Planktonic algae are cousins to filamentous algae, also sometimes referred to as string or blanket algae. Unlike filamentous algae, planktonic algae are common to ponds and are critical to a pond's food chain. They provide food for many microscopic animals that, in turn, are eaten by animals higher up the food chain such as water bugs and fish fry. Unfortunately, these algae can bloom to nuisance levels, requiring immediate control methods.

Planktonic algae or "pea soup" algae are microscopic, free-floating plants. They are generally found near the surface of a lake or pond, within the top two or three feet where sunlight can help provide food through photosynthesis. Planktonic algae consist of green algae, blue-green algae, diatoms, and euglenas. Some species of planktonic algae, primarily blue-greens, can be toxic to animals and emit an odor or foul taste to water.

Algae are most prevalent in the summer months and are extremely sensitive to water temperatures, thriving in warmer water. Cold water inhibits their reproduction and growth, resulting in a clear pond in the winter months. Algae blooms usually occur in the spring, around April or May, as the water temperature begins to rise. Depending on the algae species, the water turns various shades of green or brown.

In a natural pond or lake, microscopic animals such as the rotifers and daphnia create large populations which begin devouring the algae bloom; then the water becomes clearer. Once temperatures reach about 72 degrees Fahrenheit, the microscopic animal population declines rapidly, with decreased reproduction; this is when they become prey to fish fry, resulting in the proliferation of algae. This can be controlled by an increased number of hyacinths or bog plants to help regulate the nutrients that algae thrive on.

In addition to water plants, the single most effective control for planktonic algae is ultraviolet filter treatment. As the pond water passes through the UV light, the algae spores are killed. The deeper the pond, the cooler the water remains, and the better the chances for controlling algae growth.

Planktonic algae could create a serious threat to the fish population if it were to die off quickly. The dying algae deplete the oxygen in the water, threatening the fish. The same goes for filamentous or string algae. In small amounts, this type is beneficial as food for fish; however, it can get out of control rapidly and choke the pond. Even though string algae provide oxygen during the sunlight hours, it also consumes oxygen from dusk to dawn, canceling out the oxygen-generating benefit. In addition, if you attempt to kill off the algae all at once using herbicide or salt, the resulting effect of the dying algae depleting the oxygen could be fatal to the fish.

Great caution should be taken when handling liquid or granular fertilizer so that it never is introduced into a pond. Also, be aware of any possible contamination from groundwater run-off accessing the pond. Fertilizer, even in minute quantities, can trigger algae blooms and, in larger quantities, can poison your pond.

Most generally, an abundance of algae is a direct result of excess nutrients in the pond. If you already have a biological filter and a UV light, the source of the excess nutrients could be from (1) the watershed, (2) too many fish in proportion to the size of the pond, (3) an inadequate filter system, or (4) not cleaning or back-washing the filter often enough. These are the types of conditions that warrant an ultraviolet light in conjunction with a good biological filter for the removal of the extra nutrients, suspended particles and algae spores, as well as the microorganisms that cause the water to stink and turn the water cloudy. The UV light will need to be cleaned regularly; and if it has been over one year, it may need the bulb replaced. Or you may need to add more plants to your waterfall and pond.

Filamentous or string algae are also referred to as "moss" or "pond scum." It forms dense mats of hair-like fibers growing on the sides of the pond and submerged objects. The algae produce oxygen which becomes trapped in the strands and mat formations, causing it to float on the pond's surface. Portions of the exposed raft of floating algae become overexposed to the sun and die. The brownish-green bubbly condition gives the appearance of a disgusting scum, hence its name. However, if you were to gather up this slimy mess and wring out the water, you would quickly discover it is not slimy or scummy at all, but rather has a soft, fibrous cotton textures and smells of freshly mowed grass. As it hangs from the rocks in a waterfall, it looks like anything but that.

The best way to control string algae is by hand. Actually gather up the floating mats in the pond and hanging clumps in the falls. If you are planning to use a herbicide for control, always remove the bulk of algae by hand first. You will reduce the chance of oxygen deprivation from the dying algae.

An ultraviolet light with wiper cleaning device can reduce the amount of algae significantly. Have an expert calculate the size of your pond and how many watts you need for the proper rate of flow per hour.

The proper equipment can mean the difference between owning a water garden that can provide more joy and pleasure than anything you ever spent money on, or it can be your biggest nightmare.