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A long-term stream fertilization experiment was performed to evaluate the potential eutrophication of an arctic stream ecosystem. During 16 years of summer phosphorus (H3PO4) fertilization, we observed a dramatic change in the community structure of the Kuparuk River on the North Slope of Alaska. A positive response to fertilization was observed at all trophic levels with increases in epilithic algal stocks, some insect densities, and fish growth rates. After approximately eight years of P fertilization, bryophytes (mosses) replaced epilithic diatoms as the dominant primary producers in the Kuparuk River. The moss impacted NH4+ uptake rates, benthic gross primary production, habitat structure, and insect abundance and species composition. This study documents the long-term changes in an arctic tundra stream in response to nutrient enrichment. Predicting stream ecosystem responses to chronic perturbation requires long-term observation and experiments. [ABSTRACT FROM AUTHOR]

A comparative (15)N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds.

A recent study of hereditary prostate cancer has provided evidence for a prostate cancer-susceptibility locus, HPC20, which maps to 20q13. To assess further the potential contribution of this locus to prostate cancer susceptibility, we studied 172 unrelated families affected by prostate cancer, using 17 polymorphic markers across a 98.5-cM segment of chromosome 20 that contains the candidate region. Parametric analysis, allowing for heterogeneity, resulted in an overall HLOD score of 0.09 (P=.39) at D20S171, under the assumption of linkage in 6% of families. Mode-of-inheritance-free analysis of the entire data set resulted in a maximal Zlr score of 0.76 (LOD 0.13; P=.22) at the same location. The strongest evidence for linkage was seen in the subset of 16 black families (LOD 0.86; Zlr=1.99; P=.023) between markers D20S893 and D20S120, near the putative location of HPC20. Although some positive results were observed, our linkage study does not provide statistically significant support for the existence of a prostate cancer-susceptibility locus HPC20 at 20q13.