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

32

young of the previous year. At this station, growth was less than 2 mm
for the 6 month interval from April to October, which was about half
that observed at B-8.
Figure 10 also shows that stations deeper than 35 m have size
frequency histograms for Pontoporeia which are similar to each other.
In the 35-86 m depth zone a larger proportion of individuals fell in the
2 mm size interval than was the case in deeper regions of the lake. It
was difficult to determine how much time is required for the maturation
of Pontoporeia at depths greater than 35 m.
Monthly size frequency histograms were made for stations C-l,
C-2, C-3, C-4, and C-5 for April to November 1965 and from March
to November 1966 (Figs. 11 to 15). Pontoporeia found at station C-l,
average depth 20 m, also required two years to mature. The remaining
four C stations were positioned in waters deeper than 35 m. Stations
C-2 and C-3 lay in the 35 to 86 m depth zone and showed a higher proportion of individuals in the 2 mm size class than did stations C-4 and
C-5 which were in deeper waters.
PATTERNS OF SPATIAL DISTRIBUTION
Short-Term Study Area
Patterns of spatial distribution in the short-term study area were
examined by two methods: by plotting the mean against the variance and
by examining the occurrence frequency of the number of individuals per
quadrat. If the mean number of individuals per quadrat is independent