Phosphorus uptake in a stream draining a watershed underlain with discontinuous permafrost in interior Alaska

Background/Question/Methods
In interior Alaska, discontinuous permafrost in watersheds plays an important role in hydrology, nutrient dynamics, vegetation, and stream ecosystem function. Thawing of permafrost may result in increased nutrient inputs to streams from soil organic matter that was previously frozen, which, in turn, will impact in-stream biota. With annual temperature in the north increasing, understanding nutrient inputs and in-stream dynamics is important for predicting future functioning of streams. To determine seasonal phosphorus uptake in streams a series of solute injections were conducted during the summer of 2008 to examine the seasonal pattern in stream phosphorus uptake. The sampling spanned a 300 m reach of a stream with ~100% underlain permafrost in the Caribou Poker Creeks Research Watershed, located 50 km NE of Fairbanks, Alaska. The solute was added as a series of slugs or “pulses” consisting of increasing masses (5, 10, 15, 20 g) of phosphate (Na₂HPO₄) with a conservative solute tracer (NaCl). Sampling stations were 100 m and 300 m downstream of injection site. Once the conductivity at a station started to increase, indicating the arrival of the slug, samples were collected at intervals of ~20 seconds and ~60 seconds until the end of slug. A series of additions were conducted throughout the summer to examine the changes in phosphorus uptake with changes in stream flow, soil and vegetation demand, and seasonal thawing of soil.
Results/Conclusions Preliminary results indicate that as soil thawed through the summer phosphorus uptake increased. As soil thaws during the summer, water flows through the upper horizons of mineral and organic soils, allowing for consumption by the microbial and autotrophic community. As climate warms in the north so will the stability and distribution of permafrost. Understanding how these processes work is essential for the upcoming scenarios of climate change and stream ecosystems.