SPATIAL VARIATION IN STABLE WATER ISOTOPES AND DISSOLVED ORGANIC CARBON ACROSS HUBBARD BROOK VALLEY STREAMS

Headwater streams play a critical role in determining downstream water quantity and quality. Streams draining forest ecosystems have received considerable attention both as an important freshwater resource and in their role in the global carbon cycle. In this study, we utilize stable water isotopes to explore variation in different source waters on stream flow generation and dissolved organic carbon export.

Synoptic surveys (100 m intervals) of stream water stable isotope and dissolved organic carbon (DOC) concentrations were conducted in six streams of contrasting geomorphology (e.g., soil depth, slope, aspect) across the Hubbard Brook Valley, NH, USA. Hubbard Brook streams exhibited two distinct longitudinal trends in δ ¹⁸O values. Five of the six streams surveyed (Paradise Brook, East Beaver, Zig Zag, W7, and Cascade) showed significant depletion in δ¹⁸O with decreasing elevation. In contrast, Cushman showed significant enrichment in δ¹⁸O with decreasing elevation. Another observation within the longitudinal patterns is the abrupt change in isotopic signature and DOC concentrations. However, these abrupt changes are not coincident. Various combinations of source water end-members (i.e., snow melt, rain, soil water, ground water) and fine scale topography were examined to interpret these longitudinal patterns. Overall, the data suggest that source water variation plays a minor role in the spatial variability of DOC.