Summer, which coincides with hurricane season, causes water in a pond or lake to form layers of varying temperatures with the warmer (or lighter) water on top. If you’ve ever been swimming in a pond during the summer you have probably noticed this phenomenon. Your upper body would be in sun- warmed water and your feet and legs in cold water. The warm water reaches to a depth that sunlight can penetrate and drive photosynthesis, which is usually around a depth of 6 feet. In some soft water lakes, the upper layer may extend to 22 feet (here is where we are assuming that you have limed your pond to increase the hardness and alkalinity). In managed ponds, sunlight should only be able to penetrate 18-24 inches. Since this top layer of water is where photosynthesis takes place, it is also where oxygen is being produced; however, warm water holds less oxygen than cold water. In the summer, the top layer of water may be saturated with 5 ppm (parts per million) dissolved oxygen commonly referred to as D.O. At 3 ppm fish will start to die. Oxygen in cold water at the bottom is quickly used by rotting detritus (remains of something that has been broken down such as leaves, plant matter, etc.) and is not replaced. The cooler temperatures that come in the fall cause the varying degrees in the water to mix until they are almost the same throughout. They remain mixed until the following summer’s heat causes the water to layer out again.

Most people think that rain water splashing on the surface of a pond is enough to add oxygen. This is true to an extent, but it isn’t enough. The problem is that when it rains for several days at a time, the sun is blocked and not able to drive photosynthesis to produce oxygen. Splashing rain is not enough to overcome this loss. A hurricane has winds strong enough to mix the layers of water in a pond, even when the temperature differences are great. If the top water has an estimated 5 ppm of D.O. and the bottom is 0 ppm D.O., then when they are mixed the combined oxygen may only be 1-3 ppm. That’s when the fish begin to die. In a low oxygen kill the larger fish usually die first. Smaller fish can sometimes get enough oxygen diffusing into the very surface of the water from the air, but not always. Fish that die from lack of oxygen will have flared gills and open mouths. The same is true of gill parasites and other diseases but these are not caused by hurricanes. If you’ve seen film of the toxic water in New Orleans, you can understand how these toxins can also cause fish kills. Fish kills from poisons will generally kill fish of any size. It is also possible that some toxins might get blown into or run into your pond, but lack of oxygen is more likely the culprit.

Just like the damage to the Gulf Coast, there is not much you can do to prevent a hurricane from hurting your pond’s fish population. If you have the capability, you may be able to pump well water into the pond for cooling. Caution must be used when doing this as well water doesn’t contain dissolved oxygen. Water can also be pumped from the bottom of your pond and discharged on the surface, but this method is just as risky as bottom water seldom contains oxygen. If you don't have an aeration system in place, you can reduce the risks associated with this problem by oxygenating water in various ways: 1) Use a piece of capped PVC pipe and drill small holes in the cap. This can then be used to form a fountain. 2) Suspend a vertical piece of pipe above the surface of the water and fill with large gravel. Hold the gravel in place by using a piece of hardware cloth and run water through the pipe. Either of these two methods will break the water into small droplets. This will break up the water into smaller drops that have more surface area and therefore can absorb oxygen quicker. 3) If you have an aerator you can run it if electricity is still available or if you have the use of a generator.