Great topic, and one on which I am NOT an expert. However, I am quickly learning by hard experience. I had a few comments that I thought might add to this discussion.
First, I want to ensure that winter aeration is given credit for being a useful technique. When you get sufficiently long winters with sufficiently productive water bodies, there is no way to sustain a fish community without aeration. So, this does NOT apply everywhere, but certainly does in the northern plains states (eastern SD is a good example). There is no way that a 3-acre pond, even 15 feet deep, would sustain a pond fish community through the winter in my locale without some assistance.
Before I continue, I must admit to knowing way too little about winter aeration. I was always taught and passed on the old idea that you put the aerator discharge into the deepest part of a water body. Heck, it made sense! The warmest water is at the bottom of the pond during the winter because water is heaviest at 39 F (4 C). [Without this very unique characteristic, ponds would freeze from the bottom up rather than the top down, and we’d really be in trouble! :-)] So, it made sense to me that you put the air bubbles into the warmest water, and then lifted that water to melt a hole in the ice above. This little factoid obviously was not correct! I also was always taught that direct transfer of oxygen from the air bubbles to the water was minimal. The open hole in the ice allows some photosynthesis and oxygen production, even from the reduced algal/plant community in winter. This little factoid apparently is correct, although I recently learned that direct transfer of oxygen also occurs from the atmosphere to the open-water surface.
The concept that winter aerators should be placed in shallow water has been a great example of how I have learned much from this forum. Let me pass on my “mistake” story.
We had two ponds, one 3 acres and one 7 acres. Both were in grassed watersheds, both were relatively deep (18 ft and 23 ft, respectively), and both were moderately productive, but not overly so. They were far from electricity, so Koender’s windmills were installed, and we put the airstones on the bottom in the deepest part of the pond. The winter of 2000/2001 was a tough one that started early, and ran late. We had snow and ice in October, and so much snow that the ice did not blow clear. Ice-out did not occur until the start of April. What a sad day that was. I walked the shorelines of both ponds, and it was hard to see all those dead 12 inch black crappies that we had developed in the largemouth bass pond. It was similarly hard to look at all those 18 inch smallmouth bass dead on the shoreline of the smallmouth-only pond. The aerators had worked well all winter. During calm periods, the open hole in the ice would freeze over, but as long as there was wind, the holes would open. By the way, a 22-acre pond with 3 windmills survived that bad winter in great shape.
I was sorely disappointed in these winterkills because the ponds were built as well as possible, in as good a watershed as they could be, and the aerators were installed. Based on good input from this Forum, I learned that during a long winter period of circulation, the aeration may result in cooling of the entire water body (under the ice). Too long at too cold temps may stress the fish too much. I also learned that too much aeration may circulate too much water, and allow too much of this cooling to occur. So this lesson is simple: don’t use more aeration than necessary for the size of your water body. There are people on this website far more qualified than I to provide information on appropriate air volume capabilities for various sizes of ponds.
So, what did we do at those two ponds? Well, I suggested to the landowner that he move the aerators up into shallow water, just as has been suggested here.
One of the Pond Boss Forum regulars passed on a publication to me. Theron G. Miller and W. C. Mackay, 2003, Optimizing artificial aeration for lake winterkill prevention, Lake and Reservoir Management, Volume 19, number 4, pages 355-363. I learned a lot from this article, and it certainly fits with the concepts expressed so far on this topic. In this study in Alberta, they found that dissolved oxygen levels actually were better in waters with surface aerators (kept a hole open in the ice) than from aeration units in which the airstone was on the lake bottom. The key was keeping that hole open in the ice. Circulating from the bottom up apparently must have moved some organic material, because dissolved oxygen was always lower in waters with those circulation systems. Finally, this paper actually had some recommendations on optimal sizes of aeration units, based on kW per unit of surface area for the water body.
This may sound funny, but I think we need some more field assessment of aeration and winterkill potential. This winter, I was hoping to get dissolved oxygen profiles from several of the area ponds we manage (ponds with windmill aerators, and some gravel/sand pits that don’t have and don’t need aeration). However, it’s already Dec 10, and there is only a skim of ice on the ponds due to this warm fall we are having. So, it will be a short winter this year! I’m not sure how much we will learn. Good old Mother Nature and her variability!
Dave
