Foodweb characteristics based on stable isotopic analyses of carbon and nitrogen from streams in the central and upper Noatak River basin: Gates of the Arctic Park and Pr eserve and the Noatak National Preserve, Alaska
Allen AR, Peterson BJ, Huryn AD, Flinn MB, Bowden WB. 2009. Foodweb characteristics based on stable isotopic analyses of carbon and nitrogen from streams in the central and upper Noatak River basin: Gates of the Arctic Park and Pr eserve and the Noatak National Preserve, Alaska. National Park Service Arctic Network Inventory and Monitoring Program. Providence, RI
1. We used stable isotope analyses of the organic matter components of 32, 1 st -3 rd order, open-canopy, arctic streams in the Noatak Na tional Preserve, Alaska, to determine the trophic structure of the stream food-webs a nd the relative importanc e of terrestrial and aquatic organic matter to stream consumers during two field seasons (2005 and 2006). 2. Noatak streams are simple systems with low taxonomic richness ( ≤ 10 taxa/site) and 2- 4 trophic levels. The mean number of trophi c positions was similar across streams and years. In 2005, the mean number of trophic positions was 2.1 and in 2006 was 2.5. The highest trophic positions, 3.5 and 3.6, were occu pied by fish species in both years of the study. 3. We used the stable isotope ratio of carbon ( δ 13 C) in macroinvertebrate taxa that are known to be grazers, to infer epilithic algal δ 13 C and compared these estimates with the δ 13 C values of terrestrial detritus in each str eam. We determined that the stable isotopic composition of autochthonous food resources we re highly variable across sites (-10 to +14‰ δ 15 N, -40 to -30‰ δ 13 C) and were much more variab le than allochthonous sources as reflected by Salix spp. (-5 to +1‰ δ 15 N, -30 to -26‰ δ 13 C). 4. Because baseline isotopic values varied widely among streams we used a site-specific approach to determine that epilithic algae fo rmed the majority of the energy base of 24 out of 32 of the stream food-webs. Moreover, a two-source mixing model showed that 4 of the 5 consumer taxa in 2005 and 9 of the 12 consumer taxa in 2006 were comprised of >50% autochthonous carbon. This is consis tent with streams draining unforested watersheds that have open-canopies. 5. Autochthonous and allochthonous resource use varied among functional feeding groups across sites and years. Variable isot opic compositions of consumers relative to terrestrial and aquati c end-members suggests that some macroinvertebrates occupy broad ecological niche spaces. We interpret this as oppo rtunistic generalism that is a result of a low diversity of food resources in these arctic streams.