Response Of Daphnia Research Articles

Polymorphisms in predator induced defences of coexisting Daphnia pulex and D. longispina

A comparison of juvenile predator-avoidance polymorphisms of Daphnia pulex and D. longispina in a shallow water body of Northern Italy is reported. The presence of the Chaoborus larvae resulted in juvenile adaptive predator-avoidance cyclomorphosis in both species. The frequency of induced morphotypes was higher in the small-sized D. longispina than in the large-sized D. pulex. Relative tail-spine size was higher in D. longispina than in D. pulex. D. longispina displayed much more responsiveness to Chaoborus than D. pulex. Both species showed at least 6 different defensive morphotypes that were classified according to the neckteeth number, the neckteeth position and the thickness of occipital epithelia under neckteeth. The frequencies of different morphotypes varied significantly between species. The developmental responses of Daphnia to the presence of predators are regulated at various levels, according to specific timelines of kairomone sensitive phases and developmental frame of defense trait formation. Variation of a specific timeline might be linked to flexibility in developmental responses of Daphnia to Chaoborus and seasonal variation in the frequencies of various morphotypes. Cyclomorphosis should not only be due to changes in environmental factors that directly affects risk, but also to ones that act as proxy signaling changes in predation risk.

Beyond monoculture stoichiometry studies: assessing growth, respiration, and feeding responses of three Daphnia species to P-enriched, low C:P lake seston

Numerous studies in ecological stoichiometry have assessed responses of Daphnia to single- and multi-species food resources of varying nutritional quality. Diets based on P-limited algae with a high biomass carbon to phosphorus (C:P) ratio impose decreased growth on Daphnia because P demands for growth are not met. More recently, laboratory studies have shown that P-rich algal food also results in decreased growth rates for Daphnia; however, no published study has tested the impact of low food C:P on Daphnia under field conditions. To address this, we measured growth rate as well as respiration and ingestion rates for D. magna, D. pulicaria, and D. pulex that were fed lake seston experimentally enriched with phosphate (PO43−). Lake seston during the experimental period successfully took up PO43− enrichment treatments and was dominated by cyanobacteria and haptophytes, primarily those from the genus Prymnesium. Growth rate reductions for D. magna were strong in response to high-P food, most likely as a result of decreased ingestion rate; however, growth rate responses for D. pulex and D. pulicaria were modest and not statistically significant, although significant responses in respiration rates were observed for all species. Our study extends laboratory findings that P-rich food can have deleterious impacts on Daphnia to field conditions. We also found diverse responses among species, however, possibly caused by variations among taxa in body stoichiometry or physiology, differences in ambient and treatment seston C:P for the various experiments, or the effects of diet diversity under high-P conditions.

Variable toxicity of silver nanoparticles to Daphnia magna: effects of algal particles and animal nutrition.

Aquatic environments vary widely in aspects other than their physicochemical properties that could alter the toxicity of novel contaminants. One factor that could affect chemical toxicity to aquatic consumers is their nutritional environment as it can strongly affect their physiology and life history. Nutrition has the potential to alter an organism's response to the toxin or how the toxin interacts with the consumer through its food. Here we determined how growth and survival responses of Daphnia to an emerging contaminant, silver nanoparticles (AgNPs), are affected by the presence of food and its stoichiometric food quality. We used a series of survival tests, each slightly modified, to determine whether variable toxicity in different nutritional environments resulted from algal sequestration of AgNPs in a nontoxic form or from changes to the nutritional status of the test animals. We found that the presence of algae, of good or poor quality, reduced the toxicity of AgNPs on animal growth and survival. However, the decrease in AgNP toxicity was greater for animals consuming P-rich compared to P-poor food. We found evidence that this effect of food quality was due to greater algal uptake of AgNPs by P-rich than by P-stressed algae. However, we also found animal nutrition, in the absence of algal AgNP binding, could affect toxicity with P-nourished animals surviving slightly better when exposed to AgNPs compared to their P-stressed counterparts. Our results show an important role for algal particles and their P content in determining the toxicity of AgNPs in natural waters primarily due to their binding and uptake abilities and, less so, to their effects on animal nutrition.

Ontogenetic shift in Daphnia-algae interaction strength altered by stressors: revisiting Jensen’s inequality

Interaction strength among species plays a crucial role in shaping the functioning of ecological communities, but it is often assumed to be insensitive to inter-individual variation in underlying parameters such as attack rates or handling time. Ecological factors including stressors exert age/size-dependent effects on such behavioral parameters, promoting shifts in the distribution of parameter values over ages. Here we analyze the effects of the pesticide methamidophos on the Daphnia-microalga interaction strength. We first analyze age-dependent effects of the pesticide on the Daphnia functional response, and then decompose the population-level effects of the stressor into contributions of shifts in elevation (i.e., vertical effect) versus shifts in nonlinearity (i.e., nonlinear effect) of the response of interaction strength over consumer age. Our results show that (1) Rogers and Holling type II functional response models best fitted the empirical functional responses of Daphnia of different ages, (2) attack rate and handling time were affected by the pesticide, (3) these effects were age-specific, shifting the average attack rate and both the mean and coefficient of variation of handling time of different age classes, (4) population level interaction strength was affected by pesticide exposure by variation in both elevation and nonlinearity of its response over consumer age. We show that both vertical and nonlinear effects were important in magnitude but opposite in sign. The consequences of factors that exert age/size dependent effects can only be evaluated through properly considering inter-individual variation.

Thermal variation and factors influencing vertical migration behavior in Daphnia populations

The antipredator behavior diel vertical migration (DVM), common in aquatic keystone species Daphnia, involves daily migration from warmer surface waters before dawn to cooler deeper waters after dusk. Plasticity in Daphnia DVM behavior optimizes fitness via trade-offs between growth, reproduction, and predator avoidance. Migration behavior is affected by co-varying biotic and abiotic factors, including light, predator cues, and anthropogenic stressors making it difficult to determine each factor's individual contribution to the variation in this behavior. This study aims to better understand this ecologically significant behavior in Daphnia by: (1) determining how Daphnia pulicaria thermal preferences vary within and among natural populations; (2) distinguishing the role of temperature verses depth in Daphnia vertical migration; and (3) defining how two anthropogenic stressors (copper and nickel) impact Daphnia migratory behavior.Simulated natural lake stratification were constructed in 8L (0.5m tall, 14.5cm wide) water columns to monitor under controlled laboratory conditions the individual effects of temperature gradients, depth, and metal stressors on Daphnia vertical migration. Three major findings are reported. First, while no difference in thermal preference was found among the four populations studied, within lake populations variability among isolates was high. Second, decoupling temperature and depth revealed that depth was a better predictor of Daphnia migratory patterns over temperature. Third, exposure to environmentally relevant concentrations of copper or nickel inhibited classic DVM behavior. These findings revealed the high variability in thermal preference found within Daphnia populations, elucidated the individual roles that depth and temperature have on migratory behavior, and showed how copper and nickel can interfere with the natural response of Daphnia to fish predator cues. Thus contributing to the body of knowledge necessary to predict how natural populations of Daphnia will be affected by climate related changes in lake temperatures and increased presence of anthropogenic stressors.

A Mini-Review on Toxicity of Triclosan in Aquatic Environment

Triclosan (TCS) is a synthetic broad spectrum antimicrobial agent that has been added in amounts of personal care products and household products. Its widespread use and resistant to degradation resulted in the inevitably release into aquatic environment and thus its potential toxicity towards aquatic organisms is of concern. To understand the risk associated with TCS in aquatic ecosystems, a min review of available literature on its acute and chronic toxicity towards aquatic organisms was conducted in this study. The acute toxicity data showed that TCS always would not show lethal effects at realistic concentration and the LC50 of TCS towards different trophic species spanned more than six orders of magnitude in concentration. Among the tested organisms, algae are the most sensitive species. The chronic toxicity data of TCS on aquatic microorganisms, microalgae, aquatic macrophytes, invertebrates and fishes were systematically collected. TCS show endocrine disruption, cytotoxic, and genotoxic effects on tested organisms, and its toxicity would be enhanced or reduced while coexisting with other pollutants or environmental parameters. Its selection and spread of multidrug resistance, microorganisms should be of concern and the potential combination effects of heavy mental and TCS on cross-resistance of microorganisms might be explored in the future due to their same selective function on multidrug resistance. Daphnia and rotifer are two major components of aquatic invertebrates; more comprehensive researches have been conduct to reveal the response of Daphnia to TCS exposure while just three studies were conducted to assess its toxicity on rotifer. Considering limited ecotoxicological information and significant TCS LC50/EC50 variations to aquatic species, further studies on potential consequences of long-term TCS exposure to more abundant and ecologically relevant species are critically need to better regulate its utilization, and the effects of environmental parameters should be taken into consideration while investigating TCS toxicity towards target species due to their significant effects.

The influence of simulated microgravity on the proteome of Daphnia magna.

Background:The waterflea Daphnia is an interesting candidate for bioregenerative life support systems (BLSS). These animals are particularly promising because of their central role in the limnic food web and its mode of reproduction. However, the response of Daphnia to altered gravity conditions has to be investigated, especially on the molecular level, to evaluate the suitability of Daphnia for BLSS in space.Methods:In this study, we applied a proteomic approach to identify key proteins and pathways involved in the response of Daphnia to simulated microgravity generated by a two-dimensional (2D) clinostat. We analyzed five biological replicates using 2D-difference gel electrophoresis proteomic analysis.Results:We identified 109 protein spots differing in intensity (P<0.05). Substantial fractions of these proteins are involved in actin microfilament organization, indicating the disruption of cytoskeletal structures during clinorotation. Furthermore, proteins involved in protein folding were identified, suggesting altered gravity induced breakdown of protein structures in general. In addition, simulated microgravity increased the abundance of energy metabolism-related proteins, indicating an enhanced energy demand of Daphnia.Conclusions:The affected biological processes were also described in other studies using different organisms and systems either aiming to simulate microgravity conditions or providing real microgravity conditions. Moreover, most of the Daphnia protein sequences are well-conserved throughout taxa, indicating that the response to altered gravity conditions in Daphnia follows a general concept. Data are available via ProteomeXchange with identifier PXD002096.

Feeding response of &lt;i&gt;Daphnia&lt;/i&gt; cf.&lt;i&gt; similis&lt;/i&gt; to different concentration gradients of &lt;i&gt;Microcystis&lt;/i&gt; and its implication for preventing algal blooming

Daphnia are important components of zooplankton communities in lakes, ponds and reservoirs. Currently freshwater ecosystems are affected worldwide by Cyanobacterial blooms through the process of eutrophication. The objective of this study was to provide experimental evidence to the response of Daphnia to various concentrations of Microcystis . The experiment contained four treatments and two controls each with three replicates. The first control contained D . cf. similis without Microcystis and the second control contained Microcystis without D .cf. similis . The remaining four treatments contained both D . cf. similis and Microcystis at different concentration of Microcystis .. The results showed a significant negative relationships between D . cf. Similis and Microcystis across the treatments (F=294.5; p<0.00). From the four Microcystis concentration gradients, we found mortality of D. cf. Similis in HMC (high Microcystis concentration) and 67% of the original HMC gradient while in the other treatments, in 43% and 22% original HMC treatment, D . cf. Similis reproduce and attain high density. Therefore, from these results it is concluded that D . cf. similis can control the growth of Microcystis if the concentration is low but they cannot reduce an already existing bloom. Keywords : Concentration gradient, D. cf. Similis, Feeding-treatment, Microcystis, Ethiopia.

Conserved transcriptional responses to cyanobacterial stressors are mediated by alternate regulation of paralogous genes in Daphnia.

Despite a significant increase in genomic data, our knowledge of gene functions and their transcriptional responses to environmental stimuli remains limited. Here, we use the model keystone species Daphnia pulex to study environmental responses of genes in the context of their gene family history to better understand the relationship between genome structure and gene function in response to environmental stimuli. Daphnia were exposed to five different treatments, each consisting of a diet supplemented with one of five cyanobacterial species, and a control treatment consisting of a diet of only green algae. Differential gene expression profiles of Daphnia exposed to each of these five cyanobacterial species showed that genes with known functions are more likely to be shared by different expression profiles, whereas genes specific to the lineage of Daphnia are more likely to be unique to a given expression profile. Furthermore, while only a small number of nonlineage-specific genes were conserved across treatment type, there was a high degree of overlap in expression profiles at the functional level. The conservation of functional responses across the different cyanobacterial treatments can be attributed to the treatment-specific expression of different paralogous genes within the same gene family. Comparison with available gene expression data in the literature suggests differences in nutritional composition in diets with cyanobacterial species compared to diets of green algae as a primary driver for cyanobacterial effects on Daphnia. We conclude that conserved functional responses in Daphnia across different cyanobacterial treatments are mediated through alternate regulation of paralogous gene families.

A low ω-3:ω-6 ratio in Daphnia indicates terrestrial resource utilization and poor nutritional condition

It has been hypothesized that terrestrial particulate organic matter (t-POM) makes important contributions to Daphnia production in some lakes. We conducted a series of feeding experiments to explore the fatty acid responses in Daphnia to diets comprised of different terrestrial resources (i.e. Alnus rubra, Phragmites australis, Betula nana and Betula pendula) and mixed diets with terrestrial and phytoplankton (Scenedesmus or Cryptomonas) resources. When fed 100% phytoplankton, Daphnia had very similar (r . 0.80) fatty acid profiles to their diets, whereas Daphnia that consumed t-POM diets had weak correlations (r 1⁄4 0.002–0.56) with the corresponding diet sources. Unusual 16 carbon chain polyunsaturated fatty acids (16:2v6, 16:3v3 and 16:4v3), linoleic acid (18:2v6) and a-linolenic acid (18:3v3) were diagnostic fatty acids for Scenedesmus and Daphnia that consumed this alga. Stearidonic acid (18:4v3) and eicosapentaenoic acid (20:5v3) were diagnostic for Cryptomonas and Daphnia that consumed this diet. All of the t-POM resources were characterized by a high content of saturated fatty acids (SAFA; 79+ 12%), especially the diagnostic long-chain SAFA (20:0, 22:0, 24:0, 26:0, 28:0). Daphnia that consumed t-POM assimilated very little of these terrestrial biomarkers, but the shorter chain SAFA 16:0 and 18:0 were very prevalent in juvenile and adult Daphnia that consumed terrestrial plant matter. The v-3:v-6 ratios were distinctive between terrestrial (0.3–1.6) and phytoplankton resources ( 3–15), and this ratio in Daphnia was strongly associated with their diets (r 1⁄4 0.88). These results suggest that Daphnia, and perhaps zooplankton in general, preferentially retain algae-derived v3 fatty acids, and low v-3:v-6 ratios in Daphnia indicate a mainly terrestrial diet or poor nutritional condition.

Differential peptide labeling (iTRAQ) in LC-MS/MS based proteomics in Daphnia reveal mechanisms of an antipredator response.

Daphnia, an important model organism for studies on ecology and evolution, has become a textbook example for inducible defenses against predators. Inducible defenses are widespread in nature, and the underlying molecular mechanisms for this plasticity in general and in particular in Daphnia are not fully understood. Here, we provide for the first time a combination of established life-history changes (LHC), which are induced by chemical cues of a predator (fish kairomones), in Daphnia with differential peptide labeling (iTRAQ) in LC-MS/MS based proteomics. The aim of the present study is the elucidation of proteins involved in specific antipredator responses in a predator-prey system of ecological relevance by high-throughput proteomics. To obtain a highly specific antifish response of Daphnia, highly purified fish kairomones were applied in the presence or absence of light. We were able to identify a set of functional proteins, which are likely to explain the kairomone-mediated and light-dependent LHC in Daphnia.

Response of Daphnia's antioxidant system to spatial heterogeneity in Cyanobacteria concentrations in a lowland reservoir.

Many species and clones of Daphnia inhabit ecosystems with permanent algal blooms, and they can develop tolerance to cyanobacterial toxins. In the current study, we examined the spatial differences in the response of Daphnia longispina to the toxic Microcystis aeruginosa in a lowland eutrophic dam reservoir between June (before blooms) and September (during blooms). The reservoir showed a distinct spatial pattern in cyanobacteria abundance resulting from the wind direction: the station closest to the dam was characterised by persistently high Microcystis biomass, whereas the upstream stations had a significantly lower biomass of Microcystis. Microcystin concentrations were closely correlated with the cyanobacteria abundance (r = 0.93). The density of daphniids did not differ among the stations. The main objective of this study was to investigate how the distribution of toxic Microcystis blooms affects the antioxidant system of Daphnia. We examined catalase (CAT) activity, the level of the low molecular weight antioxidant glutathione (GSH), glutathione S-transferase (GST) activity and oxidative stress parameters, such as lipid peroxidation (LPO). We found that the higher the abundance (and toxicity) of the cyanobacteria, the lower the values of the antioxidant parameters. The CAT activity and LPO level were always significantly lower at the station with the highest M. aeruginosa biomass, which indicated the low oxidative stress of D. longispina at the site with the potentially high toxic thread. However, the low concentration of GSH and the highest activity of GST indicated the occurrence of detoxification processes at this site. These results demonstrate that daphniids that have coexisted with a high biomass of toxic cyanobacteria have effective mechanisms that protect them against the toxic effects of microcystins. We also conclude that Daphnia's resistance capacity to Microcystis toxins may differ within an ecosystem, depending on the bloom's spatial distribution.

Longevity of Daphnia and the attenuation of stress responses by melatonin.

BackgroundThe widespread occurrence of melatonin in prokaryotes as well as eukaryotes indicates that this indoleamine is considerably old. This high evolutionary age has led to the development of diverse functions of melatonin in different organisms, such as the detoxification of reactive oxygen species and anti-stress effects. In insects, i.e. Drosophila, the addition of melatonin has also been shown to increase the life span of this arthropod, probably by reducing age-related increasing oxidative stress.Although the presence of melatonin was recently found to exist in the ecological and toxicological model organism Daphnia, its function in this cladoceran has thus far not been addressed. Therefore, we challenged Daphnia with three different stressors in order to investigate potential stress-response attenuating effects of melatonin. i) Female and male daphnids were exposed to melatonin in a longevity experiment, ii) Daphnia were confronted with stress signals from the invertebrate predator Chaoborus sp., and iii) Daphnia were grown in high densities, i.e. under crowding-stress conditions.ResultsIn our experiments we were able to show that longevity of daphnids was not affected by melatonin. Therefore, age-related increasing oxidative stress was probably not compensated by added melatonin. However, melatonin significantly attenuated Daphnia’s response to acute predator stress, i.e. the formation of neckteeth which decrease the ability of the gape-limited predator Chaoborus sp. to handle their prey. In addition, melatonin decreased the extent of crowding-related production of resting eggs of Daphnia.ConclusionsOur results confirm the effect of melatonin on inhibition of stress-signal responses of Daphnia. Until now, only a single study demonstrated melatonin effects on behavioral responses due to vertebrate kairomones, whereas we clearly show a more general effect of melatonin: i) on morphological predator defense induced by an invertebrate kairomone and ii) on life history characteristics transmitted by chemical cues from conspecifics. Therefore, we could generally confirm that melatonin plays a role in the attenuation of responses to different stressors in Daphnia.

Dynamics of a producer-grazer model incorporating the effects of excess food nutrient content on grazer's growth.

Modeling under the framework of ecological stoichiometric allows the investigation of the effects of food quality on food web population dynamics. Recent discoveries in ecological stoichiometry suggest that grazer dynamics are affected by insufficient food nutrient content (low phosphorus (P)/carbon (C) ratio) as well as excess food nutrient content (high P:C). This phenomenon is known as the "stoichiometric knife edge." While previous models have captured this phenomenon, they do not explicitly track P in the producer or in the media that supports the producer, which brings questions to the validity of their predictions. Here, we extend a Lotka-Volterra-type stoichiometric model by mechanistically deriving and tracking P in the producer and free P in the environment in order to investigate the growth response of Daphnia to algae of varying P:C ratios. Bifurcation analysis and numerical simulations of the full model, that explicitly tracks phosphorus, lead to quantitative different predictions than previous models that neglect to track free nutrients. The full model shows that the fate of the grazer population can be very sensitive to excess nutrient concentrations. Dynamical free nutrient pool seems to induce extreme grazer population density changes when total nutrient is in an intermediate range.

Limnological features of Laguna Teno (35° S, Chile): a high altitude lake impacted by volcanic activity

Laguna Teno (2549 m a.s.l.) is a high elevation oligo-mesotrophic lake located in the area of influence of the active Volcanic Complex Planchon-Peteroa (VCPP) in the central Chilean Andes. The most recent eruption of VCPP was in September 2010. This study compares limnological data registered in Laguna Teno during the summers of 2009 and 2011, preand post-eruptive periods of the VCPP activity respectively. In particular remarkable post-eruption changes of conductivity and slight changes for pH values in the water column of Laguna Teno were detected. In contrast no major variations of trophic parameters of water quality and plankton attributes were observed between the two summer periods. Phytoplankton was mainly represented by the biomass and chlorophylls attributable to diatoms, dinophytes and chlorophytes, with distinctive patterns of vertical segregation. Zooplankton was constituted by a very simple community comprising the copepod Boeckella gibbosa, the cladoceran Daphnia gr. pulex and a population of rotifers of the Hexarthra genus. This study represents the first limnological characterization of Laguna Teno and the first biogeographical report of a melanized population of the Daphnia pulex complex living in a deep high altitude Chilean lake. The limnological attributes of Laguna Teno, particularly the noticeable presence of large sized pigmented Daphnia in summer allow us to: i) infer the biological responses of Daphnia to extreme conditions; ii) support the hypothesis that Teno is currently a fishless lake. The latter presumption is consistent with the lethal effects on fish registered on the watershed of Teno and Claro rivers after the eruption of VCPP in 1991. Key-words: Boeckella gibbosa, Daphnia pulex complex, Hexarthra, volcanic lake, Andes. Fundam. Appl. Limnol. Vol. 183/4 (2013), 323–335 Article Stuttgart, December 2013 © 2013 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany www.schweizerbart.de DOI: 10.1127/1863-9135/2013/0551 1863-9135/13/0551 $ 3.25 Introduction Chile is part of the geographical zone known as the ‘Pacific Ring of Fire’. This area boasts over 2000 eruptive centers of which more than 500 are geologically active, and at least 60 of them have presented some kind of important eruptive activity in the last 450 years (Gonzalez-Ferran 1995). Along the country, from north to south, an important number of large lakes of glacial origin, distributed between 0 to 4000 m above the sea level (a.s.l.), are located at the foot of volcanoes. Unlike the large North Patagonian lakes of Chile (Thomasson 1963), generally there is scant knowledge on the limnological features of the volcanic lakes of the country. In addition the effects of active volcanism on the physical-chemical features and the ecology of limnic systems have been poorly studied. Many volcanic eruptions of different intensities and durations,

Combined exposure to cyanobacteria and carbaryl results in antagonistic effects on the reproduction of daphnia pulex

In aquatic ecosystems, Daphnia are exposed to a wide variety of natural and chemical stressors that can cause interactive effects resulting in an increased impact on aquatic ecosystems. The authors therefore investigated the interactive effects of harmful cyanobacteria (cyanoHABs) with carbaryl in Daphnia pulex, because cyanobacteria have become an important concern for aquatic ecosystems. Daphnia were exposed for 21 d to 4 selected cyanobacteria (Aphanizomenon sp., Cylindrospermopsis raciborskii, Microcystis aeruginosa, and Oscillatoria sp), carbaryl, and all binary combinations of carbaryl and each individual cyanobacterium. Results were analyzed with both the independent action and the concentration addition model. The estimated median effect concentration (EC50) for carbaryl was comparable between the experiments, ranging from 2.28 µg/L to 5.94 µg/L. The EC50 for cyanobacteria ranged from 13.45% of the total diet ratio for Microcystis to 66.69% of the diet ratio for Oscillatoria. In combination with carbaryl, the response of Daphnia to 3 of the 4 cyanobacteria demonstrated antagonistic deviation patterns (p < 0.05). Exposure to combinations of carbaryl and Cylindrospermopsis did not result in statistically significant deviations from both reference models. The results may have important implications for pesticide risk assessment, underlining the impact of interactive effects on aquatic organisms. Based on these results, the authors suggest that both the independent action and the concentration addition model can serve as a protective approach in risk assessment of carbaryl in the presence of cyanobacterial blooms.

A stoichiometric producer-grazer model incorporating the effects of excess food-nutrient content on consumer dynamics

There has been important progress in understanding ecological dynamics through the development of the theory of ecological stoichiometry. For example, modeling under this framework allows food quality to affect consumer dynamics. While the effects of nutrient deficiency on consumer growth are well understood, recent discoveries in ecological stoichiometry suggest that consumer dynamics are not only affected by insufficient food nutrient content (low phosphorus (P): carbon (C) ratio) but also by excess food nutrient content (high P:C). This phenomenon is known as the stoichiometric knife edge, in which animal growth is reduced not only by food with low P content but also by food with high P content, and needs to be incorporated into mathematical models. Here we present a Lotka–Volterra type model to investigate the growth response of Daphnia to algae of varying P:C ratios capturing the mechanism of the stoichiometric knife edge.

Predator threat assessment in Daphnia magna: the role of kairomones versus conspecific alarm cues

Studying the finely tuned mechanism of predation risk assessment allows for a better understanding of how prey organisms make key decisions under different levels of predation pressure. We studied the relative importance of conspecific alarm cues and fish kairomones as initiators of D. magna antipredator defences. By exposing a clone of D. magna to different infochemicals that simulated the presence of an active fish predator, we observed cue-specific responses in terms of altered feeding behaviour, respiration and life-history traits. Results agreed with the hypothesis that D. magna processes information from alarm cues from macerated conspecifics and from predator kairomones to assess the level of predation risk, adjusting the magnitude of their responses to the different levels of threat perceived. Results support the findings of other investigations and further show that single cues (fish kairomones or alarm cues) triggered feeding reduction and increased oxygen consumption, whereas fish kairomones only elicited D. magna life-history responses. Prey-specific alarm cues can thus modify the response of Daphnia to trout kairomones and this combination of both chemical cues appears to be necessary to trigger the full deployment of antipredator responses and avoid unnecessary costs arising from maladaptive responses.

Effects of silver carp (Hypophthalmichthys molitrix) and nutrients on the plankton community of a deep, tropical reservoir: an enclosure experiment

Summary1. An in situ enclosure experiment was conducted in a deep reservoir of southern China to examine (i) the effects of a low biomass (4 g wet weight m−3) of silver carp (Hypophthalmichthys molitrix) and nutrients on the plankton community and (ii) the response of Daphnia to eutrophication.2. In the absence of fish, Daphnia galeata dominated the zooplankton community, whereas calanoids were dominant in the fish treatments, followed by D. galeata. Silver carp stocking significantly reduced total zooplankton biomass, and that of D. galeata and Leptodorarichardi, but markedly increased the biomass of smaller cladocerans, copepod nauplii and rotifers. In contrast, nutrient enrichment had no significant effect on the plankton community except for cyclopoids.3. Chlorophyta, Cryptophyta and Bacillariophyta were dominant phytoplankton groups during the experiment. Chlorophyta with high growth rates (mainly Chlorella vulgaris in the fish enclosures and Ankyra sp. in the fishless enclosures) eventually dominated the phytoplankton community. Total phytoplankton biomass and the biomass of edible phytoplankton [greatest axial linear dimension (GALD) &lt; 30 μm], Chlorophyta, Cryptophyta, Bacillariophyta and Cyanobacteria showed positive responses to fish stocking, while inedible phytoplankton (GALD ≥ 30 μm) was significantly reduced in the fish enclosures. However, there was no significant effect on the plankton community from the interaction of fish and nutrients.4. Overall, the impact of fish on the plankton community was much greater than that of nutrients. High total phosphorus concentrations in the control treatment and relatively low temperatures may reduce the importance of nutrient enrichment. These results suggest it is not appropriate to use a low biomass of silver carp to control phytoplankton biomass in warmer, eutrophic fresh waters containing large herbivorous cladocerans.

The role of ultraviolet radiation and fish in regulating the vertical distribution of Daphnia

Previous studies have demonstrated independent effects of both solar ultraviolet radiation (UV; 280–400 nm) and planktivorous fish on the vertical distribution of Daphnia. We examined the behavioral response of adult and juvenile Daphnia to both UV and planktivorous fish simultaneously in a small temperate lake in eastern Pennsylvania by conducting a large‐scale (15 m deep) in situ mesocosm experiment with full factorial treatments (± UV and ± fish). UV induced an avoidance of the surface waters in both juvenile and adult Daphnia. In contrast, the response of Daphnia to fish depended on the presence of UV, with a clear interactive effect. In the presence of both UV and fish, Daphnia were deeper in the water column than they were in the absence of either UV or fish. Sampling of the lake also revealed a rapid upward shift in the depth distribution of both juveniles and adults following a rare and intense early‐summer storm that reduced the lake's transparency to both UV (for example, 380 nm UV‐A) and photosynthetically active radiation (400–700 nm) by 44% and 39%, respectively. Evidence of a novel benefit to UV avoidance behavior was also observed: surface avoidance of UV reduces the hazards of Daphnia getting caught in the surface air–water interface and perishing. These results highlight the interactive effects of fish and UV on Daphnia vertical distribution under near‐natural conditions in situ.