Zooplankton Grazing as a Control Mechanism in Algal Blooms. A Method for the Study of the Effect of Algal Toxins on Zooplankton Vertical Migration

摘要

A method was developed to study the effect of blue-green alga toxins on the diel vertical migration of zooplankton. The usefulness of a high-frequency echo sounder to follow the diel movements of zooplankton was first tested in the field. Zooplankton were then placed in model lake systems (2.5 m high columns) in the laboratory. The echo sounder was modified to reduce the wave length of the sound output, thereby increasing the vertical resolution characteristics. Other modifications were also made to adapt the unit for use in these shallow systems. Maximum resolution was obtained by interfacing the sonar recorder with an oscilloscope so that the vertical position of individual zooplankton (Chaoborus and Daphnia) could be determined plus or minus 2.5 cm in a 200 cm column of water. Studies were conducted to calibrate the position and width of sonar traces against visual counts of the vertical distribution of animals in the columns. A monitoring system was desinged to automatically track the movements of populations of zooplankton at 15 to 30 minute intervals over 24-hour periods. Size and shape of the columns should be matched to the sound characteristics of the sonar to avoid interference from multiple echos and artifacts caused by echos from the sides of the column. Preliminary studies of the effects of soluble toxins extracted from a bloom of blue-green algae indicate sublethal doses may alter the timing and extent of the vertical migration.