The productivity and carbon budget of a natural population of Daphnia lumholtzi Sars

D. lumholtzi in Lake Samsonvale, Queensland, Australia, is a small species (max. size approx. 7 µgC) that occurs in low abundance (max. abundance 6400 m^–3), with an average daily biomass of 3.32 mgC m^–3. Its annual rates of carbon assimilation, production and respiration, are 166, 110, and 56 mgC m^–3 y^–1 respectively. Annual biomass turnover (annual production/average daily biomass) is 33 and production efficiency is 50–66%. The population may consume 1.65–2.20 mgC m^–3 daily, equivalent to about 1% of the average daily standing crop of phytoplankton. Clutch size is small, 2 eggs, but represents 30–80% of a female's weight. A female may only produce 8–10 offspring in a full lifespan, nevertheless egg production may account for 56% of total production. The population shows autumn and spring peaks in abundance, and is believed to oversummer (4 months) as ephippia.     

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.     

Seasonality and Fecundity of Daphnia Lumholtzi Sars in Lake Phewa,Nepal

Monthly sampling of zooplankton in Lake Phewa, Nepal revealed 18 species of zooplankton excluding protozoans. The seasonality and fecundity of the largest common cladoceran Daphnia lumholtzi Sars was studied. Two distinct peaks of abundance were noted. No active stages were found during May and June. Egg ratio varied from zero to 0.88 in mature females. From the presence of only helmeted individuals of this species it is concluded that the predation pressure by fish was high.     

Microsporidiosis in a Daphnia pulex population

The impact of parasitism by Thelohania sp., an intracellular microsporidian, on Daphnia pulex inhabiting a vernal pond was studied for three years. Three issues were considered: the distribution of parasites among hosts, the effect of parasites on individuals, and the impact of parasitism on population growth. Each year, Thelohania infected members of at least half of the generations of Daphnia produced in the pond. When the frequency of infection was low, parasites were found only in large adults. As infection frequency increased, parasitism spread downward through host size classes. However, parasites were rarely found in juveniles. Parasitism reduced clutch size drastically, increased mortality to a variable extent, had little impact on egg size or on per instar growth and none on molt frequency. Interaction with other stresses, such as food limitation, exacerbated some of these effects. Parasitism reduced instantaneous birth rate much more than it elevated instantaneous death rate. Population growth was reduced significantly but it is unlikely that Thelohania alone regulates the growth of this Daphnia population.     

Population dynamics and production of Daphnia hyalina Leydig and Daphnia cucullata Sars in Tjeukemeer

As part of a research programme on the food chains in Tjeukemeer, the Daphnia hyalina and Daphnia cucullata populations were studied for three successive years. To analyse the factors regulating the production of these two species, their population parameters (density, size distribution, fecundity) and population dynamics (birth rate, mortality rate) were studied and related to environmental factors. Since Daphnia in Tjeukemeer shows continuous recruitment, the population dynamics model INSTAR was developed and used to integrate field data with laboratory data on development rates and length-weight relationships. The dynamics of the Daphnia species are mainly regulated by temperature and fish predation, the latter affects both birth rate and mortality. Total annual Daphnia production was 3.1–6.9 g org. dry wt M^–2, and annual P/B ratio ranged from 25 to 40 for D. cucullata and from 45 to 49 for D. hyalina.     

Modes and mechanisms of a Daphnia invasion

Whether exotic species invade new habitats successfully depends on (i) a change in the invaded habitat that makes it suitable for the invader and (ii) a genetic change in the invading taxon that enhances its fitness in the new habitat, or both. We dissect the causes of invasions of Swiss lakes, north of the Alps, by Daphnia galeata (a zooplankter typical of eutrophic lakes, e.g. those south of the Alps, which are also warmer) by comparing the fitness performance of eight geographically distributed clones that were fed algal-food typical of oligotrophic versus eutrophic conditions at two temperatures. Daphnia longispina, native to oligotrophic Swiss lakes, served as a reference. Daphnia galeata requires eutrophic food to persist, whereas D. longispina survives and grows on oligotrophic food but does even better on eutrophic food. Invasion by D. galeata is further explained because invading clones from the north perform better on eutrophic food and at cooler temperatures than native clones from the south, suggesting a local response to countergradient selection. Our data support the hypothesis that populations of the invader in northern lakes are dominated by well-adapted genotypes. Our results illustrate how environmental change (i.e. eutrophication) and local adaptation can act together to drive a successful invasion.     

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history alterations of coexisting Daphnia species responding to environmental temperature and predator cues. In a laboratory experiment, we measured Daphnia life history plasticity under different predation risk and temperature treatments that simulate changing environmental conditions. Daphnia pulicaria abundance and size at first reproduction (SFR) declined, while ephippia (resting egg) formation increased at high temperatures. Daphnia mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues. In Glen Elder reservoir, Kansas USA, D. pulicaria dominance shifted to D. mendotae dominance as temperature and predation risk increased from March to June in both 1999 and 2000. Field estimates of life history shifts mirrored the laboratory experiment results, suggesting that similar phenotypic responses to seasonal cues contribute to seasonal Daphnia population trends. These results illustrate species-specific differences in life history plasticity among coexisting zooplankton taxa.     

Evidence for a diurnal horizontal migration in Daphnia hyalina lacustris Sars

Diurnal sampling of a transect across a shallow, eutrophic gravel pit suggested that a diurnal horizontal migration was present in Daphnia hyalina lacustris, in which it remained in the littoral zone during the day and moved out into the open water at night. The avoidance of fish predation seemed the most likely ultimate cause of this migration.     

Embryology of Chaoborus-induced spines in Daphnia pulex

Daphnia pulex (Crustacea: Cladocera) embryos were found to be sensitive to a chemical cue (kairomone) in an extract of the predator Chaoborus americanus (Insecta:Diptera). Sensitivity of embryos to the kairomone remains throughout embryonic development. Apparently declining sensitivity as development proceeds may be due to the amount of time the embryos are exposed to the kairomone. Male embryos were also found to be sensitive to the kairomone. The smallest eggs within a brood produced small offspring, which showed the antipredator morphology to a significantly lower degree than largest eggs. The production of the neckteeth is described, at the developmental stage in the maturation of the Daphnia coinciding approximately with the escape of the embryos from the brood chamber.     

Diel horizontal migration and swarm formation in Daphnia in response to Chaoborus

Dense swarms of Daphnia longispina (up to 4000 animals l^–1) were recorded along the littoral zone in a lake where Chaoborus flavicans is considered the main predator. D. longispina coexisted with D. pulex, but there were no D. pulex in the littoral swarms. Swarms were less dense at night (about 1/10 the density), and D. longispina exhibited diel horizontal migrations by aggregating in the littoral during the day and spreading out at night. Laboratory experiments showed that Chaoborus capture efficiency on juvenile daphnids was higher in the light compared to darkness, and that Daphnia exhibited a behavioural response to water that had previously contained Chaoborus. We conclude that predation from Chaoborus can be an important factor affecting the distribution patterns of Daphnia observed in this lake. The behavioural experiments indicated that this influence might be partly mediated by chemical agents.     

Demographic costs of Chaoborus-induced defences in Daphnia pulex: a sensitivity analysis

Summary We examined the demographic costs of Chaoborus-induced defensive spine structures in Daphnia pulex. Our aim was to assess the role of resource limitation and the interaction effects of limiting food level and antipredator structures on fitness of D. pulex and to pinpoint those life stages that are most sensitive to changes in the defence regime. Chaoborus-induced and typical morphotypes of D. pulex were reared at high and low food concentrations. Instar-based matrix population models were used to quantify the effects of predator-induction, food and their interaction on fitness of D. pulex. Predator-induction caused a statistically significant reduction in fitness at low food levels, but not at high food levels. Sensitivity analyses revealed that the fitness effects were primarily due to changes in the growth rate in instars 1–5, and secondarily to small reductions in the fertility of instars 5–10. The interaction between Chaoborus exposure and low food concentration was negative, and mediated through growth and fertility components. Both these components were reduced more in the Chaoborus-exposed, low food treatment than would be expected in the absence of interaction.     

Daphnia magna as a test animal in acute and chronic toxicity tests

Daphnia magna is a commonly used test animal in aquatic toxicology.Test procedures for acute and chronic test are described, together with the influence of several variables on their results.The practicability of the methods was checked with four model compounds, viz. 1,1,2-trichloroethane, dieldrin, pentachlorophenol and 3,4-dichloroaniline. Toxicity data of these compounds for Daphnia magna are given.     

Combined effect of the insecticide carbaryl and the Chaoborus kairomone on helmet development in Daphnia ambigua

The cladoceran Daphnia ambigua was exposed to both the insecticide carbaryl and the kairomone released from the predator Chaoborus simultaneously, and its morphological changes were analyzed. Daphnia developed helmets in response to the kairomone, but not in response to carbaryl at low (sublethal) concentrations (1–3 µg 1^–1). However, the carbaryl enhanced the development of high helmets and prolonged the maintenance period of the helmets over instars in the presence of the kairomone. These results suggest that sublethal concentrations of the insecticide alter predator-prey interactions by inducing helmet formation in Daphnia, which may reduce vulnerability of the Daphnia to predation.     

Moss Responses to Elevated CO2 and Variation in Hydrology in a Temperate Lowland Peatland

We studied the effects of elevated CO₂ (180–200 ppmv above ambient) on growth and chemistry of three moss species (Sphagnum palustre, S. recurvum and Polytrichum commune) in a lowland peatland in the Netherlands. Thereto, we conducted both a greenhouse experiment with both Sphagnum species and a field experiment with all three species using MiniFACE (Free Air CO₂ Enrichment) technology during 3 years. The greenhouse experiment showed that Sphagnum growth was stimulated by elevated CO₂ in the short term, but that in the longer term (≥1 year) growth was probably inhibited by low water tables and/or down-regulation of photosynthesis. In the field experiment, we did not find significant changes in moss abundance in response to elevated CO₂, although CO₂ enrichment appeared to reduce S. recurvum abundance. Both Sphagnum species showed stronger responses to spatial variation in hydrology than to increased atmospheric CO₂ concentrations. Polytrichum was insensitive to changes in hydrology. Apart from the confounding effects of hydrology, the relative lack of growth response of the moss species may also have been due to the relatively small increase in assimilated CO₂ as achieved by the experimentally added CO₂. We calculated that the added CO₂ contributed at most 32% to the carbon assimilation of the mosses, while our estimates based on stable C isotope data even suggest lower contributions for Sphagnum (24–27%). Chemical analyses of the mosses showed only small elevated CO₂ effects on living tissue N concentration and C/N ratio of the mosses, but the C/N ratio of Polytrichum was substantially lower than those of the Sphagnum species. Continuing expansion of Polytrichum at the expense of Sphagnum could reduce the C sink function of this lowland Sphagnum peatland, and similar ones elsewhere, as litter decomposition rates would probably be enhanced. Such a reduction in sink function would be driven mostly by increased atmospheric N deposition, water table regulation for agricultural purposes and land management to preserve the early successional stage (mowing, tree and shrub removal), since these anthropogenic factors will probably exert a greater control on competition between Polytrichum and Sphagnum than increased atmospheric CO₂ concentrations.     

Migrations of haemoglobin-rich Daphnia longispina in a small,steeply stratified,humic lake with an anoxic hypolimnion

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.     

Progress towards characterization of a predator/prey kairomone: Daphnia pulex and Chaoborus americanus

Predatory larvae of the midge Chaoborus americanus induce an antipredator defense (neckteeth) in prey individuals of the cladoceran Daphnia pulex. The signal for presence of predator is a water-soluble chemical. We provide evidence that this kairomone originates in the intestinal tract of the predator. The active compound is an organic molecule of intermediate polarity which is heat stable and partially destroyed by acid and base digestion. It is stable to digestion by the general peptidase Pronase. Hydroxyl groups, but not primary amines, carbonyls or thiols are essential to activity. Low-pressure liquid chromatography on a reverse-phase silica gel (Amicon Matrex C-18) column suggests there may be more than one active component.     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Daphnia and Toxic Blooms of Microcystis aeruginosa in Bautzen Reservoir (GDR)

As a part of a whole-lake, long-term experiment in biomanipulation in. the hypertrophic Bautzen reservoir (G.D.R.), during three years (1984–1986) the dynamics of mouse-related LD 50 of Microcystis aeruginosa was compared with the biomass development of this blue-green and the grazing pressure exerted by Daphnia galeata. Since the three summer averages of the biomass of D. galeata revealed strong differences due to decreasing predation activity of fish from 1984 to 1986, the effects of different grazing pressure on Microcystis toxicity could be investigated under field conditions. Microcystis was nontoxic at the beginning of the growing season and developed high toxicity during its first strong biomass increase in summer in all three years. But this decrease of the LD 50 together with the first biomass increase of the season is found in quite different periods in different years (1984: August, 1985: July, 1986: June). It is obvious that the higher the mean effective filtration rate of D. galeata during summer is found the faster the toxicity of Microcystis is formed. If these observations are combined with findings of other authors, the conclusion can be drawn that the development of toxic Microcystis blooms seems to be promoted by a combination of five conditions: (1) Presence of a mixture of toxic and nontoxic Microcystis strains at the beginning of the growing season even if the portion of toxic strains is very low, (2) physical and chemical growth conditions which favour Microcystis over other phytoplankton, (3) high grazing pressure by zooplankton on edible food particles over a rather long period, (4) patchy distribution of the different Microcystis strains if nonselective filtrators such as Daphnia dominate the zooplankton, and (5) absence of defense mechanisms of Microcystis against grazing which are not coupled with toxicity (e.g. large colony size). These conclusions contribute to a better understanding of the possibilities and limits of in-lake eutrophication control by biomanipulation and emphasize the need to combine top-down and bottom-up control mechanisms in eutrophic and hypertrophic waters.     

Four different head shapes in Daphnia hyalina (Leydig) induced by the presence of larvae of Chaoborus flavicans (Meigen)

In many species of Daphnia spines, neck teeth, and enlarged or reshaped helmets are well-known as defences against invertebrate predators. Until now, Daphnia hyalina (Leydig, 1860), a common species in many European lakes, has appeared to be an exception to this rule.Here, we provide evidence that the larvae of Chaoborus flavicans (Meigen) also can induce morphological changes in D. hyalina. Specimens react morphologically to the presence of larvae in three ways: (a) by changes in the shape of head shield, (b) by the formation of spine(s) on the head, and (c) by the elongation of the tail spine. The frequency and intensity of these morphological changes are correlated positively with midge larvae densities. The most pronounced reactions occurred in young D. hyalina.The predator kairomone also induced changes in Daphnia body size.