The Bay’s bottom is home to many species including worms, small fish and shellfish, such as clams, oysters and mussels.
- These bottom-dwelling creatures are especially sensitive to increased pollution and decreased oxygen.
- These species serve as food for bottom-feeding fish and crabs.
- The health of these creatures is a good indicator of long-term conditions in the bottom habitat and the Bay overall because they do not move great distances and have certain predictable responses to environmental stresses.
A measurement called the 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.
*Note: The historic data featured in this indicator changed from that reported in the 2007 assessment. The 2007 data changed due to the finalization of provisional data.
Long-term trend (1996-2008)
Decreased from 52.4 percent to 41.5 percent of goal achieved. Goal achievement has averaged 46 percent and has ranged from 40.8 percent to 52.7 percent.
Short-term trend (10-year trend)
Between 1999 and 2008, decreased from 49.7 percent to 41.5 percent of goal achieved
Change from previous year (2007-2008)
No significant change (42.2 percent in 2007 to 41.5 in 2008)
Benthic Restoration Goal
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.
Jacqueline Johnson at 800-968-7229 ext. 729