Date of Award

Spring 2024

Project Type

Thesis

Program or Major

Earth Sciences

Degree Name

Master of Science

First Advisor

Ruth Varner

Second Advisor

Maria Florencia Fahnestock

Third Advisor

McKenzie Kuhn

Abstract

Arctic and subarctic ecosystems are experiencing accelerated warming compared to other regions. Increasing arctic lake sediment and water temperatures stimulate the formation and emission of methane (CH4). Methane is a potent greenhouse gas that can contribute to a global warming feedback loop, in which temperatures rise due to increasing greenhouse gas emissions, which in return further increases those emissions. Methane emitted from lakes is primarily emitted through ebullition (bubbling). Various environmental characteristics have an impact on CH4 cycling in aquatic environments, such as temperature, water depth, organic matter, and nutrient availability. Sediment CH4 production is also an important driver of CH4 flux and can be influenced by similar environmental factors. It is important to constrain the influence of CH4 production on CH4 emissions and how this relationship could be affected by climate change. To look at ebullition and sediment CH4 production rates, as well as the relationship between the two, water depth and edge/vegetation to center gradients were established across three sub-Arctic lakes in northern Sweden. Ebullition rates were measured using samples from bubble traps that were collected from each lake over the course of one month, from June to July 2023 (n = 351). Sediment cores were also retrieved and were split into two depths (0-10 cm and 10-20 cm) for measuring CH4 production rates via incubations at two temperatures, 15℃ and 20℃ (n = 108). Results indicated that ebullition rates varied spatially and temporally. The shallowest lake had the greatest average CH4 flux (59.5 ± 93.0 mg CH4 m-2 d-1). Shallow areas as well as edge areas in each of the three study lakes were shown to have greater CH4 emissions than in intermediate, deep, or center areas, however only in the absence of high quantities of emergent vegetation. Methane production rates were greater in the top 0-10 cm of sediment than the 10-20 cm of sediment and production rates were found to increase in 20℃ incubations across all coring locations. Methane production rates did not significantly vary between edge and center locations, but they were significantly different between water depths in the 0-10 cm of sediment incubated at 20℃. The top 0-10 cm of sediment production rates displayed a significant relationship with the ebullition measurements, with higher production rates correlating with greater emissions. These results suggest there is a strong relationship between sediment production rates in the top 10 cm of sediment and ebullition emissions from lakes, providing important insights into the mechanistic drivers of lake CH4 emissions which could be incorporated in ecosystem models.

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