In response to the many questions from anglers, media and those concerned about Lake Erie resources, we present the following fact sheet regarding the central basin "dead zone."
What is the central basin "Dead Zone?" The central basin "dead zone" is a layer of cold water at the bottom of the central basin that has little or no dissolved oxygen in it. The central basin is located from about Huron, Ohio, to Erie, Pennsylvania. As waters warm in the central basin during the summer, they form three specific layers. The warm surface and upper layer is called the epilimnion, it is characterized by warm water with good oxygen and plant and animal life. The second layer is called the thermocline, and it is a small layer of rapid temperature change and loss of some oxygen. The bottom layer is known as the hypolimnion; it is cold, has low to moderate oxygen, no sunlight or algae activity, and decomposition by bacteria occurs here. These conditions can occur in most offshore waters deeper than 15-18 meters (49-60 feet) to the bottom of the lake. Hot weather, precipitation and wave action all dictate the onset and length of this condition. It can vary in depth and the amount of oxygen in the hypolimnion across the lake, north and south, east and west as a variable, patchy matrix. These conditions occur every summer from mid to late July through early September. After September this zone breaks down, when the lake becomes all one temperature again in an event called a turnover.
What are the causes of this condition? The dead zone occurs because bacteria use up oxygen as they decompose dead algae and other materials that have sunk to the bottom. As they use up oxygen, there is none to replace it because of the temperature and water density gradient. The central basin is unique in that the cold hypolimnion layer at the bottom is relatively small and all or nearly all of the oxygen may be used up seasonally by bacteria in the decomposition process. The Western Basin of Lake Erie is too shallow for this cold water layer to occur; plants and algae can occur throughout the water column, so decomposition can go on freely at the bottom throughout the summer without using up all the oxygen. The Eastern Basin of Lake Erie is much deeper, so it has a greater volume of cold, well-oxygenated water to use up in the short time period. The shape and depth of the Central Basin and its many sources of nutrient inputs mean that this anoxic condition could occur or has occurred seasonally each year. In fact, research has shown that this period of low oxygen in the Central Basin hypolimnion has been recorded for centuries. This condition is made worse by heavy nutrient inputs (like phosphorous and nitrogen) via rivers and other man-made sources that give rise to large blooms of both green (good) algae and blue-green (noxious) algae. Water quality agreements made in the 1970s helped reduce nutrient inputs into the lake, but increasing development pressures have led to problems with Combined Sewer Overflows (CSOs) and non-point source pollution from agriculture and development. Certainly human-induced effects have made this problem worse.
Why is it a problem? Exotic species like zebra and quagga mussels also change the way nutrients are cycled and the timing of nutrient availability. Zebra mussels, and particularly quagga mussels, filter out nutrients and green algae early in the year and release fecal pellets (nutrients) late into the summer, and reject the blue green (bad) algae. This process causes more demand for decomposition and also partitions green algae (phytoplankton) that would normally be available to zooplankton, which is a good source of food for juvenile sport and commercial fish and other small fishes. Concentrations of blue green algae can lead to taste and odor problems with drinking water from the lake. The loss of oxygen in the lower layer of the lake will force aquatic life to move out of that volume of water to places that will be less than ideal for them, but they generally will still survive. One other problem that can occur is when strong winds blow the cold layer of water into shore, resulting in a rapid water temperature change and a rapid loss of dissolved oxygen which can cause a large fish kill.
What is being done about this condition? Collaborative research on the extent of the Central Basin dead zone is being conducted by the USEPA Research Vessel Lake Guardian. On-going research and sampling is being conducted by Lake Erie agencies (like our work on the Research Vessel Grandon out of Fairport Harbor). Members of the US Congress are acting on the National Invasive Species Act reauthorization, and Great Lakes agencies in the US and Canada are implementing projects to reduce CSO and non-point impacts and restore natural river corridors and wetlands.
What can we do as citizens? The best thing we can do is support legislation that aims to stop the spread of exotic aquatic nuisance species, support funding for improved treatment facilities that reduce CSO input, act responsibly by not spreading exotic species, and utilize smart land use practices that protect or improve watersheds and wetlands, minimize shoreline and streamside development, and reduce nutrient inputs and runoff. These practices include protecting vegetation along streamside setbacks for at least 50 feet, reducing or improving timing of fertilizer applications, reducing harbor and nearshore hard structures like seawalls and jetties, maintaining thickly vegetated buffer strips and retention basins adjacent to developed areas, not overtly clear-cutting areas for development and monoculture planting, and even recycling.
For more information contact us:
Fairport Harbor Fish Research Unit
Ohio DNR, Division of Wildlife
1190 High St.
Fairport Harbor, Ohio 44077
Sandusky Fish Research Unit
Ohio DNR, Division of Wildlife
305 E. Shoreline Dr.
Sandusky, Ohio 44870
Division of Wildlife information: 1-800-WILDLIFE