Steven M. Bartell

Consumer Regulation of Nutrient Cycling

Nutrient-cycling rates in ecosystems may be altered by translocation and transformation that result from the consumption process and/or the behavior of consumer organisms. Translocation by mobile consumers redistributes nutrients within and between subunits of the ecosystem. Migration of zooplankton and fishes carries nutrients across water-mass boundaries or retards net flux by sedimentation. Bioturbation of sediments and mixing of soil horizons by invertebrates similarly redistribute nutrients across system strata, facilitating utilization by primary producers. Transformation of particle size distribution alters rates of nutrient cycling in proportion to changes in surface/volume relationships. Through selective predation and comminution, consumers regulate rates of nutrient cycling. Nutrient pools comprised of large biomass units (e.g., tree boles, soil litter, lake zooplankton) may be transformed to smaller units, resulting in accelerated nutrient-cycling processes. Small units may be aggregated, for example, as macroarthropod feces or fish biomass, resulting in conservation of nutrients.

Evaluating the constraints of temperature, activity and consumption on growth of largemouth bass

SynopsisWe present a bioenergetics model for largemouth bass (Micropterus salmoides) which simulates growth as a function of body size, temperature, activity and consumption level. We apply the model to investigate seasonal changes in condition factor exhibited by bass in Par Pond, South Carolina, a reservoir receiving heated effluent. Previous authors have suggested that these changes occur due to bioenergetic constraints, primarily the effects of heated effluent on metabolic rate. Model simulations were used to evaluate the hypotheses that seasonal changes in condition factor were caused by the heated effluent, seasonally variable activity, seasonally variable consumption, or reproductive costs.Results indicate that temperature is not directly responsible for the seasonal changes in condition factor. Bass moderate the influence of the heated effluent via behavioral thermoregulation. Activity is not a major factor, and spawning weight-loss can account for only a small portion of the observed variation. However, the pattern of seasonal changes in body condition may be adequately explained by seasonal variations in consumption. The patterns of consumption rate and/or prey availability suggested by model simulations represent testable hypotheses.

Effects of nutrient recycling and food-chain length on resilience

Food-web resilience, or the rate at which a food web returns to steady state following a perturbation, was investigated using both a simple abstract food-chain model and a complex food-web model for an ecosystem in which nutrient limitation and recycling occur. The food-chain model demonstrated that both nutrient input, or loading, and food-chain length have important effects on resilience. For very small nutrient inputs, resilience (or the inverse of the return time to steady state TR) was in all cases an increasing function of nutrient-input rate. However, the resilience of food chains in which the autotroph was the highest level was lower and constant over a wider range of input levels than the case in which herbivores were also present. The presence of a carnivore level complicated the dependence of resilience on nutrient input, leading to cases in which increased input could actually decrease resilience. All of these behaviors are consistent with the generalization that the return time, TR, tends to be similar to the turnover time of nutrient in the system. The top-down effects in the food chain influence how the nutrient turnover time, and thus the return time, TR, respond to changes in nutrient input. To test the robustness of the results of the abstract model, similar investigations were made of a detailed aquatic food-web model, the Comprehensive Aquatic-Simulation Model (CASM). The results of the study of resilience of CASM were in relative agreement with the abstract model for systems in which autotrophs and herbivores, respectively, constituted the highest trophic levels. The situation was more complex when carnivores were added. The results of both models strongly suggest that resilience may not always increase as nutrient input is increased and that resilience may not always decrease as the number of levels in a food chain increases.

Scale in the Design and Interpretation of Aquatic Community Research

The scales employed in investigations of aquatic ecosystems can strongly influence interpretations of community patterns and processes. Some examples are obvious; in contrasting cladocerans and rotifers, assessments of biomass (an instantaneous time scale) provide strikingly different impressions than assessments of production (a broader time scale) (Makarewicz and Likens 1975). Other examples are more subtle but equally important; seasonal distributions of phytoplankton species over several years may indicate patterns with little predictability but distributions of functional groups of phytoplankton can indicate a periodic behavior (Bartell et al. 1978; Reynolds 1984). Explicit considerations of the scales that an observer uses in an investigation are fundamental to an understanding of aquatic systems.

An ecosystem model for assessing ecological risks in Québec rivers, lakes, and reservoirs

Abstract The comprehensive aquatic systems model (CASM) was adapted for estimating ecological risks posed by toxic chemicals in rivers, lakes, and reservoirs in Quebec, Canada. Populations of aquatic plants, invertebrates, and fish characteristic of these aquatic ecosystems were identified and generic food webs were constructed. Bioenergetics parameters that determine the growth dynamics of these populations were derived from published values for these same or similar species. Input values of light, water temperature, concentrations of dissolved nitrogen (N), phosphorus (P), and silica (Si) were constructed from available regional data or data from similar Canadian systems at similar latitudes. The model provides the capability to estimate the probability of changes in the biomass of multiple populations of primary producers and consumers as a function of the concentration of dissolved chemical contaminant. The CASM permits the evaluation of direct toxic effects, as well as indirect toxic effects that result from changes in competitive or predator–prey relations in complex aquatic food webs. Hypothetical risk assessments were constructed for pentachlorophenol, copper, mercury, and diquat dibromide in generalized rivers, lakes, and reservoirs in Quebec. Numerical sensitivity and uncertainty analyses were used to describe the relative contributions of direct and indirect toxic effects on overall ecological risks estimated for functional guilds of producers and consumers in these ecosystems. This aquatic ecosystem model may become one component in a decision support system for assessing ecological risks.

Modeling submersed macrophyte growth in relation to underwater light climate: modeling approaches and application potential

The underwater light climate is one of the most important determinants of submersed aquatic vegetation. Because of the recent, large-scale, declines in aquatic vegetation, largely attributed to deterioration of the underwater light climate, interest in tools to predict the wax and wane of aquatic macrophyte populations has greatly increased. This paper summarizes two modeling approaches that can be applied to assess impacts of changes in underwater light climate on submersed vegetation. The first, stand-alone, model type focuses on metabolism and biomass formation of submersed freshwater macrophytes with difference in phenologies. This type is illustrated by examples from various sites using models developed for the freshwater macrophytes Hydrilla verticillata (L.f.) Royle (HYDRIL) and Myriophyllum spicatum L. (MILFO), and also by an example ecological risk assessment. The models (HYDRIL and MILFO) track carbon flow through the vegetation in meter-squared (m2) water columns. The models include descriptions of various factors that affect biomass dynamics, such as site-characteristic changes in climate, latitude, light attenuation within the water column, carbon assimilation rate at light saturation, temperature, wintering strategies, grazing and mechanical control (removal of shoot biomass). Simulated biomass, net assimilation and maintenance respiration over a relatively short (1–5 year) period agree well with measured values. The models are, therefore, believed to be suitable for predicting plant community production, growth and survival characteristics over relatively short periods over a large range of sites. The feasibility of using a macrophyte growth model of the HYDRIL type for ecological risk assessment is demonstrated. It is used to evaluate the consequences of management changes in large rivers for the survival of submersed vegetation. The current assessment evaluates the potential impact of increased commercial navigation traffic on the growth of Potamogeton pectinatus L. in Pool 4 of the Upper Mississippi River, U.S.A. In this case, navigational traffic scenarios were translated into suspended solids concentrations and underwater light climate, with the latter being used as inputs into the aquatic plant growth model. Model results demonstrate that the scenario increases in commercial traffic cause minimal decreases in growth and vegetative reproduction. Results indicate that this growth model can be a useful tool in ecological risk assessment, since the required stress-response relationships could be established. The second, integrated, model type focuses on the role of seagrass and other primary producers in estuarine littoral zone material cycling (carbon and nitrogen) at the Goodwin Islands, Virginia, U.S.A. The latter model was used to explore the effects of changes

Ecological modeling in risk assessment : chemical effects on populations, ecosystems, and landscapes

Preface, S.E. Jorgensen and R.A. Pastorok Introduction, R.A. Pastorok Methods, R.A. Pastorok and H.R. Akcakaya Results of the Evaluation of Ecological Models: Introduction, R.A. Pastorok Population Models-Scalar Abundance, S. Ferson Population Models-Life History, S. Carroll Population Models-Individual-Based, H.M. Regan Population Models-Metapopulations, H.R. Akcakaya and H. M. Regan Ecosystem Models-Food Webs, S. Carroll Ecosystem Models-Aquatic, S.M. Bartell Ecosystem Models-Terrestrial, C.E. Mackay and R.A. Pastorok Landscape Models-Aquatic and Terrestrial, C.E. Mackay and R.A. Pastorok Toxicity-Extrapolation Models, J.A. Colton Profiles of Selected Models, R.A. Pastorok Enhancing the Use of Ecological Models in Environmental Decision-Making, L.R. Ginzburg and H. R. Akcakaya Conclusions and Recommendations, R.A. Pastorok and L.R. Ginzburg Summary, R.A. Pastorok and H.R. Akcakaya References Appendix A: Fish Population Modeling: Data Needs and Case Study, S.J. Pauwels Appendix B: Classification Systems, K.V. Root Appendix C: Results of the Initial Screening of Ecological Models, Model Analysis Team

Application of an ecosystem model for aquatic ecological risk assessment of chemicals for a Japanese lake

The Lake Suwa version of the comprehensive aquatic systems model (CASM-SUWA) was developed using field data from Lake Suwa and evaluated to examine the utility of CASM-SUWA for assessing the ecological risk of chemicals for aquatic ecosystems. The calibration of the parameters for the model provided that the established reference model simulation could reproduce complex seasonal biomass behavior of populations that were not significantly different from the general seasonal pattern for the Lake Suwa ecosystem. The sensitivity analyses revealed the potential importance of indirect effects and demonstrated that the parameter values of all the trophic levels were important in determining the biomass of each trophic level in the model. The risk estimation of linear alkylbenzene sulfonates (LAS) demonstrated that the model estimated the risks of direct toxic effects on each population and the indirect ecological effects that propagate through the food-web in the model ecosystem. The CASM-SUWA-derived benchmark levels were approximately one order of magnitude less than the field-derived NOECs in literature. The analyses of the comparison implied that the model could provide a good basis in determining an ecological protective level of a chemical of concern in aquatic ecosystem. This modeling study demonstrated that the model can be used to provide additional information for the decision-making process in the management of the aquatic ecological risk of chemicals.

Prioritizing contaminants of emerging concern for ecological screening assessments

More than 40,000 organic chemicals have been identified as contaminants of emerging concern (CECs). Compared to population numbers and national debts, this may not initially appear to be a staggering number; yet, when considering rapid and often unbridled advances in technology, manufacturing, and agricultural practices worldwide—all of which use and then discard waste into the environment—the number takes on more meaning. Even exhaled human breath includes a few hundred volatile organic compounds. Indeed, thousands of organic chemicals are produced or imported annually into the U.S. and other industrialized nations. Furthermore, 40,000 is a conservative estimate that does not account for associated break-down products in the environment.

Role of Ecological Modeling in Risk Assessment

Ecological models are useful tools for evaluating the ecological significance of observed or predicted effects of toxic chemicals on individual organisms. Current risk estimation approaches using hazard quotients for individual-level endpoints have limited utility for assessing risks at the population, ecosystem, and landscape levels, which are the most relevant indicators for environmental management. In this paper, we define different types of ecological models, summarize their input and output variables, and present examples of the role of some recommended models in chemical risk assessments. A variety of population and ecosystem models have been applied successfully to evaluate ecological risks, including population viability of endangered species, habitat fragmentation, and toxic chemical issues. In particular, population models are widely available, and their value in predicting dynamics of natural populations has been demonstrated. Although data are often limited on vital rates and doseresponse functions needed for ecological modeling, accurate prediction of ecological effects may not be needed for all assessments. Often, a comparative assessment of risk (e.g., relative to baseline or reference) is of primary interest. Ecological modeling is currently a valuable approach for addressing many chemical risk assessment issues, including screening-level evaluations.