Natural Cooling and Circulation of Water in Ponds

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

Introduction
The cooling of water can be critical to aquatic life because cooler water can contain more oxygen. Various natural processes can cause water cooling, and increased water circulation and increased dissolved oxygen.

For catfish aquaculture ponds in the southern United States, for example, they have determined the shape, orientation, and depth of ponds in order to get better pond water circulation using the wind. The rectangular ponds are oriented so that the prevailing summer winds (usually for the summer, when the water gets hot and holds less dissolved oxygen) have a greater fetch blowing with the longer dimension of the pond. They have also figured out that the usual winds, by evaporative cooling, will lead to sinking water and a turn over of the water to a depth of 3-6 feet (so their ponds are usually made no deeper).

As water cools, oxygen holding capacity increases. The least dense water will be at the surface and the most dense water will be at the bottom. We want cooling, sinking, and increased dissolved oxygen to occur as much as possible to nourish the pond life. So when does this occur naturally?

Concepts:

1. Cooling
Water, at its maximum density at the temperature of 39.2°F (or 4°C) is the standard for specific gravities, one cubic centimeter weighing one gram. Water will sink as it cools to 39.2°F or 4°C, but if it cools below 39.2°F it will rise because it is then less dense. When a uniform 39.2°F is reached throughout the pond, additional surface water cooling does not cause more sinking, as water cooler that 39.2°F becomes less dense, and stays at the surface through being chilled enough to get ice formation. Thus, water cooler than 39.2°F is at the top of the pond, and circulation driven by cooling and sinking ceases.

The basic concept is to cool part of the surface water so that it becomes heavier 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.

The amount of sinking depends on many variables, some being: wind speed and duration, cooled water initial density, density of water in pond at various levels, and the amount of mixing (and thus the decrease in density) as the cooled water sinks. Water cooled at the surface may not sink all the way to the bottom.

Thus, by cooling part of the water in a pond we can set up a circulation without the use of a pump or electricity. As long as there is a difference in water temperature (and thus density) due to cooling (or heating) from one side to the other, or top to bottom, there will be circulation. Sinking water sinks to a pond level where there is water of the same temperature, and the same density. Sinking water changes density as it mixes with less dense waters while sinking.

2. Oxygen
Water can hold a limited amount of oxygen; the oxygen content of water increases with increasing atmospheric pressure and decreasing temperature and salinity. Under natural conditions, oxygen is added to water by atmospheric diffusion at the water-air interface, by wind circulation (augmented surface diffusion), by photosynthesis (oxygen is produced by phytoplankton, algae, or aquatic plants), and by water agitation (by water falls, rapids, wave action, etc). Photosynthesis usually accounts for most of the oxygen in pond water, while agitation usually accounts for most of the oxygen in mountain streams.

The warming of water reduces its oxygen holding capacity. Cloud cover limits available light, slowing or halting photosynthetic oxygen production. No wind stops pond circulation caused by evaporative cooling and restricts the surface diffusion of atmospheric oxygen. When the sun is shining over an open pond, and there is no wind or shade, the surface water heats up, and does not sink, and no circulation occurs.

Warm water increases fish consumption of oxygen by accelerating their metabolic rate. Fish are ectotherms (cold blooded); therefore, body temperature and activities 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.

For one example, wind blowing over the water agitates the water surface and adds oxygen to the water by augmented surface diffusion, plus the evaporative cooling of the water from the wind leads to denser, sinking waters containing the dissolved oxygen. Thus, the oxygen is circulated via cooled sinking waters.

Natural processes affecting pond cooling

Following are ways water is cooled, and sinks, and can contain more oxygen; these processes generally are acting together in some way and can lead to increased cooling together, or one process can decrease the cooling effect of another process (and give a net result range of heating the water to cooling the water, depending on many variables). For example, a front with a cold north wind would cool a pond by evaporation (due to the wind) plus lose heat by radiation to the atmosphere (both processes together giving a greater total cooling than either process by itself). For another example, the sun's heat evaporates water, too; but the heating effect of the solar rays is greater than the cooling effect due to evaporation by the solar rays. The net result is water heated by the sun, as in the summer. The contribution of each process to the cooling would be difficult to calculate.

When the surface water cools and sinks, circulation continues until the pond is no longer cooled and the pond temperature (and thus density) throughout the pond is equal. The time it takes to reach this temporary state depends on many variables such as wind speed, air temperature, humidity, atmospheric pressure, solar radiation, precipitation, shading, etc. Cycles of pond heating and cooling are constantly going on, depending on the variables that affect cooling.

Cooling by evaporation
The evaporative effect removes heat from the water; thus, water is cooled by evaporation. We are most familiar with evaporative cooling in the cooling of the human body through the evaporation of perspiration. The process is more effective than convection cooling. The rate of evaporation increases with increasing temperature and wind speed, and decreases with increasing humidity. There are different evaporation rates for large and small bodies of water, soils, and vegetation. These differing evaporation rates are difficult to measure or to predict.

Cooling by evaporation occurs most frequently during windy times, and the passing of the winds of a weather front.

Cooling by radiation into atmosphere
When the air temperature drops below pond surface temperature (at night, during overcast days, and after the passage of a cooler weather front), and there is no wind, the pond will lose heat mostly by radiation into the atmosphere, and surface waters will cool, and sink.

Cooling by convection
During the warmer months of the year the pond water of deeper smaller-sized (ornamental) ponds can also lose heat by convection to the normally cooler earth surrounding the pond, leading to a zone of cooler water near the bottom that rarely turns over. When the sun is shining over an open pond (no shade), and there is no wind, the surface water heats up, and does not sink, and no circulation occurs; this warmer water will be above the cooler water level.

In the winter, if the smaller-sized (ornamental) pond is deep enough, the ground temperature can influence pond temperature. When the earth is warmer than the water, the warmer bottom water, heated by convection, can rise from the bottom to drive a circulation. Cooler surface water containing more oxygen would then sink.

Cooling by shading
Shading blocks solar rays and solar heating, thus a shaded item is cooler than areas under solar heating, and can hold more oxygen. Shading can be done in several ways (See: Lawler, Shading to Increase Water (and Oxygen) Circulation in Ponds during the Summer):

Cooling by addition of cooler water (rain, sleet, or snow)
Raindrops falling through the air undergo some evaporative cooling as they fall, and the cool water, added to a pond, can sink (if it is cooler than the pond surface water) and lead to circulation. Cooler surface water resulting from sleet or snow can also sink, and lead to circulation.

A big amount of cool rain, or strong winds and a passing cold front, can cause a complete turn-over of water in a pond. If there is stagnant, low (or no) oxygen water at the bottom, this water can be displaced upwards, causing a low oxygen problem with fish in a pond. One can avoid this problem by having shallower ponds whereby water is turned over easier by evaporative cooling or shading, etc (about 3-6 feet deep).

Ponds that have not been cleaned out on a regular basis can also have bad problems after a big rain during the warmer months. Decaying organic matter in a pond, no matter how deep it is, can utilize most, or all, of the oxygen near the pond bottom (in summer). The heat from oxidative decomposition may not warm the water enough for it to rise above the warmer summer water above it, and no new oxygen is added to the area of decomposition. Then the slower and less energy-releasing anaerobic decomposition can take over. A big rain can turn the water over and put low oxygen water where the fish are. The fish may already be stressed by low oxygen levels in the warm waters. One should clean ponds of decaying organic matter (leaves, plant parts, excess food, dead organisms, excrement, etc) on a regular basis during the warm months; this cleaning should NOT be done just prior to dark, but early in the morning. Stirring up oxygen-poor (or hydrogen sulfide/methane laden) water near dark will stress out the fish with low oxygen (or hydrogen sulfide/methane) all night. If you clean in the morning on a sunny day, then photosynthesis can add oxygen to the water throughout the day and lessen the stress on fish at night; or you can aerate the pond well after cleaning.

Having a layer of organic debris in the bottom of the pond during the winter may benefit the fish. See discussion below.

Influence of pond debris
Decaying pond debris (like compost heating up) can add warmth to pond bottom water during the winter. I have had tilapia, which usually die at temperatures below about 50°F, over-winter in ponds with decaying pecan leaves at the bottom. Some decomposing bacteria have their optimum conditions at low temperatures, 0°C to 5°C, or 32°F to 40°F; they are known as psychrophylic bacteria.

Decaying debris in ponds can warm bottom water, and this water will rise if it is warmer than the water above it, bringing down more oxygen (in the sinking denser water) to fuel more aerobic decomposition.

The chemical process that's generating the heat during decomposition is called oxidation (a great deal of energy is released in the form of heat in the oxidation of the carbon to carbon dioxide). Fish surviving the winter utilizing this heat source must also be able to tolerate possibly lower oxygen levels brought on by decomposition. But the heat of decomposition will make water rise and bring down cooler water with oxygen, which then supplies the fish with oxygen plus provides more oxygen to fuel the decomposition of the pond organics. And we have a winter pond circulation driven by heat.

This winter circulation, whether driven by convection or debris decomposition, gives one reason to explain why fish seek the deeper holes of ponds and lakes and rivers during the winter.

I suggest that one remove organic debris during the summer when it can deplete too much oxygen from the already warm water or lead to low oxygen levels during turn-overs, and keep some organic debris in the pond over winter so that the decaying debris adds some heat to the pond and drives some circulation, bringing down cooler more oxygen-laden waters from above.

Summary
Cooling of pond waters, and thus an increase in dissolved oxygen, can occur with evaporation, radiation, convection, shading, and addition of cooler water. Decaying organic debris can cause low oxygen problems during warm weather, but may provide fish a warmer place to survive through the winter.