Toledo Blade: Ponds draped in heavy snow create potential for fish kills

This Article was original published in the Toledo Blade

MALINTA, Ohio — It is dark under that heavy blanket of snow covering area ponds, and in the absence of light, death can be lurking. Over a relatively short period of time, the layer of snow can rob the plant life in the pond of vital sunlight, derail the process of photosynthesis, and halt the production of life-sustaining oxygen.

Without that oxygen, fish can suffocate under the ice.

 “With all of this snow cover we’ve had, I would expect to see quite a bit of winter fish kill,” said Bill Cody, the unofficial “Pond Doctor” of the region, from his lab here.

“If we cut the light off, the cold water species of plants start to die off, and then they’re not making oxygen at all. And with the extreme amount of snow we’ve had in January, and the outlook for the near future, we could be looking at a solid eight weeks of this heavy snow cover on our ponds. That creates some serious problems.”

January brought a record 40.2 inches of snowfall to the Toledo area, and with ice already locked in place on many area ponds and smaller impoundments, fish biologists see cause for concern.

“Just a few days of snow cover can bring on problems in some ponds,” said Mike Wilkerson, fish management supervisor at the Division of Wildlife office in Findlay. “If light is not reaching the pond, it doesn’t take long for the plant life to die back, and as those plants die, the oxygen can be depleted fairly quickly.”

Even in the worst of winter, some plants present in the pond — from microscopic forms of phytoplankton to algae and other vegetation attached to the bottom — will continue to grow and release oxygen into the water column, but light is the most critical element in that process.

“As long as some light is getting through and penetrating that ice, you will get aquatic vegetation growth,” Wilkerson said.

Both Wilkerson and Cody stressed that the circumstances that bring about winter fish kills won’t be present in every pond in the area.

“The ponds that get a lot of wind exposure, where the wind can sweep the snow off the ice and allow light to get through — those won’t experience the same kinds of problems as the more wind-protected ponds,” Cody said. The freeze-thaw cycle can also complicate matters, he said, since it makes the ice more opaque, further hindering the passage of light to the plant life below the surface.

A deeper pond — with a large area of 16-20 feet of water — will have a much larger “bank account” of oxygen, Cody added, and therefore be at a lesser risk of experiencing a rapidly developing fish kill. A shallow pond will hold much less oxygen in reserve.

The age of the pond can also exacerbate the situation, since older ponds tend to be much more laden with muck and nutrients that consume oxygen as these materials continue to decompose, Cody said.

“The older the pond, the much more careful we have to be with any snow cover,” he said, “since all of the accumulated dead algae, muck, and any other dying vegetation will suck up a lot of the oxygen. That’s where most of the oxygen is used up.”

Bob Hesterman, who runs the Fin Farm commercial fishery in Ridgeville Corners near Napoleon, said he has been clearing sections of the ice over his rearing ponds on a regular basis, to keep the oxygen-producing photosynthesis working. Biologists recommend that all pond owners remove strips or sections of snow from the ice to allow sunlight to reach the water.

“It is such an important part of the process, to keep those phytoplankton producing oxygen,” Hesterman said. “Sunlight is the engine in that process, and without it, photosynthesis just stops. If that happens, once the fish consume all of the oxygen, they will suffocate.”

Hesterman also uses aerators to pump additional oxygen into his ponds, positioning them along the shoreline in winter so he does not “super cool” the water by pumping frigid air into the depths of the pond. He has observed a secondary benefit of the shoreline aerators, which create a small section of open water, even on the coldest days.

“We’ll have wildlife come up to the edge of the pond to use that water source,” he said. “A lot of songbirds will utilize that water, especially where there’s a large rock nearby for them to land on. We’ve had cardinals and jays come down to drink, plus rabbits and deer — anything that’s out and about will take advantage of that water.”

Wilkerson said winter fish kills related to oxygen depletion are not usually a problem in the large upground reservoirs in the area, since they normally do not have substantial amounts of dead vegetation decomposing in them. Cody added that Lake Erie has a much larger “account of oxygen” stored in its water, plus the wind sweeps the snow from a lot of the lake’s surface, allowing sunlight to reach the water beneath the ice.

“But these older ponds, especially the ones that are somewhat protected from the wind and have a significant amount of snow cover, those are the ones where we’re going to see the fish kills,” Cody said. “People won’t notice it right away, because these fish will die under the ice and then decompose on the bottom, but when they start fishing in the spring, that’s when I’ll hear from them. I expect we’ll see a lot of fish kills this winter.”