Examining the role of bacteria in stream food webs using a dual-isotope tracer approach

Background/Question/Methods

In streams, organic matter sources include both autochthonous carbon, which is fixed in the stream by autotrophs, and allochthonous carbon, which is fixed terrestrially and enters the stream as detritus.  Bacteria play an important role in the processing of allochthonous carbon.  Because few trophic transfers occur between bacteria and macroconsumers in streams, it is thought that allochthonous carbon from microbial pathways comprises a substantial portion of the energy used by higher trophic levels.  However, the magnitude of fluxes of bacterial carbon to higher trophic levels in stream food webs is not well documented, in part due to methodological difficulties.  We used a novel stable isotope tracer method that uses both carbon and nitrogen tracers to track bacterial energy flows in two small, headwater streams in the Adirondack region of New York.  Initially, a hardwood riparian forest shaded both of our study streams.  We thinned the canopy of one stream by removing any large trees adjacent to the stream.  A year after canopy thinning, we repeated isotope tracer studies in both streams to determine how canopy thinning influenced the food web.  Carbon or nitrogen tracers have been used in previous studies, but our coupled tracer approach is unique in stream ecosystems.

Results/Conclusions

Before canopy manipulation, the food webs of both streams are supported mostly by terrestrial energy rather than by light inputs and in-stream photosynthesis.  Pre-manipulation data indicate that bacteria contributed up to 75% of the energy assimilated by invertebrates.  Amount of bacterial contribution to consumers varied among insects: for example, scraper mayflies (Stenomena sp.) assimilated high percentages of bacterial carbon (50-75%), while shredder caddisflies (Pycnopsyche sp.) assimilated very low percentages of bacterial carbon (<1%).  Canopy thinning led to dramatic increases in light availability and subsequent increases in primary production. Bacteria were a very important carbon source for invertebrate taxa in both streams, but all invertebrate taxa assimilated substantially less bacterial carbon in the manipulated stream after canopy thinning.  Our data indicate that bacteria can serve as an important energy link in headwater stream ecosystems.  Dual-isotope tracer methods are effective to quantify the amount of bacterial carbon that reaches consumers.