939 Ecology, 85(4), 2004, pp. 939–954  2004 by the Ecological Society of America LONG-TERM RESPONSES OF THE KUPARUK RIVER ECOSYSTEM TO PHOSPHORUS FERTILIZATION K. SLAVIK, 1 B. J. PETERSON, 2 L. A. DEEGAN, 2 W. B. BOWDEN, 3 A. E. HERSHEY, 4 AND J. E. HOBBIE 2 1 University of Michigan Biological Station, Ann Arbor, Michigan 48109-1090 USA 2 The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 48104 USA 3 School of Natural Resources, University of Vermont, Burlington, Vermont 05405 USA 4 Department of Biology, University of North Carolina, Greensboro, North Carolina 27402 USA Abstract. A long-term stream fertilization experiment was performed to evaluate the potential eutrophication of an arctic stream ecosystem. During 16 years of summer phos- phorus (H 3 PO 4 ) fertilization, we observed a dramatic change in the community structure of the Kuparuk River on the North Slope of Alaska. A positive response to fertilization was observed at all trophic levels with increases in epilithic algal stocks, some insect densities, and fish growth rates. After approximately eight years of P fertilization, bryo- phytes (mosses) replaced epilithic diatoms as the dominant primary producers in the Ku- paruk River. The moss impacted NH 4 + uptake rates, benthic gross primary production, habitat structure, and insect abundance and species composition. This study documents the long-term changes in an arctic tundra stream in response to nutrient enrichment. Predicting stream ecosystem responses to chronic perturbation requires long-term observation and experiments. Key words: arctic stream; bryophytes; community structure; ecosystem response; grayling; in- sects; Kuparuk River, Alaska (USA); nutrient enrichment; phosphorus fertilization; stream fertilization. INTRODUCTION Eutrophication can substantially alter the structure and function of freshwater ecosystems (Smith et al. 1999). Increases in nutrient loading to arctic ecosys- tems may occur through disturbance in the watershed or changes in regional climate. In the Arctic, an in- crease in local human activities, such as those asso- ciated with oil drilling and road building, will likely lead to a direct increase in nutrient loading of oligo- trophic arctic watersheds. Global climate models pre- dict global warming, and it is likely that the effects of these climate changes will not only be recognized first, but also amplified, in the Arctic (Lachenbruch and Mar- shall 1986, Abelson 1989, Chapman and Walsh 1993). Chapin et al. (1995) and Nadelhoffer et al. (1992) sug- gest that arctic tundra warming will cause the perma- frost to thaw and increase the depth of the active soil layer, leading to an increase in organic matter turnover rates and nutrient availability to tundra ecosystems. The deeper thaw layer and extended thaw season will increase nutrient availability, expose mineral soil, and increase subsurface drainage leading to an increase in nutrient loading to streams (Kane et al. 1992, Woo et al. 1992, Rouse et al. 1997, Hobbie et al. 1999). We used long-term monitoring and whole-stream nu- trient enrichment experiments to understand the im- pacts of a changing climate and the potential impacts of eutrophication, as well as to discern patterns of nat- Manuscript received 6 May 2002; revised 3 July 2003; ac- cepted 14 July 2003; final version received 14 August 2003. Cor- responding Editor: B. Downes. ural variation in arctic streams. The Kuparuk River, an arctic tundra stream, has been experimentally enriched with phosphorus every summer since 1983 as a part of the Toolik Lake Long-Term Ecological Research (LTER) project. After the first four years of fertiliza- tion, algal biomass and productivity increased dra- matically, leading to increased growth rates and den- sities of some insect species and young-of-the-year (age-0) and adult fish (Peterson et al. 1985, 1993a, b, Deegan and Peterson 1992). In this paper we extend these results and examine how the river has changed since the fertilization began in 1983. We observed a dramatic change in community structure of the Ku- paruk River during the 16-year phosphorus fertiliza- tion. METHODS Site description The Kuparuk River originates in the foothills of Alaska’s Brooks Range and flows north–northeast to the Arctic Ocean, draining an area of 8107 km 2 . It is classified as a clear-water tundra river (Craig and McCart 1975) because it has no input from glaciers and little input from springs. Our 5-km study reach strad- dles the Dalton Highway (68°38' N, 149°24' W), where the Kuparuk is a meandering, fourth-order stream (Fig. 1) draining an area of 143 km 2 of alpine and moist tundra communities. Dwarf birch and willows line sec- tions of the stream bank but do not shade the channel. Within our study reach, the river channel has a mean width of 17 m and is characterized by alternating riffles (20–30 m long and 5–30 cm deep) and pools