How important is controlling air pollution to the Bay cleanup effort?
It's a matter of perspective.

In 1987, the Bay States agreed to reduce the amount of the nutrients phosphorus and nitrogen entering the Bay 40 percent by the turn of the century to improve the Chesapeake's water quality. Those nutrients spur excess algae production in the Bay. When t he algae die and sink to the bottom, they decompose in a process that depletes the water of oxygen needed by aquatic organisms, thereby limiting habitat. Computer models in 1987 indicated that a 40 percent nutrient reduction would decrease algae productio n enough to end low-oxygen conditions in the Bay.

That goal was re-evaluated in 1992, when research showed that large amounts nutrients were "uncontrollable" - they were part of the natural "background" nutrient load; originated from parts of the watershed outside the Bay Agreement signatory states of M aryland, Virginia and Pennsylvania; or resulted from air pollution. At that point, air pollution was considered too difficult to control for Bay cleanup purposes.

The bottom line: While the Bay gets more than 300 million pounds of nitrogen a year, nearly half of that was considered uncontrollable. The 40 percent reduction - measured only from the smaller, controllable, portion of the nitrogen load - amounted to a bout 74 million pounds a year.

Computer water quality models showed that amount of reduction would not eliminate low-oxygen conditions in the Bay, but it would significantly improve water quality. In 1992, the 74 million pound reduction figure for nitrogen was adopted. It was also agr eed that after the reduction was achieved, nutrient loads would be "capped" at the reduced level (with about 230 million pounds entering the Bay a year).

The 74 million pound goal has been difficult to reach, and achieving it will push nutrient control efforts on farmlands and at sewage treatment plants in some areas of the watershed to the limit of current technology, according to reports from the Bay st ates and the Bay Program.

Maintaining the 230 million pound cap despite population growth - which will result in more sewage and air pollution - will be even more difficult. As other traditional water pollution control methods reach the "limit of technology," officials see air po llution as a potentially important tool in maintaining the cap.

For example, modeling done for the Bay Program indicates that implementing the 1990 Clean Air Act controls on cars and power plants would decrease the amount of nitrogen reaching the Bay by about 5 percent. But that is equivalent to 18 percent of the 74 million pound reduction being sought by the turn of the century. Many of the Clean Air Act controls will not take affect until after that time, but as they do, they may help maintain the nutrient cap. Seeking additional air pollution reductions, if feasible, would help even more.