Nutrient limitation of periphyton in Idaho streams: results from nutrient diffusing substrate experiments Beth L. Sanderson 1,4 , Holly J. Coe 1,5 , Chau D. Tran 2,6 , Kate H. Macneale 1,7 , Deborah L. Harstad 1,8 , AND Adam B. Goodwin 3,9 1 Northwest Fisheries Science Center, 2725 Montlake Blvd. East, Seattle, Washington 98112 USA 2 Water Resources Program, University of Idaho, College of Graduate Studies, P.O. Box 443002, Moscow, Idaho 83844-3002 USA 3 Environmental Sciences Program, University of British Columbia, Okanagan, College of Graduate Studies, 3333 University Way, Kelowna, British Columbia, Canada V1V 1V7 Abstract. Many streams and rivers in the Pacific Northwest of the US are inherently oligotrophic, and primary production in these ecosystems is assumed to be nutrient-limited. In many of these streams, reductions in the amount of marine-derived nutrients delivered by spawning salmonids could be exacerbating the degree of oligotrophication. To test whether primary producers are nutrient-limited, nutrient diffusing substrate (NDS) experiments were used to measure algal responses to amendments of N, P, and a combination of N and P (N+P) in 13 Salmon River basin streams in central Idaho, USA. Thirty- eight experiments were conducted between 2003 and 2006 to determine whether nutrient limitation varied among streams and over time within individual streams. Primary producers in most streams showed some form of nutrient limitation. Thirty-nine percent of our experiments suggested N and P colimitation, 18% suggested N limitation, 11% suggested primary N and secondary P limitation, and 32% did not indicate limitation by either N or P. The type of nutrient limitation within individual streams varied with time, and the relative importance of N or P changed seasonally or annually in 7 of the 13 streams. Algal accrual rates on control and treatment substrates were most strongly predicted by water temperature, light, and ambient concentrations of N and P. Among all of the experiments, algal accrual rates were greater on N substrates in streams with lower ambient N concentrations and greater ambient P concentrations. Our results suggest that a combination of N and P typically limits primary producers in these streams. Our efforts to characterize current nutrient limitation in these streams will be of value to managers considering nutrient additions as a tool to improve stream productivity to benefit threatened and endangered salmonids. Key words: nutrient limitation, stream, nutrient diffusing substrate, nitrogen, phosphorus. In rivers, primary producers are a vital energy source for secondary consumers, including inverte- brates and fish. In unshaded rivers, low rates of primary production and limited biomass of primary producers could have bottom-up effects on higher trophic levels in aquatic food webs (Peterson et al. 1993). Factors influencing rates of primary production (e.g., benthic algae, macrophytes, and phytoplank- ton), include nutrient and light availability, temper- ature, disturbance, and grazers (Findlay et al. 1993, Rosemond et al. 1993, Biggs 1996, 2000, Hill 1996). In lotic ecosystems with minimal shading canopy, nutrient availability is often the most important factor limiting primary productivity (Hill and Knight 1988, Kiffney et al. 2004, Von Schiller et al. 2007). What factors limit primary and secondary produc- tion of aquatic food webs has been a particularly interesting question in salmonid streams in the Pacific Northwest of the US. Because of declines in migratory salmon populations, the amount of marine-derived nutrients (MDN; nutrients delivered to freshwater ecosystems via spawning adult salmonids) returned to Pacific coastal streams has been reduced to 6 to 7% of historical levels (Gresh et al. 2000). Several recent studies have illustrated the importance of MDN subsidies for Pacific Northwest streams, especially 4 E-mail addresses: beth.sanderson@noaa.gov 5 holly.coe@noaa.gov 6 chau.tran@vandals.uidaho.edu 7 kate.macneale@noaa.gov 8 deborah.harstad@noaa.gov 9 adam.goodwin@ubc.ca J. N. Am. Benthol. Soc., 2009, 28(4):832–845 ’ 2009 by The North American Benthological Society DOI: 10.1899/09-072.1 Published online: 29 September 2009 832