The Secret Life of Ponds

Author: Eve Pranis

Water makes life on the planet not just livable, but possible. All organisms are utterly dependent on it for survival. Our bodies are 80 percent water. It covers three-quarters of the Earth's surface, but only 3 percent of that area is fresh water (the rest is oceanic salt water), and more than half of that is in the form of ice. There are lots of ways to wade into water in schoolyard gardens and habitats: exploring transpiration, experimenting with mulches and other means of conserving water, creating weather stations, restoring wetlands, and assessing the water needs of area wildlife, to name a few. Many teachers have also discovered the value of schoolyard ponds as teaching tools. They can inspire explorations of the life and chemistry of aquatic ecosystems, along with reading, writing, and quiet reflection. So, you have little space? You can create a mini-pond in a corner of a courtyard or even in a half-barrel.


A Balanced Equation
As dynamic ecosystems, ponds feature a constant interaction among living and nonliving elements. To be a steward of pond life, you should have a basic understanding of this fascinating chemistry. Here's the (very) short course. First, the players:

Algae, the organisms that form the familiar green pond scum and grow on underwater objects, are neither plant nor animal, but organisms that photosynthesize (produce food and oxygen) using sunlight. They form the basis for aquatic food chains.

Pond plants come in two forms: submerged (those that grow completely under water) and floating-leaved (their leaves lie on top and roots reach to the bottom). The submerged plants, which are the pond's greatest source of oxygen, absorb nutrients through their leaves (competing with algae for light and food) and provide places for fish to hide. The floating-leaved plants take in nutrients through their roots and provide shading that keeps floating algae in check and helps moderate the temperature.

Fish require oxygen, nutrients, and places to hide their eggs. Their waste products, such as ammonia, are absorbed as nutrients by plants (as is and when broken down into nitrates). Snails, tadpoles, and other scavengers eat algae, plant debris, and other once-living matter.

Oxygen and carbon dioxide in the air dissolve into pondwater at the surface and where bubbles cause air and water to mix. During daylight, aquatic plants and algae release oxygen as they photosynthesize. Fish, other animals, and plants, meanwhile, consume oxygen from the water and produce carbon dioxide as they respire. On sunny days, the effects of photosynthesis outweigh the effects of plant respiration, so oxygen increases and carbon dioxide decreases (the opposite is true when sunlight is not available). On very sunny days, your students can look for small bubbles of oxygen gas on the leaves of aquatic plants. Oxygen is also consumed when organic materials, such as animal waste and plants, decompose in water. The pH of pondwater rises on sunny days (when oxygen is high) and declines when sunlight is not available. (A pH of 6.5 to 7.4 is ideal for most organisms.)

The decomposing organic matter breaks down to produce ammonia and other nutrients, some of which are absorbed by aquatic plants. But a pond can have too much organic matter, such as when algae become prolific during warm, sunny weather, then begin to decay. (Runoff from human activities and animal waste animal runoff also add nutrients that cause algal "blooms.") When the abundant algae decompose, the oxygen needed by fish and plants disappears.

Creating a Pond
Before scoping out a site, check with your school district administrators to see if there are any restrictions or safety precautions you should consider. Search for a level spot, away from trees, that gets at least 5 hours of sun each day. If you're considering a very small in-ground or container-based pond, you may want some shade during the day to avoid big water temperature fluctuations. If you might someday use a pump to increase dissolved oxygen (or to create a waterfall), be sure you're not far from an electrical source.

Linings
Unless you have a pre-existing pond that you're renovating, you will need some type of liner. Rigid plastic or fiberglass liners are pre-formed and easy to install. But flexible liners made from PVC or rubber offer more options. (Bear in mind that you'll have to provide a layer of sand to protect a flexible liner.) If you have limited space or simply want to start small, consider using a half-barrel or other type of tub. Installed ponds typically run between 15 and 24 inches deep.

Plants
Whether you bring in native plants from local ponds, or purchase them from aquarium or water garden suppliers, it's important to include the two types of plants--submerged pond plants, such as anacharis and milfoil, and floating-leaved plants, such as water hyacinth and water lettuce. (Hardy types of water lilies will overwinter, unlike their tropical cousins, in temperate areas of the U.S.) In very shallow areas at the edges and beyond, you might also include marginal or bog plants such as rushes and cattails. To keep algae growth in check, plants should eventually cover about three-quarters of the surface of your pond. Initially, place plants in garden soil in containers or tubs with any holes blocked. Eventually sand and sediment will form a base in which floating-leaved or marginal plants can root.

Fish and Other Creatures
As exciting as they are to observe, you should limit the number of fish to what your pond can support. Too many fish release waste that causes algae blooms which, when decaying, remove oxygen that fish need! Some pond gardeners say that a pond can support 1 inch of fish for every 5 gallons of water without needing extra oxygenation from a pump. Native fish like goldfish and mosquito fish can overwinter in ponds, so long as there is oxygen exchange (e.g., via a hole in the ice, for example.) Non-native fish, such as koi, require highly oxygenated water, need to be fed daily, tend to eat water plants, and don't readily overwinter. Fish (and some plants) can't tolerate chlorine, so if you're filling the pond with treated water, let it sit for a few days to a week before adding organisms. If you bring in plants from other ponds, you will probably bring in scavengers, such as snails, that eat algae and once-living matter on pond liners and plant stems. You can also buy scavengers at water gardening or aquarium supply stores.

See What Comes
In the early stages of a new pond, you'll likely observe an explosion of floating green algae. Don't fret. This is part of the natural process in a healthy system. Once other plants are established that provide the balance and slow algae growth, the green scum should clear somewhat. Maintaining a good mix of water plants and removing dead or dying matter will help. But keep in mind that in a thriving ecosystem, you shouldn't expect to see crystal-clear water. Once things have settled in, your keen observers should keep their eyes peeled for frogs, toads, insects, and other life forms that appear.