Monday, February 14, 2011

The Chesapeake Bay Estuary

According to the US Fish and Wildlife Service, the Chesapeake Bay is the largest of 130 estuaries in the United States. Like all estuaries it is an incredibly complex ecosystem that we are only beginning to understand. Estuaries are productive ecosystems and habitats. The type of habitat is determined by geology, salinity and climate. The Chesapeake Bay serves as a nursery ground for the fish and shellfish industry and protects the coast from storm surges and filters pollution. The estuary filters water that is carrying nutrients and contaminants from the surrounding watershed. The nutrients in proper balance bring fertility, but excess nutrient contamination to the Chesapeake Bay has caused degradation in the habitat. As a result, US EPA has taken control of the situation (despite NOAA’s and the National Estuarine Research Reserve System mission to protect and study US costal estuaries) and has developed a new federally mandated TMDL (total maximum daily load) to try to restore the natural balance in the estuary by controlling nutrients in the local waters. The TMDL allocates a pollution budget among the states which will decrease over time.

About half of the Chesapeake's water volume comes from salt water from the Atlantic Ocean the rest is fresh water from more than 50 rivers and innumerable smaller tributaries within the enormous 64,000-square-mile watershed. As would be expected of this mix of fresh and salt water, the Bay's salinity gradually increases as you move from north to south and will change with rainfall and climate influences. 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.

Due to the coriolis force (earth’s rotation), one side in the estuary is saltier than the other side. In the Chesapeake Bay this rotation causes salt water accumulate on the Eastern Shore of Maryland, so water tends to be saltier on the eastern side of the Bay at any latitude. Overall, however, the proportions of fresh and salt water in the Bay depend largely on the amount of rainfall that flows out of the Chesapeake's major rivers. During a wet year, the entire Bay will be somewhat fresher than normal, and conversely, a dry year will result in higher-than-average salinities. Salinity is one of the most important physical 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.

Estuaries are classified by both geologic events that created them and water circulation. The Chesapeake Bay is a coastal plain estuary also called a drowned river valley. The Chesapeake Bay (and all coastal plain estuaries) was formed at the end of the last ice age. As the glaciers melted and receded, sea level rose and flooded the low lying river valleys. The deep water channel in the Chesapeake Bay is the ancient river bed of the lower Susquehanna. The Chesapeake Bay watershed is characterized by rapidly flowing rivers discharging to the bay where tidal currents are weak. This creates the most stratified or least mixed type of estuary (as classified by water circulation) - a salt-wedge. Fresh water, which is less dense than salt water floats on top of the salt water as it is pushed out to sea by the rivers. A sharp boundary with limited mixing is characteristic of salt wedge estuaries. The location of the wedge boundary varies with weather and tidal conditions.

Estuaries are fragile ecosystems that are very susceptible to disturbances both natural and those created by man. In the United States the ecology of estuaries has been severely damaged by man. Diverting fresh water from tributaries for irrigation and drinking water supplies changes flow, quantity of fresh water entering the estuary, and impacts the balance within the ecology. Excess nutrients and sediment from sewage treatment plants, farm fields and animal pastures, urban and suburban run off from roads and landscaping can cause eutrophication. As the ecosystem of estuaries declines, species die out, coastlines experience excessive erosion by wind, tidal action and ice. It is going to take knowledge, effort and resources (wealth in all forms) to restore the Chesapeake Bay.

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