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

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situated in the northern basin of the lake have, in general, more amphi2
pods/m than stations located in the southern.basin that are situated at
comparable depths.
It is difficult to compare directly the amphipod counts at the 35
stations because they were located at different depths and it has been
shown that depth strongly influences the abundance of P. affinis in an inverse manner. In order to make direct comparison possible, the effect
of depth on amphipod density was removed by first calculating the densitydepth regression equation Y = 81. 04 - O0 232X where Y represents the
average number of amphipods per station stops X the depth per station
stop, 81. 04 the intercept, and -0. 232 the regression coefficient. A
second order polynomial relationship for depth was important in explaining fluctuations in amphipod abundance when all environmental parameters
were considered. However, when the relationship between amphipod
density and depth was considered independently of the other environmental factors, the squared term for depth was not significant.
The grand average density of amphipods and the average depth
must fall on the regression line. In removing the effect of depth, the
deviations of Y about the regression line were shifted so these deviations
occurred about a regression line that had a slope of zero and an intercept equal to the grand average density of amphipods. This was accomplished by modifying the above regression equation in the following
fashion: Y = y + 0. 232(X-850 9) where y was the observed average value