Joseph S. Meyer

Estimating Uncertainty in Population Growth Rates: Jackknife vs. Bootstrap Techniques

Although per capita rates of increase (r) have been calculated by population biologists for decades, the inability to estimate uncertainty (variance) associated with r values has until recently precluded statistical comparisons of population growth rates. In this study, we used two computer- intensive techniques, Jackknifing and Bootstrapping, to estimate bias, standard errors, and sampling distributions of r for real and hypothetical populations of cladocerans. Results generated using the two techniques, using data on laboratory cohorts of Daphnia pulex, were almost identical, as were results for a hypothetical D. pulex population whose sampling distribution was approximately normal. However, for another hypothetical population whose sampling distribution was negatively skewed due to high juvenile mortality, Bootstrap and full-sample estimates of r were negatively biased by 3.3 and 1.8%, respectively. A bias adjustment reduced the bias in the Bootstrap estimate and produced estimates of r and SE(r) almost identical to those ofthe Jackknife technique. In general, our simulations show that the Jackknife will provide more cost-effective point and interval estimates of r for cladoceran populations, except when juvenile mortality is high (at least >25%). Coefficients of variation in the mean of r within laboratory cohorts of D. pulex were one-half to one-third the magnitude of the corresponding coefficients of variation in the mean of total reproduction and in the mean day to death (range of values of cv(r) = 1.6 to 3.8%). This suggests that extremes in reproductive output and survival of individuals tend to be dampened at the population level, and that within-cohort variability in r is not explosive. Moreover, between-cohort variability in r can be much greater than within-cohort variability, as indicated by a statistically significant difference of 30% (P <c; .01) between the high and low r values that were computed for four cohorts of D. pulex born during a 1 -mo period from the same laboratory stock population. Based on variability in per capita rates of increase that have been estimated for several cladoceran species, we suggest that the precision for reporting r values should in most cases be limited to two significant figures.

Bioavailability and toxicity of dietborne copper and zinc to fish.

To date, most researchers have used dietborne metal concentrations rather than daily doses to define metal exposure and this has resulted in contradictory data within and between fish species. It has also resulted in the impression that high concentrations of dietborne Cu and Zn (e.g. > 900 mg kg(-1) dry diet) are relatively non-toxic to fish. We re-analyzed existing data using rations and dietborne metal concentrations and used daily dose, species and life stage to define the toxicity of dietborne Cu and Zn to fish. Partly because of insufficient information we were unable to find consistent relationships between metal toxicity in laboratory-prepared diets and any other factor including, supplemented metal compound (e.g. CuSO(4) or CuCl(2)), duration of metal exposure, diet type (i.e. practical, purified or live diets), or water quality (flow rates, temperature, hardness, pH, alkalinity). For laboratory-prepared diets, dietborne Cu toxicity occurred at daily doses of > 1 mg kg(-1) body weight d(-1) for channel catfish (Ictalurus punctatus), 1-15 mg kg(-1) body weight d(-1) (depending on life stage) for Atlantic salmon (Salmo salar) and 35-45 mg kg(-1) body weight d(-1) for rainbow trout (Oncorhynchus mykiss). We found that dietborne Zn toxicity has not yet been demonstrated in rainbow trout or turbot (Scophthalmus maximus) probably because these species have been exposed to relatively low doses of metal ( < 90 mg kg(-1) body weight d(-1)) and effects on growth and reproduction have not been analyzed. However, daily doses of 9-12 mg Zn kg(-1) body weight d(-1) in laboratory-prepared diets were toxic to three other species, carp Cyprinus carpio, Nile tilapia Oreochromis niloticus, and guppy Poecilia reticulata. Limited research indicates that biological incorporation of Cu or Zn into a natural diet can either increase or decrease metal bioavailability, and the relationship between bioavailability and toxicity remains unclear. We have resolved the contradictory data surrounding the effect of organic chelation on metal bioavailability. Increased bioavailability of dietborne Cu and Zn is detectable when the metal is both organically chelated and provided in very low daily doses. We have summarized the information available on the effect of phosphates, phytate and calcium on dietborne Zn bioavailability. We also explored a rationale to understand the relative importance of exposure to waterborne or dietborne Cu and Zn with a view to finding an approach useful to regulatory agencies. Contrary to popular belief, the relative efficiency of Cu uptake from water and diet is very similar when daily doses are compared rather than Cu concentrations in each media. The ratio of dietborne dose:waterborne dose is a good discriminator of the relative importance of exposure to dietborne or waterborne Zn. We discuss gaps in existing data, suggest improvements for experimental design, and indicate directions for future research.

Utility of Tissue Residues for Predicting Effects of Metals on Aquatic Organisms

As part of a SETAC Pellston Workshop, we evaluated the potential use of metal tissue residues for predicting effects in aquatic organisms. This evaluation included consideration of different conceptual models and then development of several case studies on how tissue residues might be applied for metals, assessing the strengths and weaknesses of these different approaches. We further developed a new conceptual model in which metal tissue concentrations from metal-accumulating organisms (principally invertebrates) that are relatively insensitive to metal toxicity could be used as predictors of effects in metal-sensitive taxa that typically do not accumulate metals to a significant degree. Overall, we conclude that the use of tissue residue assessment for metals other than organometals has not led to the development of a generalized approach as in the case of organic substances. Species-specific and site-specific approaches have been developed for one or more metals (e.g., Ni). The use of gill tissue residues within the biotic ligand model is another successful application. Aquatic organisms contain a diverse array of homeostatic mechanisms that are both metal- and species-specific. As a result, use of whole-body measurements (and often specific organs) for metals does not lead to a defensible position regarding risk to the organism. Rather, we suggest that in the short term, with sufficient validation, species- and site-specific approaches for metals can be developed. In the longer term it may be possible to use metal-accumulating species to predict toxicity to metal-sensitive species with appropriate field validation.

Relationship between copper exposure duration, tissue copper concentration, and rainbow trout growth

Abstract Rainbow trout fry were exposed in soft water to sublethal concentrations of copper for 60 days under controlled laboratory conditions. At 20-day intervals, fish were sampled for weight, length, and whole-body copper and metallothionein concentrations. Exposures to waterborne copper concentrations as low as 4.6 μg 1−1 resulted in significantly reduced growth and significantly elevated whole-body copper concentrations after 20 days. Whole-body metallothionein concentrations did not differ significantly from controls. Fish did not recover or return to control growth rates throughout the entire exposure period; a 45% reduction in mean weight relative to controls observed on day 40 in the 9.0 μg 1−1 Cu exposure was sustained through day 60. Whole-body accumulation rates of copper in fish exposed to 4.6 μg 1−1 and higher levels of Cu increased significantly between 0 and 40 days and appeared to reach steady-state after 40 days. Copper accumulation was found to depend on dose and time. Trout exposed to higher copper concentrations accumulated more whole-body copper, with longer times to reach steady-state. Our data suggest that both accumulation capacity and copper depuration rates from a slowly exchangeable pool are concentraion-dependent. A linear model was developed for the relationship between exposure duration, copper accumulation, and fish weight: In (wet wt., μg) = 4.8 + 0.03 (exposure duration, days)—0.04 (whole-body copper, mg g−1 dry wt.) (P

Structural changes in gills of Lost River suckers exposed to elevated pH and ammonia concentrations

The Lost River sucker (Deltistes luxatus) is a federally listed, endangered fish that occurs primarily in Upper Klamath Lake-a hypereutrophic lake in southern Oregon, USA. A decline of the sucker population in the lake over the past few decades has been partly attributed to adverse water quality conditions, including elevated pH and ammonia concentrations that occur during summer cyanobacterial blooms. We quantitatively analyzed structural changes in gills of larval Lost River suckers after they were exposed to elevated pH and ammonia concentrations for 30 d. Exposure to pH as high as 10 caused no observed structural changes. However, lamellar thickness and O(2) diffusion distance increased significantly (P<0.05) at ammonia concentrations that did not significantly decrease survival, growth, whole-body ion concentrations, or swimming performance. Additionally, we qualitatively observed increases in the frequency of hyperplasic and hypertrophic mucous cells, tissue damage, epithelial lifting, and infiltration of white blood cells into paracellular lymphatic spaces at the highest sublethal ammonia concentration. These observed gill changes typically indicate compromised respiratory and ionoregulatory capacity, although such effects were not manifested in the assays we performed. Regardless, these structural gill changes appear to be a more sensitive indicator of exposure to elevated ammonia concentrations than are more traditional sublethal indices. Therefore, gill histopathology might be a relevant early-warning monitoring tool of the health of Lost River suckers in Upper Klamath Lake, and other species in similar eutrophic systems.

Toxicity of cobalt and copper to rainbow trout : application of a mechanistic model for predicting survival

Abstract We conducted 14-day laboratory toxicity tests with rainbow trout to evaluate, (1) the acute toxicity of Co and Cu, and (2) the effects of selected Co concentrations on the toxicity of Cu in Co/Cu mixtures. A one-compartment, uptake-depuration model was used to estimate incipient lethal levels (ILLs) and quantify differences in lethality responses among the Co-only, Cu-only and Co/Cu exposures. Additionally, we compared mortality percentages observed in Co/Cu mixtures to those predicted from two models of joint toxicity. Co was a slower-acting and less potent toxicant than was Cu. For Co, the ILL for 50% mortality was 346 μg l −1 ; for Cu, the ILL for 50% mortality was 14 μg l −1 . Moreover, in Co/Cu mixtures, Co acted as an antagonist during the first 48–96 h, but later acted as an additive or slightly synergistic toxicant—making it difficult to predict short-term mortality of fish in Co/Cu mixtures. Lethality thresholds for Cu were reduced by 11–26% and 37–45%, respectively, in the presence of 50 and 250 μg l −1 Co, although those differences were not statistically significant. The ordering of ILLs for the metal exposures tested was: 250 μg l −1 Co+Cu>50 μg l −1 Co+Cu>Cu-only>Co-only.

Acute and embryo-larval toxicity of phenolic compounds to aquatic biota

Because of the prevalence of phenolic compounds in various types of effluents, both acute and embryo-larval bioassays were performed on eight phenolic compounds with rainbow trout, fathead minnows andDaphnia pulicaria. In flow-through bioassays, the 96-hr LC50 values for rainbow trout and fathead minnows ranged from <0.1 mg/L for hydroquinone to >100 mg/L for resorcinol.Daphnia pulicaria was consistently the least sensitive species tested as measured in 48-hr bioassays, while fathead minnows and rainbow trout varied in their relative sensitivity to phenolics as measured in 96-hr tests. Fathead minnows were more sensitive to phenol at 25°C than at 14°C.In embryo-larval bioassays with phenol, fathead minnow growth was significantly reduced by 2.5 mg/L phenol, while rainbow trout growth was significantly reduced by 0.20 mg/L phenol. For both species the embryolarval effects concentration was 1.1% of the 96-hr LC50. Another embryolarval bioassay was attempted withp-benzoquinone, a highly toxic phenolic compound found in fossil fuel processing wastewaters, which was discontinued because the compound was rapidly degraded chemically or biologically in the headtank and aquaria.

Photooxidation of wetland and riverine dissolved organic matter: altered copper complexation and organic composition

In natural waters, the uptake of transition metals such as copper (Cu) by aquatic biota depends on the activity of the free cupric ion ({Cu2+}) rather than on total Cu concentration. Thus, an important ecological function of dissolved organic matter (DOM) in aquatic ecosystems is Cu–DOM complexation, which greatly decreases the {Cu2+}. However, Cu bioavailability is greatly modified by source and environmental history of DOM because DOM affinity for Cu varies by orders of magnitude among DOM sources; moreover, DOM is photochemically unstable. During 72-h irradiation experiments at intensities approximating sunlight with DOM from a palustrine wetland and a third-order river, we investigated photooxidative effects on DOM complexation of Cu as well as spectral and chemical changes in DOM that might explain altered Cu complexation. Irradiation decreased Cu complexation by riverine DOM, but unexpectedly increased Cu complexation by wetland DOM, resulting in 150% greater {Cu2+} in riverine DOM at the same dissolved organic carbon concentrations. The specific ultraviolet absorption (SUVa) and humic substances tracked photochemical changes in the conditional stability constants of Cu–DOM complexes, suggesting that the aromaticity of DOM influences its affinity for Cu. Carbonyl concentration in 13C nuclear magnetic resonance spectra (13C-NMR) covaried directly with Cu binding-site densities in DOM. However, no aspect of Cu–DOM complexation consistently covaried with fluorophores (i.e., the fluorescence index) or low molecular weight organic acids. Our results suggest that global increases in UV radiation will affect Cu–DOM complexation and subsequent Cu toxicity depending on light regime as well as DOM source.

Metal Mixtures Modeling Evaluation project: 1. Background

Despite more than 5 decades of aquatic toxicity tests conducted with metal mixtures, there is still a need to understand how metals interact in mixtures and to predict their toxicity more accurately than what is currently done. The present study provides a background for understanding the terminology, regulatory framework, qualitative and quantitative concepts, experimental approaches, and visualization and data-analysis methods for chemical mixtures, with an emphasis on bioavailability and metal-metal interactions in mixtures of waterborne metals. In addition, a Monte Carlo-type randomization statistical approach to test for nonadditive toxicity is presented, and an example with a binary-metal toxicity data set demonstrates the challenge involved in inferring statistically significant nonadditive toxicity. This background sets the stage for the toxicity results, data analyses, and bioavailability models related to metal mixtures that are described in the remaining articles in this special section from the Metal Mixture Modeling Evaluation project and workshop. It is concluded that although qualitative terminology such as additive and nonadditive toxicity can be useful to convey general concepts, failure to expand beyond that limited perspective could impede progress in understanding and predicting metal mixture toxicity. Instead of focusing on whether a given metal mixture causes additive or nonadditive toxicity, effort should be directed to develop models that can accurately predict the toxicity of metal mixtures.

Metal Mixture Modeling Evaluation project: 2. Comparison of four modeling approaches

As part of the Metal Mixture Modeling Evaluation (MMME) project, models were developed by the National Institute of Advanced Industrial Science and Technology (Japan), the US Geological Survey (USA), HDR|HydroQual (USA), and the Centre for Ecology and Hydrology (United Kingdom) to address the effects of metal mixtures on biological responses of aquatic organisms. A comparison of the 4 models, as they were presented at the MMME workshop in Brussels, Belgium (May 2012), is provided in the present study. Overall, the models were found to be similar in structure (free ion activities computed by the Windermere humic aqueous model [WHAM]; specific or nonspecific binding of metals/cations in or on the organism; specification of metal potency factors or toxicity response functions to relate metal accumulation to biological response). Major differences in modeling approaches are attributed to various modeling assumptions (e.g., single vs multiple types of binding sites on the organism) and specific calibration strategies that affected the selection of model parameters. The models provided a reasonable description of additive (or nearly additive) toxicity for a number of individual toxicity test results. Less-than-additive toxicity was more difficult to describe with the available models. Because of limitations in the available datasets and the strong interrelationships among the model parameters (binding constants, potency factors, toxicity response parameters), further evaluation of specific model assumptions and calibration strategies is needed.