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Format:
Print
Author:
Greenwald, Morgan Johnston
Dept./Program:
Rubenstein School of Environment and Natural Resources
Year:
2007
Degree:
M.S.
Abstract:
The North Slope of Alaska's Brooks Range is underlain by continuous permafrost, but an active layer of thawed sediments develops at the surface of the tundra and beneath streambeds during the summer. The thawed sediments facilitate hyporheic exchange. The hyporheic zone is an important site for organic matter decomposition and nutrient regeneration to the surface water. Information on the characteristics of hyporheic processes, both physical and biogeochemical, is limited in arctic stream systems.
The goal of the first study was to understand how hyporheic exchange patterns and biogeochernical processes are influenced by stream geomorphology and the extent of the thaw bulb. Two second-order arctic tundra streams of contrasting geomorphology were studied. The first was a high-gradient, cobble-bottomed alluvial stream characterized by distinct riffle-pool sequences (average thaw depth: 157 cm). The second was a lowgradient, peat-bottomed, beaded stream characterized by large deep pools connected by deep runs (average thaw depth: 67 cm).
The degree of surface-subsurface exchange decreased with depth in the subsurface in both streams, but was substantial at depths>50 cm in the alluvial stream, whereas all surface-subsurface exchange in the peat stream occurred within the top ~10 cm of the streambed sediments. While the surface waters of the two streams were not significantly different fiom one another in terms of biogeochemistry, their subsurface waters were significantly different from one another in every biogeochemical constituent measured. In the alluvial stream, subsurface concentrations of nitrate, ammonium, soluble reactive phosphorus (SRP) and DOC were not significantly different from the surface water at any depth. Dissolved oxygen (DO) concentration decreased significantly with depth, yet remained sufficiently oxic at the deepest depth sampled.
In the peat stream, however, DO and nitrate concentrations decreased significantly with depth to a point where they were not detectable at ~30 cm, while ammonium, SRP and DOC concentrations increased exponentially with depth in the subsurface. The physical differences in the geomorphology of the streams led to differences in their hyporheic exchange characteristics, which then influenced the biogeochemistry of the subsurface water. In turn, the stream ecosystems were influenced differently by nutrient regeneration fiom their hyporheic zones. The hyporheic zone served as a source of ammonium and SRP to the surface water in both streams. The hyporheic zone served as a net nitrate sink in the peat stream and as a net nitrate source in the alluvial stream.
In the second study, physical water movement and subsurface biogeochemistry were examined in a.network of well-defined subsurface flow paths through a natural gravel bar. The most utilized subsurface flow paths were approximately 1 m beneath the surface of the gravel bar. These flow paths exhibited high dissolved oxygen concentrations due to their high degree of connection to the surface water and also had lower ammonium and SRP concentrations than less-connected regions, where inorganic nutrient concentrations tend to build up. The gravel bar served as a net source of ammonium (18.01 [mu]M m⁻¹ d⁻¹) and SRP (0.37 [mu]M m⁻¹ d⁻¹) to the surface water and as a net nitrate sink ( -0.99 [mu]M m⁻¹ d⁻¹).