SAVING THE BLUE CRAB

Excess Nutrients in the Chesapeake Bay Make Survival Difficult


Recently, the blue crab has been in the headlines because of reduced catches by commercial and recreational Chesapeake Bay crabbers. While harvesting is a major factor in affecting the crab population, loss of habitat has also contributed to the problem. To understand how this loss of habitat occurs, it is first necessary to understand how excess nutrients affect the Bay. The Chesapeake Bay is like a soup. Both are composed of many ingredients. But just as too much of any ingredient can spoil the flavor of the soup, too much of a particular substance can harm the Bay. The current problem with the Bay is too many nutrients. Nutrients are substances that help plants grow. The most important nutrients are nitrogen and phosphorus. Plant and animal matter (including human waste), fertilizer, and even deposition from car exhaust and power plants all contain nutrients. If not treated, these nutrients will find their way into creeks, rivers and eventually the Bay.

How Excess Nutrients Harm the Bay


How Nutrients Harm the Bay

Once the nutrients are in the Bay, they become food for plants. But excess nutrients cause too much plant growth, especially algae (microscopic floating plants). When there is too much algae,the water becomes cloudy and blocks the light needed by underwater plants called submerged aquatic vegetation (SAV). Algae can also coat the leaves of the SAV, further reducing the amount of light received by the plants. SAV are very important to blue crabs because they provide food, shelter and nursery areas. Research has shown that the density of juvenile crabs is ten times greater in SAV beds than in unvegetated Bay areas. An excess amount of algae can also cause other problems. When the algae die, they settle to the bottom where they are naturally decomposed by bacteria. During this normal decompositional process, the bacteria use dissolved oxygen from the Bay's bottom waters. When large amounts of algae are decomposed by bacteria, the removal of dissolved oxygen is substantially increased. This dissolved oxygen is needed by blue crabs and other organisms living on and near the bottom. The resulting low dissolved oxygen concentrations drive blue crabs from their preferred habitat and kill many of the small bottom organisms on which the blue crabs feed. This situation worsens in the summer when several natural factors act to further lower the amount of dissolved oxygen in the Bay's water. The low dissolved oxygen conditions caused by excess nutrients are the primary reason large bottom sections of the Bay are unsuitable as blue crab habitat.

What is Being Done about Excess Nutrients


Reducing nutrient pollution has been a priority of the Chesapeake Bay cleanup for over a decade. Progress is being made due to the combined efforts of citizens, industry and the government. Since 1985, Maryland has greatly reduced phosphorus and nitrogen pollution. We are already seeing results in the Bay and its tributaries such as cleaner water and more Bay grasses. These improvements show that the cleanup is on the right track. Dissolved Oxygen image

What You Can Do to Help


SAV acreage in Maryland Presently, many people are trying to restore the Chesapeake Bay by reducing the nutrients that ultimately harm the habitats for blue crabs and many other organisms. Actions that help improve Bay conditions include reducing the use of lawn fertilizers, carpooling, and planting trees. Become involved in Maryland's Tributary Strategies - a program designed to involve the public and local governments in activities to reduce nutrient pollution and help the State reach a 40 percent nutrient pollution reduction goal for the Chesapeake Bay.
For more information on what you can do to help the Bay, call the Maryland Department of Natural Resource's Tidewater Ecosystem Assessment Division at (410) 260-8630; e-mail: tparham@dnr.state.md.us; The Alliance for the Chesapeake Bay, (410) 377-6270; or the Maryland Department of Natural Resources Tributary Strategies, (410) 260-8710.

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