Winfried Lampert

Laboratory studies on zooplankton-cyanobacteria interactions

Abstract Laboratory studies on cyanobacteria‐zooplankton interactions have largely focused on the inadequacy of cyanobacteria as a food source. Some features of cyanobacteria can be regarded as anti‐herbivore defences. Large colonies of Aphanizomenon, Anabaena and Microcystis cannot be handled by zooplankton, but do not interfere seriously with the filtering process. Small colonies and filaments, however, may cause severe inhibition of the feeding process by mechanical interference. This reduces zooplankton growth, reproduction, and survival. Copepods, rotifers, and Bosmina are less affected by mechanical disturbance than cladocerans. If ingested, some cyanobacteria may be poorly digested or may not provide essential nutrients. Some cyanobacteria are reported to be toxic to zooplankton. Several strains of Microcystis produce an endotoxin, but the amount of toxin produced differs among strains and with the condition of Microcystis. Zooplankton encountering toxic cells cease feeding. Some evidence of an ext...

Food thresholds in Daphnia species in the absence and presence of blue-green filaments

Three Daphnia species of different body sizes were grown on a single food resource in steady-state conditions. They differed in food threshold concentrations nec- essary for body growth and reproduction. The largest species, D. pulicaria, had the lowest thresholds and the smallest, D. cucullata, had the highest thresholds, while D. hyalina was intermediate. This implies competitive superiority of the largest species and inferiority of the smallest one. The sequence, however, was reversed in the presence of blue-green algal filaments, when the thresholds become lowest for D. cucullata and highest for D. pulicaria. Thus the inhibitory effect of filaments is body-size dependent. High densities of filaments that mechanically interfere in food collection may cause shifts in competitive ability among congeneric cladocerans. This may be the reason why, in Europe, D. pulicaria is most successful in ultraoligotrophic lakes while D. cucullata is most successful in eutrophic lakes.

Lake ecosystems: Rapid evolution revealed by dormant eggs

Natural selection can lead to rapid changes in organisms, which can in turn influence ecosystem processes. A key factor in the functioning of lake ecosystems is the rate at which primary producers are eaten, and major consumers, such as the zooplankton Daphnia, can be subject to strong selection pressures when phytoplankton assemblages change. Lake Constance in central Europe experienced a period of eutrophication (the biological effects of an input of plant nutrients) during the 1960s–70s, which caused an increase in the abundance of nutritionally poor or even toxic cyanobacteria. By hatching long-dormant eggs of Daphnia galeata found in lake sediments, we show that the mean resistance of Daphnia genotypes to dietary cyanobacteria increased significantly during this eutrophication. This rapid evolution of resistance has implications for the ways that ecosystems respond to nutrient enrichment through the impact of grazers on primary production.

NATURAL SELECTION FOR GRAZER RESISTANCE TO TOXIC CYANOBACTERIA: EVOLUTION OF PHENOTYPIC PLASTICITY?

Abstract We studied the selection response of the freshwater grazing zooplankter, Daphnia galeata, to increased abundance of cyanobacteria in its environment. Cyanobacteria are a poor‐quality and often toxic food. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from three time horizons in the sediments of Lake Constance, Europe, covering the period 1962 to 1997, a time of change in both the prevalence of planktonic cyanobacteria and levels of phosphorus pollution. We assessed whether the grazers evolved to become more resistant to dietary cyanobacteria by exposing genetically distinct clones to two diets, one composed only of the nutritious green alga, Scenedesmus obliquus (good food), and the other a mixture of S. obliquus and the toxic cyanobacterium Microcystis aeruginosa (poor food). Genotype performance was measured as the specific rate of weight gain from neonate to maturity (gj).

Phenotypic Plasticity of the Size at First Reproduction in Daphnia: The Importance of Maternal Size

I propose a mechanism of rapid phenotypic response of size at first repro- duction (SFR) that may allow relatively large species of Daphnia to survive under variable fish predation. A single clone of D. magna isolated from a shallow lake with planktivorous fish is able to express the full range of SFR observed in the field (1.8-3.0 mm) when raised under controlled temperature and food conditions in the laboratory. Control of SFR is predominantly based on size differences in the eggs of the first and the following broods of Daphnia. First-brood eggs are smaller. They hatch into smaller neonates that mature at a smaller size. In the lake, fish may selectively remove large females, and this will increase the share of first-brood females in the population. Offspring of these females will mature at a much smaller size, so that the size response to predation is intensified. When predation ceases, some females can grow to larger sizes. They will contribute many large offspring to the population because they have larger clutch sizes and larger eggs. This leads to a rapid shift toward large SFR. A single clone of D. magna was hatched from isolated eggs to measure the components of maternal size dependency, which is basic to this hypothesis. Strong positive relationships were found between female size and egg size, egg size and neonate size, neonate size and SFR, size of a female and its offsprings SFR, and female size and clutch size. The predicted pattern of SFR in relation to average body size is found in the lake, but the pattern of egg size is not clear, as egg size is strongly affected by changes in food abundance. This mechanism of phenotypic response may be a fortuitous effect of selection for maximum egg numbers in the first brood, but the increase of offspring size with maternal size may also be adaptive itself.

Growth and reproduction of migrating and non-migrating Daphnia species under simulated food and temperature conditions of diurnal vertical migration

Summary(1)The growth response of two Daphnia species coexisting in Lake Constance to constant and fluctuating conditions of temperature and food was tested in a flowthrough system.(2)In the lake D. hyalina exhibits a pronounced diurnal pattern of vertical migration, whereas D. galeata stays near the surface. The experiments were designed to measure growth and reproductive success of the species under the conditions as they are experienced by their counterparts in the field.(3)Both species grow better and produce more offspring under “non-migration” conditions (constant high temperature and high concentration of food). Thus there is no metabolic advantage by vertical migration.(4)D. hyalina is more successful than D. galeata under extreme “migration” conditions (high food level/high temperature at night and very low food/low temperature during day), but D. galeata grows slightly better under favourable conditions.(5)The results do not support the hypothesis that daphnids gain some metabolic advantage from vertical migration.

Maternal control of resting-egg production in Daphnia

Many planktonic organisms produce ‘resting’ stages when the environmental conditions deteriorate. Like seeds, resting stages can survive unfavourable conditions. The crustacean Daphnia normally reproduces by means of parthenogenetically produced normal, not resting, eggs—but occasionally switches to bisexual reproduction, which results in two resting eggs encased in a robust structure carried on the back of the female. This ‘ephippium’ is shed with the next moult, and can survive dormant for many years. The induction of resting-egg production requires multiple environmental stimuli, one of them being photoperiod. The switch from production of parthenogenetic eggs to resting eggs in Daphnia has recently been shown to be influenced by a maternal food effect. Here we present evidence that female Daphnia transmit information not only about food but also on photoperiod to their offspring, and influence the production of resting eggs in the next generation. The combined maternal effects can be relevant for the correct timing of resting-egg production—for example, in discriminating between spring and autumn conditions.

Long–term genetic shifts in a microcrustacean egg bank associated with anthropogenic changes in the Lake Constance ecosystem

Diapausing egg banks of aquatic zooplankton have the potential to remain viable for decades or even centuries, and can thus harbour potentially high levels of genetic variation. Diapausing (ephippial) eggs from the Daphnia galeata–hyalina complex (Crustacea: Anomopoda) in Lake Constance (Bodensee), Germany, were isolated from sections of dated sediment cores, hatched in the laboratory, and established as a clone bank. We used cellulose acetate electrophoresis at four polymorphic enzyme loci (Pgm, Pgi, Ao, and Got) to examine long–term temporal changes in the genetic composition of the hatchling pool. Our results indicate that significant shifts have occurred in the genetic structure of this population, which parallel concomitant shifts in the trophic state of Lake Constance during the past 25–35 years. Here we discuss the utility of egg bank propagules as good model organisms to study microevolution, as related to past environmental change.

Linking genes to communities and ecosystems: Daphnia as an ecogenomic model

How do genetic variation and evolutionary change in critical species affect the composition and functioning of populations, communities and ecosystems? Illuminating the links in the causal chain from genes up to ecosystems is a particularly exciting prospect now that the feedbacks between ecological and evolutionary changes are known to be bidirectional. Yet to fully explore phenomena that span multiple levels of the biological hierarchy requires model organisms and systems that feature a comprehensive triad of strong ecological interactions in nature, experimental tractability in diverse contexts and accessibility to modern genomic tools. The water flea Daphnia satisfies these criteria, and genomic approaches capitalizing on the pivotal role Daphnia plays in the functioning of pelagic freshwater food webs will enable investigations of eco-evolutionary dynamics in unprecedented detail. Because its ecology is profoundly influenced by both genetic polymorphism and phenotypic plasticity, Daphnia represents a model system with tremendous potential for developing a mechanistic understanding of the relationship between traits at the genetic, organismal and population levels, and consequences for community and ecosystem dynamics. Here, we highlight the combination of traits and ecological interactions that make Daphnia a definitive model system, focusing on the additional power and capabilities enabled by recent molecular and genomic advances.

Differential response of Daphnia genotypes to oxygen stress: respiration rates, hemoglobin content and low-oxygen tolerance

SummaryLaboratory respiration rate experiments using three electrophoretically identified clones of the fresh water, planktonic cladoceran, Daphnia pulex, from an eutrophic farm pond, indicated that clones acclimated to both low and high oxygen levels, regulated oxygen consumption across a wide range of oxygen concentrations (1.0–9.0 mg· liter-1). A “threshold” oxygen level of 0.5–1.0 mg·liter-1 was reached, where animals succumbed to oxygen stress, regardless of hemoglobin content. No significant clonal differences in respiration rates were found. These data suggest that members of this Daphnia population are able to regulate oxygen metabolism across a wide range of ambient oxygen concentrations, and indicate a well-adapted respiratory system.Low-oxygen tolerance experiments and hemoglobin measurements indicated further that physiological differences indeed exist between clones; one clone produced the lowest amount of hemoglobin and was least tolerant of low oxygen levels. These data imply that spatial and temporal changes in dissolved oxygen concentration may be an important selective force influencing the clonal (genotypic) composition of natural cladoceran populations.