Ecology of the burrowing amphipod, Pontoporeia affinis, in Lake Michigan / by Wayne Paul Alley.

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to the environment in a negative fashion while a high density represents
a positive environmental response. Data collected from 35 stations of
the long-term study area were used in examining these environmental
relationships.
The data were analyzed by three methods. In the first an environmental factor, such as depth, was divided into increments and a geometric
mean calculated for the combined amphipod counts of each interval. The
geometric mean of each interval was accompanied by an upper and lower
limit of one standard deviation unit. In the second method a correlation
coefficient was computed by comparing the average density of Pontoporeia
with an environmental parameter. Stepwise multiple linear regression
was used as the third method of investigating these relationships. The
effects of environmental factors on changes of amphipod density are
considered simultaneously in this method of analysis.
The geometric mean is usually calculated by (X1 X20 X3., X) /iJ
where X1 represents a sample of amphipod counts. Counts in this
analysis were converted into logarithms with the geometric mean being
determined from the antilog of logXl+ logX2 + logX3 +..0 logXj)/j.
The upper and lower limits of this mean were determined by first computing the standard deviation of the log converted data, adding this
standard deviation to and subtracting it from the long mean, and the
antilogs of these two values finally constituting the upper and lower
limits.