Lester L Yuan

Lester L Yuan
United States Environmental Protection Agency | US EPA

Doctor of Engineering

About

54
Publications
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2,381
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Publications

Publications (54)
Article
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Accurately estimating the elemental stoichiometry of phytoplankton is critical for understanding biogeochemical cycles. In laboratory experiments, stoichiometric ratios vary among species and with changes in environmental conditions. Field observations of total phosphorus (P) and total nitrogen (N) collected at regional and national scales can supp...
Article
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Interest in developing periphytic diatom and bacterial indicators of nutrient effects continues to grow in support of the assessment and management of stream ecosystems and their watersheds. However, temporal variability could confound relationships between indicators and nutrients, subsequently affecting assessment outcomes. To document how tempor...
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Observational data are frequently used to better understand the effects of changes in P and N on stream biota, but nutrient gradients in streams are usually associated with gradients in other environmental factors, a phenomenon that complicates efforts to accurately estimate the effects of nutrients. Here, we propose a new approach for analyzing ob...
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River managers strive to use the best available science to sustain biodiversity and ecosystem function. To achieve this goal requires consideration of processes at different scales. Metacommunity theory describes how multiple species from different communities potentially interact with local‐scale environmental drivers to influence population dynam...
Article
Submerged macrophytes are key components of shallow lake biological communities, and their presence has been associated with a predominantly clear-water state. Conversely, lakes lacking macrophytes are often turbid with elevated phytoplankton abundance. One main mechanism that influences the presence or absence of submerged macrophytes is turbidity...
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The empirical relationship between total phosphorus and chlorophyll has guided lake management decisions for decades, but imprecision in this relationship in individual lakes limits the utility of these models. Many environmental factors that potentially affect the total phosphorus‐chlorophyll relationship have been studied, but here we hypothesize...
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Eutrophication increases hypoxia in lakes and reservoirs, causing deleterious effects on biological communities. Quantitative models would help managers develop effective strategies to address hypoxia issues, but most existing models are limited in their applicability to lakes with temporally resolved dissolved oxygen data. We describe a hierarchic...
Article
Data collected from lakes at national (regional) scales and state (local) scales can provide different insights regarding relationships between environmental factors and biological responses, and combining these two types of data can potentially yield more precise and accurate understanding of ecological phenomena. National data can include many me...
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The elemental composition of seston provides insights into how lake food webs function and how nutrients cycle through the environment. Here, we describe a Bayesian network model that simultaneously estimates relationships between dissolved and particulate nutrients, suspended volatile and nonvolatile sediments, and algal chlorophyll. The model pro...
Article
The relationship between zooplankton biomass and phytoplankton biomass can provide insight into the structure and function of lake biological communities. We used a Bayesian network model to analyze a continental‐scale data set to estimate changes in the relationship between zooplankton (Z) and phytoplankton (P) biomasses along a eutrophication gra...
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Quantitative models that predict cyanotoxin concentrations in lakes and reservoirs from nutrient concentrations would facilitate management of these resources for recreation and as sources of drinking water. Development of these models from field data has been hampered by the high proportion of samples in which cyanotoxin concentrations are below d...
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Flow alteration is widespread in streams, but current understanding of the effects of differences in flow characteristics on stream biological communities is incomplete. We tested hypotheses about the effect of variation in hydrology on stream communities by using generalized additive models to relate watershed information to the values of differen...
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"Knowledge of current and historical conditions is needed to guide conservation and restoration policies, but such information is lacking for most taxa. This problem is especially severe for small, inconspicuous taxa, such as the thousands of invertebrate species that inhabit stream and other freshwater ecosystems. We describe a novel application o...
Article
High densities of cyanobacteria can interfere with the use of lakes and reservoirs for recreation and as sources for drinking water, and one approach for reducing the amount of cyanobacteria is to reduce nutrient concentrations in the waterbody.An approach is described for deriving numeric targets for concentrations of total phosphorus (TP) and tot...
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Excess nutrient loading increases algal abundance which can cause hypoxia in many lakes and reservoirs. We used a divisive partitioning approach to analyze dissolved oxygen profile data collected across the continental United States to increase the precision of estimated relationships between chlorophyll a (chl a) concentrations and the extent of h...
Article
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Accurate and precise estimates of relationships between stressors and environmental responses can inform management decisions most usefully when models can be easily interpreted. Here, we describe an approach for classifying lakes and reservoirs that can improve estimates of the relationships between total P (TP) and chlorophyll a (chl a) concentra...
Article
The occurrence of high cyanotoxin concentrations can severely impair the use of a waterbody for drinking water and recreational purposes. Cyanotoxins are likely to occur under specific environmental conditions, and so identifying these conditions can facilitate management of the waterbody to reduce the likelihood of high cyanotoxin concentrations.W...
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Predictions of daily flows in ungauged streams frequently rely on index gauges, where the timing of the daily flow at the index gauge is assumed to be similar to that in the ungauged stream. When some limited flow measurements are available at the ungauged sites, the strength of correlation between these flows and candidate index gauges can inform...
Article
Summary1. Stream reaches found to be impaired by physical, chemical or biological assessment generally are associated with greater extent of urban and agricultural land uses, and lesser amount of undeveloped lands. However, because stream condition commonly is influenced by multiple stressors as well as underlying natural gradients, it can be diffi...
Article
Increased nutrient concentrations in streams and rivers have altered biological structure and function. Manipulative studies have provided insights into different mechanisms by which changes in nutrient concentrations influence aquatic biota, but these studies are limited in spatial scope and in their quantification of nutrient effects on aggregate...
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1. The use of species traits to interpret biological changes in invertebrate assemblages across environmental gradients has been suggested as a method to improve discrimination over existing species composition methods. One reason for greater potential discrimination and predictive ability is the assumed universality of traits across a range of spa...
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Analyses of observational data can provide insights into relationships between environmental conditions and biological responses across a broader range of natural conditions than experimental studies, potentially complementing insights gained from experiments. However, observational data must be analyzed carefully to minimize the likelihood that co...
Chapter
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Biological monitoring and assessment methods have become indispensable tools for evaluating the condition of aquatic and terrestrial ecosystems. When an undesirable biological condition is observed (e.g., a depauperate fish assemblage), its cause (e.g., toxic substances, excess fine sediments, or nutrients) must be determined in order to design app...
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The Wadeable Streams Assessment (WSA) provided the first statistically sound summary of the ecological condition of streams and small rivers in the US. Information provided in the assessment filled an important gap in meeting the requirements of the US Clean Water Act. The purpose of the WSA was to: 1) report on the ecological condition of all wade...
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One of the biggest challenges when conducting a continental-scale assessment of streams is setting appropriate expectations for the assessed sites. The challenge occurs for 2 reasons: 1) tremendous natural environmental heterogeneity exists within a continental landscape and 2) reference sites vary in quality both across and within major regions of...
Article
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We examined the effects of different regionalization schemes on the performance of River InVertebrate Prediction and Classification System (RIVPACS)-type predictive models in assessing the biological conditions of streams of the US for the National Wadeable Streams Assessment (WSA). Three regionalization schemes were considered: a single national p...
Article
We used complementary information collected using different conceptual approaches to develop recommendations for a stream nutrient criterion based on responses of algal assemblages to anthropogenic P enrichment. Benthic algal attributes, water chemistry, physical habitat, and human activities in watersheds were measured in streams of the Mid-Atlant...
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This paper provides a brief introduction to the R package bio.infer, a set of scripts that facilitates the use of maximum likelihood (ML) methods for predicting environmental conditions from assemblage composition. Environmental conditions can often be inferred from only biological data, and these inferences are useful when other sources of data ar...
Article
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1. Observations of different organisms can often be used to infer environmental conditions at a site. These inferences may be useful for diagnosing the causes of degradation in streams and rivers. 2. When used for diagnosis, biological inferences must not only provide accurate, unbiased predictions of environmental conditions, but also pairs of inf...
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The effects of measurement errors on biological inferences of stream temperature and bedded fine sediment were investigated. Single variable and multivariate logistic regression models were used to relate the occurrences of different macroinvertebrate genera and observed temperature and fine sediment. Next, a simulation and extrapolation method (SI...
Article
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River InVertebrate Prediction And Classification System (RIVPACS)-type predictive models assess the biological conditions of streams by comparing observed assemblage composition (O) with an expected assemblage composition derived from reference site observations (E). The ratio of the 2 values (O/E) can be interpreted as a measure of taxonomic compl...
Article
Human activities are modifying the condition and character of ecosystems at a rapid rate. Because of these rapid changes, questions concerning how ecosystems and their assemblages respond to anthropogenic stressors have been of general interest. Accurate prediction of assemblage composition in ecosystems with anthropogenic degradation requires that...
Article
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Weighted averaging is widely used for inferring environmental conditions from an observed species assemblage. However, weighted average inferences are known to be systematically biased, and linear corrections (i.e., deshrinking functions) are commonly applied to adjust for this bias. In this analysis, the magnitude of the biases in weighted average...
Article
Predictions of the responses of stream ecosystems to different types of disturbance would be facilitated by accurate knowledge of the environmental preferences of different taxa. These preferences are often expressed in terms of taxon optima, or the location along an environmental gradient where a particular taxon is most likely to be observed. Sev...
Article
The identification of the causal pathways leading to stream impairment is a central challenge to our understanding of ecological relationships. Bayesian belief networks (BBN's) are a promising tool for modeling presumed causal relationships, providing a modeling structure within which different factors describing the ecosystem can be causally linke...
Article
Taxonomic optima, i.e., the environmental conditions at which peak abundance is expected, are widely used to analyze assemblage response to environmental gradients. Typically analyses consider how the mean optima for an assemblage change over a gradient. In this study we consider how other characteristics of the assemblage vary. We ask two question...
Article
Water quality monitoring data are usually used independently to report on the condition of streams and watersheds. For example, watersheds are often rated as good, fair, or poor with regard to a single stressor or with regard to an index of biotic integrity. The utility of monitoring data may be enhanced by integrating stressor-response information...
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We recognized that many health outcomes are associated with air pollution, but in this project launched by the US EPA, the intent was to assess the role of exposure to ambient air pollutants as risk factors only for respiratory effects in children. The NHANES-III database is a valuable resource for assessing children's respiratory health and certai...
Article
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Accurate estimates of stressor levels in unsampled streams would provide valuable information for managing these resources over large regions. Spatial interpolation of stream characteristics have rarely been attempted, partly because defining separation distances between distinct stream samples is not straightforward. That is, conventional Eulerian...
Article
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1. Macroinvertebrates are frequently classified in terms of their tolerance to human disturbance and pollution. These tolerance values have been used effectively to assess the biological condition of running waters. 2. Generalised additive models were used to associate the presence and absence of different macroinvertebrate genera with different en...
Article
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Metrics characterizing the benthic macroinvertebrate assemblages in wadeable streams in the Mid-Atlantic region of the United States were analyzed to explore the relative responses of the metrics to different types of anthropogenic stress. The data used in our study were collected by the US Environmental Protection Agency Environmental Monitoring a...
Article
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Methods for modeling indoor air pollution are used to determine the degree of protection offered by buildings against airborne biological agents. The factors that determine the sheltering effectiveness of a particular building (air exchange rates, particle deposition rates, environmental decay of agents, and filter efficiencies) are considered. Rep...
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This paper reports on a series of large-eddy simulations of a round jet issuing normally into a crossflow. Simulations were performed at two jet-to-crossflow velocity ratios, 2.0 and 3.3, and two Reynolds numbers, 1050 and 2100, based on crossflow velocity and jet diameter. Mean and turbulent statistics computed from the simulations match experimen...
Article
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This paper examines the trajectory and entrainment characteristics of a round jet in crossflow. A series of large eddy simulations was performed at Reynolds numbers of 1050 and 2100 and at jet to crossflow velocity ratios of 2.0 and 3.3. Trajectories, which are defined based on the mean streamlines on the centerplane, all collapse to a single curve...
Article
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This paper examines the trajectory and entrainment characteristics of a round jet in crossflow. A series of large eddy simulations was performed at Reynolds numbers of 1050 and 2100 and at jet to crossflow velocity ratios of 2.0 and 3.3. Trajectories, which are defined based on the mean streamlines on the centerplane, all collapse to a single curve...
Article
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Using simulated atmospheric data from the National Center for Atmospheric Research (NCAR) community climate model (CCM), we test the hypothesis that the global positioning system (GPS) can be used to detect global and regional climate change. We examine how the fundamental GPS variables (wet and total delays and vertical profiles of refractivity) a...
Article
We present scalar transport and mixing results from large eddy simulations of a round turbulent jet issuing into a laminar crossflow. The simulation uses domain decomposition to facilitate discretization of the flow and a dynamic subgrid scale turbulence model to account for unresolved motions. Consistent with what has been found in experiment, the...
Article
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Thesis (Ph. D.)--Stanford University, 1997. Submitted to the Department of Mechanical Engineering. Copyright by the author.

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