The seasonal biological surveys of 1971 / John C. Ayers, Samuel C. Mozley, John A. Stewart.

its heated effluent. Differences in species between this and deeper zones
indicate that the shallow water zoobenthos may be adapted to broader thermal
fluctuations and less stable substrates than the typical Great Lakes benthic
fauna, i.e. Pontoporeia, StyZodrilus, and various Sphaeriidae.
A second consideration is the year-to-year differences in numbers of
several species. Figures 7-12 show especially large fluctuations in large
Chironomidae (Fig. 12) and Tubificidae (Fig. 10). Pontoporeia seems to have
been more numerous and to have grown faster in zones 1 and 3, in 1971 than in
1970 (Figs. 9 and 13). Means of Stylodrilus were higher in 1971 as well.
Preoperational studies have continued and even intensified in 1972 and 1973,
so that the more common types of year-to-year fluctuations should be distinguishable from changes due to plant operation. Statistical approaches to this
problem are also underway as exemplified by Part XVIII in this report series
(Johnston 1973).
Large statistical fluctuations in the data from shallower benthic depth
zones particularly will also make detection of change more difficult. The
1973 annual report in this series (Seibel and Ayers 1974) deals with this
issue in more detail, including presentation of an intensive sampling experiment conducted in April 1973 to ascertain the effects of replication at
stations and within regions of the survey area on the precision of numerical
estimates. Subsequent to April 1972, replication has been performed at each
station on all surveys. Subdivision of the survey area into benthic depth
zones has been discussed at some length in earlier reports (Mozley and Garcia
1972; Mozley 1973a and b) and the bases for establishment of this approach
have not changed with the addition of further data. The unique features of
zone 0 were discussed above. Zone 1 is characterized by a large variety of
species, and increased total abundance over zone 0. Zone 2 is characterized
by the appearance of large populations of Pontoporeia, StylodriZus and
Sphaerium nitidum. Its upper boundary is particularly well marked by increases in the numbers of Pontoporeia relative to zone 1 in April and the two
Novembers; Pontoporeia is more common in zone 1 in the two Julys. Subdivision
in this way helps reduce some of the numerical variation in zoobenthos over
the survey area by focusing on more homogeneous subareas, and provides a
system for combining the single observations per station in earlier surveys
to obtain estimates of the sample size variance (see also Johnston 1973).

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