Addressing pond water quality.
Because many of the factors affecting life in a pond are not visible, we often find ourselves reacting to the results that become evident when a system is unbalanced: green algae-filled water, an unpleasant odour or the loss of fish. Water clarity alone does not indicate water quality. Toxic gases, ammonia, nitrites and high pH are invisible and best monitored with use of test kits. Poor water quality will stress the fish leaving them more vulnerable to disease. It can also result in poor plant growth and ineffective performance of biological filters.
Ecological balance is largely dependent on a balanced nitrogen cycle. This is monitored by testing the concentrations of ammonia and nitrite in the pond water. Both are toxic by-products of the nitrogen cycle and are very harmful to fish.
Ammonia is produced during the decomposition of organic matter such as plant leaves. It is also present in fish waste. If there are sufficient nitrifying bacteria in the pond, they'll convert the ammonia to nitrite and then to nitrate which is consumed by plants. These nutrients often contribute to an algae bloom in early spring, before other plants establish vigorous growth.
Other factors also contribute to poor water quality. An excess of chlorine and chloramines are harmful to fish and may also kill the nitrifying bacteria, thus interrupting the nitrogen cycle and resulting in toxic levels of ammonia and nitrites. This situation can be compounded by a pH that is not within the range of 6 to 8.5. As well, suspended single cell algae can multiple quickly if the pond is not planted with enough submerged oxygenating plants to utilize the nutrients.
Because no two ponds are alike, there is no magical formula with which to achieve ecological balance. Location influences the amount of sunshine a pond receives. Correct construction and grading limit the quantity of nutrients that accidentally drain into the pond from the surrounding lawn and garden. A pond with too great a fish population adding waste to the water, or an excess of organic debris like uneaten fish food and dead plant leaves will be more difficult to balance.
Water lilies require at least six hours bright sunlight daily. Yet sunshine is a major factor contributing to unwanted algae growth. This problem can be reduced by covering up to two thirds of the surface of the pond with floating plants. To encourage early growth, place submerged oxygenators and water lilies closer to the surface, on the plant ledge (shoulders of the pond). As the water warms and the leaves begin to spread, move these plants to the bottom of the pond. Marginal aquatic plants, or grasses and daylilies planted near the pond, can be used to shade the edge of the water, while at the same time casting beautiful reflections on its surface.
Tiny floating plants like duckweed (Lemna) and fairy moss (Azolla caroliniana) are often used to cover a portion of the pond surface early in the season. Unfortunately, both multiply very quickly and can soon cover the entire surface eliminating the reflective qualities of the water. Fish often control the duckweed; mine seem to have little taste for fairy moss. Both plants are easily controlled by netting them off the surface of the water for compost or to share with other water gardeners.
Floating plants like water hyacinth (Eichhornia crassipes) and water lettuce (Pistia stratiodes) do more than shade the pond surface. Their suspended roots use nutrients directly from the water, as well as provide a spawning mop for fish to lay their eggs and a safe refuge for tiny fry. These plants multiply quickly in warm water and resent being dropped into a cold pond too early in the spring.
A small amount of algae is vital to the pond cycle, providing food for the tiny creatures at the bottom of the food chain. The velvety green growth of benthic algae on the sides of the liner and plant containers helps to camouflage them and uses nitrates from the water. However, free floating or suspended single-celled algae must be controlled before it clouds the water a murky green. The most efficient way to do this, is to reduce the nutrients on which they feed.
Submerged oxygenating plants like hornwort (Ceratophyllum demersum), feed through their leaves and are an excellent natural algae control. Suspended single cell algae cannot compete with these more complex plant forms and will starve. During the day, hornwort releases oxygen into the water although their primary function is to remove nutrient.
Vallisneria and dwarf Sagittaria are also commonly used oxygenators. To initially control algae and balance nutrients it may be necessary to plant several bunches of oxygenator per square metre (yard); these may need thinning by mid summer.
Although some oxygenators naturally float below the surface of the water and do not root, it is best to plant them in soil, moving them deeper into the water as they grow. Like many aquatic plants, oxygenators prefer not to be taken from a warm greenhouse and dropped in cold pond water. Canada pondweed (Elodea canadensis) is a native plant that may winter over in your pond, breaking dormancy in early spring. Water hyssop (Bacopa monnierii) also tolerates cold water.
Routine maintenance including: regular removal of leaves and organic waste, regular water changes, and topping up the water by spraying it like rainfall, help limit nutrient build-up as does increasing the number of submerged oxygenation plants and not overfeeding your fish. These are the critical steps to a beautiful, healthy pond. By following them, you will be able to enjoy your pond to its fullest.
Marilyn Brown is a Saskatoon gardener with a special interest in water gardening. She is president of the Prairie Water Gardening Society and shares her enthusiasm for gardening by teaching courses.