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Re: E-M:/ RE: / manure contributing to Lake Erie dead zone

Drs. Richards, Dolan -- don't forget Michigan CAFOs are in the Maumee's headwaters, and in the River Raisin's headwaters; CAFOs in Ontario contribute to Lake Erie, etc.  Add hundreds of thousands more animals (not just cows) to your calculations.  Given the rapid rate at which liquid manure applied to fields reaches sub-surface tiles and streams, what scientific data shows less than10% of the phosphorus reaches Lake Erie?
Janet Kauffman
Environmentally Concerned Citizens of South Central Michigan

Dolan, David wrote:
This letter was sent to the Cleveland Plain Dealer in response to the article on Sunday that was posted below:
The Mathematics of Cow Pies

A recent article by Fran Henry (August 1, 2004) on the impacts of mega-dairies links nutrients from manure from these dairies with the "dead zone" in Lake Erie, and attributes to Ohio State University zoologist David Culver the statement that "[there is] a very real possibility that manure from northwest Ohio farms is contributing to Lake Erie's 6,300-square-mile 'dead zone,' an oxygen-depleted area where fish cannot live."  We think Ms. Henry has stepped into a journalistic cow pie with this assertion.  While it is undeniable that mega-dairy farms can have local ecological impacts, the implication that their manure is directly connected to Lake Erie's 'dead zone' needs to be evaluated quantitatively.

The "dead zone", a scientifically defined area of low oxygen that develops within lakes under certain conditions, does not occupy an area of 6,300 square miles, as stated.  That figure is the area of Lake Erie's Central Basin, within which the zone of low oxygen develops to a varying extent each year in the summer or early fall.  This "dead zone" is confined to the bottom 15 feet or so of the water column, and never covers the entire Central Basin area, though in recent years most of the area has suffered from low oxygen.  While the low oxygen condition is harmful or fatal to critters that must live on or in the bottom, its impact on fish is minor, because they simply move into the well-oxygenated waters overhead.

Understanding the potential impact of manure from these mega-dairies on Lake Erie requires understanding Lake Erie itself.  The productivity of Lake Erie (which is what ultimately leads to the development of the low oxygen region) is limited by the amount of phosphorus in the water.  Scientific studies in the 1970's established a target of 11,000 metric tons (12,100 short or "English" tons) per year entering the lake from all sources as an amount that would leave the lake productive enough to provide a healthy ecosystem and good fisheries, but not lead to excess algal production and consequent depletion of oxygen in summer.  The recovery of Lake Erie is in good part a history of the beneficial consequences of reducing phosphorus inputs to about that level.  Between 1996 and 2001 (the latest available data), phosphorus inputs to Lake Erie have averaged 10,590 metric tons, but have been as great as 16,850 (153% of the average) and as little as 6,610 (67% of the average).  These annual variations are due mostly to differences in weather from year to year, and consequent differences in the amount of phosphorus-bearing soil that is washed into the rivers and streams.

Ms. Henry states that there are 22,600 cows in the northwest Ohio Lake Erie watershed, and that an average cow produces 127 pounds of manure per day, which is equal to 23.2 (short) tons per year per cow. Dairy cow manure contains about 1.79 pounds of phosphorus per ton.  Put the numbers together, and you discover that all those cows together produce about 426 metric tons of phosphorus per year.  Even if all of this phosphorus were to be added to the phosphorus that's already entering Lake Erie annually, it would increase the total by less than 4%.  In all probability, less than 10% of the phosphorus from the manure actually makes it to Lake Erie, so these dairies contribute substantially less than 1% of the total amount going into the lake. This is too small an amount to have a discernable impact on the "dead zone", especially when the total input fluctuates by as much as 50% from year to year. If the number of dairies continues to increase, however, the impact on the "dead zone" will increase proportionately.

To be absolutely clear, we do not advocate increasing the phosphorus inputs to Lake Erie or the number of farm animals (dairy or otherwise) in the Lake Erie watershed.  Nor do we dispute that there have been local ecological impacts from these dairies, or that handling the manure properly and ecologically is a big challenge. We do wish that Ms. Henry would have used the available scientific data to generate a hypothesis that could stand on it own legs.

R. Peter Richards, Ph.D.                David M. Dolan, Ph.D.
Water Quality Laboratory           Dept. Natural and Applied Sciences
Heidelberg College                    University of Wisconsin
Tiffin, Ohio                     Green Bay, Wisconsin

Pete Richards has been with the Water Quality Laboratory since 1978, where he has specialized in studying the transport of phosphorus and other nutrients to Lake Erie from her Ohio tributaries.

Dave Dolan has been responsible for estimating total phosphorus inputs into the Great Lakes for various government agencies since 1976.

Dave Dolan
University of Wisconsin - Green Bay
Natural & Applied Sciences, ES-317
2420 Nicolet Dr.
Green Bay, Wisconsin 54311
(920) 465-2986 (office)
               2371 (department)
               2376 (fax)