Chesapeake Bay Benthic Monitoring Program

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How Do We Know Whether Benthos are Healthy?

The Chesapeake Bay benthic index of biotic integrity (B-IBI) is an index that measures the "health" of benthic communities in Chesapeake Bay.  The B-IBI provides a means for comparing the relative condition of benthic invertebrate assemblages across habitat types.  It also combines several benthic community measures indicative of habitat "health" into a single number that measures overall benthic community condition.  (Click here for a detailed description of the Chesapeake Bay B-IBI and how it is calculated.)

Community measures, or attributes, that are components of the B-IBI include species abundance, biomass, the Shannon diversity index, the abundance and biomass of pollution-indicative species, and the abundance and biomass of pollution-sensitive species.

The B-IBI is scaled from 1 to 5.  Sites with values of 3 or more are considered to meet the Chesapeake Bay Program's Benthic Community Restoration Goals.  The Restoration Goals describe the characteristics of benthic invertebrate assemblages expected in non-degraded habitats of Chesapeake Bay.  The index is calculated by scoring each of several attributes as either 5, 3, or 1, depending on whether the value of the attribute at a site approximates, deviates slightly from, or deviates strongly from values found at the best reference sites in similar habitats.  Attribute scores are then averaged to obtain an index value.  The criteria for assigning these scores are numeric and depend on habitat.  Application is presently limited to summer samples.

The table to the right shows the benthic community condition classified into four levels based on the B-IBI.  Values in the marginal category do not meet the Restoration Goals, but they differ from the goals within the range of measurement error typically recorded between replicate samples.

B-IBI Level Score
Severely Degraded 1.0 - 2.0
Degraded 2.1 - 2.6
Marginal 2.7 - 2.9
Meets Restoration Goals 3.0 - 5.0

Because benthic communities are permanent residents of the Bay and they are highly sensitive to poor water quality and sediment quality, the B-IBI itself is a very good indicator of the overall health of the Bay.  The B-IBI is used annually to characterize the Bay's regional health as well as trends in habitat conditions over time.  (See How's the Bay Doing ?  for the most recent results.)


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Revised: April 22, 2005.