METHODS

<< Previous | Table of Contents | Next >>

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).

Horizontal Distribution

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.

Vertical Distribution

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.

Figure 2b. A Miller sampler with a No. 12 mesh net (0.199 mm aperture).

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.

Contamination

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.

Sample Analysis

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.