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

distribution from the possible effects of temperature, we have made histograms
(Fig. 5) of averaged numbers of phytoplankters and zooplankters by half-degree
increments of station temperatures across the survey area.
In Figure 5, with one or more stations in each half-degree temperature
increment, the numbers of phytoplankters per milliliter at the stations in the
several temperature ranges indicate only that there were more phytoplankters
in the warmer waters near shore. There is no evidence in the histograms that
the thermal bar was causing any convergent concentration of phytoplankters at
the 4~C isotherm. There is, instead, evidence that the low offshore numbers
were the normal overwintering population, while the progressive increase in
numbers toward shore (with the largest numbers in the warmest water) represents phytoplankters responding to the spring warming by reproduction.
The zooplankton, however, showed highest averaged numbers in the temperature increment from 3.5 to 4~C and much less concentration in the increments
from 4.5 to 7.5~C (average 7.9 organisms per liter in the interval between
3.5 and 4.0~ as opposed to 3.5 and 4.7 per liter in the two intervals between
4.5 and 5.5~C). The shape of the histogram suggests that the abundant offshore zooplankters were probably the normal winter pre-spawning population,
while the reduced numbers found inshore of the bar probably were the unspawned adults remaining after the warmer water there had stimulated many to
spawn and die. This view is consistent with the presence in this water of
greatly larger numbers of copepod young (nauplii).
Figure 6 is an intercomparison of averaged zooplankton abundances at 5-m
depth intervals during the surveys of April 1971 (with a thermal bar), April
1972 (before the thermal bar had developed), and April 1973 (after the thermal
bar had passed). During thermal bar and post thermal bar conditions, the
greatest averaged numbers of zooplankton were found lakeward of the 18-m depth
contour. In the case of the April 1971 thermal bar, the numbers of zooplankters at the 18-m depth contour position of the bar were not significantly
greater than during pre and post bar conditions. If convergence and sinking
were taking place at the bar, one would expect the upward-swimming zooplankters to be concentrated along the bar by the convergence of water movements.
Neither Figure 5 nor Figure 6 exhibit any clear evidence of zooplankton concentration at the thermal bar; if convergence concentration of zooplankters
was indeed going on it was being masked by some other factor, possibly the

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