Matthew W. Diebel

Nitrogen stable isotopes in streams: effects of agricultural sources and transformations

The nitrogen stable isotope ratio of biological tissue has been proposed as an indicator of anthropogenic N inputs to aquatic ecosystems, but overlap in the isotopic signatures of various N sources and transformations make definitive attribution of processes difficult. We collected primary consumer invertebrates from streams in agricultural settings in Wisconsin, U.S.A., to evaluate the relative influence of animal manure, inorganic fertilizer, and denitrification on biotic delta15N. Variance in biotic delta15N was explained by inorganic fertilizer inputs and the percentage of wetland land cover in the watershed, but not by animal manure inputs. These results suggest that denitrification of inorganic fertilizer is the primary driver of delta15N variability among the study sites. Comparison with previously collected stream water NO3-N concentrations at the same sites supports the role of denitrification; for a given N application rate, streams with high biotic delta15N had low NO3-N concentrations. The lack of a manure signal in biotic delta15N may be due its high ammonia content, which can be dispersed outside the range of its application by volatilization. Based on our findings and on agricultural census data for the entire United States, inorganic fertilizer is more likely than manure to drive variability in biotic delta15N and to cause excessive nitrogen concentrations in streams.

Landscape Planning for Agricultural Nonpoint Source Pollution Reduction I: A Geographical Allocation Framework

Agricultural nonpoint source pollution remains a persistent environmental problem, despite the large amount of money that has been spent on its abatement. At local scales, agricultural best management practices (BMPs) have been shown to be effective at reducing nutrient and sediment inputs to surface waters. However, these effects have rarely been found to act in concert to produce measurable, broad-scale improvements in water quality. We investigated potential causes for this failure through an effort to develop recommendations for the use of riparian buffers in addressing nonpoint source pollution in Wisconsin. We used frequency distributions of phosphorus pollution at two spatial scales (watershed and field), along with typical stream phosphorus (P) concentration variability, to simulate benefit/cost curves for four approaches to geographically allocating conservation effort. The approaches differ in two ways: (1) whether effort is aggregated within certain watersheds or distributed without regard to watershed boundaries (dispersed), and (2) whether effort is targeted toward the most highly P-polluting fields or is distributed randomly with regard to field-scale P pollution levels. In realistic implementation scenarios, the aggregated and targeted approach most efficiently improves water quality. For example, with effort on only 10% of a model landscape, 26% of the total P load is retained and 25% of watersheds significantly improve. Our results indicate that agricultural conservation can be more efficient if it accounts for the uneven spatial distribution of potential pollution sources and the cumulative aspects of environmental benefits.

Preparing for the future : teaching scenario planning at the graduate level

Are environmental science students developing the mindsets and obtaining the tools needed to help address the considerable challenges posed by the 21st century? Todays major environmental issues are characterized by high-stakes decisions and high levels of uncertainty. Although traditional scientific approaches are valuable, contemporary environmental issues also require new tools and new ways of thinking. We provide an example of how such new, or “post-normal”, approaches have been taught at the graduate level, through practical application of scenario planning. Surveyed students reported that they found the scenario planning course highly stimulating, thought-provoking, and inspiring. Key learning points included recognizing the need for multiple points of view when considering complex environmental issues, and better appreciating the pervasiveness of uncertainty. Collaborating with non-academic stakeholders was also particularly helpful. Most students left the course feeling more positive about the potential contribution they can make in addressing the environmental challenges that society faces.

A spatial autocorrelative model for targeting stream restoration to benefit sensitive nongame fishes

Stream restoration projects often aim to benefit aquatic biota and frequently use the reappearance of sensitive nongame fish species as a measure of restoration success. However, mitigation of human influence will only benefit a given species where static habitat characteristics are suitable for that species and where potential source populations are within the range of their dispersal capability. We used spatial autoregressive habitat models to simulate the effect of watershed-scale stream restoration on the distributions of six sediment-sensitive fish species in Wisconsin, USA, streams. These models consider the probability of occurrence of a species in a given stream segment as a function of characteristics of that segment as well as the characteristics of neighboring segments. Populations of individual species are predicted to be restorable in 0.2%–2.8% of Wisconsin streams. Streams with high restoration potential for one or more species generally have high watershed human land use but are also closel...

Monitoring channel morphology and bluff erosion at two installations of flow-deflecting vanes, North Fish Creek, Wisconsin, 2000-03

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