ANNAPOLIS -- A network of 50 veteran crab potters has been established by a marine biologist to help document the deadly effects and reach of a phenomenon known to watermen as "bad water."

Bad water is water that has too little or no dissolved oxygen. When it flows over crab pots, the crabs suffocate.

Low-oxygen water is a chronic problem on the Chesapeake Bay, but there has been no real effort to assess its impact on the industry, according to William Goldsborough, biologist with the Chesapeake Bay Foundation.

The effects of bad water on the entire ecosystem, including crabs that are not trapped, cannot be gauged.

"We've just picked one small portion of one fishery where we think we can document an impact," Goldsborough said. "When you realize this is just the tip of the iceberg, it's really a tremendous ecological nightmare."

Goldsborough has set up a dead crab hot line to receive calls from his volunteers. An answering machine receives the crabbers' messages, and he calls them back to find out how many crabs were in each pot, how many were dead, where the pots were set, the depth of the water and the approximate loss to the crabber in terms of time and money.

By last week, he had received reports of bad water kills occurring on 13 days. Most of the calls came in early June. He said he was sending a letter to his observers to ensure they are continuing to forward reports to him.

Goldsborough's volunteers are posted at spots between the head of the bay and the Maryland-Virginia border.

"One of our biggest objectives is to increase watermen's involvement in the water quality issue," Goldsborough said. "What we hope to do is give them the ammunition and the impetus to really get involved."

The bad water problem occurs in the summertime when there is a stratification of the water column in the Chesapeake Bay into two distinct levels, and there is not much mixing of the two, he said.

Because there is little so-called vertical mixing, not much oxygen, which enters at the surface, reaches the deeper levels.

At the same time, rotting microscopic plants consume oxygen at the bottom. The loss of oxygen is also accelerated by enriching nutrients such as phosphorous and nitrogen from sewage treatment and farm runoff that flush into the bay.

As part of his study, Goldsborough also is getting a practical education in crab potting: He has set up a crab pot line of his own near Annapolis, with the traps set at five depths.