RESULTS

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Horizontal Distribution

During each year, the zooplankton density in Crater Lake gradually increased as the summer progressed. The greatest numbers were sampled in late August, but the zooplankton population probably did not reach a maximum density until some time after the last sample in each summer was taken (Figures 3 and 4).

Bosmina longispina was numerically the most abundant organism sampled. In 1967 it was the dominant zooplankter as the larger cladoceran Daphnia pulex was hardly present in sufficient numbers to merit graphic representation. In 1968 the density of D. pulex had increased greatly over the density observed during 1967, and in a few stations actually outnumbered B. longispina. Because of its larger size, the abundance of D. pulex during 1968 may have been sufficient to dominate the zooplankton biomass despite a greater overall number of B. longispina.

B. longispina varied consistently between the stations sampled during both years, indicating a static clumped horizontal distribution. The stations that were comparatively high and low in B . longispina density throughout the summer of 1967 showed similar results in 1968. Stations 10, 13, and 25 had high densities throughout both summers, while stations 5 and 22 (classified as 21 in 1968) were low. Even when other stations had great increases of B. longispina during late August, station 22 never exceeded 100 organisms/m cubed.

Figure 3. Changes in density of cladocerans in Crater Lake, Oregon, at six locations and five dates during the summer of 1967

Figure 4. Changes in density of cladocerans in Crater Lake, Oregon, at nine locations and five dates during the summer of 1968

The highest densities of B. longispina were recorded throughout the summer of 1968 at station 30. On August 28, 1968, station 30 had in excess of 1200 organisms. A greater number of B. longispina juveniles occurred in 1968 than 1967,although the abundance and seasonal changes in the adult population were not appreciably different.

Unlike the horizontal distribution of B. longispina, the numerical densities of neither age class of D. pulex illustrated any significant differences between stations sampled. In 1968 there were more juveniles than adults. A ratio of three D. pulex juveniles to one adult was observed on August 27 and 28, 1968, indicating an increase in population. When the population finally reached a maximum size is unknown.

An analysis of variance of the horizontal distribution data (Table 2) listed significant differences between stations and dates for B. longispina adults, with differences in years being significant only for juveniles. Differences between dates and years were listed for both adults and juveniles of D. pulex. There were no significant differences between stations for D. pulex. An analysis of the two samples per station taken in late August of 1967 and 1968 revealed an insignificantly small sampling error (Table 3). This resulted in large differences between stations, dates, age classes, and interactions.

Table 2. Analysis of variance by station, date, and year, of all age classes of cladocerons in stations 5, 10, 13, 18, 22 and 25, sampled with 100 m vertical tows during the summers of 1967 and 1968 in Crater Lake, Oregon.

Table 3. Analysis of variance of August 26, 1967 and August 27 and 28, 1968, when two samples were taken from 100 m to the surface in stations 5, 10, 13, 22, and 25.

Vertical Migrations

Before August 28 and 29, 1968, studies indicated that only a fraction of the entire populations of B. longispina and D. & underwent vertical migration. The maxima at most, moved only a distance of 12.5 to 25 m. Preliminary samples taken on July 28 and 29, 1967, showed the maxima of B. longispina to move between 75 m during the day and 50 m at night. A few organisms were found at 25 m during the night, but none were observed at this depth or above during the day.

Figure 5. The diel vertical distribution of Bosmina longispina on August 24 and 25, 1967 in Crater lake, Oregon. ( m = 100 organisms/m3)

On August 24 and 25, 1967, the vertical movement of B. longispina was still between 75 m during the day and 50 m, with a very few reaching the surface, at night (Figure 5). The initial upward movement of zooplankton began before sunset, and the downward movement happened after sunrise. Again, organisms were extremely rare in the depths sampled above 50 m during periods of intense illumination.

In 1968 the use of the Miller samplers increased the accuracy of sampling the vertical distribution of zooplankton. On July 24 and 25, 1968, the maxima of B. longispina remained at 50 m, although a movement of individuals from 62.5 m during the day to 37.5 m at night was apparent (Figure 6). A few organisms occupied the surface waters at night and early morning, but by midday B. longispina was relatively absent from waters above 37.5 m. There were no major differences between the vertical distributions of adults and juveniles.

Figure 6. The diel vertical distribution of Bosmina longispina on July 24 and 25, 1968, in Crater Lake, Oregon. (  = 1'000 organisms/10 minute horizontal tow).  * = 15 minute horizontal tow.

Vertical migrations of B, longispina were more pronounced on August 28 and 29, 1968 (Figure 7). The maximum concentration of organisms at 50 m during the day ascended to 37.5 m at night with large numbers present at 12.5 and 25 m. During this period a slight variation occurred between the vertical distribution of adults and juveniles. Not a s many juveniles were found in the 12.5 and 25 m depths at night; and while the maximum concentration of adults was at 37.5 at 2140 hours, the maximum concentration of juveniles was not at this depth until 0230 hours.

Even though D. pulex was present in such low numbers in 1967, samples seemed to indicate that their maxima remained near 75 m. There was no observable migration of this species during this year.

Because of the larger population of D, & in 1968, graphic representation of its vertical distribution was possible. On July 24 and 25, 1968, a day maxima at 62.5 m descended to 87.5 m at night while large numbers of D. pulex also ascended into the shallower depth strata (Figure 8). The largest surface population was recorded at 0602 hours, just before sunrise. Juvenile D. pulex behaved in a way similar to the adults during this diel sampling period.

Figure 7. The die1 vertical distribution of Bosmina longispina on August 28 and 29, 1968, in Crater Lake, Oregon. ( I = 1000 organisms/10 minute horizontal tow)

Figure 8. The diel vertical distribution of Daphnia eon July 24 and 25, 1968, in Crater Lake, Oregon. ( I = 500 organisms/10 minute horizontal tow). * = 15 minute horizontal tow.

Figure 9. The diel vertical distribution of Daphniapulex on August 28 and 29, 1968, in Crater Lake, Oregon. ( - = 1000 organisms/10 minute horizontal tow). * = Juveniles at N01 this depth are those in the earliest molt just released from brood pouches of the adults.

During the night of August 28 and 29, tremendous numbers of D. & were found at the surface (Figure 9). These numbers consisted primarily of large adult females whose brood pouches contained eggs or juveniles in the earliest molt. Those juveniles recorded at the surface were probably those just released from the brood pouches of the largest D. pulex females. Juveniles in the more advanced molt stages were not observed at the surface, and no changes were observed in their die1 vertical distribution.

The maximum concentration of adult D. &during the day was between 50 and 02.5 meanwhile almost the entire adult population migrated to the surface at night. If the presence of small numbers of adult D . pulex in samples taken from the lower depths can be attributed to contamination while towing up through the surface, then we may assume that almost no D. pulex adults were below the surface layers at 0230 hours.

Contamination

Estimates of contamination from depths above 125 m showed that most if not all of the organisms sampled at 125 m were contaminants (Table 4). This may also be true of the 100 m samples.

Environmental Studies

Most of the research on the Crater Lake environment during the period of this study was conducted by Larson (unpublished). Thermal and water transparency (Secchi disc) information was made available. In situ14c incubation was used to estimate the rates of primary productivity at various depths down to 140 m.

Table 4. Comparison of sample averages of zooplankton per horizontal tow at 125 m and sample averages from tows made to estimate contamination by zooplankton above 525 m.

The summer of 1967 was exceptionally warm and calm, creating conditions favorable to thermal stratification. On August 22, 1967, the thermal gradient was unusually pronounced for Crater Lake (Figure 10). The onset of thermal stratification was apparent on July 22, 1968, (Figure l l ), but further stratification was destroyed by a month-long period of extremely cold and windy weather. The thermal gradient had lessened by the last sampling period in late August (Figure 12).

Figure 10. Thermal profile and Secchi disc depth on August 22, 1967 in Crater Lake, Oregon.

Figure 11. Thermal profile and Secchi disk depth 0" July 22, 1968, in Crater Lake, Oregon

Variations between Secchi disk readings were affected more by weather conditions than optical properties of the lake. Estimates of primary production showed a maximum 14c uptake at 80 m on June 14, 1968 (Figure 13), with very little uptake of 14C above 30 m. On July 25, and August 27, 1968, this difference in primary production with depth had disappeared. An increasing uptake of 14c in the shallower depth strata and a decreasing uptake between 60 to 100 m created an apparent orthograde condition.

It is likely that conditions do exist when food is absent from the surface waters. The C data recorded on July 24, 1967, appeared similar to the data of June 14, 1968. Utterback & (1942) in their study of the distribution of phytoplankton in Crater Lake found very few cells above 30 m. Their reported maximum concentration was at 75 m with large numbers occurring down to 200 m.

Figure 12. Thermal profile and Secchi disc depth on August 27, 1968, in Crater Lake, Oregon. *Read during extremely rough weather: cold, windy, rain.

Figure 13. Primary productivity (carbon-14) in Crater Lake, 1968, represented in total counts per minute. (Courtesy of D. W. Larson)