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

Figure 11 shows Mollusca other than Pisidium and total Hirudinea.
Sphaerium nitidum was well represented in benthic zone 2 and usually in zone
3 as well, but samples sizes were extremely variable as shown by broad
standard errors. There was no detectable seasonal variation in distribution
or abundance of this viviparous fingernail clam in 1970-71. Apparent changes
in zone 3 were probably due to the irregularity of population densities (Note
large standard errors), rather than real changes in population size. The
larger but much rarer Sphaerium striatinum was present in the three deeper
zones in all seasons with little if any seasonal changes in abundance. Gastropoda, which were mostly Valvata but included some Lymnaea Bulimus (
Bithynia) appeared to have increased in 1971 over 1970 and to have been most
numerous in benthic zone 2 in November, but their numbers were still very
low. Hirudinea were composed mostly of Helobdella stagnalis. They were
generally very consistent in occurrence and rare, but apparently increased
in benthic zone 2 in July 1970 and November 1971.
Patterns for the larger kinds of Chironomidae larvae are shown in Figure
12. Although ProcZadius and Chironomus larvae were occasionally present in
moderate numbers, there was no repeated seasonal pattern in their depth distribution and occurrence from year to year. The same months in the two successive years had different depth distributions and densities of each kind
of larva. Perhaps multiple species with two-year overlapping life cycles in
each of the types of larvae produced this irregularity. Cryptochironomus
sp. 2, however, was more consistent. It was always most abundant in benthic
zoae 1 and its numbers changed little from season to season.
Size and Sex Frequencies in Pontoporeia affinis
Figure 13 provides size and sex frequency histograms for P. affinis in
the major surveys of July and November 1970 and April, July and November
1971. Unfortunately a previous presentation of these data (Mozley 1973a,
Fig. 44) has the benthic zones mislabelled, and benthic zone 3 was divided
into samples from greater and less than 40 m depths. The present Figure 13
is based on the same data but arranged strictly according to benthic zones.
Benthic zone 0 is omitted from the figure because too few amphipods were
present (<30) to indicate population size/sex frequencies. The November 1971
data are shown in Figure 13 for the first time.

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