The occurrence and degree of injury are determined primarily by the relation between the amount of dissolved oxygen in the water and the amount of oxygen required by the cranberry vines.

Water in contact with the air normally contains oxygen in solution. The greatest amount of oxygen that water can hold in solution at 32 F. is about 14 parts per million, at 40 about 12.4, and at 50 about 11.

The oxygen content of the water of the flooded bog tends to remain uniform at a given temperature because of the action of diffusion, convection, and wind. Wind sets the surface water in motion and causes it to mix with water below. The stronger the wind, the faster the mixing proceeds and the greater the depth to which it extends.

Two physiological processes, respiration and photosynthesis, carried on by plants and other living organisms, often cause great variations in the oxygen content of the water. Mosses, algae, and other plants, besides the vines, grow in the bogs. Besides, bacteria and other micro-organisms exist in organic matter on the surface of bog soils and in the soil itself.

Respiration releases chemical energy mainly by the oxidation of carbohydrates. The energy is used for the performance of the physiological processes necessary to maintain life and goes on in every living cell. The process requires oxygen, and carbon dioxide is given off.

The oxygen used in respiration by the cranberry vines and other plants on a flooded bog is taken from that in solution in the water and the oxygen given off in photosynthesis goes into solution in it. Consequently, respiration reduces the amount of dissolved oxygen and photosynthesis increases it.

The oxygen content of water on winter-flooded bogs not covered with ice usually undergoes only relatively small, brief changes, as there is nearly always enough wind to cause mixing, so that the water is kept at or near its oxygen capacity.

On winter-flooded bogs covered by ice, circulation of the water by wind is prevented. The oxygen content of the water is then determined by the relative rate of oxygen consumption by the cranberry vines and the other organisms, as compared with the rate of liberation of oxygen in photosynthesis, mainly in the cranberry vines themselves. Since respiration goes on continuously, while photosynthesis occurs only in light, the oxygen content of the water increases or decreases at a rate proportional to the amount by which the oxygen given off in photosynthesis is greater or less than that used in respiration. Light, therefore, becomes the controlling factor in determining the oxygen content of the water on an ice-covered bog.

The amount of light received by the cranberry vines in water under ice depends primarily on the degree and duration of cloudiness, the thickness of the ice, the inclusion of snow in the ice,and the presence or absence of a snow cover.

Snow on the ice is the most important factor in causing a reduction in the oxygen content of water on winter-flooded bogs, since by excluding light, it prevents the liberation of oxygen in photosynthesis. The inclusion of snow in the ice may sometimes cause almost as great a reduction in the oxygen content of the water under ice as does snow on the ice.

The amount of oxygen required by vines frozen into the ice probably becomes negligible at the low temperature of ice in cold weather. It is always less than that required by vines in water under the ice. That may be an important factor in determining the probability of injury from oxygen deficiency and in determining the severity of injury when an oxygen deficiency occurs. Numerous observations have shown that shallowly flooded vines, which are frozen into the ice during the winter, produce larger crops and bear more regularly than those deeply flooded.

Different parts of the plant vary greatly in their oxygen requirement. The more active the part, the more oxygen it requires. The most active parts of cranberry vines in their winter condition are the flower buds, young leaves, and the growing point of the stem within the terminal buds. They are the first to be injured or killed, therefore, when the oxygen content of the water reaches a low level. The old leaves are much less active. The stems are least active and accordingly are injured only under extreme conditions of oxygen deficiency.

Injury occurs only when the oxygen content of the water reaches a level at which the oxygen requirement of the more active parts of the cranberry vines cannot be supplied. Evidence indicates that this level is about 5.7 parts per million, since injury occurs when the oxygen content of the water reaches that level and remains there for a day or two. The injury is more severe at a lower oxygen content or for a greater length of time. Usually the oxygen content of the water falls to that level only when there are several inches of snow on the ice, but it may do so in very cloudy weather when there is little or no snow on the ice, but much snow is included in it.

Injury from oxygen deficiency on winter-flooded bogs may be greatly reduced or prevented by changes in the flooding practice. Bogs are often flooded too deeply or for too long a time. Bogs should be flooded for as short a time as weather permits and should be flooded as shallowly as possible. Shallowly flooded vines are soon frozen into the ice when ice forms and then are injured less. On bogs that are much out of grade, some parts must be deeply flooded if the higher parts are covered. Then it usually would be better (in Massachusetts and New Jersey) to flood the bog shallowly and run the risk of some winter-killing on the higher parts. Moreover, the loss in yield as a result of oxygen deficiency injury to vines in water under ice on deeply flooded parts of a bog in most years and on most bogs is much greater than the loss from winter-killing on parts of the bog not flooded.

When the water supply is ample, bogs may be flooded in the usual way. Then after 5 to 6 inches of ice have formed, if the oxygen content of the water drops to near 5.7 parts per million, the water should be drawn out from under the ice. As long as ice remains, the bog need not be reflooded, but as soon as the ice melts from any considerable part of the bog it should again be flooded. This procedure has been used with success in Wisconsin and has been used on many bogs in Massachusetts when winters were cold enough to make ice of the desired thickness.

HERBERT F. BERGMAN has degrees from Kansas State College and the University of Minnesota. He joined the Bureau of Plant Industry, Soils, and Agricultural Engineering in 1917.