Installing a small water garden or pond has been a home landscaping trend for the past several years, but homeowners should be aware that maintaining an aquatic environment means more than filling a tub and tossing in some fish.
"The root of all evil in a garden pond is excess algae, and the key to controlling algae is controlling nutrients," says Thomas Martin, assistant professor of aquatic ecology in Penn State's School of Forest Resources. "The most common nutrients are nitrogen compounds and phosphorus, which are the main ingredients in lawn fertilizer."
Martin says ornamental ponds should be installed so that lawn runoff will not drain into the pond. He also recommends using plastic pond liners so nutrients will not soak into the pond from surrounding soil. "Some homeowners plant a buffer strip of plants around the pond to soak up those nutrients," Martin says.
Algae must have light, warm water temperatures and adequate nutrients to grow -- climatic conditions that are met from about April to October in the Northeast. These factors also are harder to control as the surface area of water is reduced.
"It's a problem of scale," he says. "The smaller the pond, the more you have to treat it like an aquarium. Homeowners should also be aware that adding any plant or animal to a pond requires some sort of management."
For example, introducing fish into a pond can cause an increase in algae. All fish produce ammonia as a waste product, Martin explains. In a lake or a river, that ammonia is diluted, but in a small pond, levels can quickly build up and kill all the fish. Luckily, all ponds contain bacteria that can break down ammonia, but the compounds ammonia breaks down into are nitrates -- or more fertilizer to promote algae growth.
One way to counteract algae is to introduce rooted plants, which can absorb nutrients that would otherwise feed algae. The plants also shade the surface of the water, which prevents algae growth. "To really control algae, you'll have to shade 70 to 80 percent of the water surface," Martin says. "This means if you have any fish in the pond, you won't be able to see much of them."
Martin says homeowners who want a very low maintenance pond can use floating leaf plants and a water dye that turns the surface black, preventing light and temperature from encouraging algae growth.
For most pond owners, Martin recommends installing a filter system that is adequate for the size of the pond and adding water plants to absorb nutrients. "You should add a couple of fish to prevent any insect problems," Martin adds. "You'll never see mosquitoes where there are fish -- they don't co-exist."
Martin also recommends cleaning out the pond liner at least once a summer to eliminate waste and other material that builds up on the bottom.
Professionals also say pond owners should change the water about 10 percent per week, although Martin says it might be easier to change 40 percent of the water every month. "If you are on a municipal water system, the water will be chlorinated," Martin warns. "Chlorine will kill the bacteria that breaks down ammonia and other compounds. You can either add chemicals to eliminate the chlorine or aerate the water to dissipate the chlorine before adding it."
If a pond has an outbreak of algae, Martin says homeowners should remove as much of the algae as possible, check to see if the filter is clogged, and then seek help at the local garden center. "If you leave algae in a pond system, it uses more oxygen than it produces. When algae is competing for oxygen with other plants and fish, the fish usually die. A fountain can help this by adding oxygen to the water."
Martin also advises against adding wildlife such as snails or fish that feed by filtering food along the bottom. "Whatever algae the snails eat probably is offset by the amount of waste they produce," Martin says. "Filter-feeding fish are extremely difficult to sustain in a man-made environment."
Martin says if an average homeowner changes 20 to 40 percent of the water every two to four weeks and filters the pond, few problems should develop. As winter approaches, smaller ponds should be drained, cleaned and left empty until spring.
"Although it seems like a lot of work, maintaining a pond is pretty easy," Martin says. "After all, most people who install them don't do it just to look at the fish and plants. They also want to have a hobby they can tinker with."
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