Date of Award

Summer 1992

Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences


Ecological Sciences

Committee Director

Frank P. Day

Committee Member

Ray Alden

Committee Member

Colbert Cushing


Stable carbon isotope analysis, a technique that measures the ratio of stable carbon-12 to stable carbon-13 in abiotic and biotic components of ecosystems, was used to assess the differential utilization of autochthonous and allochthonous carbon by aquatic insects of two cold desert spring-streams in the Columbia Basin of Washington State. The aquatic insects of Snively Springs, a heavily overgrown and densely shaded stream, has isotopic values close to that of stream aufwuchs (substrata scrubate) indicating substantial autochthonous carbon dependence. Aquatic insects of Douglas Creek, the more open and less shaded of the two streams, were equally dependent upon autochthonous and allochthonous carbon. In addition to insects, isotopic values of several other consumers were determined at both streams. The stable carbon isotopes of insects, combined with the isotopic values of the additional consumers, strongly suggest that the food webs of both streams are principally based on autochthonous energy.

Allochthonous carbon isotope values at both streams were consistently in the expected -27.0 0/00 -28.0 0/00 range. Autochthonous carbon isotopic values varied widely within each stream among the various types of producers: epilithic aufwuchs, filamentous green algae, emergent macrophytes, and submerged macrophytes. The wide isotopic variance among autochthonous carbon sources, some of which were consistently in the expected allochthonous isotopic range, made it impossible to quantify differential utilization of autochthonous and allochthonous carbon at either stream. However, within the study assumptions, stable carbon isotope technology did enable a limited qualitative indication of relative dependence on in-stream and out-of-stream carbon.

The application of stable carbon isotope technology to cold desert spring-stream carbon utilization has substantial problems. Many of the assumptions upon which this study and similar studies in the literature have been based lead to incomplete and/or erroneous data interpretations. I conclude that stable carbon isotope technology is not helpful in unambiguously assessing the differential utilization of allochthonous and autochthonous carbon in the study streams and question its value in similar studies of all freshwater streams.


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