Preliminary Studies of Zooplankton Distribution with the Continuous Plankton Recorder

Abstract

During the months of July, August, and September of 1966, preliminary studies were directed toward biogeographical distributions of some of the principal zooplankters in Lakes Superior and Michigan. These analyses were based on continuous transect samples taken by means of the Continuous Plankton Recorder, an instrument developed for oceanographic work by the Scottish Marine Biological Association of Edinburgh, Scotland. The recorders sampled a transect segment of water one-half inch square and 300-500 miles in length in both Lakes Michigan and Superior. The zooplanklers were concentrated on a continuously moving belt of silk eighteen and one-half feet long. These silks were of two mesh sizes, #60 grit and #15 silk bolting cloth, respectively. Tows were made aboard the S. S. Casun J. Callaway, a commercial ore carrier which followed the normal navigation routes in both lakes. Zooplankton collections were made on two principal shipping routes: Duluth, Minnesota - Chicago, Illinois, and Two Harbors, Minnesota -Gary, Indiana. All tows were made at a depth of 10 meters and at velocities between 14-16 knots ( 15-19 mph). Zooplankton collections were made along two separate lake sections in each lake on each run, one on the downbound course and one on the upbound course. During the sampling period, a total of nearly 300 feet of silk were utilized, representing more than 6.000 miles of tow. Tows were completed at five day intervals with two exceptions. On one occasion a ten day interval occurred between cruises. and on another. two runs were combined with no interval between. Significant differences were noted between the characteristic species of the two lakes. In addition, a numerical difference was evident. Marked diurnal variations in both numbers and in species were observed at the 10 meter depth, with the highest concentrations occurring in the nighttime hours. Marked regional differences were noted which contributed to "patchiness" of zooplankton distribution.