 |
|
||
| MD-DE-DC Water Science Center |
U.S. GEOLOGICAL SURVEY
Fact Sheet 213-96
The Chesapeake Bay, the Nation's largest estuary, has experienced serious environmental degradation during the past century. Symptoms of degradation are large declines in sea grass acreage and in finfish and shellfish (oysters and crab) populations, seasonal depletions in dissolved oxygen, and increases in sedimentation. These environmental changes raised serious concern in the 1970's because they threatened major commercial and recreational activities by damaging key habitats and reducing water quality necessary for bay species to survive and reproduce. Most scientists attribute these changes, at least indirectly, to ecological stress due to human activities, especially land use changes in the bay watershed related to deforestation, agriculture, use of fertilizers, and more recently, urbanization, pollution, and sewage. Future stress on bay ecosystems will potentially worsen, as the Chesapeake Bay Commission predicts that the population in the bay watershed will swell to 17.4 million by the year 2020.
 |
Sites where sediment cores are being collected to study nutrient and sedimentation trends in Chesapeake Bay. |
 |
A scientist collects sediment cores from |
Another critical problem in the Chesapeake Bay is sedimentation. Over the last 100 years, the net deposition of sediment that entered the Maryland portion of the bay was 155 million metric tons. Scientists believe that sedimentation rates have increased since colonial time due to land use changes and have been disruptive to the health of the bay; sediment can cloud the water so much that SAV cannot survive.
Responding to these environmental problems, members of the Chesapeake Bay Program (a regional partnership among the States of Maryland, Pennsylvania, and Virginia; the District of Columbia; the Chesapeake Bay Commission; and the Environmental Protection Agency) have developed bay-wide efforts to monitor water quality and biotic resources in order to assess the bay's ecological health on a regular basis and to produce computer models to predict watershed quality and ecosystem response to nutrient loading. The bay program modeling studies were used to establish a goal of a 40% reduction from 1985 levels of nutrient input into the bay by the year 2000. Biotic resource management plans have also been developed to restore and preserve key species for commercial use and recreation.
 |
Scientists in research vessels collect sediment cores from the bay floor. |
Despite the extensive bay restoration effort, large gaps exist in our understanding of the bay ecosystem. Certain trends in discharge, oxygen, and SAV underscore our need to better understand the bay ecosystem as a whole.
Extreme fluctuations in interannual freshwater discharge entering the bay, are the result of changing precipitation and climatic patterns over the last few decades and have become acute during the last 5 years. Trend data show a clear and significant influence of discharge on the total nutrients entering the bay and on bay salinities, which in turn influence how bay phytoplankton utilize the nutrients and lead to low oxygen levels. Climatic factors may, in fact, play a larger role in controlling bay environmental health and determining recent trends in water quality than previously realized; for example, the drought years of the 1950's and 60's caused low tributary discharge into the bay.
 |
Interannual and decadal variations in river flow are obtained from seven tributary monitoring stations, unimpeded by dams and other structures. River flow strongly influences nutrient input and dissolved oxygen in Chesapeake Bay. Climatic factors in turn control to a great extent interannual and decadal flow variations. |
Since restoration efforts began in the early 1980's, SAV surveys have indicated that grasses have begun to return to several tributaries of the bay. In theory, this reflected a response to the improved water quality. However, there were areas where SAV did not return despite an improvement in water quality in 1994 and 1995. This continued absence of SAV concerns scientists and resource managers, in part because SAV provides key habitat for young blue crabs. The bay provides 50% of the Nation's total blue crab harvest, and the bay crab harvest was worth $126.6 million in 1993, according to the National Marine Fisheries Service.
What do recent trends in discharge, anoxia, sea grasses, and blue crabs mean? Are they caused by habitat change, overharvesting, or natural mortality related to long-term, external climatic factors? These questions remind us of how complex the bay ecosystem is and of how little we know about the relations among individual species, their environment, and climate. They also illustrate how difficult it is for scientists to separate natural versus human-induced effects on critical species and their habitats based solely on monitoring. Detection of a trend can be easy, attribution of cause is much more difficult. While events like these can not yet be predicted by water-quality or ecosystem models, the effects of past events can be studied in Chesapeake Bay history archived in its sediments.
The U.S. Geological Survey (USGS) Chesapeake Ecosystem Response Project is designed to improve our under-standing of large-scale environmental changes that influence water quality and living resources in the Chesapeake Bay. In particular, project workers will investigate the links among changes in climate, precipitation, discharge, salinity and dissolved oxygen over interannual, decadal, century-scale periods. To do this, USGS scientists and other researchers from the University of Maryland, the Maryland Geological Survey, and the Virginia Institute of Marine Science are collaborating to study the bay's sediments, which capture the history of its water, plants, and animals during the period before monitoring began in the 1980's. Using ecosystem "indicators" (microfossils) and geochemical data preserved in the bay's sediment and historical and reconstructed discharge data, members of the Chesapeake Ecosystem Response Project will reconstruct trends and response in the bay since 1950 and will determine the natural conditions in the bay over the last few millenia, including periods prior to 17th-18th century colonial agriculture and 19th-20th century industrialization and urbanization. In the next 5 years, the USGS will lead a multi-institution research program to carry out integrated coring, sedimentological, environmental, and ecological investigations of the Chesapeake Bay ecosystem. Trends in oxygen, nitrogen, and phosphorus levels, phytoplankton, benthic invertebrates, sedimentation, and biodiversity will be studied with emphasis on separating natural versus human causes of and responses to extreme events.
From U.S. Department of the Interior, U.S. Geological Survey, Fact Sheet FS-213-96
For more information about USGS activities in Maryland, Delaware and the District of Columbia contact:
Director
MD-DE-DC Water Science Center
U.S. Geological Survey
8987 Yellow Brick Road
Baltimore, MD 21237
Telephone: (410) 238-4200
Fax: (410) 238-4210
or access the USGS Water Resources of Maryland, Delaware, and District of Columbia home page at: http://md.water.usgs.gov/.
U.S. Department of the Interior | U.S. Geological Survey
Persistent URL: https://pubs.water.usgs.gov/fs21396
Page Contact Information: USGS Publishing Network
Page Last Modified: Tuesday, 29-Nov-2016 18:10:14 EST
