/ Problems arising from application of the representative criterion for conservation and natural heritage evaluation are discussed. An ecological basis to this criterion is suggested that focuses on those key environmental factors dominating biotic response. A methodology is proposed that utilizes computer-based methods of establishing and interrogating spatial data bases (geographic information systems), environmental modeling, and numeric analysis. An example is presented illustrating some of the advantages and limitations of classification and dimension reduction techniques in both defining bioenvironments and displaying their spatial distribution. The advantages of this method for representativeness evaluation are that it maximizes the utility of available data, is explicit and repeatable, and enables large areas to be analyzed at relatively fine scales.Representativeness is one criterion used for assessing the suitability of a place for inclusion in a nature conservation reserve system or for listing on a heritage register. While other criteria, such as species diversity, focus assessment on the condition of the land cover within a nominated place, the representativeness criterion is intended to provide environmental context; that is, to demonstrate how a place is related to the surrounding region, continent, or globe.The term representativeness implies that a subset of a population is taken such that all or most of the characteristics found in the total population are present. The concept of representativeness in conservation was reviewed by Austin and Margules (1984), who suggested that, generally, the term refers to a method for assessing how adequately a reserve or system of reserves represents the range of biological variation in a given region. As defined by common usage, representativeness has definite biological and ecological implications. However, exactly what it is that samples are sought from is ill-defined. The targets for assessment have included biogeographic regions, ecosystems, habitats, plant and animal species, and plant communities/ assemblages/alliances. These definitional problems are compounded by the dynamic nature of many impor-

Draining 31 states and roughly 3 million km 2 , the Mississippi River (MSR) and its tributaries constitute an essential resource to millions of people for clean drinking water, transportation, agriculture, and industry. Since the turn of the 20 th century, MSR water quality has continually rated poorly due to human activity.Acting as first responders, microorganisms can mitigate, exacerbate, and/or serve as predictors for water quality, yet we know little about their community structure or ecology at the whole river scale for large rivers. We collected both biological (16S and 18S rRNA gene amplicons) and physicochemical data from 38 MSR sites over nearly 3000 km from Minnesota to the Gulf of Mexico. Our results revealed a microbial community composed of similar taxa to other rivers but with unique trends in the relative abundance patterns among phyla, operational taxonomic units (OTUs), and the core microbiome. Furthermore, we observed a separation in microbial communities that mirrored the transition from an 8 th to 10 th Strahler order river at the Missouri River confluence, marking a different start to the lower MSR than the historical distinction at the Ohio River confluence in Cairo, IL. Within MSR microbial assemblages, we identified subgroups of OTUs from the phyla Acidobacteria, Bacteroidetes, Oomycetes, and Heterokonts that were associated with, and predictive of, the important eutrophication nutrients nitrate and phosphate. This study offers the most comprehensive view of MSR microbiota to date, provides important groundwork for higher resolution microbial studies of river perturbation, and identifies potential microbial indicators of river health related to eutrophication.

This article analyzes the effect of agricultural cost sharing for cover crops on the acres of three conservation practices. A survey of farmers from Maryland is used to estimate the direct effect of cover crop cost sharing on the acres of cover crops, and the indirect effect of cover crop cost sharing on the acres of two other practices: conservation tillage and contour/strip cropping. A two‐stage simultaneous equation approach is used to correct for voluntary self‐selection into cost‐sharing programs, and to account for substitution effects among conservation practices. Using model parameters from the U.S. Environmental Protection Agency's Chesapeake Bay Program, the estimated effects of cost sharing are then translated to pollution reduction in order to quantify water quality benefits. The results indicate that the large cover crop cost sharing effort in Maryland had considerable effects on cover crop acreage, substantially reducing nitrogen and phosphorus runoff. Moreover, after accounting for the indirect effects on conservation tillage, the cost per pound of phosphorus abatement in the Chesapeake Bay decreased by between 60–67%.

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