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Bottom Habitat

In 2012, 45 percent of the area of the Bay and its tidal tributaries met the restoration goals, which is not significantly different from the  previous year.

Bottom Habitat 2012

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Benthic Habitat (Index of Biological Integrity) (2012) Annual Average Score by Sampling Station Mar 28 2013 / Download

Benthic Habitat (Index of Biological Integrity) (2012) Percent of Goal Achieved Mar 28 2013 / Download


The Bay’s bottom is home to many small creatures such as worms, clams and tiny crustaceans. These creatures, called benthic macroinvertebrates, live on or in the bottom sediments.

Benthic macroinvertebrates are especially sensitive to increases in chemical contaminants and decreases in oxygen. Since they cannot move to avoid poor environmental conditions, the health of benthic macroinvertebrates is an excellent indicator of the Bay’s health.

The Benthic Index of Biotic Integrity makes a good integrator of long-term environmental conditions because benthic organisms have limited mobility and their responses to stress are well documented.


The health of the Bay’s bottom habitat is measured on a scale of 1 (very degraded) to 5 (least impaired). The goal is for all scores to be at least a 3.

Long-term Trend (1996-2012)

Goal achievement has averaged 47 percent and has ranged from 41 percent to 56 percent.

Change from Previous Year (2011-2012)

As a whole, bottom habitat in the Bay's tidal waters did not change significantly relative to the previous year. In 2012, there were declines in pass rates in all areas in Maryland water except the Choptank River. In 2012, increases in pass rates were observed in most Virginia waters except the lower west.  PLEASE NOTE-These finding are based on PROVISIONAL monitoring data. Final results with more definitive finding will be available after May 2013.

Additional Information

Benthic Restoration Goal

A measurement called the Benthic Index of Biotic Integrity is used to rate the health of bottom habitats on a scale of 1 to 5. Each year, 250 random samples are collected throughout the Bay and its tributaries. The goal is for all scores to be at least a 3.

Attainment of the benthic restoration goal was determined by examining:

  • Benthic biodiverstiy measures
  • Measures of assemblage abundance and biomass
  • Life history strategy measures
  • Activity beneath the sediment surface
  • Feeding guild measures

These metrics are examined at each site and compared to several reference sites.

Causes of Bottom Habitat Degradation

Low levels of dissolved oxygen, which are ultimately caused by excessive nutrient inputs to the Bay, are the primary cause of benthic degradation. However, small embayments (such as the Elizabeth and Pagan rivers in Virginia) have good bottom dissolved oxygen but stressed benthos due to toxic contaminants. Other areas, such as Baltimore Harbor, have a combination of problems.

Benthic Monitoring Data

Data and program information are available to the research community and the general public through the Chesapeake Bay Benthic Monitoring Program website.

Source of Data

Versar Corporation

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