The Vertical Distribution of Volvox in the Plankton of Lake Monona.

Abstract

On the morning of July 6, I9I6, Messrs. Birge and Juday, of the Wisconsin Geological and Natural History Survey, noted a very peculiar condition, while collecting plankton samples from Lake Monona, Madison, Wisconsin. They found that there was a very decided stratum of Volvox colonies at a depth of three meters, while above and below this stratum the colonies were very scarce. Mr. Juday called my attention to this condition, and suggested that I make plankton catches at various times of the day to see if the position of the belt changed. Although the data presented herewith are, at best, fragmentary, they seem worthy of record since nothing is known concerning the vertical distribution of Volvox as a limnetic organism. The bloom of this alga is so sporadic that it may be some time before so favorable a condition for a study of this kind again presents itself. The ordinary method of pumping a measured amount of water (ten liters) from a desired depth and then straining through a filter net was used for collecting the samples, which were then preserved in alcohol until counted. In cotunting, the volume of the catch was reduced to ten cubic centimeters, then a one-cubic-centimeter sample was taken, put in a trough, and the number of colonies counted through a binocular microscope. The station where the alga was first discovered is about a kilometer northwest of Winnequah point, at the deepest part of the lake. My samples were collected about a half kilometer out from the Wirka Boat Livery where the water is between ten and thirteen meters deep. This station is about a kilometer and a half from the deepest part of the lake. Sunset occurred at 7:40 on July 6 and the sample from the surface was taken at 7:45. Since it takes about five minutes to adjust the hose, pump the water, label and preserve the sample, the interval between two catches is about five minutes. The same order was followed in every case, the first sample taken at the top and others on down at intervals of a meter. Twilight lasts about an hour at this