Shading to Increase Water (and Oxygen) Circulation in Ponds during the Summer

by Adrian Lawler, Ph.D.
(retired) Aquarium Supervisor (l984-l998) J. L. Scott Aquarium Biloxi, Ms 39530
Original to Aquarticles

Introduction and Background
For ponds where electricity is too far away, or is not available, or is too expensive, one needs a way to get reliable partial or complete turn over (circulation) of the water in the pond in order to distribute dissolved oxygen to various levels in the pond to maintain healthy fish during hot still summer days and the following nights.

When the sun is shining over an open pond, and there is no wind, the surface water heats up, and does not sink, and no circulation occurs. This is the type of weather one would have during hot, still summer days; there is less oxygen in the water due to increased water temperature. No wind stops pond circulation and restricts surface diffusion of atmospheric oxygen.

Water can hold a limited amount of oxygen; the oxygen content of water increases with increasing atmospheric pressure and decreasing temperature and salinity. The amount of oxygen in water is measured as milligrams per liter (mg/l) dissolved oxygen (DO). The amount of dissolved oxygen required for fish varies per species. Under natural conditions, oxygen is added to water by atmospheric diffusion at the water-air interface, by wind circulation (augmented surface diffusion), and by photosynthesis (oxygen produced by phytoplankton, algae, and other aquatic plants). Photosynthesis usually accounts for most of the oxygen in pond water.

Higher water temperature (86 degrees F or greater; or 30 C) increasingly reduces oxygen holding capacity. Cloud cover limits available light, slowing or halting photosynthetic oxygen production.

Warm water increases fish consumption of oxygen by accelerating their metabolic rate. Fish are ectotherms (cold blooded); thus, body temperature and functions are regulated by water temperature. Fish biomass (total weight in pond) and oxygen needs are usually greatest during the hot (and sometimes cloudy and still weather) months of late summer.

As water cools, oxygen holding capacity increases. We want cooling, sinking, and increased dissolved oxygen to occur as much as possible, not just mostly when the wind is blowing or there is natural shading or it rains. We want to get more oxygen into water during hot still summer days so there is less chance of low oxygen stress on fish during the warm summer nights when no oxygen is produced by photosynthesis, and both the fish and plants are using oxygen.

For the smaller ponds used for enjoyment, or rearing various ornamental fishes, another non-electric way is needed to help circulate the pond water during the summer, as winds do not usually blow enough to give much cooling and turnover. As water cools, oxygen holding capacity increases; and we want this to occur more of the time. Cooling increases density and increases sinking and mixing (until maximum water density is reached, and it, and temperature become constant throughout the pond).

The following methods were initially used by me in the 1970's when I was rearing fathead minnows in small ponds (1000-3000 gallons) for toxicity testing. I wanted to cool pond waters to prolong spawning of the minnows. Methods were needed that would cool the ponds when the wind was not blowing and when the sun was shining on hot, still days.

The basic concept is to cool part of the surface water so that it becomes denser and sinks. The sinking water displaces lighter warmer water below which then goes upward, where it is then cooled and sinks, setting up a circulation of water being cooled and sinking, mixing and getting warmer, and rising to get cooled again.

Surface Water Cooling by Shading
Water can be cooled by using an effective shade to stop the sun's heat before it reaches the water. Shaded water is generally cooler than unshaded water and consequently holds more oxygen.

Thus, by shading and cooling part, or all, of the surface water in a pond we can set up a circulation (and get a higher oxygen carrying capacity) without the use of a pump or electricity. As long as there is a difference in water temperature due to cooling from one side to the other, and/or top to bottom, there will be circulation. A portion of the pond is covered (as below), and thus cooled by shading. This can be done in several ways:

1---Put untreated plywood, roofing tin, tarp, shading fabric, or other non-toxic cover, suspended over the water, over one end of the pond. Cover should be reflective and not absorb heat, not sag from pooled rain water, and rest on edges of pond.

Make it so the cover can be removed for pond cleaning, etc. Vary the amount of the pond covered to suit your designs (larger area covered means more water cooled quicker in the daytime). Oxygen is added to water via atmospheric diffusion and photosynthesis when there is no wind.

2---Cover one end of the pond with a thick layer of floating plants (duckweed, water lettuce, water hyacinths, water lettuce, water lilies, etc) which are contained inside a floating ring or barrier (to keep them together and in a compact cluster) to make a more complete cover and shading.

This method is not as efficient as the totally shaded (covered) method as there may be holes in the area shaded due to spaces between the plants. These spaces may heat up and partially decrease the water cooling effect of shading by the surrounding plants.

3---Put a vertical barrier up at each end (E and W) of the pond (can be shrubs or other plants, a wall of wood or bricks, tin, etc.) so that shade from the barriers extends over pond surface in both the morning and the afternoon.

This is the least efficient way, and causes no cooling or increase in circulation from shading when there is no shade (when sun is overhead) thrown over the pond

Summary
When the sun is shining, the surface water in the shaded area will be cooler (and thus more dense), and will contain more dissolved oxygen, and will sink, leading to circulation and distribution of oxygen to areas of the pond below the surface. The cooler water will also retain more of the oxygen derived from photosynthesis. If there is not an overload of fish and plants in the pond, then the cooler waters derived from shading and their increased dissolved oxygen will sustain the fish through summer nights until oxygen production via photosynthesis starts up again in the mornings.

NOTE:
---The use of shaded areas also has the benefit of giving a temperature differential plus a shaded area in the pond for fish and other organisms to utilize.
---The use of shaded areas also can help reduce the occurrence of algae blooms.