Autochthony in Karst Spring Food Webs

Trophic dynamics and the relative importance of allochthonous versus autochthonous production in supporting invertebrate consumers have been studied from headwaters to large rivers but rarely in springs. Our goals were to examine spatial and temporal invertebrate feeding pathways in three U.S. karst springs differing in canopy coverage and type (coniferous, open, or deciduous). Based on current food web models for headwaters, we initially hypothesized that: (a) overall biomass production would primarily depend on allochthonous carbon; (b) terrestrial carbon would be most important following leaf litter input; and (c) autochthonous carbon would be responsible for most of the biomass production in open canopy areas. To test these predictions, we analyzed trophic pathways using carbon and nitrogen stable isotopes, biomass measurements, and stoichiometric analyses. Contrary to our initial hypotheses, the proportional contribution of autochthonous carbon to consumer biomass exceeded that of allochthonous in each spring along temporal and spatial scales. Autochthony increased spatially along the gradient between spring sources and lower springbrook reaches and was highest in the open canopy spring. Allochthony increased slightly during the winter following litter fall. Despite the reported importance of periphyton to zoobenthic consumers, macroinvertebrate biomass was primarily supported by moss and water cress in these springs.