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Multi-scale Landscape Factors Influencing Stream Water Quality in the State of Oregon
Citation:
NASH, M. S., D. T. HEGGEM, D. W. EBERT, T. G. WADE, AND R. K. HALL. Multi-scale Landscape Factors Influencing Stream Water Quality in the State of Oregon. ENVIRONMENTAL MONITORING AND ASSESSMENT. Springer, New York, NY, 145:343-360, (2008).
Impact/Purpose:
A need has been recognized to develop a method(s) for the assessment and evaluation of surface water nutrient and pathogen levels in the Western United States as a result of urbanization, livestock grazing and agriculture activities. Because information on location and number of cattle grazing is limited, this effort focuses on creating a grazing potential index (GPI) map of Oregon as a case study (Wade et al., 1998). The GPI uses, a combination of land cover, land ownership, distance to water, and slope to map the relative likelihood of presence of grazing cattle. GPI values range from 0 to 100, with higher values representing greater grazing potential. Objectives of this study are to investigate the following: Does the enterococci concentration in streams reflect the condition and composition of the surrounding landscape? Will sites with geometric mean of enterococci less than 35 cfu/100 ml be correlated to different constituents than sites with a geometric mean of more than 35 cfu/100 ml? What is the likelihood that a site with a high concentration of enterococci is due to land use and anthropogenic sources?
Description:
Enterococci bacteria are used to indicate the presence of human and/or animal fecal materials in surface water. In addition to human influences on the quality of surface water, a cattle grazing is a widespread and persistent ecological stressor in the Western United States. Cattle may affect surface water quality directly by depositing nutrients and bacteria, and indirectly by damaging stream banks or removing vegetation cover, which may lead to increased sediment loads. This study used the State of Oregon surface water data to determine the likelihood of animal pathogen presence using enterococci and analyzed the spatial distribution and relationship of biotic (enterococci) and abiotic (nitrogen and phosphorous) surface water constituents to landscape metrics and others (e.g. human use, percent riparian cover, natural covers, grazing, etc.). We used a grazing potential index (GPI) based on proximity to water, land ownership and forage availability. Mean and variability of GPI, forage availability, stream density and length, and landscape metrics were related to enterococci and many forms of nitrogen and phosphorous in standard and logistic regression models. The GPI did not have a significant role in the models, but forage related variables had significant contribution. Urban land use within stream reach was the main driving factor when exceeding the threshold (≥35 cfu/100 ml), agriculture was the driving force in elevating enterococci in sites where enterococci concentration was < 35 cfu/100 ml. Landscape metrics related to amount of agriculture, wetlands and urban all contributed to increasing nutrients in surface water but at different scales. The probability of having sites with concentrations of enterococci above the threshold was much lower in areas of natural land cover and much higher in areas with higher urban land use within 60m of stream. A one percent increase in natural land cover was associated with a 12% decrease in the predicted odds of having a site exceeding the threshold. Opposite to natural land cover, a one unit change in each of manmade barren and urban land use led to an increase of the likelihood of exceeding the threshold by 73%, and 11%, respectively. Change in urban land use had a higher influence on the likelihood of a site exceeding the threshold than that of natural land cover.