Contrasting migration behaviour of Daphnia pulicaria and D. galeata × hyalina, in avoidance of predation by 0+perch (Perca fluviatilis)

In spring and early summer, a small population of the large-bodied Daphnia pulicaria coexists with a much larger population of the medium-sized hybrid Daphnia galeata × hyalina in the epilimnion of Lake Maarsseveen (The Netherlands). When large shoals of juvenile perch (Perca fluviatilis) appear in the open water, both species start to migrate vertically. Since D. pulicaria has a larger body-size than D. galeata × hyalina, and is therefore competitive dominant over the hybrid, it is unlikely that both species interact via their common food resource, but they react both to fish predation. However, since they differ in size, and therefore in vulnerability for fish predation, both species adopt different strategies. The smaller bodied, and less vulnerable D. galeata × hyalina exhibits diel vertical migration ascending to the surface at dusk, and staying there during the night. In this way, benefiting from the higher temperatures of the surface layers. In contrast, the large-bodied, and more vulnerable D. pulicaria selects the deep cold hypolimnion water layers as refuge against fish predation. In this way it benefits from a safe habitat, free from fish predators, but on the other hand suffers from low water temperatures, which decrease its fitness. It is likely that the relatively higher temperature in the upper water layers serves as a proximate factor for the downward migration of D. pulicaria [KEYWORDS: diel vertical migration, depth selection behaviour, predator avoidance, inducible defences, Daphnia, juvenile fish]

Increased phototaxis in the field leads to enhanced diel vertical migration.

Downward and upward displacement velocity of diel vertical migration in Lake Maarsseveen by the hybrid Daphnia gaZeata x hyalina, was correlated with the relative change in light intensity at dawn and dusk, indicating a phototactic basis for these migrations. The significance of the regression function increased over a period of 4 yr. Changes in environmental factors such as water transparency and food concentration are not responsible. Since 1989, a diel vertical migration (DVM) of Daphnia in Lake Maarsseveen, The Neth-erlands, has been intensively studied. This mi-gration is confined to 5-6 weeks in June and July when large shoals of juvenile perch (Perca fluviatilis) are present in the open water (Rin-gelberg et al. 199 1). Before 1989, day and night vertical distributions of Daphnia were occa-sionally determined, but differences in dept

Life-time contributions of Joop Ringelberg to new approaches in aquatic ecology, father of modern aquatic ecology in the Netherlands

Two studies of complicated ecological phenomena in Lake Maarsseveen (The Netherlands) are presented to illustrate that a combination of field and laboratory analysis might be a successful approach. In the first one, the yearly varying ratio of population abundance of two diatoms, Asterionella formosa and Fragilaria crotonensis during early spring is explained by a combination of abiotic and biotic factors. A fungal parasite blocks population development of the first species at temperatures higher than 3–5 °C . The second species then competes successfully for phosphate and develops a large population. The interaction between Asterionella and the fungus proved to be intricate. Several physiological mechanisms operate together and combined with variable climatic factors makes the prediction of what species will be dominant, impossible. In the second example diel vertical migration of the hybrid Daphnia galeata × hyalina is analysed in the field and the laboratory. Migration is confined to six to seven weeks in June–July when large shoals of juvenile perch are present in the pelagic zone of the lake. These fish exudate a kairomone that enhances the reaction of Daphnia to relative increases in light intensity of dawn. Especially accelerations in rate of relative increases enhance swimming velocity. The extent of enhancement depends, however, on the concentration of the kairomone and on food concentration. A ’Decision Making Mechanism‘ is introduced and the relation between the response mechanism and the adaptive relevance of diel vertical migration is made. Finally considerations about research strategies and the importance of fundamental research are made. It is concluded that the development of limnology and aquatic ecology might be in danger when the present tendency to give prevalence to applied research continues. [KEYWORDS: strategies of plankton research, field observations, laboratory experiments, phytoplankton succession, fungus epidemics, diel vertical migration]