Rate of Movement of Populations of Daphnia in Relation to Changes in Light Intensity

Abstract

A knowledge of the rate of vertical movement of a planktonic population is a prerequisite for the interpretation of the role that various factors play in depth control. A method for estimating this characteristic of a planktonic population of Daphnia is described. Several estimates of the net rate of movement have been determined within each of the four periods that characterize the migratory behavior of Daphnia in Lake Mendota, Wisconsin; a series of rates were calculated for each of 11 cruises of approximately 24-hr duration. In most cases, a linear relationship exists between the rate of vertical movement and the rate of change in the logarithm of the light intensity; a number of such relationships are summarized for three of the four periods of movement. An increase in the slope of this relationship with increasing temperature is evidence that the rate of movement in Daphnia is proportional to temperature. The consistent pattern of the rates of movement during the dawn rise and the subsequent regularity in timing of the morning maximum in relation to sunrise together provide the most striking aspect of the diel migration in Mendota. From the onset of the dawn rise, the rate of upward movement increases to a maximum at or slightly preceding sunrise and hence decreases to zero at the time of the morning maximum. Typically, the maximal rates do not exceed 1.5 m/hr. The mean morning maximum at the surface occurs 60.2 min following sunrise, with a standard deviation of 37.2 min; the mean evening maximum at the surface occurs 19.4 min before sunset, with a standard deviation of 61.0 min. Those morning maxima which deviate substantially from the mean were commonly formed under conditions of moderate to heavy cloud-cover. Behavioral evidence suggests a threshold of visual sensitivity of 10−4 to 10−5 lux for Daphnia. The mean amplitude of vertical movement characteristic of Daphnia in Mendota is 0.87 m, with a range from 0.29 m to 1.52 m. Comparative evidence supports the conclusion that the magnitude of the observed amplitudes of vertical movement are logical for an eutrophic lake such as Mendota. Aggregations of Daphnia at various levels present a problem in the design of a program to determine the periodicity of diel migration. Various types of aggregations are summarized within a framework of related environmental factors and with a description of the methods employed to meet the problem.