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University of Nevada, Reno

Carbon and Nitrogen Stable Isotopes on the Truckee River: Results of March 2004 Sampling

Prepared By
Laurel Saito, Ph.D., P.E., Christa Fay, and Kristin Kvasnicka
Department of Natural Resources and Environmental Science, University of Nevada Reno
1000 Valley Road
Reno, NV 89512-0013

Prepared For
Karen Vargas, Environmental Specialist
Nevada Division of Environmental Protection
July 27, 2004

Dr. Laurel Saito and her students at the University of Nevada Reno (UNR) have been collaborating with the United States Geological Survey (USGS), the Pyramid Lake Paiute Tribe (PLPT), the Desert Research Institute (DRI), and the Nevada Division of Environmental Protection (NDEP) to investigate the use of stable carbon and nitrogen isotopes to understand anthropogenic impacts on the aquatic ecosystem in the Truckee River. Previous work included stable isotope sampling and analysis of the Truckee River aquatic food web (i.e., fish and macroinvertebrates, and periphyton) in the summers of 2002 and 2003 during relatively low flows, and in the spring of 2003 during higher flows. The scope of the current study involved collecting another set of aquatic food web samples in March 2004 on the Truckee River for carbon and nitrogen stable isotope analysis. This report presents the methods and results of this sampling.

The Truckee River is a vital resource to Nevadans in the northwest region of the state. It provides public water supplies to the cities of Reno and Sparks, and while little irrigated agriculture occurs directly adjacent to the river, about one-third of its flow is diverted to the Lahontan Valley for irrigation purposes. The river terminates into Pyramid Lake, which has experienced severe declines in water level because of the heavy water diversions along its length. In addition, there are numerous resort and recreational activities throughout the basin, and the river and Pyramid Lake provide valuable water and habitat for endangered Lahontan cutthroat trout and cui ui species. In 1998, the USGS’s Nevada Basin and Range (NVBR) National Water-Quality Assessment (NAWQA) Program reported that while stream habitat at all sites (based on degradation indices related to riparian vegetation, stream modification, bank stability, and bank erosion) on the Truckee River system was better than the national median, fish communities in the lower reaches of the Truckee River were more degraded than the national median (Bevans et al. 1998). Furthermore, nutrients in the river and trace elements in its sediments increased 3 to 10 times downstream of the discharge from sewage treatment plants and the entrance of Steamboat Creek to the river. Thus, it appears that downstream influences on water quality and associated biological activity are detrimentally affecting the food web in the Truckee River.

The current work involves the use of stable carbon and nitrogen isotopes to gain insight into the aquatic food web. The use of stable isotopes in trophic studies employs the fundamental concept that ‘you are what you eat.’ Stable isotopes incorporate two kinds of information: origin and fractionation. The isotopic signature of an individual will reflect the signature of the sources of the isotopes (i.e., where the isotopes first entered the food web) and the change in the isotopic signature due to isotopic fractionation by consumption and metabolism in the food web (Peterson and Fry 1987). Because isotopes accumulate in body tissues over time, a one-time analysis of stable isotopes provides a time-integrated measure of the diet (Fry and Sherr 1984; Hesslein et al. 1993; Vander Zanden et al. 1998). Stable isotope analysis can even be used in food webs with omnivory because isotope values can be measured in all levels of the food web, including phytoplankton, zooplankton, and aquatic insects (Michener and Schell 1994; Vander Zanden and Rasmussen 1996; France 1997). Carbon and nitrogen ratios are the most commonly used stable isotope ratios in food web studies. Carbon ratios (?13C ) are used because the slight (0.2 – 1.1000) increase of ?13C in animals relative to their diet means that the ?13C signature of the primary producer (first organic food source) is likely to be preserved through several trophic levels (Peterson and Fry 1987; Michener and Schell 1994; Yoshioka et al. 1994; France and Peters 1997). Thus, carbon isotope analysis can be used to identify and distinguish the influence of different primary food sources if the isotopic signatures of those food sources are distinctive enough (Forsberg et al. 1993; Michener and Schell 1994). The nitrogen ratio (?15N ) is often used as an indicator of trophic position of a consumer (Fry 1988; Kling et al. 1992; Yoshioka et al. 1994) because the increase of ?15N with trophic level is much greater than with carbon (~3-4000 per trophic level) (Michener and Schell 1994).

Stable carbon and nitrogen isotopes have value in potentially detecting anthropogenic influences on aquatic food webs. Human- and animal-derived wastewater should have higher ?15N values because of the volatilization of 15N depleted ammonia which occurs during the hydroloysis of urea, and because humans tend to eat higher in the food chain, which elevates their waste nitrogen signatures (Heaton 1986; Silva et al. 2002; Wayland and Hobson 2001). On the other hand, synthetic fertilizers are typically derived by industrial fixation of atmospheric nitrogen (which has a reference signature of 0000), so waters draining fields using these fertilizers tend to have lower nitrogen signatures (Heaton 1986; Silva et al. 2002). Distinctive carbon signatures may be detected when aquatic-terrestrial interactions are altered (e.g. due to alteration of the stream channel and/or flooding regime) because terrestrial plants may have significantly different ?13C signatures than their aquatic counterparts. Such approaches have been used to detect the importance of autochthonous versus allochthonous material in streams (Rounick and Winterbourn 1986; Finlay et al. 1999). In addition, shifts in food web dynamics such as shifts in diets or elimination of species may be detectable with stable isotopes; if the food chain shortens, we should see shifts in nitrogen signatures in the top predators, and if a food source is eliminated at the base of the food web, we may see shifts in the carbon signature.

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