Endogenous and exogenous control of ecosystem function: N cycling in headwater streams
- ORNL
- Virginia Polytechnic Institute and State University (Virginia Tech)
- University of Nebraska
- University of New Mexico, Albuquerque
- Griffith University, Nathan, Queensland, Australia
- Loyola University
Allochthonous inputs act as resource subsidies to many ecosystems, where they exert strong influences on metabolism and material cycling. At the same time, metabolic theory proposes endogenous thermal control independent of resource supply. To address the relative importance of exogenous and endogenous influences, we quantified spatial and temporal variation in ecosystem metabolism and nitrogen (N) uptake using seasonal releases of {sup 15}N as nitrate in six streams differing in riparian-stream interaction and metabolic character. Nitrate removal was quantified using a nutrient spiraling approach based on measurements of downstream decline in {sup 15}N flux. Respiration (R) and gross primary production (GPP) were measured with whole-stream diel oxygen budgets. Uptake and metabolism metrics were addressed as z scores relative to site means to assess temporal variation. In open-canopied streams, areal uptake (U; {micro}g N {center_dot} m{sup -2} {center_dot} s{sup -1}) was closely related to GPP, metabolic rates increased with temperature, and R was accurately predicted by metabolic scaling relationships. In forested streams, N spiraling was not related to GPP; instead, uptake velocity (v{sub f}; mm/s) was closely related to R. In contrast to open-canopied streams, N uptake and metabolic activity were negatively correlated to temperature and poorly described by scaling laws. We contend that streams differ along a gradient of exogenous and endogenous control that relates to the relative influences of resource subsidies and in-stream energetics as determinants of seasonal patterns of metabolism and N cycling. Our research suggests that temporal variation in the propagation of ecological influence between adjacent systems generates phases when ecosystems are alternatively characterized as endogenously and exogenously controlled.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Oak Ridge National Environmental Research Park
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 944101
- Journal Information:
- Ecology, Vol. 89, Issue 12; ISSN 0012-9658
- Country of Publication:
- United States
- Language:
- English
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