Temperature affects the limitation of Daphnia magna by eicosapentaenoic acid,and the fatty acid composition of body tissue and eggs

1. Poikilothermic animals incorporate more polyunsaturated fatty acids (PUFAs) into their cellular membranes as temperature declines, suggesting an increased sensitivity to PUFA limitation in cool conditions. To test this we raised Daphnia magna at different temperatures and investigated the effect of varying dietary PUFA on life history parameters (i.e. growth, reproduction) and the PUFA composition of body tissue and eggs. 2. Upon a PUFA‐rich diet (Cryptomonas sp.) females showed higher concentrations of several ω3 PUFAs in their body tissue at 15 °C than at 20 °C and 25 °C, indicating a greater structural requirement for ω3 PUFAs at low temperature. Their eggs had an equal but higher concentration of ω3 PUFAs than their body tissue. 3. In a life history experiment at 15 and 20 °C we supplemented a diet of a PUFA‐free cyanobacterium with the ω3 PUFA eicosapentaenoic acid (EPA). The growth of D. magna was more strongly EPA limited at low temperature. A greater requirement for structural EPA at 15 °C was indicated by a steeper increase in somatic EPA content with dietary EPA compared to 20 °C. 4. At 20 °C the development of eggs to successful hatching was high when EPA was supplied to the mothers. At 15 °C the hatching success was generally poor, despite of a higher maternal provision of EPA to eggs, compared to that at 20 °C, suggesting that EPA alone was insufficient for proper neonatal development at the low temperature. The growth of offspring from mothers raised at 20 °C without EPA supplementation was very low, indicating that the negative effects of EPA deficiency can be carried on to the next generation. 5. The fatty acid composition of Daphnia sp. in published field studies shows increasing proportions of saturated fatty acids with increasing environmental temperature, whereas ω3 PUFAs and EPA show no clear pattern, suggesting that variations in dietary PUFA may mask temperature‐dependent adjustments in ω3 PUFA concentrations of cladocerans in nature.     

Temperature and the effects of elemental food quality on Daphnia

1. We examined the responses of two species of Daphnia to changes in food phosphorus (P) content, with animals reared at three different water temperatures. Specifically, we measured mass‐specific growth rate (MSGR), body P content and respiration rate of Daphnia magna and Daphnia pulex acclimatised to 10, 17.5 and 25 °C and fed food carbon : phosphorus (C : P) ratios of either 150 or 500. 2. The responses of these three physiological variables to temperature–food quality interactions were species‐specific. There was a significant interactive effect of temperature and food quality on D. magna, as the greatest proportional effect of food quality on growth was observed at 10 °C and reductions in body P because of low food P content were relatively greater at 25 °C. These effects may reflect the temperature dependence of mechanisms that reduce elemental constraints associated with food quality in D. magna. By contrast, there were no interactive effects between food quality and temperature on MSGR, body P or mass‐specific respiration of D. pulex. 3. It thus appears that temperature can alter food quality effects on Daphnia but the nature of these alterations depends upon the daphniid species and its thermal adaptability. Significant temperature–food quality interactions will complicate efforts to understand zooplankton nutrition in nature and warrant future consideration.     

Transgenerational effects of poor elemental food quality on Daphnia magna

Environmental effects on parents can strongly affect the phenotype of their offspring, which alters the heritability of traits and the offspring’s responses to the environment. We examined whether P limitation of the aquatic invertebrate, Daphnia magna, alters the responses of its offspring to inadequate P nutrition. Mother Daphnia consuming P-poor algal food produced smaller neonates having lower body P content compared to control (P-rich) mothers. These offspring from P-stressed mothers, when fed P-rich food, grew faster and reproduced on the same schedule as those from P-sufficient mothers. In contrast, offspring from P-stressed mothers, when fed P-poor food, grew more slowly and had delayed reproduction compared to their sisters born to control mothers. There was also weak evidence that daughters from P-stressed mothers are more susceptible to infection by the virulent bacterium, Pasteuria ramosa. Our results show that P stress is not only transferred across generations, but also that its effect on the offspring generation varies depending upon the quality of their own environment. Maternal P nutrition can thus determine the nature of offspring responses to food P content and potentially obfuscates relationships between the performance of offspring and their own nutrition. Given that food quality can be highly variable within and among natural environments, our results demonstrate that maternal effects should be included as an additional dimension into studies of how elemental nutrition affects the physiology, ecology, and evolution of animal consumers.     

Essential fatty acid content and the phosphorus to carbon ratio in cultured algae as indicators of food quality for Daphnia

1. A series of experiments examined growth of Daphnia magna on three algal diets ( Rhodomonas minuta , Scenedesmus acutus and Synechococcus sp.) at varying physiological states [nitrogen and phosphorus (P) limitations] to test whether variation in algal fatty acid and/or elemental composition can predict Daphnia growth.
2. These algae differed widely in their essential fatty acid (EFA) composition while phosphorus (P) or nitrogen limitation had only a small influence on their ω3-polyunsaturated fatty acid (PUFA) content.
3. Individually, algal ω3-PUFA content explained 69% of the variation in the normalised growth of D. magna, while algal phosphorus content explained 11% of the variation. Quantitative models for D. magna growth used both algal ω3-PUFA content and algal C : P ratio as food quality indices. Together, algal ω3-PUFA content and C : P ratio explained 70% of the variation in the normalised growth rate of D. magna .
4. Our results indicate that EFA influenced algal food quality much more strongly than P. The EFA and mineral P impacts appear to be independent.     

The influence of food quality on the phototactic behaviour of Daphnia magnaStraus

We examined the influence of food quality on the phototactic behaviour of Daphnia magna. Cohorts of a positively phototactic D. magna clone were fed nine diets differing in quality. Diets were obtained by substitution of a fraction of unicellular algae ( Scenedesmus acutus) by a biomass-equivalent of fresh yeast ( Saccharomyces cerevisiae). Some of the diets were enriched with an inoculum of ciliates and organic compounds added as a hay infusion filtrate. Animals fed with a diet containing at least 25% algae showed a similar phototactic behaviour as animals fed with a diet that contained 100% algae. Addition of ciliates to the yeast diets resulted in a lower mortality and a higher reproductive rate compared to diets without a ciliate supplement. The presence of ciliates did not influence phototactic behaviour. Experiments testing for a maternal effect showed that the phototactic behaviour of the animals was strongly influenced by diet during the early developmental stages, but was not influenced by the diet of the mother. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of algal food quality on sexual reproduction of Daphnia magna

The objective of our study was to investigate sexual reproduction of Daphnia magna associated with mating behaviors and hatching rates, according to different algal food sources. Since a diatom is known to contain more abundant long‐chain poly unsaturated fatty acids (PUFAs), we hypothesized that the diatom‐consuming D. magna would exhibit more successful reproduction rates. Upon the hypothesis, we designed three experiments using two algal species, a green alga (Chlorella vulgaris) and a diatom (Stephanodiscus hantzschii). From the results, we found that the mating frequency and copulation duration increased in the treatment with S. hantzschii, resulting in a significant increase of hatching rates of resting eggs. In the other two repetitive mating strategies (e.g., one female vs. multiple males, and one male vs. multiple females), we found that the hatching rates of resting eggs were greater in the S. hantzschii treatment. In addition to the mating strategy, male body size significantly increased in the diatom treatment, hence average diameter of penis was also statistically different among the treatments (greater diameter in the S. hantzschii treatment). To examine the effect of algal food quality, we estimated quantity of fatty acids in the two algal species. Our result showed that S. hantzschii had a higher proportion of long‐chain PUFAs than C. vulgaris. Furthermore, a stable isotope analysis revealed that carbon and nitrogen originated from S. hantzschii were more assimilated to D. magna. In summary, our study manifested that diatom consumption of D. magna leads to more successful sexual reproduction. We then discussed how the diatom consumption of zooplankton influences food web dynamics in a freshwater ecosystem.     

Negative effects of UVB-irradiated phytoplankton on life history traits and fitness of Daphnia magna

SUMMARY 1. We tested the effect of ultraviolet-B (UVB)-irradiated phytoplankton on life history characteristics of Daphnia magna . Two phytoplankton species were used, Chlamydomonas reinhardtii and Cryptomonas pyrenoidifera . The phytoplankton species were cultured under photosynthetically active radiation (PAR) conditions, and under PAR supplemented with ultraviolet-A and ultraviolet-B radiation, and fed to Daphnia .
2. Life history traits of Daphnia were negatively affected when fed on UVB-irradiated Cryptomonas . Size at maturity was depressed and fewer juveniles with lower fitness were produced in the UVB treatments. In the Chlamydomonas experiment, no significant effects were found.
3. The cause of the observed UVB effects is likely to be constraints in food quality. Ultraviolet-B radiation thus has the potential of inhibiting energy transfer from the first to the second trophic level.     

Seston composition and the potential for Daphnia growth

A field survey was conducted to study the relationships amongst the composition of the seston, the nutritional value of the seston for herbivorous zooplankton (Daphnia), and selected water clarity parameters. Sixteen ponds in a wetland area and seven larger lakes, all located in south central Saskatchewan, Canada, were sampled for seston. The phytoplankton species were identified, and various biochemical seston variables were measured. A biotest using the zooplankter Daphnia magna, was employed to assess the nutritional value of the seston. The best seston variable to explain Daphnia growth was the phospholipid content (simple linear regression analysis: R ² _adj = 0.50). The water absorbance ratio A250/A365 was a good predictor of lipid content of the seston. Both the absorbance ratio A250/A365 and the dissolved organic carbon (DOC) concentration were negatively correlated with Daphnia growth. We hypothesize that the penetration of visible and ultraviolet radiation is an important determinant of seston quality, especially the phospholipid content, and that this has important implications for determining ultimate growth rates of herbivorous zooplankton.     

Kinetics of nitrogen and phosphorus release in varying food supplies byDaphnia magna

Rates of nitrogen and phosphorus release from individualDaphnia magna were determined by measuring ammonia and soluble reactive phosphorus in successive 10-min incubations in small (0.05 ml) vessels after the animals were removed from their food. Release rates of both nutrients were generally highest initially and decreased with time after removal. The ratio of nitrogen to phosphorus released increased with time after animals were removed from an artificial detritus/bacterial food; ratios were lower and changed with time less for animals fed algae. These data suggest errors may be introduced by assumptions of constant stoichiometry for nutrient release in varying environments.GLERL Contribution No. 268     

Associating particulate essential fatty acids of the ω3 family with phytoplankton species composition in a Siberian reservoir

1. We studied variation in the composition of fatty acids in the seston of a small freshwater reservoir with changes in phytoplankton composition during four growth seasons. We focused on the dynamics of the ω3 fatty acids because of their potential importance for zooplankton nutrition. 2. Total diatoms were related to the 20:5ω3 fatty acid (eicosapentaenoic, EPA) content in seston. Among two dominant diatom genera, Cyclotella was not associated with EPA content. In contrast, there was a significant correlation between Stephanodiscus and the percentage contribution and content of EPA throughout the study. Hence, freshwater diatoms can differ strongly in content of the essential EPA. 3. We considered abundant cyanobacteria as a potential source of 18:3ω3 fatty acid (linolenic, ALA) to aquatic food webs. Among four dominant cyanobacteria species, two (Anabaena flos‐aquae and Planktothrix agardhii) showed significant correlation with the ALA content of the seston, while the other two (Aphanizomenon flos‐aquae and Microcystis aeruginosa) did not. 4. Dinophyta had a relatively high level of 22:6ω3 (docosahexaenoic, DHA) for freshwater species and can be also a source of EPA to aquatic food webs. 5. Our results show that various species of diatoms as well as cyanobacteria can be of contrasting nutritional value for zooplankton because of their different content of the essential PUFAs. Diatoms, which are low in EPA, could not be considered as a valuable food, while some field populations of cyanobacteria might be valuable sources of essential ALA.     

Maternal consumption of non‐toxic Microcystis by Daphnia magna induces tolerance to toxic Microcystis in offspring

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Effect of food concentration on the respiration of Daphnia magna

The relationship between respiration rate and dry weight forDaphnia magna was investigated in the absence of food. In addition the respiration and feeding rates of matureDaphnia magna females were measured simultaneously at differentChlorella concentrations. It was found that, besides the expected effect of food concentration on the feeding rate, there also was an effect of food concentration on the respiration rate. For food concentrations above a certain critical level a negative correlation between respiration rate and food concentration was found. Below this critical level the effect of the food concentration on the respiration rate was not significant.     

An in situ test of the effects of food quality on Daphnia population growth

The temporal variation of the surface deposit feeders in a mudflat of Chuwei Mangrove Forest showed a significant input of populations from the recruitments in the spring. The chemical analysis of the sediment in the top 8 cm showed that the sedimentary organic content had a subsurface maximum value in the deeper layer in most cases. Most of the temporal variations in the organic carbon and nitrogen contents occurred in the top few centimeters. The high C:N ratio and low organic nitrogen content in the sediment suggests the supply of carbon (or energy) to the benthos was low. The increase of labile protein content in the deep layer might be due to the downcore transport of sediment nutrients by the bioturbation. Laboratory growth experiment suggests that some the chemical characters examined in this study may represent the sediment nutrients to the animals.     

Niche overlap between herbivorous cladocerans; the role of food quality and habitat homogeneity

In spite of resource limitation, five abundant species of herbivorous metazoan zooplankton in a humic lake exhibited extensive niche overlap both with regard to seasonal and spatial occurrence, time of reproductive maxima, juvenile release and food choice. Their coexistence could not be explained by modifying predation, environmental oscillations or recolonization.Laboratory bottle experiments indicated only weak interspecific interactions between the three tested species at low food levels, but negative interactions were induced at elevated food levels. Bosmina appeared as competitively inferior during enrichment with cultured algae, but as the superior species during starvation. At low nutrient levels, all species coexisted for several generations with low reproduction, in accordance with the lake situation. It was concluded that the observed niche overlaps would be promoted if; 1) Intraspecific competition is more important than interspecific competition. 2) All species are co-adapted to low nutrient availability, food is quantitatively in surplus, but qualitatively deficient (mainly recycled detritus). During such conditions, no species would be capable of obtaining a population increase until extinction of the other species. This situation may be typical of oligotrophic humic lakes, and of other localities with low predation pressure and high inputs of allochthonous particulate carbon.     

Seston food quality and Daphnia production efficiencies in an oligo-mesotrophic Subalpine Lake

Because of major biochemical imbalances between plants and animals, ecological efficiency at this interface may have a major impact on overall energy flow in ecosystems. In order to study relationships between seston food quality and energy transfer between primary producers and herbivores, we conducted five microcosm experiments in Castle Lake, California, USA during the summer of 1996. We simultaneously performed life table experiments to determine the effects of highly unsaturated fatty acids (HUFA) on Daphnia rosea growth, reproduction and survival. The results of these experiments suggest strong energy limitation of D. rosea growth in Castle lake during the study. D. rosea production was coupled with primary production in Castle Lake and in the microcosm experiments. D. rosea production efficiencies, i.e., the ratios of D. rosea productivity to primary productivity, decreased towards the end of the summer. A food quality index based on phytoplankton species composition and seston carbon to phosphorus (C:P) ratio were good predictors of D. rosea production efficiencies. The predicted D. rosea production pattern based on phytoplankton composition and primary productivity matched the zooplankton biomass dynamics in Castle Lake during 1991. Life table experiments showed HUFA effects on D. rosea population growth rates, reproduction and survival in support of the HUFA limitation hypothesis.     

Effects of water hardness and diet on productivity of Daphnia magna Straus. in laboratory culture

The effects of diet and water hardness, alone and in combination, on life history characteristics of Daphnia magna were determined in two laboratory tests. Number of young on the first day of reproduction, total young and the number of generations were greater with increasing hardness. At the maximum test hardness of 350 mg/l (as CaC0₃), approximately 65% more young were produced than at the lowest hardness of 50 mg/l (as CaCO₃). Furthermore, time to sexual maturity was about one day shorter in the harder culture water. Daphnids fed the combination of a green alga, trout chow and dehydrated alfalfa were over three times more productive than daphnids fed only the alga or only trout chow and alfalfa. The combination of the algae-reinforced diet with hard culture water provided for optimal productivity.     

Temperature reaction norms of Daphnia magna: the effect of food concentration

1. The effects of temperature and food concentration on the fitness of Daphnia magna were tested in a 4×4 factorial flow-through design. Food ranged from 0.1 to 1.0 mg C L⁻¹ and temperature ranged from 15 to 30 °C. 2. The juvenile growth rate ( g _j) was used to construct reaction norms for temperature at varying food concentrations. Two clones isolated from the same pond at different seasons did not differ with respect to their temperature responses. Reaction norms had the shape of an optimum curve with highest values around 20 °C. There was a significant temperature–food interaction as the temperature response was most pronounced when the food was not limiting. 3. Differences in fitness were a consequence of different responses of physiological parameters to food and temperature. Age and size at first reproduction, as well as egg numbers, decreased with increasing temperature and decreasing food concentration. 4. As the temperature effect was strongest at the highest food concentrations, it can be concluded that environmental warming may affect D. magna more through a temperature rise earlier in spring rather than in summer.     

Egg size and egg mass of Daphnia magna: response to food availability

The influence of different food availability on egg size and egg mass in Daphnia magna Straus was studied in long-term experiments using a flow-through system. Daphnia were either kept a constant high or low food levels or subjected to alternating periods of high food and starvation. Some animals were starved continuously after they had deposited their first clutch of eggs. Eggs were measured and weighed and their density (dry mass per volume) was determined. The results support the model of Glazier (1992), which defines a region of reproductive constraint at very low food concentrations and a region of adaptive response as food concentration increase. Egg sizes were largest under continuously low food concentrations (0.1 mg Cl^–1), which indicates that the maximum of Glazier's non-linear response curve is at very low food levels. Eggs produced during starvation were small, probably as a result of reproductive constraints. Egg density was about 0.37 mg dry weight mm^–1 and did not differ between treatments.