The role of instream vs allochthonous N in stream food webs: modeling the results of an isotope addition experiment

• Published in: Journal of the North American Benthological Society 2004-09, Vol.23 (3), p.429-448
• Main Authors: Hamilton, Stephen K., Tank, Jennifer L., Raikow, David F., Siler, Edward R., Dorn, Nathan J., Leonard, Norman E.
• Format: Article
• Language: English
• Subjects: Gammarus; Marine; Orconectes propinquus; Pleurobema sintoxia; Psephenus; Stenelmis
• ISSN: 0887-3593; 1937-237X
• DOI: 10.1899/0887-3593(2004)023<0429:TROIVA>2.0.CO;2

Stable isotope enrichment experiments offer a potentially powerful way to examine the base of aquatic food webs, but interpretation of the data from these experiments may be confounded by problems such as selective ingestion/assimilation of bulk food sources by consumers, variable tracer enrichment over time, and the failure of consumers to approach isotopic equilibrium with the tracer in their diets over the course of the experiment. Our study examined data from a stable isotope addition experiment in which 15N-labeled NH4+ was added to a midwestern US stream. A compartment model was used to provide insights into the kinetics of 15N uptake and release from algae, heterotrophic microbes colonizing detritus, and invertebrate consumers. The model accounted for temporal variation in the degree of isotopic enrichment and did not require the assumption of isotopic equilibrium between consumers and their diets. The importance of instream production (i.e., growth of algae and microbes within the study reach during the experiment) relative to allochthonous and upstream inputs was similar to 38 to 50% for heptageniids and Psephenus, 10 to 20% for Orconectes propinquus, Gammarus, hydropsychids, and larval Stenelmis, and