David P. Larsen

Regional reference sites: a method for assessing stream potentials

Field assessments of impacted streams require a control or at least an unbiased estimate of attainable conditions. Control sites, such as upstream/downstream or wilderness sites, have proven inadequate for assessing attainable ecological conditions where the control streams differ naturally from the impacted streams to a considerable degree or where different disturbances exist than those being studied. Relatively undisturbed reference sites with watersheds in areas having the same land-surface form, soil, potential natural vegetation, and land use as are predominant in large, relatively homogeneous regions are suggested as alternative control sites. These areas are considered typical of the region and therefore the sites also are considered typical of the region because their watersheds exhibit all the terrestrial variables that make that region a region. The logical basis for developing regional reference sites lies in the ability to group watersheds and common stream types into regions by integrating available maps of terrestrial variables that influence streams. Relatively undisturbed reference sites can be selected from typical areas of the regions and from transition zones where one or two of the terrestrial variables are not the predominant one(s) of the region. These reference sites are useful for estimating attainable conditions, for evaluating temporal and spatial changes in ecological integrity, for classifying attainable uses of streams, and for setting biological and environmental criteria.

Rare species in multivariate analysis for bioassessment: some considerations

BRIDGES is a recurring feature of J-NABS intended to provide a forum for the interchange of ideas and information between basic and applied researchers in benthic science. Articles in this series will focus on topical research areas and linkages between basic and applied aspects of research, monitoring policy, and education. Readers with ideas for topics should contact Associate Editors, Nick Aumen and Marty Gurtz. Multivariate analyses are used commonly in bioassessment studies examining the degree of human impact on aquatic ecosystems. However, these analyses may have shortcomings with respect to how well they address the presence or absence of rare species. Researchers may delete rare species explicitly, or ignore them implicitly by the use of small sample sizes. The motivation for exclusion of rare species may be related to sampling or analytical resource limitations. The authors provide an overview of the importance of rare species, the sensitivity of the newer multivariate techniques to rare species, and the need for careful evaluation of the potential influences of the inclusion or exclusion of rare species from analyses in light of each study’s objectives and spatial scale. Nick Aumen,nick_aumen@nps.gov Marty Gurtz,megurtz@usgs.gov Co-editors

MONITORING FOR POLICY‐RELEVANT REGIONALTRENDS OVER TIME

The term trend describes the continuing directional change in the value of an indicator, generally upward or generally downward. Many policy questions concern trend across a number of sites, such as lakes in a region, rather than trend at a single site. Power to detect regional trend seldom is discussed, and monitoring designs suitable for detecting such trends rarely are explored. Components of variance and temporal sampling designs play central roles in characterizing trend detection. We present relative numerical values of important components of variance, developed from the Surface Waters component of U.S. EPA’s Environmental Monitoring and Assessment Program (EMAP) field data, and use them as a basis for further assumptions of values. We discuss power curves in general and present them in relation to temporal designs, years of field observation, components of variance, and the level of trend detected. Revisit designs give adequate power for moderate trend in 10–15 yr, even when revisits are less frequent than annually.

Designing a Spatially Balanced, Randomized Site Selection Process for Regional Stream Surveys: The EMAP Mid-Atlantic Pilot Study

In 1993, the U.S. Environmental Protection Agency (EPA), as part of the Environmental Monitoring and Assessment Program (EMAP), initiated a sample survey of streams in the mid-Atlantic. A major objective of the survey was to quantify ecological condition in wadeable streams across the region. To accomplish this goal, we selected 615 stream sites using a randomized sampling design with some restrictions. The design utilized the digitized stream network taken from 1:100,000-scale USGS topographic maps as the sample frame. Using a GIS, first- through third-order (wadeable) stream segments in the sample frame were randomly laid out in a line and sampled at fixed intervals after a random start. We used a variable probability approach so that roughly equal numbers of first-, second-, and third-order stream sites would appear in the sample. The sample design allows inference from the sample data to the status of the entire 230,400 km of wadeable stream length in the mid-Atlantic study area. Of this mapped stream length, 10% was not in the target population because no stream channel existed (4%), the stream channel was dry (5%), or the stream was not wadeable (1%). We were unable to collect field data from another 10% of the mapped stream length due to lack of access (mostly landowner denials). Thus, the field data we collected at 509 sites allows inference to the ecological condition for 184,600 km of the mapped stream length in the region.

Condition of stream ecosystems in the US: an overview of the first national assessment

Abstract 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 wadeable, perennial streams and rivers within the conterminous US, 2) describe the biological condition of these systems with direct measures of aquatic life, and 3) identify and rank the relative importance of chemical and physical stressors affecting stream and river condition. The assessment included perennial wadeable streams and rivers that accounted for 95% of the length of flowing waters in the US. The US Environmental Protection Agency, states, and tribes collected chemical, physical, and biological data at 1392 randomly selected sites. Nationally, 42% of the length of US streams was in poor condition compared to best available reference sites in their ecoregions, 25% was in fair condition, and 28% was in good condition. Results were reported for 3 major regions: Eastern Highlands, Plains and Lowlands, and West. In the West, 45% of the length of wadeable flowing waters was in good condition. In the Eastern Highlands, only 18% of the length of wadeable streams and rivers was in good condition and 52% was in poor condition. In the Plains and Lowlands, almost 30% of the length of wadeable streams and rivers was in good condition and 40% was in poor condition. The most widespread stressors observed nationally and in each of the 3 major regions were N, P, riparian disturbance, and streambed sediments. Excess nutrients and excess streambed sediments had the highest impact on biological condition; streams scoring poor for these stressors were at 2 to 3× higher risk of having poor biological condition than were streams that scored in the good range for the same stressors.

Comparison of ecological communities: The problem of sample representativeness

Obtaining an adequate, representative sample of ecological communities to make taxon richness (TR) or compositional comparisons among sites is a continuing challenge. Although randomization in the collection of sample units is often used to assure that sampling is representative, randomization does not convey the concept of how well samples represent the community or site from which they are drawn. In ecological surveys, how well a sample represents a community or site literally means the similarity in taxon composition and relative abundance between a sample and the community from which it is drawn. Using both field and simulated data, we show that the proportion of the total taxon richness at a site (%TTR) achieved with a fixed sample size varies across sites, which in turn causes changes in site-to-site differences in observed TR with sample size. This means that equal-sized samples may differentially represent the communities from which they are drawn. However, the similarity of a sample to the community from which it is drawn cannot be measured directly because the taxon composition and relative abundance of the community is usually unknown. We propose to estimate it by measuring the average similarity among replicate samples randomly drawn from a community, i.e., autosimilarity, which is measured with Jaccard Coefficient in this study. Using the same data sets, we found that: (1) samples of equal size from different sites or communities achieved different levels of autosimilarity, with lower levels achieved in taxon richer sites, indicating variation in how well samples of equal size represent their respective communities; (2) %TTR was positively and almost linearly correlated with autosimilarity, indicating that autosimilarity might be a good predictor of TTR; and (3) when samples were compared at the same level of autosimilarity, similar %TTRs across different sites were achieved (i.e., the relative differences in taxon richness among sites became independent of sample size). We conclude that standardization on autosimilarity, rather than on sample size, can improve the accuracy of taxon richness comparisons.

Comparing strengths of geographic and nongeographic classifications of stream benthic macroinvertebrates in the Mid-Atlantic Highlands, USA

The US Environmental Protection Agency’s (USEPA) Environmental Monitoring and Assessment Program (EMAP) sampled ∼500 wadeable streams in the Mid-Atlantic Highlands region of the US during the late spring of 1993 to 1995 for a variety of physical, chemical, and biological indicators of environmental condition. Eighty-eight sites that were minimally affected by human activities were chosen to determine the extent to which geographic and stream-based classifications accounted for variation in the composition of riffle macroinvertebrate assemblages. Bray–Curtis similarities among sites were calculated from the relative abundance of macroinvertebrates to assess the strength of classifications based on geography (ecoregions and catchments), habitat (slope and stream order), and water chemistry (conductivity). For comparison, a taxonomic classification (two-way indicator species analysis, TWINSPAN) and a gradient analysis (correspondence analysis, CA) were performed on the macroinvertebrate data. To assess the effect of taxonomic resolution, all analyses were completed at the family level and to lowest practical taxon. The large overall variation within and among ecoregions resulted in a low average classification strength (CS) of ecoregions, although some ecoregions had high CS. Stream order had the highest CS of the habitat and water chemistry classifications. Ecoregion CS increased, however, when stream sites were 1st stratified by stream order (ecoregions nested within stream order). Nested ecoregion CS did not increase within 1st-order streams, yet increased within 2nd- and 3rd-order streams. CA ordinations and TWINSPAN classification showed a clear gradient of streams along stream size (order), with a clear separation of 1st- and 3rd-order streams based on macroinvertebrate composition. The ordinations did not, however, show a distinct clustering of sites on the basis of ecoregions. Overall, the lowest practical taxon level of identification resulted in a clearer pattern of sites in ordination space than did family-level identification, yet only a slight improvement in the different classifications (geographic, habitat, and water chemistry) based on average similarity.

Selecting reference sites for stream biological assessments: best professional judgment or objective criteria

Abstract Selection of reference sites is a critical component in the process of developing and applying biological indicators of ecological condition. Some evidence suggests that despite its importance the rules by which reference sites are selected have not always been evaluated critically to assure that the sites represent least-disturbed conditions. We present a comparison of physical and chemical disturbance measures and biotic indices at handpicked reference sites provided by resource agencies and at sites selected by a probability design from a 12-state survey of western streams and rivers. In most cases, the distributions of disturbance measures and biotic index scores were essentially the same for both types of sites; that is, only a subset of the handpicked reference sites represented least-disturbed conditions. We recommend that all agencies that use reference sites critically review those sites with a set of explicit criteria, using field-collected data as well as mapped information.

A null model for the expected macroinvertebrate assemblage in streams

Abstract Predictive models such as River InVertebrate Prediction And Classification System (RIVPACS) and AUStralian RIVer Assessment System (AUSRIVAS) model the natural variation across geographic regions in the occurrences of macroinvertebrate taxa in data from streams that are in reference condition, i.e., minimally altered by human-caused stress. The models predict the expected number of these taxa at any stream site, assuming that site also is in reference condition. A significant difference between the ratio of observed (O) and expected (E) taxa (O/E) and 1.0 indicates that the site is not in reference condition. The standard deviation (SD) of O/E values estimated for a set of reference sites is a measure of predictive-model precision, with a small SD indicating that the model accounts for much of the variability in E that is associated with natural factors such as stream size and elevation. We propose a null model for E that assumes fixed occurrence probabilities for individual taxa across reference sites. The null model explains none of the variability in E caused by natural factors, so the SD of its O/E predictions is the upper limit attainable by any predictive model. We also derive a theoretical lower limit for SD of O/E that is caused only by replicate-sampling variation among predictions from a perfect model. Together, the null-model and replicate-sampling SDs estimate the minimum and maximum precision, respectively, attainable by any predictive model for a given set of reference-site data. A predictive model built from data at 86 reference sites in the Mid-Atlantic Highlands region, USA, had SD = 0.18 for O/E across those sites, while the corresponding null model had SD = 0.20, indicating relatively little gain from the predictive-model effort. In contrast, a model built from 209 sites in North Carolina, USA, had predictive- and null-model SDs of 0.13 and 0.28, respectively, indicating that the North Carolina predictive model had relatively high gain in precision over the null model. Replicate-sampling SDs of O/E for the Mid-Atlantic and North Carolina data were 0.09 and 0.11, respectively, suggesting that the North Carolina predictive model had little room for further improvement, in contrast to the Mid-Atlantic model. The precisions of null-model estimates were lower than those of predictive models, so null models somewhat underestimated the percentages of 447 and 1773 test assemblages from the Mid-Atlantic region and North Carolina, respectively, that differed significantly from reference conditions. The estimates illustrate how a simple and easily built null model provides a lower bound for the prevalence of impaired streams within a region.

The effect of wastewater phosphorus removal on shagawa lake, Minnesota: phosphorus supplies, lake phosphorus and chlorophyll a

Abstract In early 1973, the phosphorus supply to Shagawa Lake, Minnesota, was reduced by about 80% when a tertiary wastewater treatment plant began operating. Significant reductions in total and soluble reactive phosphorus concentrations have occurred in the lake since that time. By 1976 the average (volume weighted over the entire lake) total and soluble reactive phosphorus concentrations had declined from about 51 and 21 μg l −1 to about 30 and 4.5 μg l −1 , respectively, corresponding to 40 and 80% reductions. During 1975 and 1976, chlorophyll a (averaged over the top 5 m) had decreased to less than 50% of the pretreatment level during May-June but during July-August little change had occurred. A phosphorus residence time model projected equilibrium total phosphorus concentrations of about 12 μg l −1 within 1.5 years. The fact that this level was not reached is attributed to a feedback of phosphorus from the sediments, primarily during summer. This phenomenon has been incorporated into a modified total phosphorus mass balance model which projects the phosphorus pattern through 1976 quite accurately. The close fit of the modified mass balance model implies that the phosphorus supply from the sediments has not diminished since treatment since treatment began, and that further recovery of the lake will depend upon how long this feedback of phosphorus from the sediments continues.