The Trophic Significance of Bacteria in a Detritus-based Stream Food Web

We compared relative use of streamwater dissolved organic carbon (DOC) by bacteria and the trophic significance of bacteria to invertebrates in two headwater streams at Coweeta Hydrologic Laboratory in North Carolina: a stream with all leaf litter inputs excluded for 1 yr, and a reference stream. Leaf litter standing crop in the treatment stream was ,1% that of the reference stream, and fine benthic organic matter (FBOM) was 50% lower than the reference. We used a whole-stream tracer addition of 13C-1 sodium acetate for 3 wk to label bacteria and hence their consumers during both July and December. Bacterial d13C was measured by collecting respired bacterial carbon. We estimated the contribution of bacterial carbon to consumers using a mixing model for invertebrates and bacteria. The acetate label declined exponentially downstream with a 10-m uptake length in each stream and season. FBOM and biofilm were the only detrital samples to show a strong label; both were more labeled in the litter-excluded stream. Bacteria in the litterexcluded stream had 7–10 times more label than those in the reference stream during both seasons, showing their higher relative use of streamwater DOC. The percentage of invertebrate carbon derived from bacteria was .20% for many taxa. This was significantly related to the percentage of amorphous detritus in invertebrate guts, suggesting that bacterial carbon supporting higher trophic levels was associated with amorphous detrital particles. Predatory invertebrates were labeled, showing that bacterial carbon was important for higher trophic levels. Some invertebrates were more highly labeled than the bacteria. Stenonema in the treatment stream contained eight times more label than measured bacteria. This suggests that they were using an unmeasured bacterial source such as bacteria in exposed epilithic biofilms, which had higher d13C than all other detrital components. Invertebrates in the treatment stream did not appear to use more bacterial carbon than in the reference stream despite a lower standing crop of detritus. Tallaperla, a shredding stonefly, derived 20–40% of its carbon from bacteria in both streams, even though it was more labeled in the treatment stream. Our estimates of the percentage of invertebrate carbon derived from bacteria were higher than those found in laboratory-based studies. To investigate reasons for this difference, we examined the possibility that bacterial carbon was principally found in exopolymers, as our labeling method would have labeled exopolymers. We found 6 g/ m2 of colloidal carbohydrates in the reference stream, which was five times greater than bacterial biomass; thus the high use of bacterial carbon by invertebrates may be a consequence of the availability of these polymers.