Discontinuous permafrost affects the hydrology of boreal forest streams by restricting flow path depth where permafrost is present. Stream water chemistry, such as flux of inorganic nutrients and dissolved organic matter (DOM), is subsequently affected by permafrost distribution. Headwater streams in the boreal forest are dependent on allochthonous inputs of energy and nutrients, and as a result are dominated by heterotrophic processes. A warming climate may change the distribution of permafrost in high latitude ecosystems, with consequences for nutrient availability. Herein, we investigated how nutrient stoichiometry interacts with the availability of labile carbon to control nutrient uptake. We hypothesized that 1) the carbon, nitrogen, and phosphorus (C:N:P) ratio in DOM affects nutrient uptake due to stoichiometric constraints on autotrophic and heterotrophic nutrient assimilation, and 2) labile DOM affects nutrient uptake by increasing heterotrophic production. We tested these hypotheses using a series of instantaneous nutrient additions with a factorial design manipulating both C:N:P ratios and DOM source (acetate or birch leaf leachate) in nine headwater streams in interior Alaska. We added N and P (as NH4+ and PO43-) alone, simultaneously, and in combination as DOM. Uptake velocities (Vf-amb) for C, N, and P were within the upper range of literature values from other ecosystems, ranging from 4.1 – 67.2 mm/min for N, 4.0 – 25.0 mm/min for P, and 4.2 – 34.5 mm/min for acetate. Vf-amb was in part explained by ambient stream chemistry, with N Vf-amb weakly positively correlated with ambient P concentration, while uptake velocity for SRP and acetate was weakly negatively correlated with ambient inorganic N and ambient dissolved organic carbon (DOC), respectively. In addressing our nutrient stoichiometry hypothesis, we found uptake efficiency (UE) was similar for N and P added alone, in co-additions, and with DOM. For our labile DOM hypothesis, acetate and birch-derived DOM had similar effects on N and P UE during in-stream additions. During laboratory incubations both carbon sources were very labile with ≥ 70% of acetate and birch-DOM mineralized over 30 days. Overall, our results were inconclusive as to whether DOM stoichiometry or source affected uptake, but the addition of inorganic nitrogen increased DOC loss from birch DOM. High demand for nutrients in boreal forest headwater streams suggests that uptake could increase concurrently with greater inorganic nutrient flux following a loss in permafrost extent, and that response in stream nutrient uptake is relatively flexible to changes in DOM source and nutrient stoichiometry.
|Advisor:||Jones, Jeremy B.|
|Commitee:||Harms, Tamara K., Kielland, Knut|
|School:||University of Alaska Fairbanks|
|Department:||Biology & Wildlife|
|School Location:||United States -- Alaska|
|Source:||MAI 55/02M(E), Masters Abstracts International|
|Keywords:||CPCRW, DOM, Nutrients, TASSC, Uptake velocity|
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