Wednesday, June 6, 2007 - 11:45 AM
238

Quantifying the Roles of Neotropical Tadpoles in Headwater Stream Nitrogen Cycling: the Influence of Starvation and Stress on Excretion Estimates

Matt R. Whiles1, Alex D. Huryn2, Robert O. Hall3, Karen R. Lips1, Catherine M. Pringle4, Susan S. Kilham5, Walter K. Dodds6, and Chad E. Montgomery7. (1) Department of Zoology and Center for Ecology, Southern Illinois University, Carbondale, IL 62901-6501, (2) Biological Sciences, University of Alabama, 2620 University Blvd, Tuscaloosa, AL 35401, (3) Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, (4) Institute of Ecology, University of Georgia, Athens, GA 30602-2202, (5) Department of Biosciences and Biotechnology, Drexel University, Philadelphia, PA 19104-2875, (6) Division of Biology, Kansas State University, 116 Ackert Hall, Manhattan, KS 66506, (7) Department of Biology, University of Wisconsin-La Crosse, La Crosse, WI 54601

Excretion by consumers can be a significant component of N cycling in streams, particularly in low N environments.  Hence, excretion estimates are important for quantifying roles of consumers in N cycling.  As part of the TADS (Tropical Amphibian Declines in Streams) project, we measured excretion rates of a variety of common tadpoles in headwater streams in Panama.  Tadpoles were incubated in 60 ml containers for periods ranging from 15 min. to 3 hours, and changes in ammonium concentrations were measured.  Mass-specific estimates ranged from 0.003 - 2.1 μgNH4/mg/hr, indicating abundant tadpole assemblages can mineralize significant quantities of N in these oligotrophic streams.  However, as has been observed by others, estimates varied greatly with incubation time, with shorter incubations producing higher rates.  To test the hypothesis that starvation during incubation reduced excretion, we starved groups of tadpoles for 0, 1, 2, and 6 hr. before measuring excretion and found that excretion rates consistently declined with time, regardless of starvation period.  This indicates that handling stress, which causes initially higher excretion rates, is responsible for observed declines over time.  Hence, incubation time is a critical factor in these studies and initially high values from handling stress should be factored out of estimates.

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