Invasibility of a reservoir to exotic Daphnia lumholtzi: experimental assessment of diet selection and life history responses to cyanobacteria

SUMMARY 1. In reservoirs of the south-central United States, the exotic cladoceran Daphnia lumholtzi is common during warm midsummer conditions, when cyanobacteria are abundant and native Daphnia are rare. In the current study, we employed surveys, field experiments, and a life table experiment to investigate the role of food quality in explaining the distribution and phenology of D. lumholtzi , relative to two native species ( Daphnia parvula and Daphnia mendotae ).
2. During May–September 2000 in eutrophic McDaniel Lake, Missouri U.S.A., cyanobacteria (primarily Oscillatoria ) first appeared at 6-m depth and then became abundant throughout the epilimnion.
3. During the May field experiment, D. lumholtzi , D. parvula and D. mendotae all consumed a similar diet of algae, showing positive selection for small greens (chlorophytes and cryptophytes <20 μm). During the July experiment, when the epilimnion exceeded 25 °C and cyanobacteria were common in the lake, D. lumholtzi consumed significantly more total algae and more cyanobacteria than the two native species. Although the Daphnia selected against cyanobacteria, all three species consumed about 25% of this food in their diet.
4. A life table experiment compared the responses of D. lumholtzi and D. parvula with variation in density of high-quality food ( Ankistrodesmus ) and concentration of a toxic strain of cyanobacteria ( Anabaena flos-aquae ). Both Daphnia species showed reduced survivorship, fertility and intrinsic rates of increase in response to elevated concentrations of cyanobacteria, particularly at the higher food level.
5. The results suggest that D. lumholtzi shows similar inhibition from cyanobacteria as does the native Daphnia . However, their continued high in situ feeding rates imply that D. lumholtzi is less affected by midsummer conditions in warm-water reservoirs than are native Daphnia .     

Periphyton as alternative food source for the filter-feeding cladoceran Daphnia magna

1.  Daphnia magna , a well-studied primary consumer, is mainly known as a filter feeder. In this study, we investigated the ability of D. magna to use periphyton as an alternative food source to phytoplankton. We examined the development of laboratory populations fed with different food sources ( Desmodesmus subspicatus and/or periphyton or neither) over a period of 42 days, and observed the behaviour of the daphnids.
2.  The addition of periphyton to phytoplankton food led to an increase of daphnid population biomass. When fed with periphyton as the only food source, a small but stable D. magna population developed.
3.  The behaviour of daphnids fed with both food sources revealed a preference for feeding on D. subspicatus . Only below a concentration of D. subspicatus of approximately 0.05 mg C L⁻¹ (0.4 × 10⁷ cells L⁻¹) did D. magna use periphyton as an alternative food source.
4.  Periphyton showed distinct reactions to grazing by D. magna . The thickness of the periphyton layer was reduced from about 4 to 1 mm and we observed a change in species composition due to grazing.
5.  The ability of D. magna to graze on periphyton could serve to stabilize its population density and reinforce its competitive advantage over other cladocerans. By switching between food sources, D. magna can act as a coupler between pelagic and benthic habitats and food webs.     

Constitutive mineralization of low concentrations of the herbicide linuron by a Variovorax sp. strain

The mineralization of the herbicide linuron at concentrations of μg and mg L⁻¹ was studied in liquid batch experiments with Variovorax sp. strain SRS16. The strain was highly efficient at mineralizing a range of linuron concentrations (0.002–10 mg L⁻¹) with 20–60% of the added ¹⁴C-ring-labeled linuron metabolized to ¹⁴CO₂ within hours to days depending on the initial linuron concentration and incubation period. At mg L⁻¹ linuron concentrations the mineralization activity by SRS16 was inducible and a shift to constitutive mineralization activity was apparent with a reduction in the linuron concentration to μg L⁻¹ levels. This study revealed that strain SRS16 is a promising candidate for bioaugmentation of water or soil resources contaminated with low linuron concentrations.     

Effects of artificial ultraviolet-B radiation on experimental aquatic microcosms

1. The effects of prolonged ultraviolet-B (UVB) radiation on freshwater communities were studied in indoor microcosms (600 L) with artificial light sources, simulating a clear, shallow, mesotrophic aquatic ecosystem. A range of six intensities (in duplicate) of UVB radiation, ranging from 0 (control) to 9.56 kJ m⁻² day⁻¹ at the water surface, was applied for 8 weeks. The UVB radiation levels, attenuation, shading and scattering were comparable to those in Dutch shallow freshwater systems. Physical, chemical and biological variables were monitored weekly.
2. The UVB treatment did not affect the abundance, species composition or biovolume of the phytoplankton or zooplankton communities, nor did it affect the periphyton or the macroinvertebrate community. A few species showed a significant response on some of the sampling dates, but there was no negative UVB effect at the community level. Overall, the ecosystems in the microcosms were not affected by the UVB treatment.
3. In a bio-assay, a laboratory clone of Daphnia pulex , not subjected to UVB radiation, was fed with seston from the microcosms. Daphnia pulex feeding on seston from the control microcosms grew faster, had better survival and better reproduction than D. pulex feeding on seston from the UVB treated microcosms. The phytoplankton–zooplankton interaction may have been influenced by the UVB treatment.
4. The dissolved oxygen content (DOC) concentrations in the microcosms were around 5 mg L^−1. The DOC levels in Dutch systems rarely fall below 10 mg L^−1. This might provide sufficient protection against the detrimental effects of increased UVB radiation.     

Impact of sediment and nutrient inputs on growth and survival of tadpoles of the Western Toad

1. Sediment and nutrient loading in freshwater systems are leading causes of aquatic habitat degradation globally. We investigated the impacts of fine-sediment and nutrient additions on the growth and survival of western toad ( Bufo boreas ) tadpoles and emergent metamorphs in mesocosm and exclosure experiments.
2. Mesocosm tanks received weekly pulses of fine sediments to create initial concentrations of 0, 130 and 260 mg L⁻¹ of suspended sediment and either bi-weekly additions of nutrients (N = 160 μg L⁻¹, P = 10 μg L⁻¹) or no additions in a factorial design. Within mesocosms, tadpole exclosures allowed for quantification of tadpole grazing pressure on periphyton biomass, chlorophyll- a and sediment deposition.
3. Tadpoles receiving sediment additions experienced slower growth rates and reduced survival to metamorphosis, although no effects of treatment were detected on size at metamorphosis or time to metamorphosis. Nutrient additions also lowered survival, but had no impact on other measured parameters of tadpole fitness. Dissections and gut content analysis revealed that tadpoles ingested sediment in large quantities altering the proportion of the organic content of ingested food.
4. Together these results suggest that although sediment was readily consumed by tadpoles, its presence in the larval environment had an overall negative effect on tadpole growth and survival, although not as severe as predicted.     

The effect of an emergent macrophyte (Typha angustifolia) on sediment resuspension in a shallow north temperate lake

1. The effects of emergent macrophytes on water turbidity and sediment resuspension in the shallow Kirkkojärvi basin of Lake Hiidenvesi were studied with sediment traps, and concomitant sediment and water samples. The study was conducted during May–August in three different zones of a stand of the emergent Typha angustifolia .
2. Within the stand (5 m from the edge), both the concentration of suspended solids and the rate of sediment resuspension were significantly lower than at the edge and outside the stand (5 m from the edge). The differences between the zones increased towards the end of summer together with the growing stem density. During the study period (82 days), 2210 g dw m⁻² of sediment was resuspended in the outer zone. At the edge and in the inner zone, the corresponding numbers were 1414 and 858 g dw m⁻², respectively.
3. With the resuspended sediment, 39.4 mg P  m⁻² day⁻¹ was brought to the water column outside the stand, 22.4 mg P  m⁻² day⁻¹ at the edge and 13.4 mg P  m⁻² day⁻¹ within the stand.
4. In early summer, the concentration of suspended solids had a highly significant positive effect on soluble reactive phosphorus (SRP) concentration in the water, whereas in late summer no effect was found. During the study period, phosphorus retention by emergent macrophyte stands corresponded to 3–5% of the present annual external phosphorus loading of the Kirkkojärvi basin.     

Factors which affect the abundance of an invasive cladoceran, Daphnia lumholtzi, in U.S. reservoirs

1.  Daphnia lumholtzi is a subtropical, Old World species which is rapidly spreading throughout reservoirs in the southern U.S.A. It was first recorded in Lake Texoma (Oklahoma–Texas) in September 1991.
2. Southern U.S. reservoirs typically have strong spatial and temporal gradients in temperature, conductivity, turbidity and in the distribution of organisms. Therefore, the present experiments examined the reproductive and moulting rates, and survival of D . lumholtzi in relation to extremes of food concentration, temperature, conductivity and turbidity.
3. Increases in temperature (range = 15–29 °C) and decreases in turbidity (range = 0.6–470 NTU) increased reproductive and moulting rates, whereas food concentration only affected the rate of reproduction, while conductivity had no effects. Survival was affected only by temperature.
4.  Daphnia lumholtzi tolerates high temperatures (27–30 °C) at which other cladocerans in Lake Texoma disappear. Therefore, D . lumholtzi may exploit resources in midsummer, when there are few potential competitors.     

Mortality and growth responses of Daphnia carinata to increases in temperature and salinity

1.  Daphnia carinata King, a freshwater cladoceran, occurs in Lake Waihola, a tidal lake with seasonal fluctuations in temperature (4–21 °C), and salinity (30–2000 mg L^–1 Cl). We hypothesise that these fluctuations influence the seasonal changes in D. carinata abundance.
2. To test this hypothesis, adults and juveniles were exposed to combinations of temperature and salinity. We measured mortality of adults and juveniles, growth of juveniles over 20 days, and their age at first reproduction.
3. The salinity tolerance of adult D. carinata was temperature-dependent. Juveniles were more sensitive to salinity, but more tolerant of temperature increases.
4. Growth rates were higher at elevated temperatures, but reduced by elevated salinities. The onset of reproduction was earlier at elevated temperatures.
5. We conclude that seasonal changes in temperature and salinity contribute to seasonal population changes of D. carinata .     

Longevity of the freshwater anostracan Streptocephalus mackini (Crustacean: Anostraca) in relation to food (Chlorella vulgaris) concentration

SUMMARY 1. Temporary ponds are inhabited by a variety of invertebrates, of which anostracans are an important group. We studied the lifetables of male and female anostracan Streptocephalus mackini at 3 algal concentrations (0.5 × 10⁶, 1.0 × 10⁶ and 1.5 × 10⁶ cells mL⁻¹).
2. Regardless of sex, S. mackini showed better survivorship at lower food levels. The longest average lifespan observed was 85 ± 2 days for males fed Chlorella at 0.5 × 10⁶ cells mL⁻¹.
3. Both net reproductive rate and generation time decreased with increasing food level. The highest net reproductive rate was about 120 cysts per female. The longest generation time of about 40 days, observed at 0.5 × 10⁶ cells mL⁻¹, was more than three times that at 1.5 × 10⁶ cells mL⁻¹.
4. The rate of population increase ( r ) was nearly the same (0.31 ± 0.06) at high (1.5 × 10⁶ cells mL⁻¹) and intermediate (1.0 × 10⁶ cells mL⁻¹) food levels. The r -value at low food level (0.5 × 10⁶ cells mL⁻¹ of Chlorella ) was 0.20 ± 0.01 per day.     

Effect of Lactobacillus gasseri BNR17 on blood glucose levels and body weight in a mouse model of type 2 diabetes

Aims:  To investigate the effect of Lactobacillus gasseri BNR17 isolated from human breast milk on blood glucose and body weight in type 2 diabetic animals.
Methods and results:  db/db mice were divided into one control group and five sample groups; the sample groups received BNR17 (10⁷, 10⁸, 10⁹ and 10¹⁰ CFU) or rosiglitazone (8 mg kg⁻¹) orally twice a day for 12 weeks. BNR17 groups had a dose-dependent reduction in food, water intake and amount of excrement. Body weight loss was not seen in the BNR17 groups. Fasting and postprandial 2 h blood glucose levels were significantly lower in the BNR17 (10¹⁰ CFU) group compared with the control group. HbA1c decreased in the BNR17 group, although it was not statistically significant. During the oral glucose tolerance test, the BNR17 groups exhibited dose-dependent improvement in glucose sensitivity.
Conclusions:  Lactobacillus gasseri BNR17 has a suppressing effect on blood glucose levels and improved diabetic symptoms in db/db mice.
Significance and Impact of the Study:  Blood glucose-lowering lactic acid bacteria are expected to be useful as a therapeutic for treating type 2 diabetes in humans.     

Empirical relationships between profundal macroinvertebrate biomass and environmental variables in boreal lakes of Alberta, Canada

1. We empirically modelled profundal macroinvertebrate biomass (PMB) from collections of macroinvertebrates and a suite of trophic, water chemistry and morphometric variables from 26 lakes located within the Boreal Mixedwood and Boreal Subarctic ecoregions of Alberta, Canada.
2. Hypolimnetic dissolved oxygen (DO) concentrations (mg L⁻¹) explained 37% of the variance in PMB ((g m⁻²)^0.1) as a quadratic function, whereas water temperature (°C) explained 23% of variance in a linear relationship.
3. In contrast with other studies based on oligotrophic systems, indirect indicators of food availability (i.e. chlorophyll a and total phosphorus concentrations and Secchi depth) were not significant predictors of PMB in this study.
4. The predictive power of our models may be improved by obtaining more frequent (seasonal) estimates of PMB and hypolimnetic DO concentration and by quantifying biotic factors such as competition, predation, life history phenologies and recruitment success.     

Long-term effects of successive Ca(OH)2 and CaCO3 treatments on the water quality of two eutrophic hardwater lakes

1. Whole-lake experiments were conducted in two hardwater lakes (Halfmoon and Figure Eight) in Alberta, Canada, to investigate the effectiveness of repeated lime (slaked lime: Ca(OH)₂ and/or calcite: CaCO₃) treatments (5–78 mg L^–1) for up to 7 years.
2. Randomized intervention analysis of intersystem differences between the experimental and three reference lakes demonstrated a decline in euphotic total phosphorus and chlorophyll a concentrations in the experimental lakes after repeated lime treatments.
3. After the second lime application to Halfmoon Lake, mean winter total phosphorus release rates (TPRR) decreased to < 1 mg m^–2 day^–1 compared with 3.6 mg m^–2 day^–1 during the winter after initial treatment. In the final year of lime application, mean summer TPRR decreased to 4.5 mg m^–2 day^–1 compared with 7.6 mg m^–2 day^–1 in the pre-treatment year.
4. Mean macrophyte biomass declined and species composition was altered at 1 and 2 m depths in Figure Eight Lake during lime application. Over the first 6 years of treatment, macrophyte biomass at 2 m declined by 95% compared with concentrations recorded during the initial treatment year. In the last year of the study, macrophyte biomass at 2 m reached initial treatment concentrations, which coincided with the greatest water transparency. Over the treatment period, macrophyte species shifted from floating to rooted plants.
5. Multiple lime applications can improve water quality in eutrophic hardwater lakes for periods of up to 7 years.     

Stimulation of biomethanation by Clostridium sp. PXYL1 in coculture with a Methanosarcina strain PMET1 at psychrophilic temperatures

Aim:  Bioaugumentation of low temperature biogas production was attempted by addition of cold-adapted Clostridium and a methanogen.
Methods and Results:  A psychrotrophic xylanolytic acetogenic strain Clostridium sp. PXYL1 growing optimally at 20°C and pH 5·3 and a Methanosarcina strain, PMET1, growing optimally on acetate and producing methane at 15°C were isolated from a cattle manure digester. Anaerobic conversion of xylose at 15°C with the coculture of the two strains was performed, and batch culture methane production characteristics indicated that methanogenesis occurred via acetate through 'acetoclastic' pathway. Stimulation studies were also undertaken to evaluate the effect of exogenous addition of the coculture on biogas yields at 15°C. Addition of 3 ml of PXYL1 at the rate of 12 × 10² CFU ml⁻¹ increased the biogas 1·7-fold (33 l per kg cowdung) when compared to control (19·3 l per kg cowdung) as well as increased the volatile fatty acid (VFA) levels to 3210 mg l⁻¹ when compared to 1140 mg l⁻¹ in controls. Exogenous of addition of 10 ml PMET1 inoculum at the rate of 6·8 ± 10² CFU ml⁻¹ in addition to PXYL1 served to further improve the biogas yields to 46 l kg⁻¹ as well as significantly brought down the VFA levels to 1350 mg l⁻¹.
Conclusions:  Our results suggest that the rate-limiting methanogenic step at low temperatures could be overcome and that biogas yields improved by manipulating the population of the acetoclastic methanogens.
Significance and Impact of the Study:  Stimulation of biomethanation at low temperature by coculture.     

Calcium content of crustacean zooplankton and its potential role in species distribution

SUMMARY 1. The occurrence and species composition of crustacean zooplankton in Norwegian lakes was related to ambient Ca concentrations, pH, total phosphorus and chlorophyll a , and to the presence/absence of other zooplankters and a planktivorous fish (roach: Rutilus rutilus ).
2. Ambient Ca concentrations appeared to influence the distribution of Daphnia species: Daphnia longispina often occurred in Ca-rich lakes with low fish-predation pressure while the smaller Daphnia cristata often occurred in opposite conditions.
3. Body Ca contents were measured in zooplankters from a wide range of localities, to examine Ca requirements and thus the potential for Ca-limitation of common species.
4. All Daphnia species had relatively high specific Ca contents [ranging from 0.8 to 4.4% Ca dry weight (DW)⁻¹] compared with other cladocerans and also copepods (ranging from 0.1 to 0.4% Ca DW⁻¹). Within the Daphnia genus, the specific Ca content increased with increasing body size of the species, and thus the large-bodied species had especially high Ca demands.
5. Because of their high Ca demands, species of Daphnia could be competitively disadvantaged in softwater lakes relative to less Ca-demanding species.     

Mutagenic analysis of putative domain II and surface residues in mosquitocidal Bacillus thuringiensis Cry19Aa toxin

The growing resistance against antibiotics demands the search for alternative treatment strategies. Photodynamic therapy is a promising candidate. The natural intermediate of chlorophyll biosynthesis, protochlorophyllide, was produced, purified and tested as a novel photosensitizer for the inactivation of five model organisms including Staphylococcus aureus, Listeria monocytogenes and Yersinia pseudotuberculosis , all responsible for serious clinical infections. When microorganisms were exposed to white light from a tungsten filament lamp (0.1 mW cm⁻²), Gram-positive S. aureus, L. monocytogenes and Bacillus subtilis were photochemically inactivated at concentrations of 0.5 mg L⁻¹ protochlorophyllide. Transmission electron microscopy revealed a disordered septum formation during cell division and the partial loss of the cytoplasmic cell contents. Gram-negative Y. pseudotuberculosis and Escherichia coli were found to be insensitive to protochlorophyllide treatment due to the permeability barrier of the outer membrane. However, the two bacteria were rendered susceptible to eradication by protochlorophyllide (10 mg L⁻¹) upon addition of polymyxin B nonapeptide at 50 and 20 mg L⁻¹, respectively. The release of DNA and a detrimental rearrangement of the cytoplasm were observed.     

Waterborne metals impair inducible defences in Daphnia pulex: morphology, life-history traits and encounters with predators

1.  Inducible defences may be temporary and favoured where predation is intermittent and have been demonstrated in several invertebrates and vertebrates when prey detect chemical cues (kairomones) released by predators. Daphnia pulex (a water flea) exposed to Chaoborus (midge larvae) kairomones produce small neckteeth on the dorsal surface of the head as a defence against this gape-limited predator and survive better in the presence of Chaoborus . Recent studies have shown that waterborne copper (Cu) impairs the induction of neckteeth which could lead to lower survival.
2.  Here, we examined the effects of Cu on morphological changes and shifts in life-history traits in D. pulex exposed to kairomone from Chaoborus americanus . We exposed D. pulex mothers to chemical cues of C. americanus fed on either D. pulex neonates or on brine shrimp Artemia salina , the same Chaoborus cues combined with an environmentally relevant concentration of copper (10 μg L⁻¹), or dechlorinated tap water. We examined several morphological characteristics of neonates and life-historical characteristics of adults as well as assessing survival of neonates by staging encounters with predators.
3.  Neonates from mothers exposed to kairomone plus copper had fewer and shorter neckteeth than neonates from mothers exposed to kairomone alone. Moreover, neonates exposed to Cu had lower survival during encounters with predators than neonates exposed to kairomone without Cu.
4.  Adult female Daphnia exposed to kairomones released more neonates within the first 24 h of brood release and emptied their brood pouches quicker than mothers not exposed to kairomones, irrespective of the presence of Cu.
5.  Impairment by metals of morphological defences in zooplankton could lead to a decline in population density and alter community structure.     

Potassium dependence and phytoplankton ecology: an experimental study

SUMMARY 1. Unialgal cultures of three species common in the freshwater phytoplankton were used to test limitation of specific growth rate and final yield in defined media of low K⁺ concentration (range <0.3–6 μmol L⁻¹ or mmol m⁻³).
2. Growth rate of the diatom Asterionella formosa was independent of K⁺ concentration above 0.7 μmol L⁻¹. Final yield was dependent on initial concentration when accompanied by K⁺ depletion below this concentration, but not by lesser depletion with more residual K⁺. Analyses of particulate K in the biomass indicated a mean final cell content of 2.8 μmol K 10⁻⁸ cells, approximately 1.0% of the organic dry weight.
3. Less detailed work with the diatom Diatoma elongatum showed no dependence of growth rate or final yield upon the initial K⁺ concentration in the range 0.8–3.2 μmol L⁻¹. The phytoflagellate Plagioselmis nannoplanctica suffered net mortality in the lowest concentration tested, 0.8 μmol L⁻¹.
4. Comparison with the range of K⁺ concentration in natural fresh waters, including a depletion induced by an aquatic macrophyte, suggests that K⁺ is unlikely to limit growth of phytoplankton. Nevertheless, there can be correlation of K⁺ with lake trophy.     

Predicting zebra mussel fouling on native mussels from physicochemical variables

1. Predictive models of impact are needed for the risk assessment of invasive species. One such species is the Eurasian zebra mussel ( Dreissena polymorpha ), a fouling bivalve that overgrows and kills native mussels (Unionidae) in many North American lakes and rivers. The level of mortality in a native mussel population increases with the fouling intensity – i.e. the mean number of zebra mussels attached to each native mussel.
2. We conducted a multi-site survey within the St Lawrence and Richelieu rivers (Quebec, Canada) to determine whether zebra mussel fouling intensity can be predicted from environmental variables. We found fouling intensity to be positively correlated with calcium concentration [Ca²⁺] and negatively correlated with sediment size, but not affected by local macrophyte cover.
3. A multiple regression model that includes calcium concentration and sediment size explains 86% of variation in fouling intensity across all sites.
4. Analysis of data from invaded sites in North America and Europe revealed a nonlinear relationship in which fouling intensity increases with calcium concentration up to an asymptotic threshold of 24 mg L⁻¹ Ca²⁺.
5. Our results suggest that the community-level impacts of zebra mussels are mediated by abiotic environmental variables and gradients in these variables may provide local habitat refugia for native mussels.     

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.     

Stable isotope variability of meso-zooplankton along a gradient of dissolved organic carbon

1.  The δ¹³C and δ¹⁵N signatures of zooplankton vary with dissolved organic carbon (DOC), but inconsistent and limited taxonomic resolution of previous studies have masked differences that may exist among orders, genera or species and are attributable to dietary and/or habitat differences. Here we investigate differences among the isotopic signatures of five zooplankton taxa ( Daphnia , Holopedium , large Calanoida, small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of DOC concentration.
2.  δ¹³C signatures of Daphnia , small calanoids and large calanoids became more depleted with increasing lake DOC, whereas Holopedium and cyclopoid δ¹³C became enriched with increasing DOC concentration.
3.  The variability of δ¹³C and δ¹⁵N isotopic signatures among zooplankton groups was reduced in high-DOC, compared to low-DOC lakes, especially for δ¹³C. Differences in δ¹³C and POM-corrected δ¹⁵N accounted for up to 33.7% and 19.5% of the variance, respectively, among lakes of varying DOC concentration.
4.  The narrow range of signatures found in higher DOC lakes suggests that different taxa have similar food sources and/or habitats. In contrast, the wide range of signatures in low-DOC lakes suggests that different taxa are exploiting different food sources and/or habitats. Together with the variable trends in zooplankton isotopic signatures along our DOC gradient, these results suggest that food web dynamics within the zooplankton community of temperate lakes will change as climate and lake DOC concentrations change.