Increased Light Availability Reduces the Importance of Bacterial Carbon in Headwater Stream Food Webs
- 550 Downloads
Many ecosystems rely on subsidies of carbon and nutrients from surrounding environments. In headwater streams that are heavily shaded by riparian forests, allochthonous inputs from terrestrial systems often comprise a major part of the organic matter budget. Bacteria play a key role in organic matter cycling in streams, but there is limited evidence about how much bacterial carbon is actually assimilated by invertebrate and fish consumers, and how bacterial carbon assimilation varies among streams. We conducted stable isotope tracer additions of 13C-acetate, that is assimilated only by bacteria, and 15N-ammonium, that is assimilated by both bacteria and algae, in two small, shaded streams in the Adirondack region of New York State, USA. Our goal was to determine whether there is an important trophic link between bacteria and macroconsumers, and whether the link changes when the light environment is experimentally altered. In 2009, we evaluated bacterial carbon use in both streams with natural canopy cover using 10-day dual-isotope tracer releases. The canopy was then thinned in one stream to increase light availability and primary production and tracer experiments were repeated in 2010. As part of the tracer experiments, we developed a respiration assay to measure the δ 13C content of live bacteria, which provided critical information for determining how much of the carbon assimilated by invertebrate consumers is from bacterial sources. Some invertebrate taxa, including scraper mayflies (Heptagenia spp.) that feed largely on biofilms assimilated over 70% of their carbon from bacterial sources, whereas shredder caddisflies (Pycnopsyche spp.) that feed on decomposing leaves assimilated less than 1% of their carbon from bacteria. Increased light availability led to strong declines in the magnitude of bacterial carbon fluxes to different consumers (varying from −17 to −91% decrease across invertebrate taxa), suggesting that bacterial energy assimilation differs not only among consumer taxa but also within the same consumer taxa in streams with different ecological contexts. Our results demonstrate that fluxes of bacterial carbon to higher trophic levels in streams can be substantial, that is over 70% for some taxa, but that invertebrate taxa vary considerably in their reliance on bacterial carbon, and that local variation in carbon sources controls how much bacterial carbon invertebrates use.
Keywordsmicrobial loop stream ecology bacteria food web light availability stable isotope tracer
This research was supported by funding from the Cornell Biogeochemistry and Environmental Bioxomplexity IGERT program and the Kieckhefer Adirondack Fellowships program. Fumika Takahashi assisted with sample collection in the field and Kim Sparks assisted with preparing equipment for gas sampling. Dan Josephson, Cliff Kraft, the Adirondack League Club, and the Little Moose Field Station provided access to field sites and logistical support. Walter Dodds developed the dynamic compartment model used to calculate turnover time. This manuscript was improved by suggestions from Nelson Hairston Jr., Stuart Findlay, Cliff Kraft, and two anonymous reviewers.
- Barbour MT, Gerritsen J, Snyder BD, Stribling JB. Rapid bioassessment protocols for use in streams and wadeable rivers: periphyton, benthic macroinvertebrates and fish, 2nd ed. EPA 841-B-99-002. Washington DC: U.S. Environmental Protection Agency; Office of Water; 1999.Google Scholar
- Dodds WK, Collins SM, Hamilton SK, Tank JL, Johnson S, Webster JR, Simon KS, Whiles MR, Rantala HM, McDowell WH, Peterson SD, Riis T, Crenshaw CL, Thomas SA, Kristensen PB, Cheever BM, Flecker AS, Griffiths NA, Crowl T, Rosi-Marshall EJ, El-Sabaawi R, Marti E. 2014. You are not always what we think you eat: selective assimilation across multiple whole-stream isotopic tracer studies. Ecology 95:2757–67.CrossRefGoogle Scholar
- Hynes HBN. 1975. The stream and its valley. Int Verein Theor Angew Limnol Verh 19:1–15.Google Scholar
- Jespersen AM, Christoffersen K. 1987. Measurements of chlorophyll-a from phytoplankton using ethanol as extraction solvent. Arch Hydrobiol 109:445–54.Google Scholar
- Mulholland PJ, Helton AM, Poole GC, Hall RO Jr, Hamilton SK, Peterson BJ, Tank JL, Ashkenas LR, Cooper LW, Dahm CN, Dodds WK, Findlay SEG, Gregory SV, Grimm NB, Johnson SL, McDowell WH, Meyer JL, Valett HM, Webster JR, Arango CP, Beaulieu JJ, Bernot MJ, Burgin AJ, Crenshaw CL, Johnson LT, Niederlehner BR, O’Brien JM, Potter JD, Sheibley RW, Sobota DJ, Thomas SM. 2008. Stream denitification across biomes and its response to anthropogenic nitrate loading. Nature 452:202–6.CrossRefPubMedGoogle Scholar
- Mulholland PJ, Tank JL, Sanzone DM, Wollheim WM, Peterson BJ, Webster JR, Meyer JL. 2000. Nitrogen cycling in a forest stream determined by a N15 tracer addition. Ecol Monogr 70:471–93.Google Scholar
- Newell SY. 1984. Bacterial and fungal productivity in the marine environment: a contrastive overview. Colloq Int Cent Natl Rech Sci 331:133–9.Google Scholar
- Nusch EA. 1980. Comparison of different methods for chlorophyll and pheopigment determination. Arch Hydrobiol Bull 14:14–36.Google Scholar
- R Core Team. 2013. R: a language and environment for statistical computing. Vienna: R Foundation for Statistical Computing.Google Scholar
- Tank JL, Bernot MJ, Rosi-Marshall EJ. 2007. Nitrogen limitation and uptake. In: Hauer FR, Lamberti GR, Eds. Methods in stream ecology. San Diego: Academic Press. Google Scholar
- Thimijan RW, Heins RD. 1983. Photometric, radiometric and quantum light units of measure: a review of procedures for interconversion. Hortic Sci 18:818–22.Google Scholar