Water clarity and oxygen levels in the St. Mary's River in Southern Maryland are at failing levels, a condition activists and scientists attribute mainly to development.

Data collected by the St. Mary's Watershed Association and other scientists show that between 2000 and 2008, water quality has decreased in streams that feed into the St. Mary's River near the Lexington Park development district, including fewer macro-invertebrates, the tiny organisms that live in the bottoms of streams and rivers.

"Generally speaking, most of the streams in the upper area of the development district have gone down in quality," while pavement, roofs and other impervious surfaces have increased, said Bob Lewis, executive director of the watershed association.

The St. Mary's and Wicomico rivers are considered to be the most degraded of lower Potomac River tributaries, according to a 2008 report by the Interstate Commission on the Potomac River Basin, Lewis said.

Robert Paul, biology professor and director of the St. Mary's River study project at St. Mary's College of Maryland, said conditions at Church Point are typical of those throughout the river. The college has 10 years of data from the point.

The studies found that, during summers, the levels of dissolved oxygen at the bottom of the river is nonexistent and that aquatic vegetation is dying at high rates, Paul said.

That's especially true in wet summers, when more harmful nutrients wash off lawns, pavement and developed areas and into water ways, creating algae blooms and sediment that block sunlight, he said.

"Not only are you killing the organisms outright due to lack of oxygen, you are also killing [fish and plant] habitat," Paul said.

Another indicator of the river's health is its oyster population, he said. In 1974, there were about 90,000 bushels of oysters harvested from the St. Mary's River. Since 2000, about 1,500 bushels were harvested.

In about 20 years, the population of the Lexington Park development district - which borders busy Patuxent Naval Air Station - should double, doubling the pressure on the stream systems, Lewis said.

The Watershed Association is working on an "action strategy" to combat the onslaught of sediment and other harmful impacts on the river. Association President Joe Anderson said that once the plan is completed, the county will be at the forefront of others in the area and will have a solid plan to back grants from local, state and federal sources.

Because the St. Mary's River watershed covers more than 45,000 acres and is entirely in St. Mary's, the county has the ability to protect and improve the watershed. The county can make changes and "essentially act as a model [of how] to do things the right way," Anderson said.

The association said creating larger buffers to streams, maintaining wetlands, improving erosion controls and setting development standards could lead to a healthier waterway and "protect the biological integrity of the river," Lewis said.

The group also asked the St. Mary's County Commissioners to support incentives for developers to build some of those controls into their projects. Association members said that the state's new storm-water management laws should help, but that without enforcement, fixing the issues would be difficult.

Commissioner Lawrence D. Jarboe, a Republican, suggested a moratorium on oystering, which President Francis Jack Russell, a Democrat and former waterman, opposed.

"We all need to keep working together to all get a grip on this stuff," Russell said. He noted that more work should be done above St. Mary's County on the Potomac and Patuxent rivers.

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