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Low Oxygen Raises Ponds Threat
Our 'January thaw' is welcome news to most Iowans. However, weeks of snow cover and bitter cold conditions have done a number on many of the state's 80,000-plus ponds. Fish in those ponds process oxygen in the water to 'breathe.' In warm weather, that oxygen is produced through photosynthesis as sunlight reaches underwater vegetation. Diffusion of oxygen also comes as air meets the water surface. Springs and a current through any body of water provide oxygen, too. With the double whammy of early winter ice and snow, Mother Nature has nailed a lid on many ponds. "We went out to monitor levels in some ponds for an early season base; to compare late winter readings to them. We found that they were already at levels we often find late in the winter," explains Scott Gritters, fisheries biologist with the Department of Natural Resources. Levels below three parts per million cause stress on fish. If it gets below two parts per million for an extended period, fish start dying. In northeast Iowa, Gritters has seen readings of 1 to 2.5; at the bottom and top of a pond's water column. "When we drop a camera in that water, we see fish just sitting there motionless. It's a big concern; we have a lot of winter left," cautions Gritters. It's a concern that crosses the state, especially where snow cover is thicker; in central and western Iowa. There are solutions. An aerator is a good one; though it should be installed before cold weather blows in. The quickest solution is to simply shovel off a section of the pond. The larger the clearing, the more sunlight is allowed to beam through to the vegetation below. The downside, of course, is that the cleared section gets covered each time it snows. Long term solutions include land management around the pond. A buffer strip above it will reduce runoff of leaves, detritus from crop fields and manure from livestock in the vicinity. Building the pond deep enough to stand up to years of runoff is also an effective choice. The toll usually doesn't show up until the spring thaw. Of course, some fish die in just about any Iowa winter. When it is serious enough that large numbers of game fish wash up on shore, though, it means a part of a valuable investment is lost...as well as some great outdoor recreation.
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