Effects of competitors and Chaoborus predation on the cladocerans of a eutrophic lake: an enclosure study

The role of large laboratory grown food competitors ofthe genus Daphnia as well as the predationimpact of Chaoborus on the cladoceran communityof an eutrophic lake was assessed in five insitu enclosure experiments. The hypothesis tested wasthat the outcome of competition and gape-limitedpredation on cladocerans is size dependent. Accordingto the generally accepted assumptions on competitionand invertebrate predation, large-bodied cladocerantaxa were expected to be less affected by competingcongeners and by Chaoborus than weresmall-bodied taxa. Effects of the predator upon anassemblage of differently sized cladoceran taxa weremuch more pronounced than effects of competition.There was a tendency of predation and competitionimpact to decrease with cladoceran size, but predationpressure was also low for some small cladocerans andhigh for some large cladocerans. The general trendswere further obscured by factors not or indirectlylinked to body size.     

Predator–prey instability: individual-level mechanisms for population-level results

1. In previous work we established that increasing temperature led to a destabilization of the population dynamics of the invertebrate carnivore Mesostoma ehrenbergii and its prey Daphnia pulex , which ultimately resulted in the local extinction of Daphnia at higher temperatures. Two mechanisms are proposed to explain the population-level phenomena: (1) quantitative changes in carnivore vital rates with increasing temperature led to stronger functional and numerical response and (2) qualitative changes in the dynamic allocation of energy to reproduction by the predator with increasing temperature introduces inverse density dependence in the predator's response.
2. The growth of individual M. ehrenbergii was monitored under various food conditions to determine the effect of two temperatures (18 and 24 °C) and five food levels on rates of growth, prey consumption and reproduction and on reproductive allocation patterns.
3. The first mechanism was supported by both higher consumption rates (stronger functional response) and faster growth rates with earlier age at maturity and shorter generation time (stronger numerical response).
4. Evidence for mechanism two was also provided by an alteration of the reproductive allocation pattern with temperature. Viviparous (subitaneous) eggs were more likely to be produced by this carnivore at low food levels at 24 °C, while at 18 °C, high food levels were required before individuals made this switch. This shift actually introduces inverse density dependence in the predator's numerical response which is highly destabilizing.
5. Based on the results of this study, the differential effect of M. ehrenbergii on the dynamics and structure of its D. pulex prey populations can be attributed to changes in both physiological rates and reproductive allocation patterns with temperature.     

On the way to eternity; the success of an aquatic laboratory microecosystem

A microecosystem consisting of three subsystems roughly representing the trophic levels of autotrophs, herbivores and decomposers was developed. A recirculating flow of water connected the three subsystems. Analysis of the data over a period of 7500 days indicates that the system has remained in the same state and has the prospect of being immortal. The Daphnia population showed periods of large oscillations, but also long periods of almost constant numbers. External factors were responsible for the initiation of the oscillations. The threshold food level for population increase was lower than mostly is found for individual growth, indicating adaptation to low food concentrations.     

Zooplankton-phytoplankton interactions: the case for refining methods, measurements and models

Many of the classic experiments on the interactions between animals and their food supply were performed using laboratory cultures of Daphnia but comparable predator-prey cycles have seldom been recorded in the field. In this paper, we report the results of a field experiment designed to examine the effect of seasonal variations in the supply of edible algae on the population dynamics of Daphnia hyalina var lacustris. The experiment was performed in a large (45 m diameter) enclosure that was fertilised at regular intervals to sustain phytoplankton production. The results demonstrate that predator prey interactions of the kind recorded in culture can be detected in the field if large numbers of samples are collected and the results analysed using appropriate methods. Analyses using the numbers of Daphnia collected at a single station and chlorophyll a as a measure of food availability produced inconclusive results. Similarly, Daphnia birth rates calculated using the total number of individuals provided a poor measure of the animals reproductive response to changes in the supply of food. The clearest evidence of systematic plant-herbivore interactions was obtained when improved estimates of Daphnia numbers and adult birth rates were combined with estimates of edible rather than total algal biomass. Daphnia birth rates were then positively correlated with the estimated amount of ingestible carbon (r = 0.77) and the proportion of gravid adults decreased dramatically when the concentration of food fell below 0.10 mg C l^-1. Measurements and models of phytoplankton loss rates were then used to assess the impact of Daphnia grazing on the growth and decline of three species of edible algae. The results demonstrated that most species were able to sustain positive growth rates despite short-term increases in the population filtration rate. For much of the summer, the Daphnia appeared to behave as non-interactive grazers and had very little effect on the growth rate of their main phytoplankton food. Their grazing activity did, however, arrest the growth of a late summer population of Chlorella when the Daphnia were estimated to be filtering all the water contained in the enclosure in less than a day.     

Defenses in phytoplankton against grazing induced by nutrient limitation, UV-B stress and infochemicals

It is becoming increasingly evident that the efficiency of zooplankton grazing on algae is not only a matter of quantity of the grazer relative to its food. Planktonic primary producers are not defenseless food-particles that are easily harvested by the consumers. Several algal species are able to adjust their phenotype (colony formation, spines, size) in such a way that it results in a reduced grazing pressure. It was recently demonstrated that morphological changes in the cell wall of green algae, induced by nutrient limitation and UV-B stress, may reduce their digestibility. A high fraction of induced cells pass intact and viable through the gut of the zooplankters, such that the grazing impact on the population is strongly reduced. It was also found that the presence of exudates (infochemicals) released by daphnids may change the morphology of algae. Unicellular green algae of the genus Scenedesmus were induced to form eight-cell coenobial types, heavily armed with spines, within three to five days after adding filtered water from an algal culture with Daphnia present. Both defence mechanisms may play an important role in zooplankton production and competition, and may serve as an example of highly efficient strategies to resist heavy grazing pressure.     

Timing of predation by rainbow trout controls Daphnia demography and the trophic status of a Minnesota lake

1. Stocking of lakes with rainbow trout is a common practice that presents a potential conflict for lake managers who must balance the interests of anglers with those concerned that zooplanktivory by trout may trigger a trophic cascade and result in decreased water clarity. 2. This study examined how the timing of trout stocking (autumn versus spring) in a Minnesota (U.S.A.) lake affected (i) the population dynamics of their zooplankton food supply (Daphnia pulicaria), (ii) phytoplankton biomass and water clarity and (iii) trout survival. Sizes of both Daphnia and trout populations were estimated acoustically with high‐frequency (192 kHz) sonar. 3. Daphnia were nearly eliminated from the lake during winters after trout were stocked in autumn. In both of these years (1996 and 1997), the Daphnia population was small in the spring, and grew during the summer and into the autumn as the trout population diminished. 4. The lake was then stocked in spring for 2 years (1998 and 1999). This fisheries manipulation alleviated predation over the winter, but increased predation on D. pulicaria during the spring, summer and autumn. However, the high mortality caused by the spring‐stocked trout was offset by even higher rates of reproduction by the relatively large populations of fecund Daphnia that survived the winter in 1998 and 1999. 5. Grazing by these dense populations of Daphnia produced clear‐water phases during May and June that were inhibited in autumn stocking years. In addition, the large Daphnia populations present during the spring and early summer of 1998 and 1999 provided abundant forage for trout. 6. This fisheries manipulation achieved seemingly mutually exclusive management objectives: a robust planktivorous sport fishery, and clear water for other forms of recreation.     

An estimation of the heritability of phototaxis in Daphnia magna Straus

Summary The phototactic responses of four clones of Daphnia magna were experimentally analysed. Broad-sense heritability of this behavioural character was estimated through an analysis of variance, and it was very high under the standardised experimental conditions of this study.     

The energy budget of two sympatric Daphnia species in Lake Constance: productivity and energy residence times

Summary Two Daphnia species, D. hyalina and D. galeata, are living in Lake Constance. Both populations show logistic growth, with the phase of exponential increase in May and the phase of steady state during the summer. In spring, food is not limited and both daphnids reveal an exploitative strategy: females mature early, primiparae are small, egg weights are small, but clutch sizes are large. From June on, when food limiting conditions prevail, D. hyalina starts to migrate vertically with a diurnal rhythm whereas D. galeata keeps on living in the epilimnion over the whole season. The differing thermal environments of the populations lead to short generation times (age of first reproduction) of 11 days for D. galeata in mid-summer, and to 47 days (July) for D. hyalina. The annual production (P) of D. hyalina and D. galeata were 18 and 24 g DW/m², respectively. The average standing stocks (B) were 1.6 and 0.9 g DW/m². This corresponds to turnover rates of 11 x and 27 x per season (200 days), and to turnover times of 18 days and 7.5 days, respectively. From 1980 to 1982 the cumulative annual primary production (PPR) varied between 260 and 330 g C/m². The common productivity (P) of the functional component Daphnia (=sum of both Daphnia species) followed the PPR, but the Daphnia stading stock was constant at 2.5 g DW/m². The increase of secondary production (P) was a consequence of a shift in abundance between the two Daphnia species: at low PPR, D. hyalina was 4 x more abundant, but at high PPR, the two species were equally abundant. Such internal regulatory mechanism within a system component is in accordance with the hypothesis of Cheslak and Lamarra (1981): increase of energy residence time (B/R) with declining nutrients and increasing strength of competitive interrelationships within a given functional component of an ecosystem.This research was supported by the Deutsche Forschungsgemeinschaft within Sonderforschungsbereich Stoffhaushalt des Bodensees (SFB 248)     

Über die Ultrastruktur des Komplexauges von Daphnia pulex

Zusammenfassung Die Komplexaugen von erwachsenen Daphnien (Daphnia pulex) und deren Verankerung wurden elektronenmikroskopisch untersucht. Das Auge enthält 4 Bau-Elemente: 1. Deckzellen, 2. Interzellularsubstanz, 3. Linsenzellen und 4. pigmenthaltige Receptorzellen. Ihre Ultrastruktur wird beschrieben.Da die lichtbrechende Substanz als kompakter Körper innerhalb der Linsenzellen liegt, gehört der Linsenkörper zum euconen Typ. Er wird von aufgereihten Mitochondrion umgeben, ist nicht durch eine Membran vom Cytoplasma getrennt und zeigt eine nach innen zunehmende Elektronendichte.Die zentral gelegene, sehr dichte Zone grenzt basal an das Rhabdomer, nur durch die Zellmembran von ihm getrennt. Das Rhabdomer besteht aus Mikrovilli, die durch close contacts miteinander verbunden sind. Desmosomen-ähnliche Haftstrukturen zwischen Linsen- und Receptorzellen kommen in der Nähe der Rhabdomere vor. Aus den jeweils 8 Receptorzellen eines Ommatidiums entspringen basal 8 Neuriten, die zunächst durch eine Basalmembran, im Bereich des Hilus außerdem durch einen Gliafortsatz aus dem Ganglion opticum gebündelt werden. Das ganze Auge ist an der Körperoberfläche (Cuticularzellen und Cuticula) durch einen Halteapparat aufgehängt, der aus spezialisierten Zellen und Basalmembran-artigem Material besteht.

---

On the ultrastructure of the compound eye of daphnia pulex

Summary Compound eyes of adult Daphnia pulex and their connection with the integument have been examined electron microscopically. The eye contains four elements: 1. covering cells, 2. intercellular substance, 3. lense cells and 4. receptor cells containing pigment. Their ultrastructure is described. Since the light refracting medium forms a compact lense body within the cytoplasm of lense cells it belongs to the eucone type. Though being surrounded by threaded mitochondria the lense body is not separated from the cytoplasm by a membrane. Electron density increases towards its center. At its basal part the central zone being covered only by a plasma membrane borders on the rhabdomer, the microvilli of which show close contacts between each other. There are desmosome-like contacts between lense and receptor cells at the periphery of the rhabdomer. 8 receptor cells each giving off 1 neurite build up the ommatidium. The 8 neurites of one ommatidium are bundled by a basement membrane and a glial process originating from the optic ganglion. The eye in toto is tied to the integument (cuticular cells, cuticula) by a complicated supporting apparatus, which consists of specialized cells and basement membrane-like material.

     

The lack of vertical migration in Daphnia: the effect of homogenously distributed food

Neither Daphnia hyalina, Daphnia cristata, nor Daphnia cucullata vertically migrated in Lake Mikoajskie and Lake Majcz. We suggest that even under strong fish predation pressure there is no reason to migrate when seston is relatively homogenously distributed throughout the water column.