🦀 Blue Crabs & The Ecology of the Chesapeake Bay

To survive, the Chesapeake Bay, its rivers and creeks, must endure an array of assaults from air, water and land. The watershed’s worst problem is caused by the overabundance of the nutrients, nitrogen and phosphorus (fertilizer).

Other problems are related to toxic chemicals, air pollution and landscape changes, along with sedimentation and the over-harvesting of living resources. PCB’s for example have been found in blue crabs in the Patapsco River: Middle & Northwest Branch Only. Nutrients such as nitrogen and phosphorus, come from two types of sources: point and non-point. Excess nitrogen and phosphorus are harmful, because they cause algae blooms: uncontrolled growth of microscopic plants in the water. Algae blooms cloud water and block sunlight, causing submerged aquatic vegetation to die. When algae die and decompose, they use up oxygen needed by other plants and animals living in the water.

The fertilizer put on lawns will eventually seep into the ground. The run off during a rainstorm will find its way into storm sewers of streams feeding directly into the bay. Sewers also carry trash of all sorts into the waters where bay creatures may swallow it or become hopelessly entangled and die.

With few exceptions, brooks and streams join major rivers that eventually reach the ocean. Because the Bay receives constantly varying amounts of salt water and fresh water from tides and rainfall, it acts as large mixing area. Nutrients vital to plant and animal life drain from the land and reach estuaries where they are used by these living resources.

Also presenting problems for the future is the rising tide level in bay. This will have a dramatic effect in the future.

Because salt water is heavier than fresh water, estuaries like Chesapeake Bay contain two layers: a saltier layer that lies on the bottom and a freshwater layer above. Mixing occurs where the two layers meet. Further mixing takes place as a result of wind, tides, temperature changes and rainfall. The waters of the Chesapeake are saltiest near the mouth of the Bay and gradually become fresher northward. The force of the earth’s rotation makes salt water accumulate on the Eastern Shore, so water tends to be saltier on the eastern side of the Bay. The amount of fresh and salt water depends largely on the how much rainfall flows from the Chesapeake’s rivers. During a wet year, the entire Bay will be somewhat fresher than normal. Salt content is one of the most important features in determining what lives in a particular part of the Bay, so plant and animal populations in the Bay differ north to south, west to east, and from year to year.

Temperature and bottom sediment also determine the distribution and abundance of organisms. The shallow water is home for the crustaceans known as crabs. In warm weather they can be found close to intertidal flats in grassy areas where the tides rise and fall and alternately uncover and cover the shore. If the grasses where crabs molt and feed disappear, they will not survive. When more are caught than are being produced by natural reproductive means they have been over fished.

Blue crabs have been a mainstay of the Bay for decades. The blue crab population is on the decline. In an effort to stimulate a larger crab population, toss your females back into the water.