Like a real-life Sherlock Holmes, oceanography professor George Luther recently put his sleuthing talents to work to help solve a marine mystery: What killed more than 2 million bait fish in Delaware's Rehoboth Bay?

When masses of dead menhaden began floating to the surface of Torquay Canal and Bald Eagle Creek in the northern part of the bay in July 2000, local residents feared that the toxic microbe Pfiesteria might be the culprit. However, laboratory analyses disproved that notion.

"Many people were very concerned about the fish kills--so many dead fish is a sobering sight--and there was some uncertainty about what had caused their deaths," says Luther, who is Maxwell P. and Mildred H. Harrington Professor of Marine Studies and an expert in marine chemistry. "Low oxygen was suspected, but we wanted to explore the problem further."

To find the answer to this local "whodunit," Luther drew on his knowledge of a waterway thousands of miles across the globe--the Black Sea--and verified his hunch with the help of a novel chemical sensor he and his research team developed.

"An unusual feature of the Black Sea is that oxygen is found only in the upper levels of its waters," Luther says. "As you move deeper, the sea is permeated by a high concentration of dissolved hydrogen sulfide, which forms a 'dead' zone that can be inhabited only by specially adapted bacteria."

While Rehoboth Bay averages less than 6 feet deep, Luther's sampling operations and discussions with local residents and environmental officials led to the identification of more than a dozen "deep holes" in the Bald Eagle Creek and Torquay Canal system, where the fish kills occurred. Some of the holes are as deep as 18 feet and as large as a football field.

"We learned that these holes were created when developers used sand and mud from the creek and the man-made canal to build up the soil for housing developments in the 1960s," Luther says.

Next, Luther and his team worked with Bob Scarborough and Mike Mensinger of the Delaware Department of Natural Resources and Environmental Control (DNREC) to make real-time measurements of the sites' water chemistry, using a gold-tipped microelectrode created at UD. Once inserted into bay waters or bottom sediments, the needle-like probe can simultaneously measure a number of highly reactive chemicals and metals that are key indicators of environmental health.

Using the probe, the scientists found that when the weather is calm, oxygen is measurable in the surface waters of the Bald Eagle Creek area but not in deep waters. In the deep holes, only toxic hydrogen sulfide is measurable--and at values up to five times higher than in the Black Sea, which is the largest water body on the planet that contains hydrogen sulfide.

"These deep holes have restricted water flow and become stagnant or stratified in the summer when the weather is calm," Luther says. "Nutrients from fertilizers, sewage treatment and other sources fuel an overgrowth of algae. When the algae die, they sink into the holes. Bacteria then decompose the dead algae, using up the oxygen.

"After the oxygen is consumed, another group of bacteria starts to decompose the algae. These bacteria are found in the sediments and use the sulfate in seawater instead of oxygen. The chemical byproduct of this activity is hydrogen sulfide, which is even more toxic than the hydrogen cyanide used in gas chambers."

As long as the hydrogen sulfide stays at the bottom of the deep holes, all seems well, Luther says. However, when storms and strong winds occur in the summer, over a period of a few days, the water in the holes overturns and the poisonous hydrogen sulfide comes to the surface.

Currently, Luther is working with colleagues at DNREC to help solve the toxic problem, which may have a relatively simple solution.

"The best approach is to simply fill the holes with sand and mud to an even water depth of 6 feet," he says.

"We expect that better water quality will result if the holes are filled," Luther says. "We know the fish will be a lot happier."

A member of the UD faculty since 1986, Luther conducts a variety of marine chemistry research at home and abroad, on phenomena ranging from marshes to deep-sea hydrothermal vents. In 1996, the University of Cardiff in Wales awarded him its highest honorary title of Distinguished Visiting Fellow.

--Tracey Bryant