Red Alder (Alnus rubra) Distribution Influences Nitrate Discharge to Coastal Estuaries: Comparison of Two Oregon Watersheds

摘要

We determined nutrient export from the Yaquina and Alsea Rivers as part of a larger program for evaluating nutrient sources to coastal waters. The Yaquina and Alsea data indicated that one river typically contained twice the amount of dissolved nitrate-N, although temperature, conductivity and the concentrations of other nutrients were similar. We developed a nitrate export model using multiple linear regression (MLR) to analyze the discriminating variables that included nutrient concentrations and hardwood cover containing approximately 90% red alder (Alnus rubra), a nitrogen-fixing tree species. Using data from the Coastal Landscape Analysis and Modeling Study (CLAMS), hardwood cover was found to be most prevalent in the upper (gaged) Yaquina watershed. Estimated nitrate export was 2.02 Mg km-2 y-1 in the Yaquina and 1.24 Mg km-2 y-1 in the Alsea for 2006. However, the annual nitrate-N exported from the entire Alsea basin (1560 Mg) was slightly greater than that of the Yaquina basin (1320 Mg) since the Alsea is about twice as large with proportionally greater discharge. Various factors, including the relatively low nitrate concentrations in local rainfall and the lack of local primary anthropogenic sources, indicated that red alder density may be the primary source of higher concentrations in the Yaquina, relative to the Alsea River. The regression model developed in this study can provide a rapid estimate of nitrate-N export based on water discharge data and hardwood distribution in the Oregon Coast Range.