Biogeochemical Dynamics and Response to Permafrost Degradation in McMurdo Dry Valley Streams, Antarctica

Torrens, Christa L.

Graduate Thesis Or Dissertation

Biogeochemical Dynamics and Response to Permafrost Degradation in McMurdo Dry Valley Streams, Antarctica Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/3x816p41b

Abstract

Streams are biogeochemical connectors that transport and transform nutrients and carbon throughout their watershed. This ability can be altered temporarily or permanently by anthropogenic disturbances like climate warming. The ephemeral streams of the McMurdo Dry Valleys, Antarctica [MDV], are ‘model systems’, with relatively simple ecology and hydrology: they are fed only by glacial meltwater, and perennial microbial communities drive stream ecosystem function. As Antarctic climate warming continues, it is important to understand both current stream biogeochemical processes and how these may alter under a warmer climate regime.

My research goals are to quantify two key MDV stream biogeochemical processes: carbon fluxes and nutrient (N, P) uptake; and to assess how these ecosystems respond to a warming-related thermokarst disturbance. First, I quantified concentration-discharge relationships for dissolved organic carbon [DOC], and found that, despite low organic carbon stocks and large diel changes in discharge, these streams exhibit DOC chemostasis. To explain this behavior, I developed a new conceptual model for DOC generation and storage. Next, I used pulse additions to determine nutrient uptake dynamics for NO₃-N, NH₄-N and PO₄-P in six streams across the Taylor Valley, at nutrient concentrations from ambient to saturation. These streams demonstrated efficient uptake even at concentrations 2-3 orders of magnitude above typical background levels, indicating biotic ability to adjust to large and rapid changes in nutrient levels. Finally, I quantified stream biotic response to a 2012 thermokarst event, which loaded sediment and nutrients into an MDV stream, by using high-resolution satellite imagery to map stream microbial mat activity from 2010 to 2019. Surprisingly, biotic activity increased the year after the thermokarst event, indicating that MDV mat communities are resilient to this type of disturbance. We hypothesize that significantly-higher post-thermokarst N and P loads may have aided this rapid recovery.

As a whole, my research advances understanding of essential stream biogeochemistry in this polar desert environment, including how these systems may respond to a warmer future. Findings from these model systems can also advance our understanding of more complex systems, e.g. temperate and tropical streams, where the microbially-driven processes I elucidate are often hidden by other, larger fluxes.

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