Winterkill prevention in lakes and ponds using artificial aeration

Abstract

Winterkill is a common phenomenon in many eutrophic lakes subject to prolonged ice and snow cover. These conditions can lead to greatly reduced oxygen absorption and generation under ice‐cover, compared with oxygen consumption, and to dissolved oxygen (DO) depletions below concentrations required for fish survival. DO requirements of fishes under ice‐cover are not well defined, but are much reduced compared with summer conditions, with many fishes surviving at DO concentrations <1.0 mg/1 at water temperatures of 0 to 4°C. A lower DO limit of 2 mg/1 for gamefish has been suggested. Typically, as under‐ice DO concentrations become progressively depleted, the more valued gamefish succumb first, leaving less desirable fishes; and, in extreme cases, no fish survive. Frequencies, causes, and preventions of winterkill have been most thoroughly studied in Michigan, Minnesota, Wisconsin, and the Great Plains region of the U.S. and Canada. Long‐term records from these areas indicate that winterkill frequency is related to duration and intensity of winter weather conditions; but some lakes experience winterkill almost every year regardless of the weather.

Many approaches have been tried for winterkill prevention or manipulation, including: snow removal, mechanically cutting holes in ice, pumping water onto ice, water level manipulation, fish population manipulation, and artificial aeration. The most common method of artificial aeration for winterkill prevention involves air injection from a line or point sources. Resultant bubble plumes upwell deep water, which has higher temperatures than water just under the ice. This results in ice melt and creates an open water area. This type of aeration requires suitable heat reserves in deep waters to maintain an open water area for extended periods. Oxygen is absorbed from injected air and from the atmosphere, as well as from photosynthetic DO production due to greater light penetration.

Aeration also can be applied before ice‐cover forms to oxidize substances in water and sediments and thereby reduce under‐ice DO demands and increase DO reserves in the water at the time of ice formation. Under ice‐cover, water densities are slight, and very little energy is required to circulate the entire water mass. Often, under‐sized “aeration”; systems fail, or even worsen, winterkill conditions by circulating waters and increasing DO consumption, without adding much DO to the water.

Aeration alternatives include pump‐and‐baffle systems, and partial aeration. Pump‐and‐baffle aerators pump water from the lake, aerate it on‐shore, and return aerated water to the lake. They are independent of the lake's heat reserve. Partial aeration would create a DO sanctuary for fish, without attempting to aerate the entire water mass. Other aeration systems involve surface‐aspirating pumps, surface aerators, wind‐powered aerators, use of molecular oxygen, and hypolimnetic aeration under ice‐cover. Better methods are needed for winterkill prediction, predicting DO demands, and sizing aeration systems for a given lake.

Key words:

• Winterkill
• fish kill
• lake aeration
• pond aeration
• fishery management
• lake management