It’s a cool, misty morning, after one of our famous summer evening thunderstorms. You’ve gone out into the backyard to visit your ornamental pond and you see dead fish, possibly several, when just yesterday they seemed fine. Yikes, what happened? This is what the pond folks call summerkill.
Summerkill is an oxygen depletion event. Most fish prefer an oxygen concentration in the water of about 4 parts per million (ppm), but sometimes, during the heat of late spring and early summer, a layer of oxygen poor water may form at the bottom of the pond. This bottom layer will be cooler, darker, and may smell bad. If the water on the bottom contains less than 2 ppm of dissolved oxygen the fish will try to stay out of it, even if they are a species that normally prefers cooler temperatures (i.e. goldfish, koi). Bacterial decomposition of organic material causes this "bad" water. In the decay process, bacteria consume dissolved oxygen in the water to aid in breaking down plant and animal material, further dropping oxygen levels.
This condition occurs more frequently in deeper ponds, where excess water flows off of the surface layer. The bottom layer slowly accumulates while any overflow comes from the top.
If you happened to be back there during the event you would see the fish swimming with their heads poking out of the water, opening and closing their mouths rapidly, as though they were gasping for breath. This actually does help them to take in some oxygen as it diffuses in from the air, into the surface film of the water. If your pond has sloping sides, the poor things might be perched on their stomachs in the shallows as well. This helps them to conserve oxygen that would normally go to driving the muscles in their tails. From there, they continue to “gasp” heavily, trying to draw in as much oxygen as possible. If this situation is not soon corrected, the fish will begin to die. Larger fish, with their higher oxygen demand, tend to die before smaller ones. Small fish may be able to hold out longer, in those shallow areas where the big ones just can’t fit.
Summerkill begins with overcast skies, hot weather and the return of rainy days. Low water levels, brought on by one of our normal, dry springs contribute to the problem. A heavy rain, flowing into the pond, can cause the top and bottom layers of the water to mix. After mixing, the pond may change color from its normal green to brown. This mixing may happen a day or two following the rain, and the subsequent drop in dissolved oxygen need only occur for a short time to kill fish. It might even happen during the night, so the oxygen concentration could be on its way back up toward normal levels by the time you wander out there.
The best strategy for dealing with this situation is to prevent the buildup of that lower, oxygen poor layer in the first place by keeping the pond water circulating. This can be done by adding a submerged pump, a decorative fountain or an actual filter. If you discover an oxygen depletion event as it is happening, stirring the surface or adding water with a garden hose may help. This exposes more of the surface to the air above, so that more oxygen diffuses into the pond. Once the oxygen levels begin to rise, fish will stop acting like they just swam a marathon.
If you don’t have them already, adding live plants to the pond is helpful. Plants use sunlight to produce and add dissolved oxygen to the water, a process known as photosynthesis. When there are not enough plants, or when overcast skies keep sunlight from reaching the pond for several days, oxygen levels will drop. These plants can be anything from that suspended, microscopic algae that makes the water appear green, to broad-leafed aquatic plants, rooted in containers and submerged. Conversely, plants whose leaves float on the surface (i.e., water lilies) and other plants whose leaves are exposed to open air, do not contribute as much to the oxygen supply below the water surface.
Rain and wind from thunderstorms may also exacerbate the problems by stirring up bottom sediments, muddying the pond and further blocking the amount of sunlight penetrating the water. Rain can also wash in even more of the decaying organic material that our hungry bacteria in the bottom layer were decomposing to begin with.
You may experience oxygen depletion events for several nights running, until the pond stabilizes again. In this case it might be wise to add water or stir the surface each night until you are no longer greeted by gasping fish in the morning.