Daphnia enhances relative reproductive allocation in response to toxic microcystis: Changes in the performance of parthenogenetic and sexual reproduction

Eutrophication and warming lead to frequent occurrence of cyanobacterial blooms, which significantly impact on zooplankton. Freshwater zooplankton Daphnia adopts two distinct ways of reproduction: asexual (parthenogenetic) reproduction for rapidly reproducing many offspring in favorable environment and sexual reproduction for producing resting eggs as seed bank to survive in harsh environments. Daphnia pulex has worse performance in growth and reproduction under the exposure to toxic cyanobacteria Microcystis aeruginosa and tends to allocate less energy to reproduction in the case of insufficient food. However, the relative reproductive allocation strategy (energy allocation) of D. pulex individuals exposed to toxic M. aeruginosa is still unclear. Here we tested the relative reproductive performance of D. pulex fed on solely Chlorella pyrenoidosa (high quality food) or Chlorella mixed with toxic M. aeruginosa (low quality food), based on the parthenogenetic reproduction (life-history experiments) and sexual reproduction (population experiments). The results showed that under low quality food conditions, D. pulex reproduced fewer offspring which were also smaller and thus led to a reduced absolute output in parthenogenetic reproduction, but produced ephippia in the same size and quantity compared to those cultured under high quality food conditions. However, as the body size of maternal D. pulex cultured under low quality food conditions decreased, the relative reproductive allocation significantly increased in both parthenogenetic and sexual reproduction, compared to those cultured under high quality food conditions. In conclusion, D. pulex tend to allocate relatively more energy to reproduction under Microcystis conditions, which is a reasonable strategy for it to decentralize the risks from low-quality food.

Evolution of size-dependent intraspecific competition predicts body size scaling of metabolic rate

Growth in body size is accompanied by changes in foraging capacity and metabolic costs, which lead to changes in competitive ability during ontogeny. The resulting size-dependent competitive asymmetry influences population dynamics and community structure, but it is not clear whether natural selection leads to asymmetry in intraspecific competition.We address this question by using a size-structured consumer-resource model, in which the strength and direction of competitive asymmetry between different consumer individuals depends on the scaling of maximum ingestion and maintenance metabolism with consumer body size. We use adaptive dynamics to study selection on the scaling exponents of these processes.Selection leads to an identical scaling of maximum ingestion and maintenance metabolism with consumer body size. Equal scaling exponents neutralize strong competitive differences within the consumer population, because all consumer individuals require the same amount of resources to cover maintenance requirements. Furthermore, the scaling exponents respond adaptively to changes in mortality such that biomass production through growth or reproduction increases in the life stage that is subject to increased mortality. Also, decreasing size at birth leads to increased investment in juvenile growth, while increasing maximum size leads to increased investment in post-maturation growth and reproduction.These results provide an explanation for observed variation in the ontogenetic scaling of metabolic rate with body size. Data of teleost fish are presented that support these predictions. However, selection towards equal scaling exponents is contradicted by empirical findings, which suggests that additional ecological complexity beyond this basic consumer-resource interaction is required to understand the evolution of size-dependent asymmetry in intraspecific competition. A plain language summary is available for this article.

Integrating lipid storage into general representations of fish energetics

Fish, even of the same species, can exhibit substantial variation in energy density (energy per unit wet weight). Most of this variation is due to differences in the amount of storage lipids. In addition to their importance as energy reserves for reproduction and for survival during unfavourable conditions, the accumulation of lipids represents a large energetic flux for many species, so figuring out how this energy flux is integrated with other major energy fluxes (growth, reproduction) is critical for any general theory of organismal energetics. Here, we synthesize data from a wide range of fish species and identify patterns of intraspecific variation in energy storage, and use these patterns to formulate a general model of energy allocation between growth, lipid storage and reproduction in fishes. From the compiled data we identified two patterns: (1) energy density increases with body size during the juvenile period, but is invariant with body size within the adult size range for most species, and (2) energy density changes across seasons, with depletion over winter, but increases fastest in periods of transition between favourable and unfavourable conditions for growth (i.e. fall). Based on these patterns we propose DEBlipid, a simple, general model of energy allocation that is closely related to a simplified version of Dynamic Energy Budget theory, DEBkiss. The crux of the model is that assimilated energy is partitioned, with κ fraction of energy allocated to pay maintenance costs first, and the surplus allocated to growth, and 1 - κ fraction of assimilated energy is allocated to accumulating storage lipids during the juvenile phase, and later to reproduction as adults. This mechanism, in addition to capturing the two patterns that motivated the model, was able to predict lipid dynamics in a novel context, the migration of anadromous fish from low-food freshwater to high-food marine environments. Furthermore, the model was used to explain intra and interspecific variation in reproductive output based on patterns of lipid accumulation as juveniles. Our results suggest that many seemingly complex, adaptive energy allocation strategies in response to ontogeny, seasonality and habitat quality can emerge from a simple physiological heuristic.

Relating suborganismal processes to ecotoxicological and population level endpoints using a bioenergetic model

Ecological effects of environmental stressors are commonly evaluated using organismal or suborganismal data, such as standardized toxicity tests that characterize responses of individuals (e.g., mortality and reproduction) and a rapidly growing body of "omics" data. A key challenge for environmental risk assessment is relating such information to population dynamics. One approach uses dynamic energy budget (DEB) models that relate growth and reproduction of individuals to underlying flows of energy and elemental matter. We hypothesize that suborganismal information identifies DEB parameters that are most likely impacted by a particular stressor and that the DEB model can then project suborganismal effects on life history and population endpoints. We formulate and parameterize a model of growth and reproduction for the water flea Daphnia magna. Our model resembles previous generic bioenergetic models, but has explicit representation of discrete molts, an important feature of Daphnia life history. We test its ability to predict six endpoints commonly used in chronic toxicity studies in specified food environments. With just one adjustable parameter, the model successfully predicts growth and reproduction of individuals from a wide array of experiments performed in multiple laboratories using different clones of D. magna raised on different food sources. Fecundity is the most sensitive endpoint, and there is broad correlation between the sensitivities of fecundity and long-run growth rate, as is desirable for the default metric used in chronic toxicity tests. Under some assumptions, we can combine our DEB model with the Euler-Lotka equation to estimate longrun population growth rates at different food levels. A review of Daphnia gene-expression experiments on the effects of contaminant exposure reveals several connections to model parameters, in particular a general trend of increased transcript expression of genes involved in energy assimilation and utilization at concentrations affecting growth and reproduction. The sensitivity of fecundity to many model parameters was consistent with frequent generalized observations of decreased expression of genes involved in reproductive physiology, but interpretation of these observations requires further mechanistic modeling. We thus propose an approach based on generic DEB models incorporating few essential species-specific features for rapid extrapolation of ecotoxicogenomic assays for Daphnia-based population risk assessment.

Predicting climate change impacts on polar bear litter size

Predicting the ecological impacts of climate warming is critical for species conservation. Incorporating future warming into population models, however, is challenging because reproduction and survival cannot be measured for yet unobserved environmental conditions. In this study, we use mechanistic energy budget models and data obtainable under current conditions to predict polar bear litter size under future conditions. In western Hudson Bay, we predict climate warming-induced litter size declines that jeopardize population viability: ∼28% of pregnant females failed to reproduce for energetic reasons during the early 1990s, but 40–73% could fail if spring sea ice break-up occurs 1 month earlier than during the 1990s, and 55–100% if break-up occurs 2 months earlier. Simultaneously, mean litter size would decrease by 22–67% and 44–100%, respectively. The expected timeline for these declines varies with climate-model-specific sea ice predictions. Similar litter size declines may occur in over one-third of the global polar bear population.

Predicting ecological impacts of climate change is complicated, because key biological parameters are unknown for future conditions. Using a mechanistic energy budget model to relate sea ice to polar bear reproduction, Molnár et al. predict decreases in litter size with anticipated changes in sea ice.

Dynamic energy budget theory and population ecology: lessons from Daphnia

Dynamic energy budget (DEB) theory offers a perspective on population ecology whose starting point is energy utilization by, and homeostasis within, individual organisms. It is natural to ask what it adds to the existing large body of individual-based ecological theory. We approach this question pragmatically--through detailed study of the individual physiology and population dynamics of the zooplankter Daphnia and its algal food. Standard DEB theory uses several state variables to characterize the state of an individual organism, thereby making the transition to population dynamics technically challenging, while ecologists demand maximally simple models that can be used in multi-scale modelling. We demonstrate that simpler representations of individual bioenergetics with a single state variable (size), and two life stages (juveniles and adults), contain sufficient detail on mass and energy budgets to yield good fits to data on growth, maturation and reproduction of individual Daphnia in response to food availability. The same simple representations of bioenergetics describe some features of Daphnia mortality, including enhanced mortality at low food that is not explicitly incorporated in the standard DEB model. Size-structured, population models incorporating this additional mortality component resolve some long-standing questions on stability and population cycles in Daphnia. We conclude that a bioenergetic model serving solely as a 'regression' connecting organismal performance to the history of its environment can rest on simpler representations than those of standard DEB. But there are associated costs with such pragmatism, notably loss of connection to theory describing interspecific variation in physiological rates. The latter is an important issue, as the type of detailed study reported here can only be performed for a handful of species.

Complete compensation in Daphnia fecundity and stage-specific biomass in response to size-independent mortality

1. Recent theory suggests that compensation or even overcompensation in stage-specific biomass can arise in response to increased mortality. Which stage that will show compensation depends on whether maturation or reproduction is the more limiting process in the population. Size-structured theory also provides a strong link between the type of regulation and the expected population dynamics as both depend on size/stage-specific competitive ability. 2. We imposed a size-independent mortality on a consumer-resource system with Daphnia pulex feeding on Scenedesmus obtusiusculus to asses the compensatory responses in Daphnia populations. We also extended an existing stage-structured biomass model by including several juvenile stages to test whether this extension affected the qualitative results of the existing model. 3. We found complete compensation in juvenile biomass and total population fecundity in response to harvesting. The compensation in fecundity was caused by both a higher proportion of fecund females and a larger clutch size under increased mortality. We did not detect any difference in resource levels between treatments. 4. The model results showed that both stages of juveniles have to be superior to adults in terms of resource competition for the compensatory response to take place in juvenile biomass. 5. The results are all in correspondence with that the regulating process within the population was reproduction. From this, we also conclude that juveniles were superior competitors to adults, which has implications for population dynamics and the kind of cohort cycles seen in Daphnia populations. 6. The compensatory responses demonstrated in this experiment have major implications for community dynamics and are potentially present in any organisms with food-dependent growth or development.

Effects of spinosad and Bacillus thuringiensis israelensis on a natural population of Daphnia pulex in field microcosms

Spinosad, a candidate biological larvicide for mosquito control, was evaluated for its effects on a field population of Daphnia pulex, using Bacillus thuringiensis serovar israelensis (Bti) as a reference larvicide. Microcosms (125L enclosures) were placed in a shallow temporary oligohaline marsh where D. pulex was present. Three concentrations of spinosad (8, 17 and 33 microg L(-1)) and two concentrations of Bti (0.16 and 0.50 microL L(-1)) were applied (5 replicates per concentration, including the controls). Effects of larvicides on D. pulex were evaluated after 2, 4, 7, 14 and 21d of exposure, through measurements of abundance and individual size. Dissipation of spinosad from the water phase was rapid. Four days after treatment, residue concentration represented 11.8%, 3.9% and 12.7% of the initial exposure level for the nominal concentrations of 8, 17 and 33 microg L(-1), respectively. Spinosyns A and D dissipated at similar rates. Analysis of abundance and size structure of the D. pulex population showed an impact of spinosad. Both survival and size structure were affected. However, at the lowest concentration (8 microg L(-1)), population recovered after the first week. In microcosms treated with Bti, the abundance of D. pulex was not affected but the size structure of the population changed after 21d. As compared to laboratory tests, the use of in situ microcosms improved the environmental risk assessment of larvicides, taking into account the influence of environmental factors (e.g., temperature, light, salinity) and intrinsic capacity of recovery of D. pulex under field conditions.

Scaling up the functional response for spatially heterogeneous systems

Scale transition theory is a framework for predicting regional population dynamics from local process functions and estimates of spatial heterogeneity. Using this framework, we estimated regional scale functional responses for a benthic predator-prey system in the Baltic Sea. Functional responses were based on laboratory experiments or field observations of stomach contents, and prey densities measured at a local scale (0.1 m(2)) or a regional scale (300 km(2)). Laboratory data overestimated consumption at high prey densities, whereas predictions based on local scale data tallied closely with consumption observed at the regional scale. The predicted regional functional response was different for increasing and decreasing prey densities, reflecting that predator and prey densities, as well as the covariance between them, exhibit oscillatory dynamics. We conclude that it is important to validate laboratory data with small-scale field observations and that scale transition is a powerful tool for scaling-up process functions in heterogeneous systems.

A model for energetics and bioaccumulation in marine mammals with applications to the right whale

We present a dynamic energy budget (DEB) model for marine mammals, coupled with a pharmacokinetic model of a lipophilic persistent toxicant. Inputs to the model are energy availability and lipid-normalized toxicant concentration in the environment. The model predicts individual growth, reproduction, bioaccumulation, and transfer of energy and toxicant from mothers to their young. We estimated all model parameters for the right whale; with these parameters, reduction in energy availability increases the age at first parturition, increases intervals between reproductive events, reduces the organisms' ability to buffer seasonal fluctuations, and increases its susceptibility to temporal shifts in the seasonal peak of energy availability. Reduction in energy intake increases bioaccumulation and the amount of toxicant transferred from mother to each offspring. With high energy availability, the toxicant load of offspring decreases with birth order. Contrary to expectations, this ordering may be reversed with lower energy availability. Although demonstrated with parameters for the right whale, these relationships between energy intake and energetics and pharmacokinetics of organisms are likely to be much more general. Results specific to right whales include energy assimilation estimates for the North Atlantic and southern right whale, influences of history of energy availability on reproduction, and a relationship between ages at first parturition and calving intervals. Our model provides a platform for further analyses of both individual and population responses of marine mammals to pollution, and to changes in energy availability, including those likely to arise through climate change.

A continuous model of biomass size spectra governed by predation and the effects of fishing on them

A new time-dependent continuous model of biomass size spectra is developed. In this model, predation is the single process governing the energy flow in the ecosystem, as it causes both growth and mortality. The ratio of predator to prey is assumed to be distributed: predators may feed on a range of prey sizes. Under these assumptions, it is shown that linear size spectra are stationary solutions of the model. Exploited fish communities are simulated by adding fishing mortality to the model: it is found that realistic fishing should affect the curvature and stability of the size spectrum rather than its slope.

Trade-offs in the vertical distribution of zooplankton: ideal free distribution with costs?

Zooplankton vertical migratory patterns are a classic example of optimal habitat choice. We hypothesize that zooplankton distribute themselves vertically in the water column according to an ideal free distribution (IFD) with costs such as to optimize their fitness. In lakes with a deep-water chlorophyll maximum, zooplankton are faced with a trade-off, either experiencing high food (high reproductive potential) but low temperature (slow development) in the hypolimnion or high temperature and low food in the epilimnion. Thus, in the absence of fish predation (e.g. at night) they should allocate the time spent in the different habitats according to fitness gain dependent on the temperature gradient and distribution of food. We tested this hypothesis with a Daphnia hyalina x galeata clone in large indoor columns (Plön Plankton Towers) and with a dynamic energy budget model. In the tower experiments, we simulated a deep-water algal maximum below the thermocline with epilimnetic/hypolimnetic temperature differences of 2, 5 and 10 degrees C. Experimental data supported the model. We found a significantly larger proportion of daphniids in the hypolimnion when the temperature difference was smaller. Our results are consistent with the concept of IFD with costs originally developed for stream fishes. This concept can be applied to predict the vertical distribution of zooplankton in habitats where fish predation is of minor importance.

Population growth rate as a basis for ecological risk assessment of toxic chemicals

Assessing the ecological risks of toxic chemicals is most often based on individual-level responses such as survival, reproduction or growth. Such an approach raises the following questions with regard to translating these measured effects into likely impacts on natural populations. (i) To what extent do individual-level variables underestimate or overestimate population-level responses? (ii) How do toxicant-caused changes in individual-level variables translate into changes in population dynamics for species with different life cycles? (iii) To what extent are these relationships complicated by population-density effects? These issues go to the heart of the ecological relevance of ecotoxicology and we have addressed them using the population growth rate as an integrating concept. Our analysis indicates that although the most sensitive individual-level variables are likely to be equally or more sensitive to increasing concentrations of toxic chemicals than population growth rate, they are difficult to identify a priori and, even if they could be identified, integrating impacts on key life-cycle variables via population growth rate analysis is nevertheless a more robust approach for assessing the ecological risks of chemicals. Populations living under density-dependent control may respond differently to toxic chemicals than exponentially growing populations, and greater care needs to be given to incorporating realistic density conditions (either experimentally or by simulation) into ecotoxicological test designs. It is impractical to expect full life-table studies, which record changes in survival, fecundity and development at defined intervals through the life cycle of organisms under specified conditions, for all relevant species, so we argue that population growth rate analysis should be used to provide guidance for a more pragmatic and ecologically sound approach to ecological risk assessment.

Higher-tier laboratory methods for assessing the aquatic toxicity of pesticides

Registration schemes for plant-protection products require applicants to assess the potential ecological risk of their products using a tiered approach. Standard aquatic ecotoxicity tests are used at lower tiers and clearly defined methodologies are available for assessing the potential environmental risks. Safety factors are incorporated into the assessment process to account for the uncertainties associated with the use of lower-tier single-species ecotoxicity studies. If lower-tier assessments indicate that a substance may pose a risk to the environment, impacts can be assessed using more environmentally realistic conditions through the use of either pond mesocosms, artificial streams or field monitoring studies. Whilst these approaches provide more realistic assessments, the results are difficult to interpret and extrapolation to other systems is problematic. Recently it has been recognised that laboratory approaches that are intermediate between standard aquatic toxicity tests and field/mesocosm studies may provide useful data and help reduce the uncertainties associated with standard single-species tests. However, limited guidance is available on what tests are available and how they can be incorporated into the risk-assessment process. This paper reviews a number of these higher-tier laboratory techniques, including modified exposure studies, species sensitivity studies, population studies and tests with sensitive life stages. Recommendations are provided on how the approaches can be incorporated into the risk-assessment process.

Risk assessment approaches for ecosystem responses to transient pollution events in urban receiving waters

Alternative risk assessment approaches are reviewed for the evaluation of the ecological status and health of urban receiving waters subject to intermittent pollution events. Performance-based criteria founded on exceedance probabilities and related to the end-of-pipe discharge of chemical-specific substances comprise the conventional basis for setting regulatory standards in both North America and Europe. The difficulties and limitations of this approach, particularly in identifying realistic chronic, sub-lethal toxic risks arising from complex effluents are discussed. The potential role of Toxicity Based Criteria (TBC) for setting ecological consent limits for stormwater effluents is considered and the capabilities and limitations of Direct Toxicity Assessment (DTA) are identified. The inability of DTA procedures to satisfactorily evaluate chronic, sub-lethal risks has led to increasing interest in the potential use of in-situ biomarker techniques for the fingerprinting of stress-response properties as a means of diagnosing risk assessment for integrated urban runoff management.

Ontogenetic scaling of foraging rates and the dynamics of a size-structured consumer-resource model

The ontogenetic scaling of foraging capacity strongly influences the competitive ability of differently sized individuals within a species. We develop a physiologically structured model to investigate the effect of different ontogenetic size scalings of the attack rate on the population dynamics of a consumer-resource system. The resource is assumed to reproduce continuously whereas the consumer only reproduces at discrete time instants. Depending on the ontogenetic size scaling, the model exhibited recruit-driven cycles, stable fixed point dynamics, non-recruit juvenile-driven cycles, quasiperiodic orbits, or chaotic dynamics. The kind of dynamics observed was related to the maintenance resource levels required of differently sized individuals. Stable fixed point dynamics was, besides at the persistence boundary, only observed when the minimum resource levels were similar for newborns and mature individuals. The tendency for large population fluctuations over a wide range of the parameter space was due to the consumer's pulsed reproduction. Background mortality and length of season were major determinants of cycle length. Model dynamics strongly resembled empirically observed dynamics from fish and Daphnia populations with respect to both patterns and mechanisms. The non-recruit juvenile-driven dynamics is suggested to occur in populations with size-dependent interference or preemptive competition like cicada populations.