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

on 11 June 1972 surface temperatures at all stations were greater than 4~C.
On the survey date 25 April 1973 (yet to be reported) a thermal bar was
present, but it had already moved offshore beyond the 7-mile limits of our
sample grid. It thus appears that, within the area we are investigating,
thermal bars are regular but short-lived phenomena.
Stoermer (1968) has reported distinct phytoplankton assemblages on
either side of the thermal bar. These differences he attributed to the pooling of nutrient-rich runoff water inside the bar. It is less likely that the
effects of temporary enrichment could be detected in the populations of zooplankton crustaceans since these animals occupy a higher position in the food
chain and increase their numbers more slowly. However, responses to water
temperature changes would be more immediate, hence the consequences of earlier
warming inshore may be noticeable. Data from the surveys of April 1971 and
May 1972 indicate the following: 1) Copepods are reduced in numbers inside
the thermal bar (ca. 1000/m3 versus ca. 5000/m3 offshore). 2) Nauplii are
more plentiful in the inshore water (ca. 3000/m3 versus ca. 500/m3 outside
the bar). 3) Highest concentrations of zooplankton are found at stations
located closest to the thermal bar (Fig. 4). We speculate that these results
are attributable to selective predation of the larger copepods by fish moving
into the inshore regions, to increased hatching of eggs in the warmer water,
and to a possible tendency for upward-swimming zooplankters to become concentrated in regions where there is convergence and downwelling of the water.
The Survey of 9 July 1971
Of the four surveys, highest numbers of zooplankton were found in July —
several stations having over 100,000 individuals per cubic meter. These
numbers are equivalent to those found in July 1972 but are higher than those
reported for July 1970. The difference can be attributed to a change from
the #5-mesh net used in 1970 to the #10-mesh net used thereafter. Since
Bosmina longirostris accounts for upwards of 50% of the assemblage during
this part of the year, and since the 282 p aperatures of a #5 net are larger
than the length of a first-instar Bosmina, more complete catches of this
species probably account for most of the difference. Examination of Figure 4
reveals a random and fairly even spacial distribution of total zooplankton
abundances. However, if the totals are broken down into their three principal

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