John H. Gentile

University of Miami, Coral Gables, FL, United States

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Publications (26)29.92 Total impact

  • Mark A Harwell, John H Gentile
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    ABSTRACT: The Exxon Valdez oil spill occurred more than two decades ago, and the Prince William Sound ecosystem has essentially recovered. Nevertheless, discussion continues on whether or not localized effects persist on sea otters (Enhydra lutris) at northern Knight Island (NKI) and, if so, what are the associated attributable risks. A recent study estimated new rates of sea otter encounters with subsurface oil residues (SSOR) from the oil spill. We previously demonstrated that a potential pathway existed for exposures to polycyclic aromatic hydrocarbons (PAHs) and conducted a quantitative ecological risk assessment using an individual-based model that simulated this and other plausible exposure pathways. Here we quantitatively update the potential for this exposure pathway to constitute an ongoing risk to sea otters using the new estimates of SSOR encounters. Our conservative model predicted that the assimilated doses of PAHs to the 1-in-1000th most-exposed sea otters would remain 1-2 orders of magnitude below the chronic effects thresholds. We re-examine the baseline estimates, post-spill surveys, recovery status, and attributable risks for this subpopulation. We conclude that the new estimated frequencies of encountering SSOR do not constitute a plausible risk for sea otters at NKI and these sea otters have fully recovered from the oil spill.
    Human and Ecological Risk Assessment 06/2014; 20(4):889-916. · 1.29 Impact Factor
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    ABSTRACT: Ecosystem management requires understanding society's goals for an ecosystem and managing for some optimal solution. Unlike terrestrial ecosystem managers, coastal and marine ecosystem management seldom integrates across sectors or scientific disciplines to achieve desired social benefits. An Integrated Ecosystem Assessment (IEA) considers the ecosystem (including humans) as a unit and can assist in setting goals, determining an ecosystem's ability to support ecological processes and society's desires, and predicting the outcome of alternatives. The use of Coupled Ecological-Societal Systems Models utilised within the Integrated Assessment and Ecosystem Management Protocol (IAEMP) allows managers to extend a graphical picture of risk hypotheses to forecast scenarios that can be analysed relative to management goals. Scenarios predicted to meet management goals are evaluated against management constraints to select the "optimal" option for a management action in an adaptive management process. The IAEMP thus helps characterise potential causes of environmental problems, select appropriate response options, and implement and evaluate the selected option, thereby either addressing the concern or improving the ecosystem model for future decisions.
    Journal of Environmental Assessment Policy and Management 03/2013;
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    ABSTRACT: Harlequin Ducks (Histrionicus histrionicus) were adversely affected by the Exxon Valdez oil spill (EVOS) in Prince William Sound (PWS), Alaska, and some have suggested effects continue two decades later. We present an ecological risk assessment evaluating quantitatively whether PWS seaducks continue to be at-risk from polycyclic aromatic hydrocarbons (PAHs) in residual Exxon Valdez oil. Potential pathways for PAH exposures are identified for initially oiled and never-oiled reference sites. Some potential pathways are implausible (e.g., a seaduck excavating subsurface oil residues), whereas other pathways warrant quantification. We used data on PAH concentrations in PWS prey species, sediments, and seawater collected during 2001-2008 to develop a stochastic individual-based model projecting assimilated doses to seaducks. We simulated exposures to 500,000 individuals in each of eight age/gender classes, capturing the variability within a population of seaducks living in PWS. Doses to the maximum-exposed individuals are ∼400-4,000 times lower than chronic toxicity reference values established using USEPA protocols for seaducks. These exposures are so low that no individual-level effects are plausible, even within a simulated population that is orders-of-magnitude larger than exists in PWS. We conclude that toxicological risks to PWS seaducks from residual Exxon Valdez oil two decades later are essentially non-existent.
    Human and Ecological Risk Assessment 03/2012; 18(2):261-328. · 1.29 Impact Factor
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    Mark A Harwell, John H Gentile, Keith R Parker
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    ABSTRACT: Ecological risk assessments need to advance beyond evaluating risks to individuals that are largely based on toxicity studies conducted on a few species under laboratory conditions, to assessing population-level risks to the environment, including considerations of variability and uncertainty. Two individual-based models (IBMs), recently developed to assess current risks to sea otters and seaducks in Prince William Sound more than 2 decades after the Exxon Valdez oil spill (EVOS), are used to explore population-level risks. In each case, the models had previously shown that there were essentially no remaining risks to individuals from polycyclic aromatic hydrocarbons (PAHs) derived from the EVOS. New sensitivity analyses are reported here in which hypothetical environmental exposures to PAHs were heuristically increased until assimilated doses reached toxicity reference values (TRVs) derived at the no-observed-adverse-effects and lowest-observed-adverse-effects levels (NOAEL and LOAEL, respectively). For the sea otters, this was accomplished by artificially increasing the number of sea otter pits that would intersect remaining patches of subsurface oil residues by orders of magnitude over actual estimated rates. Similarly, in the seaduck assessment, the PAH concentrations in the constituents of diet, sediments, and seawater were increased in proportion to their relative contributions to the assimilated doses by orders of magnitude over measured environmental concentrations, to reach the NOAEL and LOAEL thresholds. The stochastic IBMs simulated millions of individuals. From these outputs, frequency distributions were derived of assimilated doses for populations of 500,000 sea otters or seaducks in each of 7 or 8 classes, respectively. Doses to several selected quantiles were analyzed, ranging from the 1-in-1000th most-exposed individuals (99.9% quantile) to the median-exposed individuals (50% quantile). The resulting families of quantile curves provide the basis for characterizing the environmental thresholds below which no population-level effects could be detected and above which population-level effects would be expected to become manifest. This approach provides risk managers an enhanced understanding of the risks to populations under various conditions and assumptions, whether under hypothetically increased exposure regimes, as demonstrated here, or in situations in which actual exposures are near toxic effects levels. This study shows that individual-based models are especially amenable and appropriate for conducting population-level risk assessments, and that they can readily be used to answer questions about the risks to individuals and populations across a variety of exposure conditions.
    Integrated Environmental Assessment and Management 01/2012; 8(3):503-22.
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    ABSTRACT: Prince William Sound (PWS) is a semi-enclosed fjord estuary on the coast of Alaska adjoining the northern Gulf of Alaska (GOA). PWS is highly productive and diverse, with primary productivity strongly coupled to nutrient dynamics driven by variability in the climate and oceanography of the GOA and North Pacific Ocean. The pelagic and nearshore primary productivity supports a complex and diverse trophic structure, including large populations of forage and large fish that support many species of marine birds and mammals. High intra-annual, inter-annual, and interdecadal variability in climatic and oceanographic processes as drives high variability in the biological populations. A risk-based conceptual ecosystem model (CEM) is presented describing the natural processes, anthropogenic drivers, and resultant stressors that affect PWS, including stressors caused by the Great Alaska Earthquake of 1964 and the Exxon Valdez oil spill of 1989. A trophodynamic model incorporating PWS valued ecosystem components is integrated into the CEM. By representing the relative strengths of driver/stressors/effects, the CEM graphically demonstrates the fundamental dynamics of the PWS ecosystem, the natural forces that control the ecological condition of the Sound, and the relative contribution of natural processes and human activities to the health of the ecosystem. The CEM illustrates the dominance of natural processes in shaping the structure and functioning of the GOA and PWS ecosystems.
    Human and Ecological Risk Assessment 01/2010; 16(4):672-726. · 1.29 Impact Factor
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    ABSTRACT: A comprehensive, quantitative risk assessment is presented of the toxicological risks from buried Exxon Valdez subsurface oil residues (SSOR) to a subpopulation of sea otters (Enhydra lutris) at Northern Knight Island (NKI) in Prince William Sound, Alaska, as it has been asserted that this subpopulation of sea otters may be experiencing adverse effects from the SSOR. The central questions in this study are: could the risk to NKI sea otters from exposure to polycyclic aromatic hydrocarbons (PAHs) in SSOR, as characterized in 2001-2003, result in individual health effects, and, if so, could that exposure cause subpopulation-level effects? We follow the U.S. Environmental Protection Agency (USEPA) risk paradigm by: (a) identifying potential routes of exposure to PAHs from SSOR; (b) developing a quantitative simulation model of exposures using the best available scientific information; (c) developing scenarios based on calculated probabilities of sea otter exposures to SSOR; (d) simulating exposures for 500,000 modeled sea otters and extracting the 99.9% quantile most highly exposed individuals; and (e) comparing projected exposures to chronic toxicity reference values. Results indicate that, even under conservative assumptions in the model, maximum-exposed sea otters would not receive a dose of PAHs sufficient to cause any health effects; consequently, no plausible toxicological risk exists from SSOR to the sea otter subpopulation at NKI.
    Human and Ecological Risk Assessment 01/2010; 16(4):727-761. · 1.29 Impact Factor
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    ABSTRACT: Ecological risk assessments evaluate the likelihood of adverse ecological effects caused by stressors related to human activities such as draining of wetlands or release of chemicals. The term stressor is used to describe any chemical, physical, or biological entity that can induce adverse effects on ecological components (i.e., individuals, populations, communities, or ecosystems). In this review article, a historical perspective on ecological risk assessment activities at the U.S. Environmental Protection Agency (EPA) is followed by a discussion of the EPA's “Framework Report,” which describes the basic elements for conducting an ecological risk assessment. The “Framework Report” is neither a procedural guide nor a regulatory requirement within the EPA. Rather, it is intended to foster a consistent approach to ecological risk assessments within the Agency, identify key issues, and define terminology.
    Environmental Toxicology and Chemistry 10/2009; 11(12):1663 - 1672. · 2.62 Impact Factor
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    Mark A Harwell, John H Gentile
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    ABSTRACT: An ecological significance framework is used to assess the ecological condition of Prince William Sound (PWS), Alaska, USA, in order to address the current management question: 17 y following the Exxon Valdez oil spill (EVOS), are there any remaining and continuing ecologically significant exposures or effects on the PWS ecosystem caused by EVOS? We examined the extensive scientific literature funded by the Exxon Valdez Trustees or by ExxonMobil to assess exposures and effects from EVOS. Criteria to assess ecological significance include whether a change in a valued ecosystem component (VEC) is sufficient to affect the structure, function, and/or health of the system and whether such a change exceeds natural variability. The EVOS occurred on 24 March 1989, releasing over 250,000 barrels of crude oil into PWS. Because PWS is highly dynamic, the residual oil was largely eliminated in the first few years, and now only widely dispersed, highly weathered, or isolated small pockets of residual contamination remain. Many other sources of polycyclic aromatic hydrocarbons (PAHs) exist in PWS from past or present human activities or natural oil seeps. Multiple-lines-of-evidence analyses indicate that residual PAHs from EVOS no longer represent an ecologically significant exposure risk to PWS. To assess the ecological significance of any residual effects from EVOS, we examined the literature on more than 20 VECs, including primary producers, filter feeders, fish and bird primary consumers, fish and bird top predators, a bird scavenger, mammalian primary consumers and top predators, biotic communities, ecosystem-level properties of trophodynamics and biogeochemical processes, and landscape-level properties of habitat mosaic and wilderness quality. None of these has any ecologically significant effects that are detectable at present, with the exception of 1 pod of orcas and possibly 1 subpopulation of sea otters; however, in both those cases, PWS-wide populations appear to have fully recovered. Many other stressors continue to affect PWS adversely, including climate and oceanographic variability, increased tourism and shipping, invasive species, the 1964 earthquake, and overexploitation of marine resources, with associated cascading effects on populations of PWS fish and predators. We conclude that the PWS ecosystem has now effectively recovered from EVOS.
    Integrated Environmental Assessment and Management 08/2006; 2(3):204-46.
  • J H Gentile
    Human &amp Experimental Toxicology 11/2001; 20(10):513-5; discussion 529-31. · 1.45 Impact Factor
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    ABSTRACT: The Everglades and South Florida ecosystems are the focus of national and international attention because of their current degraded and threatened state. Ecological risk assessment, sustainability, and ecosystem and adaptive management principles and processes are being used nationally as a decision and policy framework for a variety of types of ecological assessments. The intent of this study is to demonstrate the application of these paradigms and principles at a regional scale. The effects-directed assessment approach used in this study consists of a retrospective, eco-epidemiological phase to determine the causes for the current conditions and a prospective predictive risk-based assessment using scenario analysis to evaluate future options. Embedded in these assessment phases is a process that begins with the identification of goals and societal preferences which are used to develop an integrated suite of risk-based and policy relevant conceptual models. Conceptual models are used to illustrate the linkages among management (societal) actions, environmental stressors, and societal/ecological effects, and provide the basis for developing and testing causal hypotheses. These models, developed for a variety of landscape units and their drivers, stressors, and endpoints, are used to formulate hypotheses to explain the current conditions. They are also used as the basis for structuring management scenarios and analyses to project the temporal and spatial magnitude of risk reduction and system recovery. Within the context of recovery, the conceptual models are used in the initial development of performance criteria for those stressors that are determined to be most important in shaping the landscape, and to guide the use of numerical models used to develop quantitative performance criteria in the scenario analysis. The results will be discussed within an ecosystem and adaptive management framework that provides the foundation for decision making.
    Science of The Total Environment 08/2001; 274(1-3):231-53. · 3.26 Impact Factor
  • John H. Gentile
    Human and Ecological Risk Assessment 03/2001; 7(2):227-230. · 1.29 Impact Factor
  • John H. Gentile, Mark A. Harwell
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    ABSTRACT: Assessing and managing the ecological risks from multiple stressors is becoming increasingly important as our environmental and regulatory focus moves from managing point sources to one of managing and trading risks from multiples sources over large geographic areas. There are important corollaries to this shift in focus and scale, one is the increased role and importance of non-chemical stressors in shaping and controlling ecological systems, the fact that these categories of stress are, for the most part, not regulated under the traditional legislative mandates, that we have limited knowledge regarding the interaction of chemical and non-chemical stressors, that at large scales we are faced with the integration of and trading of risks to multiple resource categories. This trend of increased attention to regional-scale environmental issues requires the development of new analysis and interpretive strategies and highlight the need for a systematic framework for addressing the ecological effects of multiple stressors at regional ecological scales. We propose that such a framework have three essential properties: it be risk-based; it be effects driven; and it have the flexibility to be used in both a retrospective and prospective manner. While several approaches have been proposed we believe that an ecological risk assessment framework satisfies these criteria, has been used successfully in regional assessments, and can readily be modified and adapted to serve a wide variety of problem settings.
    Human and Ecological Risk Assessment 03/2001; 7(2):239-246. · 1.29 Impact Factor
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    ABSTRACT: A benthic index of estuarine condition was constructed for the Virginian Biogeographic Province (from Cape Cod, Massachusetts, to the mouth of Chesapeake Bay, Virginia) with data collected during summers of 1990 through 1993 by the US EPA’s Environmental Monitoring and Assessment Program (EMAP). Forty-eight metrics, based on attributes of the macrobenthos, were considered for the index, including measures of biodiversity, community condition, individual health, functional organization, and taxonomic composition. Salinity was correlated significantly with some of the metrics. Therefore, some metrics were normalized for salinity. The data used to develop the index (the calibration data) included equal numbers of reference and degraded sites, distributed equally across three salinity zones (<5, 5–18, >18‰). An independent set of data was used for validation. Linear discriminant analysis identified combinations of metrics that could best discriminate reference from degraded sites. The targets for correct classification were 90% of the sites for the calibration data and 80% for the validation data. Six combinations of metrics were identified. The final index was based on the ecological interpretation and relevance of the individual metrics and the ability to meet the calibration and validation targets. The final index consisted of three metrics: a positive contribution from salinity-normalized Gleason’s D (a biodiversity metric), and negative contributions from two taxonomic composition metrics, abundances of spionid polychaetes and of salinity-normalized tubificid oligochaetes. The index correctly classified 87% of reference and 90% of degraded sites in the calibration data and 88% of reference and 81% of degraded sites in the validation data. The index correctly classified sites over the full range of salinity (tidal-fresh to marine waters) and across grain sizes (silt–clay to sand).
    Ecological Indicators 01/2001; · 2.89 Impact Factor
  • John H. Gentile
    Human and Ecological Risk Assessment - HUM ECOL RISK ASSESSMENT. 01/2000; 6(2):223-226.
  • John H. Gentile, William H. van der Schalie
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    ABSTRACT: Hormesis is a widespread phenomenon across occurring many taxa and chemicals, and, at the single species level, issues regarding the application of hormesis to human health and ecological risk assessment are similar. However, interpreting the significance of hormesis for even a single species in an ecological risk assessment can be complicated by competition with other species, predation effects, etc. In addition, ecological risk assessments may involve communities of hundreds or thousands of species as well as a range of ecological processes. Applying hormetic adjustments to threshold effect levels for chemicals derived from sensitivity distributions for a large number of species is impractical. For ecological risks, chemical stressors are frequently of lessor concern than physical stressors (e.g., habitat alteration) or biological stressors (e.g., introduced species), but the relevance of hormesis to non‐chemical stressors is unclear. Although ecological theories such as the intermediate disturbance hypothesis offer some intriguing similarities between chemical hormesis and hormetic‐like responses resulting from physical disturbances, mechanistic explanations are lacking. While further exploration of the relevance of hormesis to ecological risk assessment is desirable, it is unlikely that hormesis is a critical factor in most ecological risk assessments, given the magnitude of other uncertainties inherent in the process.
    Human and Ecological Risk Assessment - HUM ECOL RISK ASSESSMENT. 01/2000; 6(2):227-236.
  • W H van der Schalie, J H Gentile
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    ABSTRACT: Hormesis is a widespread phenomenon across many taxa and chemicals, and, at the single species level, issues regarding the application of hormesis to human health and ecological risk assessment are similar. For example, convincing the public of a 'beneficial' effect of environmental chemicals may be problematic, and the design and analysis of laboratory studies may require modifications to detect hormesis. However, interpreting the significance of hormesis for even a single species in an ecological risk assessment can be complicated by considerations of competition with other species, predation effects, etc. Ecological risk assessments involve more than a single species; they may involve communities of hundreds or thousands of species as well as a range of ecological processes. Applying hormetic adjustments to threshold effect levels for chemicals derived from sensitivity distributions for a large number of species is impractical. For ecological risks, chemical stressors are frequently of lessor concern than physical stressors such as habitat alteration or biological stressors such as introduced species, but the relevance of hormesis to non-chemical stressors is unclear. Although ecological theories such as the intermediate disturbance hypothesis offer some intriguing similarities between chemical hormesis and hormetic-like responses resulting from physical disturbances, mechanistic explanations are lacking. Further exploration of the relevance of hormesis to ecological risk assessment is desirable. Aspects deserving additional attention include developing a better understanding of the hormetic effects of chemical mixtures, the relevance of hormesis to physical and biological stressors and the development of criteria for determining when hormesis is likely to be relevant to ecological risk assessments.
    Journal of Applied Toxicology 01/2000; 20(2):131-9. · 2.60 Impact Factor
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    ABSTRACT: The Everglades and associated coastal ecosystems of South Florida are unique and highly valued ecosystems. One of the world's largest water management systems has been developed in South Florida over the past 50 years to provide flood control, urban and agricultural water supply, and drainage of land for development. However, this system has inadvertently caused extensive degradation of the South Florida environment, resulting in the loss of more than half the historical Everglades system and elimination of whole classes of ecosystems. The U.S. Man and the Biosphere Program (US MAB) instituted a project to develop ecosystem management principles and identify requirements for ecological sustainability of South Florida. A strategic process developed by the US MAB Project illustrates how ecosystem management and ecological risk assessment principles apply to South Florida, including the development of societal goals and objectives of desired sustainable ecological condition, translation of these goals/objectives into scientifically meaningful ecological endpoints, creation of a regional plan designed to meet the sustainability goals, and development of a framework for evaluating how well the plan will achieve ecological sustainability of South Florida. An extensive federal, state, and tribal interagency process is underway to develop a restoration plan for restructuring the regional management system, essentially following the elements in the US MAB project process. The Florida Governor's Commission was established as an institution to reflect societal values and define regional sustainability goals. The U.S. Army Corps of Engineers is developing a science-based plan for Congressional approval to restructure the water management system to achieve the societal goals. Thus, South Florida may become the prototype example of successful regional-scale ecosystem management.
    Urban Ecosystems 09/1999; 3(3):201-222. · 1.74 Impact Factor
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    ABSTRACT: The measure of whether a management scenario is capable of establishing regional-scale ecosystem sustainability is the degree to which it recovers the historical characteristics of the regional landscape mosaic. This study examines the ability of alternate management scenarios to recover the defining ecological features of the Everglades and South Florida landscape. Five conceptual scenarios are evaluated for recovering and sustaining the ecological characteristics of the wetland systems in South Florida. First, the regional-scale physical characteristics are identified that created and supported the major organizing and driving forces in the predrainage Everglades and Big Cypress basins. Eight hypotheses are proposed to explain how human-caused modifications to these defining characteristics have been responsible for the substantial level of ecological deterioration that has been documented in South Florida wetlands during the last century. The restoration scenarios are evaluated on their proposed ability to correct the physical and biological problems identified by the hypotheses. Our assessment of the five scenarios shows that all would improve the problems addressed by the eight hypotheses, as all could more effectively move increased volumes of water across broader expanses of contiguous wetlands than do existing management programs. This would result in longer hydroperiods over larger areas, reflecting historical patterns. Two of the scenarios would be successful in increasing flows into Florida Bay and the Gulf coast estuaries because removing internal structures increases the spatial extent of the upstream areas that could be devoted to natural hydropatterns.The benefits of eastern boundary buffer zones include improved flow into the Taylor Slough basin. Using Lake Okeechobee as a site for increased water storage, followed by the addition of eastern buffer zones and portions of the Everglades Agricultural Area, would produce increased flexibility in providing the storage capacity required to meet sustainability goals. Scenarios with maximum areas of buffer not only are more successful in reducing groundwater seepage losses to the east but also are more likely to reduce the level of nutrients and other contaminants entering the natural wetlands.
    Urban Ecosystems 09/1999; 3(3):279-303. · 1.74 Impact Factor
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    ABSTRACT: ecosystems
    BioScience. 01/1999; 49:543-556.
  • John H. Gentile, Mark A. Harwell
    Human and Ecological Risk Assessment 08/1998; 4(4):815-828. · 1.29 Impact Factor