Influence of low and decreasing food levels on Daphnia-algal interactions: Numerical experiments with a new dynamic energy budget model

Aging of nanosized titanium dioxide modulates the effects of dietary copper exposure on Daphnia magna - an assessment over two generations

Nanosized titanium dioxide (nTiO2) is widely used in products, warranting its discharge from various sources into surface water bodies. However, nTiO2 co-occurs in surface waters with other contaminants, such as metals. Studies with nTiO2 and metals have indicated that the presence of natural organic matter (NOM) can mitigate their toxicity to aquatic organisms. In addition, "aging" of nTiO2 can affect toxicity. However, it is a research challenge, particularly when addressing sublethal responses from dietary exposure over multiple generations. We, therefore exposed the alga Desmodesmus subspicatus to nTiO2 (at concentrations of 0.0, 0.6 and 3.0 mg nTiO2/L) in nutrient medium aged for 0 or 3 days with copper (Cu) at concentrations of 0 and 116 µg Cu/L and with NOM at concentrations equivalent to 0 and 8 mg total organic carbon (TOC) per litre. Subsequently, the exposed alga was fed to Daphnia magna for 23 days over two generations and survival, reproduction and body length were assessed as endpoints of toxicity. In parallel, Cu accumulation and depuration from D. magna were measured. The results indicate that the reproduction of D. magna was the most sensitive parameter in this study, being reduced by 30% (at both parental (F0) and filial (F1) generations) and 50% (at F0 but not F1) due to the dietary Cu exposure in combination with nTiO2 for 0 and 3 days aging, respectively. There was no relationship between the effects observed on reproduction and Cu body burden in D. magna. Moreover, D. magna from the F1 generation showed an adaptive response to Cu in the treatment with 3.0 mg nTiO2/L aged for 3 days, potentially due to epigenetic inheritance. Unexpectedly, the presence of NOM hardly changed the observed effects, pointing towards the function of algal exopolymeric substances or intracellular organic matter, rendering the NOM irrelevant. Ultimately, the results indicate that the transferability of the impacts observed during the F0 to the responses in the F1 generation is challenging due to opposite effect directions. Additional mechanistic studies are needed to unravel this inconsistency in the responses between generations and to support the development of reliable effect models.

Characteristics of phytoplankton-zooplankton communities and the roles in the transmission of antibiotic resistance genes under the pressure of river contamination

Insight into the distribution of antibiotic resistance genes (ARGs) in phytoplankton-zooplankton communities (PZCs) is essential for the management and control of antibiotic resistance in aquatic ecosystems. This study characterized the profiles of PZCs and their carried ARGs in a typical urban river and ranked the factors (water physicochemical parameters, PZCs, bacterial abundance, and mobile genetic elements) influencing the dynamic of ARG profiles by the partial least squares path modeling. Results showed Cyanobacteria, Bacillariophyta and Chlorophyta were dominant phyla of phytoplankton, and Rotifera was with the highest abundance in zooplankton. River contamination markedly altered the structure of PZCs, increasing the abundance of phytoplankton and zooplankton, decreasing the diversity of phytoplankton while elevating in zooplankton. PZCs harbored large amounts of ARGs with average relative abundance of 2.35 × 10^-2/copies nearly an order magnitude higher than the living water and most ARGs exhibited significant accumulation in PZCs with the aggravated environmental pollution. The partial least squares path modeling predicted the water parameters as the most important factor mainly playing indirect effects on ARGs via PZCs and bacterial communities, followed by mobile genetic elements as the most essential direct factor for ARGs profiles. Besides, PZCs were also important drivers for the carried ARGs via direct effects on the ARGs' composition and indirect effects on host bacterial communities of ARGs and their mobile genetic elements. The present study fills the gaps in knowledge about the distribution of ARGs in PZCs and provided a new perspective to decipher the key roles of PZCs in the maintenance and dissemination of ARGs in urban river ecosystems.

Norfloxacin pollution alters species composition and stability of plankton communities

Despite recent advances in assessing lethal effects of antibiotics on freshwater organisms, little is known about their potential consequences on community composition and function, which are essential for assessing the ecological risk of these pollutants. Here, we investigated the impact of norfloxacin (NOR) on the short-term (≤ 6 days) dynamics of co-cultured Scenedesmusquadricauda-Chlorella vulgaris and Scenedesmusobliquus-C. vulgaris, and the long-term (≤ 70 days) dynamics of co-cultured S.obliquus-C. vulgaris in experiments with or without grazer Daphnia magna at sublethal antibiotic concentrations (0, 0.5, 2 and 8 mg L^-1). NOR increased the relative abundance of Scenedesmus species in the absence of grazers but exerted opposite effects when Daphnia was present in both short- and long-term experiments due to reduced colony size. Meanwhile, increasing NOR concentrations led to quickly increased total algal density in the initial stage, followed by a sharp decline in the long-term experiment in the absence of grazers; when Daphnia was present, population fluctuations were even larger for both prey and predator species (e.g., grazer extinction at the highest concentration). Thus, NOR affected the outcome of species interactions and decreased temporal stability of plankton ecosystems, suggesting that antibiotics have more extensive impacts than presently recognized.

Modelling growth performance and feeding behaviour of Atlantic salmon (Salmo salar L.) in commercial-size aquaculture net pens: Model details and validation through full-scale experiments

We have developed a mathematical model which estimates the growth performance of Atlantic salmon in aquaculture production units. The model consists of sub-models estimating the behaviour and energetics of the fish, the distribution of feed pellets, and the abiotic conditions in the water column. A field experiment where three full-scale cages stocked with 120,000 salmon each (initial mean weight 72.1  ± SD 2.8 g) were monitored over six months was used to validate the model. The model was set up to simulate fish growth for all the three cages using the feeding regimes and observed environmental data as input, and simulation results were compared with the experimental data. Experimental fish achieved end weights of 878, 849 and 739 g in the three cages respectively. However, the fish contracted Pancreas Disease (PD) midway through the experiment, a factor which is expected to impair growth and increase mortality rate. The model was found able to predict growth rates for the initial period when the fish appeared to be healthy. Since the effects of PD on fish performance are not modelled, growth rates were overestimated during the most severe disease period. This work illustrates how models can be powerful tools for predicting the performance of salmon in commercial production, and also imply their potential for predicting differences between commercial scale and smaller experimental scales. Furthermore, such models could be tools for early detection of disease outbreaks, as seen in the deviations between model and observations caused by the PD outbreak. A model could potentially also give indications on how the growth performance of the fish will suffer during such outbreaks.

STATEMENT OF RELEVANCE

We believe that our manuscript is relevant for the aquaculture industry as it examines the growth performance of salmon in a fish farm in detail at a scale, both in terms of number of fish and in terms of duration, that is higher than usual for such studies. In addition, the fish contracted a disease (PD) midway through the experiment, thus resulting in a detailed dataset containing information on how PD affects salmon growth, which can serve as a foundation to understanding disease effects better. Furthermore, the manuscript describes an integrated mathematical model that is able to predict fish behaviour, growth and energetics of salmon in response to commercial production conditions, including a dynamic model of the distribution of feed pellets in the production volume. To our knowledge, there exist no models aspiring to estimate such a broad spectre of the dynamics in commercial aquaculture production cages. We believe this model could serve as a future tool to predict the dynamics in commercial aquaculture net pens, and that it could represent a building block that can be utilised in a future development of knowledge-driven decision-support tools for the salmon industry.

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.

Relationship between trans-generational effects of tetracycline on Daphnia magna at the physiological and whole organism level

The effects of pharmaceuticals have been underestimated during single generation exposure. Therefore, in this study, we investigated toxic responses at the physiological and whole organism level in tetracycline-exposed Daphnia magna over four consecutive generational lifecycles. The results showed that tetracycline affected energy-related physiological functions in concentration- and generation-dependent manners, and especially maintenance costs increased. Consequently, multigenerational exposure to tetracycline induced changes in energy balance, resulting in the change of higher levels of biological responses. In contrast, D. magna acclimated to tetracycline exposure over multiple generations, as evidenced by the increased LC50 values. Transgenerational adaptation was related to the neonatal sensitivity and energy reserves of the organism. The results also emphasized the idea that the number of generation is an important factor for toxicity. The present study confirmed that toxic stress induces metabolic changes in an organism, thereby leading to increased energy consumption that results in adverse effects on reproduction.

Evidence for links between feeding inhibition, population characteristics, and sensitivity to acute toxicity for Daphnia magna

A population experiment with Daphnia magna tested the hypothesis that short-term feeding inhibition provokes a shift in population structure that will vary with conspecific pressure (e.g., pressure occurring from individuals of the same species due to competition for food and space) and increases population sensitivity to a xenobiotic exposure due to size-dependent toxicity (e.g., decreasing sensitivity with increasing body length). Populations were exposed for one week to a feeding inhibitor (imidacloprid, 0.15 or 12.0 mg/L) followed by one week of recovery and one day of exposure to an acute toxin (carbaryl, 0.0098 mg/L). Identical exposure under low and high conspecific pressure was studied by delaying the start of exposure for half of the populations by two weeks; thus populations were in a different stage of population development when exposure occurred. Feeding inhibition of 97% (12.0 mg/L imidacloprid) caused a shift in population structure toward smaller individuals but also reduced population abundance by up to 56 ± 7% with a strong influence of conspecific pressure. Increased population sensitivity to carbaryl was observed after feeding inhibition of 97% as hypothesized. Carbaryl exposure for one day resulted in population decline of up to 23 ± 6% when populations were not previously exposed to imidacloprid. Identical carbaryl exposure provoked a four times stronger decline in population abundance when exposure occurred following feeding inhibition of 97%. In conflict with the hypothesis, this was at least in part due to changes in the reproductive strategy of daphnids following exposure to imidacloprid rather than driven by the shift in population structure. The differences in population sensitivity to additional stress (carbaryl) occurring one week after feeding inhibition caused by exposure to imidacloprid adds a further challenge to understanding potential impacts from multiple stressors as occurring in the field at the population level.

Feeding inhibition explains effects of imidacloprid on the growth, maturation, reproduction, and survival of Daphnia magna

Effects of some xenobiotics on aquatic organisms might not be caused directly by the compound but rather arise from acclimation of the organism to stress invoked by feeding inhibition during exposure. Experiments were conducted to identify effects of imidacloprid on individual performance (feeding, growth, maturation, reproduction, and survival) of Daphnia magna under surplus and reduced food availability. Concentrations inhibiting feeding by 5, 50, and 95% after one day of exposure were 0.19, 1.83, and 8.70 mg/L, respectively. Exposure with imidacloprid at ≥ 3.7 mg/L reduced growth by up to 53 ± 11% within one week. Surplus food availability after inhibition allowed recovery from this growth inhibition, whereas limited food supply eliminated the potential for recovery in growth even for exposure at 0.15 mg/L. A shift in the distribution of individual energy reserves toward reproduction rather than growth resulted in increased reproduction after exposure to concentrations ≤ 0.4 mg/L. Exposure to imidacloprid at ≥ 4.0 mg/L overwhelmed this adaptive response and reduced reproduction by up to 57%. We used the individual based Daphnia magna population model IDamP as a virtual laboratory to demonstrate that only feeding was affected by imidacloprid, and that in turn this caused the other impacts on individual performance. Consideration of end points individually would have led to a different interpretation of the effects. Thus, we demonstrate how multiple lines of evidence linked by understanding the ecology of the organism are necessary to elucidate xenobiotic impacts along the effect cascade.