Date of Award

2015-01-01

Degree Name

Doctor of Philosophy

Department

Environmental Science and Engineering

Advisor(s)

Vanessa L. Lougheed

Abstract

The Arctic is known for containing large stocks of soil organic carbon, which exists frozen in permafrost in a greenhouse inert state. With predicted future warming in these high northern latitudes, the mobilization of stored soil organic carbon and release to the atmosphere may increase and induce further positive climatic feedbacks. Previous studies have shown that Arctic wetlands and ponds cover a large percentage of the Arctic Coastal Plain and contribute large amounts of carbon to the atmosphere; however, it remains largely unknown how these systems are responding to a warming climate and how this change will impact regional carbon budgets. Therefore, it is of urgent interest to better assess and monitor the effects of climate change on Arctic wetlands and their role in the fate and transport of carbon to the atmosphere.

The overall focus of this Dissertation is to identify decade time scale changes in the structure and function of Arctic tundra ponds, to understand how these changes are driven by warming and nutrient enrichment, and to advance new technologies to remotely track environmental change. Specifically, this study aims to:

* Assess decadal time scale changes in surface hydrology of tundra wetland ponds and its relationship with landscape geomorphological evolution.

* Determine decadal time scale changes in methane emissions through modeling of environmental drivers of methane efflux from aquatic vegetation.

* Develop and evaluate novel near-surface remote sensing tools for monitoring phenology and productivity of Arctic aquatic macrophytes.

* Investigate the impacts of nutrient enrichment on Arctic aquatic plant productivity and spectral properties.

Our study was carried out on the Arctic coastal plain, more specifically on the Barrow Peninsula, Alaska. This region is underlain by continuous permafrost and dominated by drained thaw lake basins containing numerous wetland ponds. The comparison of historical aerial imagery from 1948 to modern high resolution satellite imagery revealed a net decrease in area and number of ponds. This contradicts geomorphological succession processes in the Arctic coastal plain and provides evidence that climate change can reverse millennial-scale processes affecting landscape evolution and surface energy balance for this region.

During the summers of 2010-2013, re-sampling of historical research sites established in the 1970's demonstrated a deepening of the active layer and an increase in aboveground biomass and cover of the dominant aquatic plants Carex aquatilis and Arctophila fulva. This is attributed to an increase in nutrients and warmer, longer growing seasons. Plant-mediated methane efflux was observed to be a function of thaw and water depth for C. aquatilis and plant biomass for A. fulva. Based on the increase in biomass and thaw depth, we modeled past and present methane emissions and estimated an increase efflux of 240% for C. aquatilis and 80% for A. fulva over the past 40 years. Although both plant species only cover 11% of the land surface in the Barrow peninsula, we found that they account for approximately 78% of the total estimated methane emissions for this region. These results emphasize the effects of climate change on soil-atmosphere methane emissions and the importance of C. aquatilis and A. fulva as key species for methane efflux.

Timing and intensity of plant primary production and phenology has implications for land-atmosphere carbon exchange. The need for high-frequency monitoring of plant phenology and productivity in Arctic wetlands motivated the development of a novel technology that employs digital repeat photography as a near-surface remote sensing method to track seasonal plant biomass and phenology. We took advantage of the red, green and blue (RGB) color space of commercial digital cameras to estimate seasonal greenness based on a multi-channel RGB index. Digital repeat photography observations at 9 sites over 4 growing seasons (2010-2013) revealed (i) interannual variation in greening, (ii) species-specific and (iii) site-specific greening patterns. Nutrient availability and temperature determined the timing and magnitude of aquatic plant green-up. This novel automated method proven to be a reliable and cost-effective alternative for continual monitoring of productivity in aquatic plants, important for assessing carbon and energy balance in arctic wetlands.

Nutrient release from thawing permafrost and warming soils likely has significant effects on primary productivity in aquatic systems and thus, carbon exchange. However, no studies have assessed how Arctic aquatic plants vary in nutrient status among sites with different levels of nutrient enrichment, and how this may impact spectral responses important for remote sensing monitoring. We sampled water, plant and soil nutrients as well as plant biomass and spectral reflectance in (i) relatively pristine reference ponds and (ii) urban and thermokarst ponds as our nutrient enriched sites. Multivariate analyses indicated that measures of greenness were strongly related to gradients in nutrient enrichment and plant nutrients.

Combined these findings reveal substantial changes in the structure and function of Arctic tundra ponds over the past 40 or more years, which have major implications for carbon efflux as thawing of permafrost continues enriching aquatic ecosystems. They also demonstrate the utility of remote sensing technologies, including novel near-surface methods, in monitoring key components of Arctic tundra ecosystems such as: pond extent and productivity.

Language

en

Provenance

Received from ProQuest

File Size

121 pages

File Format

application/pdf

Rights Holder

Christian Gerardo Andresen

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