Ben J. G. Flik

The mortality effect of ultraviolet radiation in a translucent and in a red morph ofAcanthodiaptomus denticornis (Crustacea, Copepoda) and its possible ecological relevance

Ultraviolet radiation induced a lower mortality in morphs ofAcanthodiaptomus denticornis, coloured red by carotenoid pigments, than in translucent morphs. Calculations showed that ultraviolet penetration in a lake of low chlorophyll-a content may cause a hazardous dose in the surface layers. Experiments confirmed this.

Estimation of the primary production in Lake Maarsseveen I with an incubator technique

Summary1.A description of an incubator for14C primary production measurement is given.2.Once and sometimes twice a month, during two years, estimations of primary production and chlorophyll contents were made for Lake Maarsseveen I.3.The incubator technique and thein situ method for the measurement of primary production are compared.

Contrasting migration behaviour of Daphnia pulicaria and D. galeata x 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.

Diel vertical migration ofEudiaptomus gracilis during a short summer period

Several aspects of a diel vertical migration (DVM) of adultEudiaptomus gracilis in Lake Maarsseveen (The Netherlands) are described. The period of DVM lasted from the end of May until the middle of August. On May 21, 1989, the population was found divided into a deep dwelling part and a part in the upper five meter. Large shoals of juvenile perch were observed in the open water for the first time. On June 7, the whole population was down below 10 m and concentrated in a zone of high chlorphyll-a concentrations. One week later, a regular DVM was performed. The amplitude of this migration gradually decreased towards the end of the migration period. The ascent in the evening and the descent in the morning took place after sunset and before sunrise, respectively. The movements coincided with high relative changes in light intensity. Population size increased rapidly during the period of DVM but decreased again before the end of this period.

Underestimation of primary production as indicated by measurements with size-fractionated phytoplankton in Lake Maarsseveen I (The Netherlands)

Primary productivity of four size classes of phytoplankton (<150μm, <50μm, <20μm and <5μm) was measured from March through October 1986 in Lake Maarsseveen I with an incubator technique. The mean column production was approximately 400 mg C.m−2.day−1, with a range of values between 150 and 750 mg C.m−2.day−1. The mean contribution of the size fractions <50μm, <20μm and <5μm to the size fraction<150μm was 80%, 60% and 35%, respectively. During their appearance the grazing impact of small herbivorous zooplankton,e.g. rotifers, can give an underestimation of the size fraction <150μm. An indication of this phenomenon is given.

Analysis of the shape of P vs I curves of a natural phytoplankton assemblage during the year in the mesotrophic Lake Maarsseveen (I), The Netherlands

Production versus light response curves were made of natural phytoplankton assemblages during the year in an incubator at 8 different light intensities ranging from 60 to 1500 μEinstein.m−2.sec−1. The shape of these curves is analyzed in relation to the sensitivity for photo-inhibition. At the end of the month of April, there is a sudden shift in this sensitivity at higher light intensities. The algal assemblage becomes less sensitive for photo-inhibition. The influence of light-adaptation, temperature, nutrient limitation and species composition is discussed.

Diel and annual oxygen regimes in a small lake

An extensive ecosystem analysis program was conducted at the “Barlosche Kolk”, a small eutrophic lake in the eastern Netherlands, from September 1971 to September 1973. As part of this program, the oxygen concentration at different depths was determined continuously over several 24 hour periods. Although no well defined thermocline was observed, a marked oxygen stratification persisted during summer. Detailed pictures of the diel oxygen regime were constructed, showing strong dynamics especially during early autumn. Diel column-mean ranges were highly correlated with time of the year and diel patterns showed to be highly predictable.

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

This dedication marks the retirement of Prof. Dr J. Ringelberg from the University of Amsterdam. In this article his contributions to the aquatic ecology in the Netherlands are reviewed. After his Ph.D. study on ‘The positively phototactic reaction of Daphnia magna Straus, a contribution to the understanding of diurnal vertical migration’ (1964), Joop Ringelberg became associate professor at the Laboratory of Animal Physiology of the University of Amsterdam. He started at the university the research group of Experimental Hydrobiology. From 1965 to the 1980s he played a very important role in the development of experimental ecology and ecosystem research in the aquatic habitats in the Netherlands. During the last ten or twelve years of his scientific career (1985- to date) Dr Ringelberg returned to his old hobby horse the ‘Diel Vertical Migration (DVM)’ of zooplankton, especially daphnids. Intensive field and laboratory studies during these later years, helped him and his students to get a deeper insight into DVM behaviour of the daphnids, in response to the rates of changes in light intensity in close conjunction with release of predator (fish) kairomones. In addition, Ringelberg initiated the use of micro-ecosystems and was leader of a team that has developed a flowcytometer, especially for quantitative and pigment analyses of phytoplankton. His other landmark achievements, national and international, included the chairmanship of the Dutch Hydrobiological Society and of the Aquatic Ecology division of BION (Biological Research in the Netherlands). He was a national representative of SIL (International Association of Theoretical and Applied Limnology) and during ten years a member of the Scientific Advisory Board of the Max Planck Institute for Limnology at Plön, Germany. As a guest scientist he is still continuing his scientific pursuits at the Centre of Limnology, Nieuwersluis.