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 Preserve 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 Preserve and the Noatak National Preserve, Alaska. Fairbanks, AK
SUMMARY 1. We used stable isotope analyses of the organic matter components of 32, 1st-3rd order, open-canopy, arctic streams in the Noatak National Preserve, Alaska, to determine the trophic structure of the stream food-webs and the relative importance 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 trophic 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 occupied by fish species in both years of the study. 3. We used the stable isotope ratio of carbon (δ13C) in macroinvertebrate taxa that are known to be grazers, to infer epilithic algal δ13C and compared these estimates with the δ13C values of terrestrial detritus in each stream. We determined that the stable isotopic composition of autochthonous food resources were highly variable across sites (-10 to +14‰ δ15N, -40 to -30‰ δ13C) and were much more variable than allochthonous sources as reflected by Salix spp. (-5 to +1‰ δ15N, -30 to -26‰ δ13C). 4. Because baseline isotopic values varied widely among streams we used a site-specific approach to determine that epilithic algae formed 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 consistent 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 isotopic compositions of consumers relative to terrestrial and aquatic end-members suggests that some macroinvertebrates occupy broad ecological niche spaces. We interpret this as opportunistic generalism that is a result of a low diversity of food resources in these arctic streams.