I worked on this answer for a couple days. Post was enhanced again on Dec 13th.
This discussion of winter aeration has been interesting.
Bruce brings up the phenomonon of super-cooling of the entire water column during ice cover when diffusers are operated in the deep pond basin. Pottsy reports that his experiences lead him to believe that thicker ice is not clear enough to allow penetration of adequate light to maintain photosynthesis for plants to enhance the pond�s dissolved oxygen. PaPond remarks that the geo-thermal affect also aids the warming of the water near the mud water interface. Wood�s experiences lead him to believe that fish density, pond age, overall nutrient content and BOD of the pond are important in whether a pond needs aerated. D.Willis reminds us that oxygen transfers into water from diffused air bubbles and when the unfrozen open water area is exposed to atmospheric air. Ted provides some information about his DO and temperature measurements during ice cover and experiments of diffuser types and their location or placement. Brian, Dave and Cecil reported their experiences of the affects of supercooling on their fish.
Here are my experiences and homework results on this topic:
My temperature tests have proven that deep water mixing from aeration does lower the temperature below the normal 39F in the overall water column i.e. super-cooling or chilling. I have measured temperatures of 34F-36F through much of the water column and near the bottom even in a pond where the aerator was shut off a week or two before ice formed. Extended aeration and wind action supercooled the water in this situation before ice formation.
The warm water layer near the pond bottom is due primarily to the transmission or release of heat that was absorbed by the sediments during the warm periods of the year (Wetzel 1983, 2001). Geothermal affect has been measured to occur but is a low amount compared to the overall release of the summer heat sink from the sediments. I think the amount of heat release from the sediments can be even greater during winter when the pond has been aerated to a point during the summer where the water column is a fairly uniform temperature top to bottom (homoiothermic). I have measured aerated bottom water to be 80F+ during summer. I think this condition allows a greater amount of heat to be absorbed by sediments and then it is transmitted or releaed back to the water during winter. In these homiothermic ponds I have measured bottom water temperatures of as high as 41F to 41.4F during ice cover; truly a warm water refuge during winter.
In almost all cases, I think aeration is not needed during winter. McComas 2003 recommends about 30% of the ice surface can be cleared of snow cover and it can be removed in alternating strips of snow strip, clear strip, snow strip , clear strip. As a rule I try to not let snow cover the ice more than 7 to 10 days before I remove some of it. I usually try to remove the snow in north-south strips to take advantage of the angle of sunlight penetration through the ice. I am not sure if this is a more effective method, but I do it just in case. Often many of the wind exposed ponds in NW OH get fairly large areas cleared of snow by blowing wind and snow shoveling is not necessary.
Light stimulates underwater plants to produce oxygen that dissolves into the water. Three main types of underwater plants � phytoplankton, attached algae, and rooted submerged �weeds�, produce the majority of the dissolved oxygen (DO) in ponds and lakes. Light penetration through ice cover is very important in maintaining dissolved oxygen (DO) levels during winter. Light will pass through clear ice similar to that through water (Wetzel (2001). However the condition or type of ice and snow cover can greatly affect the amount of light that penetrates the ice cover. Snow cover reduces light penetration through ice the most. Snow at 5� deep stops about all light transmission through ice. Cloudy surface ice layer, white ice or ice with air bubles has an intermediate light transmission affect. When light can shine through 17� of clear ice about 72% of light will pass through and 23% will pass through 5 ft of clear ice (Wetzel 2001). This was verified by Wood when he kept snow off his Alberta pond with up to 37� of ice and his trout survived the extended ice cover season without aeration. Trout require at least 5 ppm DO for survival. Phytoplankton under the ice did their job of producing adequate DO all winter long for Wood and his trout pond.
I have noticed that ice will freeze very clear even in relatively cloudy water that was murky from suspended solid particulates. When I chop ice holes for fishing in a pond with fairly cloudy water, the lower chunks of ice are composed of very clear ice that look like glass. It looks very similar to ice made from distilled water � very clear. I think this occurs because as ice crystals form it forces the particulates away from the forming ice crystals and the ice forms clear. This clear ice allows lots of light transmission.
DO has been measured to increase about 3 ppm per day for 3 days under snow free ice due mostly to activity of phytoplankton (Bennett 1970). The high saturation of DO that Ted Lea measured under ice was no doubt due to phytoplankton activity under the ice. Certain phytoplankton taxa or forms have a survival advantage to thrive in cold water with low light intensities (Wetzel 2001). Some algae types have been reported to actually form bloom conditions under the ice. More research will provide more information in these areas. I think that the cold water species of phytoplankton which are usually composed of very small forms can do an excellent job of producing DO, especially during sunny days, if they get a small amount of light though the surface ice cover.
Alternatively, in extreme cases DO has been measured to decrease very rapidly under ice cover in a Michigan lake where DO decreased from 12.3 ppm to 2.4 ppm in 2 days (5ppm/day). Normally DO loss is not normally this rapid when a fresh snow blankets the ice cover. Some lakes can withstand several weeks to a several months of snow covered ice conditions and still have adequate DO for fish survival. The primary cause of DO loss in winter is due to the amount of microbial activity and decomposition of organic particulates in the water column and organic sediments (BOD biochemical oxygen demand).
Obviously the more organically enriched and shallower the water body the more rapid the loss of DO will be in the dark conditions under snow covered ice. Keep in mind that in dark conditions, everything living consumes oxygen including bacteria decomposing the already dead materials. When conditions have been dark long enough that the plants die or begin to deteriorate and then illuminated conditions reoccur there will be a lag or delay time for plants to either recolonize, reestablish or recover before substantial oxygen production can begin again.
If removing snow from ice is not economically or physically feasible then use of aeration can be an option for enhancing the DO of a pond or small lake. If DO levels in WINTER cold water drop to 2 or 1ppm or less the pond has a potential for winter kill and aeration at that point in time will probably not prevent winterkill for that year. Fish will tolerate lower DO in winter than summer due to a lower metabolism. Too little - too late. The intent is not to oxygenate the entire lake or pond but rather create an oxygen rich refuge in part of the lake to allow fish to survive the low oxygen periods under ice and snow cover. The goal of winter aeration should be to aerate 10% of the lake volume to minimize the chance of winter kill (McComas 2003).
Last edited by Bill Cody; 01/19/20 06:35 PM.
