Complete Report - Limnetic Zooplankton in Crater Lake, Oregon

Complete Report

During the summers of 1967 and 1968 the horizontal distribution and diel vertical migrations of zooplankton were studied within the unique environment of Crater Lake. Sampling of the horizontal distribution was done by towing plankton nets vertically in different locations. The vertical distribution was sampled by towing at different depths with a standard tow net and Miller samplers. During both summers, the greatest numbers of zooplankton were sampled in late August. Bosmina longispina was the most numerous zooplankter, while Daphnia e, insignificant in 1967, increased in abundance during 1968. The horizontal distribution of B. longispina was clumped, being consistently more abundant in some locations than others. D. pulex had a random, or near-uniform, distribution. Vertical migrations were not consistent and seem to occur only during certain times of the year. The depth of the maximum concentration of B. longispina was found to vary between distances of 12.5 and 25 m, and was located at depths of 75 to 50 m during the day and between 50 to 37.5 m at night. A few B. longispina, however, did migrate to the surface at night. On August 28 and 29, 1968, the entire adult population of D. pulex migrated from 62.5 m during the day to the surface at night. This migration appears to have reproductive advantages. Prior to this time, only a nocturnal scattering of a small portion of the total population of D. occurred. Various ideas are given as to why these different migrations occurred, but based on the information that is presently available, any direct relationships between the variations of vertical migrations and environmental factors are difficult to make.

A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science, June 1969

I would like to acknowledge the U.S. National Park Service for its help and assistance in making this study possible. I am especially grateful to Mr. Robert C. Bruce, Chief Naturalist; Mr. Richard M. Brown, Research Biologist; and Mr. Rudy Wilson of Crater Lake Lodge, Inc. The field assistance of Mr. Harold V. Kibby, Mr. Douglas W. Larson, Mr. James Malick, and Mr. Ethan Schumacher was greatly appreciated, along with the supervision, guidance and inspiration of Dr. John R. Donaldson.

1. Selected morphometric, physical and chemical characteristics of Crater Lake, Oregon

2. Analysis of variance by station, date, and year, of all age classes of cladocerans 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

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

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

A standard 112-m diameter tow net, with a No. 6 nylon mesh (0. 239 mm aperture), equipped with a T. S. Slow-meter

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

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

The diel vertical distribution of Bosmina longispina on August 24 and 25, 1967 in Crater Lake, Oregon

The diel vertical distribution of Bosmina longispina on July 24 and 25, 1968 in Crater Lake, Oregon

The diel vertical distribution of Bosmina longispina on August 28 and 29, 1968 in Crater Lake, Oregon

The diel vertical distribution of Daphnia pulexon July 24 and 25, 1968 in Crater Lake, Oregon

The diel vertical distribution of Daphnia pulex on August 28 and 29, 1968 in Crater Lake, Oregon

13. Primary productivity (carbon-14) in Crater Lake, 1968, represented in total counts per minute

There is extensive literature on the horizontal distribution and vertical migrations of zooplankton in fresh water lakes. An adequate review is given by Hutchinson (1967). Studies in a wide variety of lakes indicate that water currents are the main environmental factor involved with the horizontal distribution of zooplankton, but vertical migrations may also mediate the horizontal distribution by utilizing different water currents at different depths.

In general, vertical migrations are best observed in lakes that are deep, transparent and unproductive. Worthington (1931) in the Lake of Lucerne found vertical migrations with amplitudes in excess of 50 meters. Past observations and experiments reviewed by Hutchinson suggest light to be the initiating, controlling, and orienting factor affecting die1 migrations, but chemical and thermal gradients may also determine the position of zooplankton.

Although the exact causes of vertical migration are not fully understood, predator avoidance (Manteufel, 1959), navigation (Mackintosh, 1937), increased gene flow (Carson, 1957), and a decreased "social stress" (Wynne-Edwards, 1962) have all been proposed as possible adaptive advantages. These are discussed by McLaren (1963) who offers the most recent and complete treatment of the adaptive significance of vertical migrations, McLaren, however, stresses that reproductive and growth advantages are gained through vertical migrations. He suggests that all necessary feeding is done at the surface while a greater size and fecundity are attained in the lower temperature of deeper waters. Because of its formation, location, and extreme depth, Crater Lake has unusual optical and thermal properties, and offers an unique environment for zooplankton investigations. It was the intent of this study to record the horizontal distribution of zooplankton by determining variations in density with time and location in the lake. Observations of the die1 vertical distribution of zooplankton were made in an attempt to record the timing and extent of vertical migrations under the unusual conditions of light penetration in Crater Lake.

With the exception of some preliminary studies, summarized by Nelson (1961), little is known about the horizontal and vertical distribution of limnetic zooplankton in Crater Lake. Brode (1938) reported Daphnia pulex maxima at 38 and 53 m, and Bosmina longispina maxima at 100 and 150 m. It was his opinion that D. pulex might undergo daily or seasonal vertical migrations. Kernrnerer et al. (1923) observed D. pulex maxima a t 60 to 80 m in August and at 50 to 60 m in September. He also found B. longispina at 100 to 150 m. Hasler (1938) reported only on D. pulex whose maxima he found between 50 to 100 m during the summer of 1937. During the summer of 1940, Hasler and Farner (1942) found no g. pulex in tow-net samples from 100 m to the surface.

Crater Lake is located within the collapse caldera of Mt. Mazama on the crest of the Cascade Mountains in southern Oregon. The near circular lake is the deepest in the United States and is fed entirely by natural precipitation. Its extreme depth, high surface elevation, and isolation with no permanent inflowing streams account for water that is quite low in minerals and unusually transparent (Table 1).

Throughout most of the year, the lake is isothermal, stratifying only during the warm summer months. This generally occurs from late June through late September. During thermal stratification the epilimnion is very shallow, the metalimnion existing between 10 to 20 m. Because of strong wind action, which gives rise to a high summer heat increment, Crater Lake seldom completely freezes over. The only major freeze was during February through April of 1949.

Table 1. Selected morphometric, physical and chemical characteristics of Crater Lake, Oregon.

All field data were collected during the summers of 1967 and 1968. Sampling before mid-June or after early September was virtually impossible because of the extreme weather conditions during the long winter season that impair access to the lake. Most of the data obtained in 1967 were the results of exploratory efforts. For convenience in sampling, the lake was divided into one mile square sections. These were numbered consecutively and referred to a s stations (Figure 1).

During the investigations of the horizontal distribution of zooplankton, six stations in 1967 and nine in 1968 were arbitrarily selected to represent a variety of conditions throughout the lake and give an even distribution to the sampling effort. Stations 5, 10, 13, 18, 22, and 25 were sampled in 1967. Stations 3, 5, 10, 13, 18, 21, 23, 25, and 30 were sampled in 1968. During each summer the sampling was repeated five times at intervals of two to three weeks.

Figure 1. Sampling grid of Crater Lake showing the zooplankton sampling stations

Samples were obtained with a standard plankton tow net 1 /2-m in diameter, with a No, 20 nylon mesh (0. 076 mm aperture). The net was towed vertically from 100 m to the surface a t each station sampled. Two samples at each station were taken on August 26, 1967 and August 27 and 28, 1968, to obtain some estimate of sampling error. Only one sample per station was collected on all other dates.

When sampling the vertical distribution it was necessary to tow the net horizontally in order to catch sufficient quantities. of zooplankton. At first, vertical tows were made with a 112-m diameter closing net (No. 20 mesh nylon). But, too few organisms were caught to give meaningful results, and the use of the closing net was discontinued. Exploratory horizontal tows were taken on July 24, 1967. A standard 112-m diameter tow net with a No. 6 nylon mesh (0. 239 mm aperture) equipped with a T. S. flow-meter (Tsurumi-Seiki Kosakusho Go., Ltd.) was used to sample different depths at several locations in the lake (Figure 2a).

The sampling process involved lowering the net vertically to the desired depth. Then, more cable was slowly released to maintain the net at depth as the forward motion of the boat brought the angle of the towing cable to 60 degrees. The cable length at 60 degrees is twice the vertical depth.

The first meaningful series of horizontal tows was made on July 28, 1967. All sampling was restricted to station 13 (Figure 1). Two series of seven tows were made at a range of depths from 200 to 25 m a t 1020 to 1255 hours, and at 2200 to 2435 hours.

Figure 2a. A standard 1/2-m diameter two net, with a No. 6 nylon mesh (0.239 mm aperture), equipped with a T. S. flow-meter.

On August 24 and 25, 1967, five series of six tows were taken at 1341 to 1555 hours, 1824 to 2000 hours, 2408 to 230 hours, 0655 to 0855 hours, and 11 31 to 1321 hours. The depths sampled were in 25 m intervals from the surface to 125 m. A sample from 125 m was lost from the second series of tows.

In 1968, a sampling device designed by Miller (1961), a modified small Hardy plankton sampler with a No. 12 mesh net (0. 199 mm aperture), was used in place of the standard tow net (Figure 2b). Its small size, light weight, and increased efficiency made it very effective for sampling the depth strata simultaneously at high towing speeds.

Unlike the standard tow nets, the Miller samplers did not have flow meters. All horizontal tows taken with the Miller sampler were towed exactly 10 minutes in order to standardize the volume of water sampled. The only exception was the first series of tows on July 24, 1968, which were towed for 15 minutes.

During a sampling series four Miller samplers were simultaneously towed horizontally at 12.5 m depth intervals from 25 to 62.5 m and 75 to 125 m (omitting a sample from 112.5 m). Two samplers were simultaneously towed at 1 and 12.5 m. A cable angle of 70 degrees was maintained after a sufficient cable length was attained to reach the desired depth. The length of cable needed to reach the desired depth was calculated from the following relationship:

Six series of horizontal tows were made during July 24 and 25, 1968, at 1340 to 1415 hours, 1700 to 1728 hours, and 2013 to 2100 hours, 2408 to 2457 hours, 0536 to 0639 hours, and 0928 to 1013 hours. No samples were taken from 1 and 12. 5 m during the first two series because previous results showed that few if any organisms were to be found at those depths during periods of high illumination. One sample each was lost on the fifth series at 125 m and on the sixth from 12.5 m.

On August 28 and 29, 1968, six additional series of horizontal tows were made at 1200 to I245 hours, 1617 to 1659 hours, 2115 to 2158 hours, 0205 to 0245 hours, 0739 to 0817 hours,and 1118 to 1159 hours. No samples were lost during this series.

Since none of the sampling equipment employed in 1967 or 1968 had a closing apparatus, attempts were made to estimate the extent of contamination of the samples by zooplankton encountered while towing back through depths above 125 m. Immediately after sampling the vertical distribution on August 25, 1967, two vertical tows using the No. 6 mesh standard tow net were made from 125 m to the surface to estimate the contamination from depths above 125 m.

After sampling the vertical distribution on August 29, 1968, a Miller sampler was lowered to 125 m to duplicate the upward path of samples taken at this depth, the cable length was increased and cable angle brought to 70 degrees before the Miller sampler was hauled to the surface.

All samples collected from Crater Lake were immediately preserved in 3 percent formalin solution. In the laboratory an aliquot was taken from a thoroughly mixed sample and the organisms counted under a dissecting scope. A graduated bulb pipette was used in extracting the aliquot from the sample.

The relationship between aliquot volume to be counted and sample volume was determined by weight. By dividing the weight of the aliquot into the weight of the sample and multiplying the quotient times the number of organisms counted, an estimate of the number of organisms in the entire sample could be obtained. This relationship is represented as follows:

The organisms were enumerated by species and two age classes (adults and juveniles) to observe any differences in their horizontal and vertical distributions. Identification of juveniles was determined by size. All forms having no eggs and being no larger than one fourth the size of the adults were classified as juveniles.

Two aliquots per sample were counted for the vertical tows in 1967 and the horizontal tows in 1968. Three aliquots per sample were counted for the horizontal tows in August 1967, and the vertical tows in 1968.

Since the T. S. flow-meter is capable of measuring the volume of water sampled, samples taken with nets equipped with a flow meter were computed in organisms per cubic meter of water sampled. Horizontal calibration of the flow meter was done in the Men's Pool at Oregon State University. Calibration of the vertically towed flow meter was done a t Crater Lake in 1968. Vertical tows at 100 m were made with both standard and closing nets (No. 20 mesh) with and without a flow meter. Averages of flow meter revolutions per 100 m vertical tow were used for tows made without a flow meter. The standard tow net with a No. 6 mesh was always equipped with a flow meter.

All calculations were programmed for computerization. Calculations were made of sample means and the number of organisms/cubic meter of water sampled (these data are included in the Appendix). The horizontal distribution data were analyzed in a multiple analysis of variance by comparing samples taken in the same stations during both 1967 and 1968. Station 21, sampled in 1968, was compared with station 22, sampled in 1967, because of their relatively similar positions on the lake (Figure 1).

Since sampling error from within station variance might bias the results of the previous analysis, the effect of sampling error was determined by analyzing common stations sampled during August 26, 1967, and August 28, 1968, when two samples per station were taken.

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.

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.

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.

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)

The rapid increase in the numbers of zooplankton during late August of both summers appears to indicate a monocyclic seasonal variation that usually occurs during spring in most temperate lakes (Hutchinson, 1967). There seems to be some correlation between the warming of the surface water and the increase of zooplankton populations, but only the adult D. pulex and their contained juveniles show any movement into these warmer waters. It is not certain when the maximum population size of either species is reached, but it apparently peaks some time in the summer or fall after the last samples were taken.

Despite the dominance of D. pulex that has consistently been reported by past investigators, only during the latter part of the summer of 1968 were its numbers comparable to the numbers of B, longispina. The depths at which the maximum numbers of these two species occurred were also shallower than those reported in earlier studies. Seasonal succession through competition could be responsible for the variation in species composition, however, no decrease in the population of D. longispina was observed with the concurrent increase of D. pulex in 1968.

In general, B. longispina has a static clumped dispersal. High and low densities occurred in the same respective stations during both 1967 and 1968. The highest densities occurred in the central and eastern stations, and the lowest densities occurred in the more northerly and south-westerly stations that are less than one-half mile (800 m) from shore. D. pulex, however, does not have any definite dispersal pattern. A random or near uniform dispersal seems likely. Where B. longispina definitely seems to avoid an area like station 22 near Wizard Island there is no corresponding absence of D. pulex.

It is possible that the differences in the vertical migrations of the two species could account for their differences in horizontal distribution. A greater vertical movement of D. pulex would expose it to a greater diversity of water currents both on the surface and below, dispersing the population in different directions. In contrast, B. longispina which has only a slight vertical movement, maintains an almost uniform depth. If upwelling occurs B. longispina could become locally concentrated, according to Ragotzkie and Bryson (1953). Strong upwelling was suspected by Kibby St. (1968) during their study of the surface temperatures and currents in Crater Lake.

Even though Crater Lake is exceptionally deep, clear, and unproductive, the observed vertical migrations were not as marked as those reviewed by Hutchinson (1967) in similar lake environments. Except for the migration of the entire adult population of D. pulex in late August 1968, die1 vertical migrations are limited to a partial upward scattering of both D. pulex and B. longispina during the night. Evidently true diel vertical migrations are not consistent and occur only during certain times of the year in Crater Lake. Seasonal and annual variations in the vertical distribution are apparent. Differences within the reported depths of the day maxima of zooplankton in previous studies by Kemmerer &a. (1923), Hasler (1938), and Brode (1938), indicate that these variations may be even more pronounced than those observed in this study. Of course, these observations may be biased by differences in sampling methods and equipment.

It is difficult to explain the vertical migration of zooplankton in Crater Lake based on presently considered theories for such phenomenon. Since the depth of the maximum zooplankton population by day is dependent on transparency (Kikuchi, 1930), high surface illumination must be the main environmental factor that determines the vertical distribution during the day. However, there are no obvious explanations for what determines the nocturnal distribution of zooplankton.

A direct relationship between the nocturnal distribution and environmental variations is not clear. Variations in weather conditions affected Secchi disc depths more than any variation in water transparency, so it is impossible to say anything about the relationship of water transparency and the nocturnal distribution based on Secchi disc information.

A relationship between the nocturnal distribution and water temperature is also difficult to make. Different thermal gradients occurred during each sampling of the die1 vertical distribution, yet B. longispina underwent a nocturnal upward scattering regardless of these differences. However, a full migration of adult D. pulex was observed to occur in August 1968 when there was the least change of temperature between the surface waters and the depths of the day maxima.

Primary production was observed to have increased in the surface water, and has a similar curve during both July and August 1968, but the migrations of zooplankton tend to favor the conditions that existed in August. Vertical movements of B. longispina were more pronounced in August, but only the adult D. pulex migrated en masse to the surface. Juvenile D. pulex scattered upward like the adults in June, but no upward vertical movement of juvenile D. pulex was observed in August.

An explanation of the 1968 migration of adult D. pulex from 62.5 m during the day to the surface at night may be offered by McLaren's (1963) theory on the adaptive value of vertical migration. McLaren concludes that migrations are favorable to reproduction and growth when a thermal gradient exists between the depths of the day and night zooplankton maxima. Assuming that all feeding is done at or near the warmer surface waters, the energy gained by "resting" in the colder lower depths during the day would result in a more efficient growth, larger size, and greater fecundity.

Since the migrants are mainly large adult D. pulex carrying juveniles and eggs, there appears to be a definite reproductive advantage in this migration. This migration was observed to take place when there is a thermal gradient and when food appears to be more abundant in the shallower depths. However, a marked thermal gradient and increased primary production at the surface occurred in July of 1968, but only a few adult D. pulex were found at the surface during the night. The only major difference between July and August 1968 is that D. pulex adults and juveniles were more abundant in August.

It could be possible that either the full migration of D. pulex does not occur until a maximum fecundity is reached, or that a new generation of die1 vertical migrants are being produced during this July 1968 period. McLaren states that both phenomenon are possible. As the surface temperature begins to warm and as food becomes abundant at the surface, those few organisms migrating to the surface will have a higher reproductive rate producing a greater number of offspring (Hutchinson, 1967 and Pennak, 1953). Eventually the greater proportion of the entire zooplankton population will be made up of descendants from those few July vertical migrants. The great increase in the density of juvenile D. pulex during August 1968 seems to support this idea (Figure 4).

Primary productivity studies (Larson, unpublished) along with phytoplankton studies (Utterback et al., 1942) seem to indicate that at certain times of the year the food source in Crater Lake is concentrated at lower depths coinciding with the depths of the day zooplankton maxima. This could explain the initial inhibition of migration as all upward movement would be away from the food source.

Although an increase in numbers at 12.5 and 25 m is observed with B. longispina at night in August 1968, the population does not generally experience much of a temperature change during its slight upward movement. However, full migrations of B. longispina may occur later in the season. Marked migrations of Bosmina sp, have been shown by Worthington (1931) who found the amplitudes of migration of B. coregoni to exceed that of Daphnia longispina in the Lake of Lucerne.

These ideas suggest that greater study is needed on the relation of the physical and biotic environment of Crater Lake to the horizontal and vertical distribution of zooplankton. Correlations of nutrient availability and phytoplankton density should be made with the difference in the abundance and diel vertical distribution of zooplankton. Primary productivity studies done in Crater Lake have just been estimates of photosynthesis and are not necessarily a true indicator of food density.

Similarities between current patterns and the horizontal distribution should be done to try to solve the problem of the apparent static clumped distribution of B. longispina. Optical properties should be studied with a light photometer in relation to vertical migrations. Finally, sampling earlier and later in the season should be done, especially when the maximum density of both D. pulex and B. longispina is reached. Unfortunately, the period of conventional access to Crater Lake is extremely limited.

Nowhere are the exact causes of vertical migrations fully understood. But, it is my opinion that Crater Lake would be an ideal environment in which to attempt to solve this problem. Here, the environmental variables are few, and the numbers of species to observe are limited to B. longispina and D. pulex. I hope that this study will inspire future investigations of Crater Lake zooplankton to attempt to answer the question of why there are diel vertical migrations.

1 . Studies of the horizontal distribution and die1 vertical migrations of zooplankton were initiated because of the unique environment created by the unusual optical and thermal properties of Crater Lake.

2. Plankton nets were towed vertically in different locations and horizontally at different depths in order to sample, respectively, the horizontal distribution and vertical migration of zooplankton during the summers of 1967 and 1968.

3. Bosmina longispina was numerically the most abundant zooplankter sampled during the course of the study. Daphnia pulex, insignificant in 1967, increased in abundance during 1968, and in some locations may have dominated the zooplankton biomass by August.

4. During both summers the greatest numbers of zooplankton were sampled in late August. However, it is not known when the zooplankton reach a maximum density.

5. A monocyclic seasonal variation seems to be indicated by the rapid increase of zooplankton numbers during late August of both 1967 and 1968,

6. Studies of the horizontal distribution indicated that B. longispina was clumped, and was consistently more abundant in some locations than others. D. pulex did not have any definite dispersal pattern, and a random or near uniform dispersal seems likely.

7. Upwelling as well as vertical migrations may have affected the horizontal distribution of both D. and B. longispina.

8. Seasonal and annual variations in the vertical distribution are apparent. Vertical migrations are not consistent and may only occur during certain times of the year.

9. Vertical migrations of B. longispina were only represented by a fraction of the entire population. The depth of their maximum concentration was found to vary only a distance of 12.5 to 25 m. No major differences were observed between the vertical distribution of adults and juveniles.

10. Vertical migrations of D. pulex were represented by the entire adult population in late August 1968. No apparent migration of juveniles was observed at this time. Prior to August 28 and 29, 1968, only a portion of the entire population underwent vertical migrations.

11. The range of the vertical distribution of zooplankton, contrary to previous investigations, was shallower than 125 m. Estimates of contamination of samples by zooplankton encountered in the shallower depths indicated that few zooplankters, if any, were present at 125 m.

12. It was not possible to make direct relationships between the different vertical migrations and the variations of water transparency, water temperature, and primary production.

13. An explanation of what causes the different vertical migrations in Crater Lake is difficult, but there appears to be a definite reproductive significance in the migration the entire adult population of D. pulex on August 28 and 29, 1968. A greater size and fecundity can be attained by remaining in cooler waters, but as food and temperatures increase in the surface waters, then migrations are advantageous.

14. It might also be possible that the August 1968 migrations of D. pulex are descendants of the few migrants in July whose reproductive rate was increased by the warmer surface waters.

15. The initial inhibition of vertical migrations could be explained by the concentration of primary production at depths coinciding with the depths of the diurnal zooplankton maxima during early summer. When primary production was concentrated at lower depths, any migration towards the surface would be away from the food source.

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Table 1. 100 m vertical tows taken during five dates at different locations in Crater Lake, Oregon, using a No. 20 mesh 1/2-m diameter net during the summer 1967.

Table 2. 100 m vertical tows taken during five dates at different locations in Crater Lake, Oregon, using a No. 20 mesh 1/2-m diameter net during the summer 1968.

Table 3. The vertical distribution of cladocerans in Crater Lake, Oregon, sampled with a No. 6 mesh 1/2-m diameter standard tow net in July, 1967, using a towing angle of 60 degrees while towing horizontally for 15 minutes.

Table 4. The vertical distribution of cladocerans in Crater Lake, Oregon, sampled with a No. 6 mesh 1/2-m diameter standard tow net in August, 1967, using a towing angle of 60 degrees while towing horizontally for 15 minutes.

Table 5. The vertical distribution of cladocerans in Crater Lake, Oregon, sampled with Miller samplers in July, 1968, using a towing angle of 70 degrees while towing horizontally for a duration of 10 minutes.

Table 6. The vertical distribution of cladocerans in Crater Lake, Oregon, sampled with Miller samplers in August, 1968, using a towing angle of 70 degrees while towing horizontally for a duration of 10 minutes.