A Strategy for Algae

If you’re a pond-owner, then you’ve likely had to cope with the messy problem of algae from time to time. In my 15+ years of managing a small garden pond, I’ve been faced with excesses of both the “pea soup” type of planktonic algae and the long, stringy filamentous algae.

A sudden increase in planktonic algae concentration is called a “bloom” or colloguially “pond scum”. It’s an apt word, given that millions of microscopic organisms all seem to be visible at the same time in water that was clear just a few days earlier.

In fact, “algae” is a catch-name for lots of different organisms (including salt-water seaweeds and kelps, and some that live in snow or soil). They are not plants, i.e. members of the Kingdom Plantae, but more primitive members of the Kingdom Protista. However, many algae have chloroplasts and use sunlight to photosynthesize, in the same way that true plants do.

Algae spores are everywhere: in the air, on the wind, carried in rain, in pond soil and on aquatic plants, fish and snails. They are a natural element of life on earth and are present even in “clear” ponds. In fact, aquatic algae are actually vital to the health of your pond, providing oxygen and food for fish and other pond life. Brown or smooth algae form the velvety growthon liner walls and plant pots. They make excellent fish food and are not a problem

Free-floating algae is called “planktonic” algae, and causes the murky green water that often appears a few weeks after a pond is filled. Though a big algae bloom looks terrible, in a well-planted pond with the correct number of fish, it often only lasts a few weeks until submerged oxygenating plants begin to absorb nutrients that are keeping the algae fed and floating aquatic plants such as waterlilies or water hyacinths grow and shade the water, lowering its temperature (freshwater algae like warm water) and also depriving the algae of the sunlight they need to photosynthesize. That’s why it’s usually recommended that 60-70% of a pond’s surface be shaded by the leaves of floating aquatic plants.

In a pond with fish, apart from calculating the correct number of fish for your water volume (called “fish load”) it’s also important to keep water moving with a pump. Don’t overfeed fish, since excess fish food also feeds algae. If you have a large fish population, especially koi, you will likely need to use a bio-filter which contains a filtration medium seeded with nitrifying bacteria that clear the nutrient-rich fish waste from the pumped water. Even with a bio-filter, a pond with a large fish population might benefit from extra bacteria being added regularly. (Microbe-rich solutions can be found at pond supply dealers.)

All of these things, however, are designed to help keep algae growth in check. The best way to actually clear algae mechanically in a pond with lots of fish is with an ultra-violet clarifier (UVC), a special light past which pond water is pumped as part of the filtration process. When the light shines on the water, it will kill the algae.

Occasionally, if a pond is out of balance ecologically, algae blooms can last indefinitely and mar the aesthetics of the water garden. This can happen for reasons other than those already listed. For example, golf courses and botanical gardens often have sprawling ponds with large open surface areas, thus increasing the amount of algae-friendly sunlight being absorbed. In addition, these places often use lawn fertilizers which can run off into the pond, increasing the nutrient level in the water and promoting algae growth. Or waterfowl like Canada geese can become a persistent nuisance around the pond’s edge where their excrement will be carried via rainfall into the pond, again increasing nutrient levels in the water.

Although it’s possible to curb severe blooms with algaecides, most pond owners don’t want to use chemicals if they can avoid it. Besides, algaecides harm aquatic plants.

One interesting approach to preventing planktonic algae growth rather than dealing with it after it’s become a problem is used by the Montreal Botanic Garden in the large pond in their Japanese garden. They use a pond dye that turns the water a turquoise-blue. It is non-toxic and safe for fish and humans. When used in the correct concentration (usually 1 quart per acre of pond water 4-8 feet deep) and dispersed well throughout the pond, pond dye will not stain or burn plants or fish. However, rubber gloves should be used to mix it into the pond and it should not be poured directly on or near fish or pond plants. There are several good pond dyes on the market, including Aquashade®, Aquashadow®, Algae Blocker® and Blue Lagoon®.

For more on water gardening, read Water, Water Everywhere.

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