Preliminary Investigation of Nutrients and Isotopic Nitrogen in Oregon Chub Habitat Adjacent to Oakridge Sewage Treatment Plant

摘要

Oregon chub (Oregonichthys crameri) is a minnow endemic to the Willamette River drainage, with a current distribution limited to 25 natural and nine introduced populations (Scheerer et al. 2003). The species was listed as endangered in 1993 over its entire range under the Endangered Species Act of 1973, as ammended. Oregon chub prefer slow-moving backwater habitats, such as sloughs, flooded marshes, and beaver ponds, that are common along the Willamette River and its tributaries. While some chub populations are stable or have increased in recent years, others are declining or have been extirpated (Scheerer et al. 2003). In the Oakridge Slough in the Middle Fork Willamette River drainage, the chub population density is low and the population has shown a recent decline in abundance from about 500 fish in 1999 to less than 50 fish in 2002 (Scheerer et al. 2003). Oakridge Slough also has an approximately 1-m thick layer that consists of primarily suspended organic material in various states of decay overlying a hard substrate. Sloughs in the area with more stable chub populations typically have less than 10 cm of organic material accumulated over a solid substrate. Reports from U.S. Forest Service indicate that the slough is undergoing natural eutrophication rates, and reduced water velocity and disconnection from the active flood plan because construction of the upstream dam has reduced flushing events and led to a net increase in organic material. However, the consistency and appearance of the material in Oakridge Slough was observed to be very different from nearby sloughs.