Ecological Effects on Streams from Forest Fertilization-Literature Review and Conceptual Framework for Future Study

By Chauncey W. Anderson

Fertilization of forests with urea-nitrogen has been studied numerous times for its effects on water quality. Stream nitrogen concentrations following fertilization are typically elevated during winter, including peaks in the tens-of-thousands of parts per billion range, with summer concentrations often returning to background or near-background levels. Despite these increases, water-quality criteria for nitrogen have rarely been exceeded. However, such criteria are targeted at fish toxicity or human health and are not relevant to concentrations that could cause ecological disturbances.

Studies of the responses of stream biota to fertilization have been rare and have targeted either immediate, toxicity-based responses or used methods insensitive to ongoing ecological processes. This report reviews water-quality studies following forest fertilizations, emphasizing Cascade streams in the Pacific Northwest and documented biological responses in those streams. A conceptual model predicting potential ecological response to fertilization, which includes effects on algal growth and primary production, is presented. In this model, applied fertilizer nitrogen reaching streams is mostly exported during winter. However, some nitrogen retained in soils or stream and riparian areas may become available to aquatic biota during spring and summer. Biological responses may be minimal in small streams nearest to application because of light limitation, but may be elevated downstream where light is sufficient to allow algal growth. Ultimately, algal response could be greatest in downstream reaches, although ambient nutrient concentrations remain low due to uptake and benthic nutrient recycling. Ground-water flow paths and hyporheic processing could be critical in determining the fate of applied nitrogen. A framework is provided for testing this response in the Little River watershed, a tributary to the North Umpqua River, Oregon, at basic and intensive levels of investigation.

Abstract
Introduction
Literature Review
    Forest Processes
    Water-Quality Effects on Streams from Forest
           Fertilization
    Ecological Effects on Streams from Forest
           Fertilization
    Stream Nutrient Dynamics
    Hyporheic Processing
Little River Watershed
    Physiographic Setting
    Water-Quality Issues in the Little River
           Watershed
    Land uses in the Little River Watershed and
           Potential Effects on Water Quality
    Water-Quality Conditions
Framework for Fertilization Study in Wolf Creek
          and Little River Watershed
Summary
References Cited

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