MODIFICATION OF MIGRATORY BEHAVIOR OF PLANKTONIC CRUSTACEA IN THE PRESENCE OF A BLOOM OF MICROCYSTIS-AERUGINOSA (CYANOBACTERIA)

Nycthemeral vertical migrations of the planctonic Crustaceans (Daphnia longispina, Cyclops vicinus, C. strenuus & Eudiaptomus gracilis) through different stages of the development, have been studied in laboratory. The goal of the study was to specify the role of the exogenous factors, especially the effects of Cyanobacteria bloom, and also those of photoreceptors of the Crustaceans. We used a transparent column (3 meter high and with a 0,25 m base diameter) in a controled enclosure. Organisms were subjected to different conditions: various light conditions L.D. 16-8 cycles or under permanent light; either organisms could interact directly with the Cyanobacteria or were separated from the latter. The Cyanobacteria were themselves at various stages of age: growing, declining or dead. They were placed in a plexiglas bark on top of the column in contact with the water. The different bottoms of the used barks permit either a total chemical isolation or a partial contact or even a complete contact with the subjects in the experiment. The results are as follows: The migrations are synchronized by the light variations and this is always the case except for red and blue light. The Crustaceans do not react to red light. Migrations are observed under red light (L.D. or L.L. cycles) only among adults and can be understood as the reaction of an internal oscillator. Blue radiations strongly inhibit climbing. The green ones stimulate displacements. Microcystis aeruginosa in growing phase do not alter behaviours, even under the 'direct contact' condition. Declining or dead Cyanobacteria stop however the migrations of Daphnia and D. Eudiaptomus, even under the no direct contact condition. Under the contact condition, animals die quickly because of the toxines emitted by M.a. Cyclops vicinus and C. strenuus maintain their displacements in these conditions. This behavioral difference seems to be linked to the diet. The absence of a direct relationship entails a visual appraisal of the physiological fitness of the Cyanobacteria, by the species with stopped migration. The information is carried by the quality of the light transmitted through the colonies. The amplitude of the displacements at dusk, depends on the relative importance of the blue and green radiations in the diurnal light. Declining M.a. spill in the water a blue pigment, while when growing, they absorb most of the blue radiations and not at all the green ones. Thus, the herbivorous planktonic crustaceans seem to be capable of adjusting their vertical displacements in accordance with the light information they received during the day. The stopping of the migrations when M.a. declines, help them avoid contact with the toxins. In the column, whose height is limited, this postpone their death; in situ a large number will survive in deep.