Tuesday, May 27, 2008 - 11:15 AM

Drainage-area thresholds for shifting trophic interactions in the South Fork Eel River watershed

Camille McNeely1, Jacques C. Finlay2, Mary E. Power3, and Michael Limm3. (1) Sci 258, Eastern Washington University, Department of Biology, Cheney, WA 99004, (2) Ecology Evolution and Behavior, University of Minnesota, 100 Ecology Building, 1987 Upper Buford Circle, St. Paul, MN 55104, (3) Integrative Biology, University of California Berkeley, 3060 Valley Life Sciences Building #3140, Berkeley, CA 94720

We have identified shifts in major trophic interactions associated with increasing drainage area (DA) in the South Fork Eel River watershed, Mendocino, Co. CA, USA. As stream drainage areas increase from <1 km2 to 17 km2, we observe increasing dominance of algae in food webs. We observe major shifts in major trophic interactions at two drainage area thresholds. Algal grazers, although present and consuming algae, do not limit algal accrual in streams with DA < 1 km2. Above a threshold at 1-2 km2 DA, food webs are dominated by armored invertebrate grazers that limit algal growth and compete with invertebrates that support predators, which are fueled by terrestrial detritus. We observe a second threshold at a DA of about 10 km2. In streams smaller than 10 km2, predators are supported primarily by terrestrial production, while in larger streams they are supported primarily by aquatic production. Shifts in both algal production and detrital inputs clearly underlie this general trend. To better understand the processes that mediate these strong spatial patterns, we are using stable isotope analyses, experiments and diet studies to examine controls of transfer of terrestrial and algal production to top predators in stream food webs.

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