Why Does the Lake Erie Dead Zone Exist?
In the summer, the water in the Great Lakes separates into two layers. The top layer is warmer than the bottom one, it receives the sunlight, and it mixes with oxygen from the air. The bottom layer is cooler than the top layer, it is usually dark, and it is cut off from the air so it cannot re-supply its oxygen.
The Western Basin of Lake Erie is shallower than the typical thickness of the upper layer of water. Winds are able to keep the whole water column stirring, so there is plenty of opportunity for oxygen to dissolve into the water from the air. The bottom of the Central Basin is a broad, shallow plain that is slightly deeper than the typical thickness of the upper layer. Therefore, the bottom layer is relatively thin and contains a relatively small volume of water. In the deeper Eastern Basin , the bottom layer is much thicker and contains more water.
If there is much phosphorus in the water, it acts like fertilizer, and more algae will grow in the warm, sunlit top layer. Eventually these algae will sink into the dark bottom layer, where they stop growing and begin dying. Bacteria and fungi then decompose the plant organic matter. Bacteria and fungi also need oxygen to live, and they use what is available in the water. Because the bottom layer is cut off from the air, over the summer, less and less oxygen remains in the water. If there is a small volume of water and a lot of decomposition going on, as exists in the Central Basin , the oxygen will be used up faster than if there is much water and/or little decomposition. This is not a problem in the Eastern Basin largely because of the much greater volume of water in the bottom layer.
Each autumn, the top layer cools, and the wind mixes it deeper and deeper into the bottom layer. Eventually the whole water column is the same temperature, and the wind can again mix it from top to bottom and the oxygen can be restored from the air. In the Central Basin , this phenomenon typically occurs in early to mid-September, when oxygen is restored from top to bottom.
I am a retired news journalist who now operates a sportfishing charter business on Lake Erie.
