THE DIMENSIONS are uncertain and the explanations complex, but the Chesapeake Bay, at the over-ripe old age of 10,000, is showing sad signs of wear. Scientific data being gathered in a federal study point to disturbing trends: phosphorus and nitrogen are pouring into the waters mainly above the Bay Bridge, while toxic metals and hazardous synthetic chemicals pile up and various fish, grasses and other aquatic life decline sharply. The big question, as a report in this paper noted the other day, is how seriously the federal and state governments respond to this evidence.

Participants in the federal study emphasize that they have come to no official conclusions yet--and still other scientists note that the Chesapeake Bay may be cleaner and less disturbed than many other estuaries where salt and fresh waters mix. There are some bright signs, too: the lower portion of the Potomac River has been found cleaner than it was in the recent past; and some fish that spawn in salt water--bluefish and menhaden, for example--have been increasing.

Where does this leave matters? The study is scheduled to end in six months, and groups are pressing for continued federal financing of the project, as well as for participation by the governments of Maryland, Virginia and Pennsylvania. So far, no federal money has been set aside for the bay study after current allotments run out. Officials at the Environmental Protection Agency haven't opposed a continuation; but they have suggested that some interest on the part of state governments--a share of the cost--might help.

It's worth it--not to keep a study going or to postpone corrective measures until that day when the bay is totally ruined, but rather to learn more about the the many and mysterious causes and effects of changes in the Chesapeake. Sen. Mathias of Maryland supports an appropriation to continue the scientific monitoring, and it is in the country's interest to approve this modest but important project. Much more needs to be known about the effects of plants and animals on each other and how they, in turn, are affected by seasonal changes in rainfall, dam-building and discharges of pollutants by industries and boaters.

The biggest waste of money would occur if the study were simply dropped at this point. If the federal government is at all serious about any marine research, there is no more fascinating place for it than right here on the bay.