Fixed Stations
Trends in condition at fixed stations are identified using the nonparametric technique of
van Belle and Hughes (1984). This procedure is
based on the MannKendall statistic and consists of a sign test comparing each value
(the BIBI score)
with all values measured in subsequent periods. The ratio of the
MannKendall statistic
to its variance provides a normal deviate that is tested for significance. Alpha is
set to 0.1 for these tests because of the low power for trend detection for biological
data. An estimate of the magnitude of each significant trend is obtained using
Sen's (1968) procedure which is closely related to
the MannKendall test. Sen's
procedure identifies the median slope among all slopes between each value and all values
measured in subsequent periods.
Baywide Estimations
To estimate the amount of area in the entire Bay that
fails to meet the Chesapeake Bay Benthic Restoration Goals (p), we define for every site
i in stratum h a variable y_{hi} that has a value of 1 if the
benthic community fails the goals, and 0 otherwise. For each stratum, the estimated
proportion of area failing to meet the goals, p_{h}, and its variance is
calculated as
the mean of the y_{hi}'s and their variance. Estimates for strata are
combined in a weighted average to achieve a statewide estimate. For combined strata,
the 95% confidence intervals are estimated as the proportion plus or minus twice the
standard error. For individual strata, (e.g., each of the 10 strata in 2001)
the exact
confidence interval is determined from tables.


The document Methods for Calculating
The Chesapeake Bay Benthic Index of Biotic Integrity contains the
information necessary to calculate the Chesapeake Bay BIBI, including the
latest updates.
If Acrobat Reader is set up on your machine, this document will open when
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The Chesapeake Bay benthic index of biotic integrity (BIBI) was developed
to assess benthic community health and environmental quality in Chesapeake
Bay. The BIBI evaluates the ecological condition of a sample by comparing
values of key benthic community attributes (“metrics”) to reference values expected
under nondegraded conditions in similar habitat types. It is therefore
a measure of deviation from reference conditions.
The development of the Chesapeake Bay BIBI has been described in
Weisberg et al. (1997). In addition, a series
of statistical and simulation studies were conducted to evaluate and optimize
the BIBI (Alden et al. 2002). The results of
Alden et al. (2002) indicated that the BIBI
is sensitive, stable, robust, and
statistically sound. New sets of metric and threshold combinations for the tidal
freshwater and oligohaline habitats were also developed in Alden et al. (2002)
with a larger dataset than was available to Weisberg et al. (1997) for these two
habitats.
The Chesapeake Bay BIBI is calculated by scoring each of several
attributes of benthic community structure and function (abundance, biomass, Shannon
diversity, etc.) according to thresholds established from reference data
distributions. The scores (on a 1 to 5 scale) are then averaged across
attributes to calculate and index value. Samples with index values of
3.0 or more are considered to have good benthic condition indicative of good
habitat quality.
