Dissolved organic carbon composition and reactivity in arctic Canadian lakes

Abstract

Freshwater systems are active components of the global carbon cycle and contribute to global CO2 emissions. Freshwater studies in the Arctic are underrepresented, especially regarding lakes and their Dissolved Organic Carbon (DOC) reactivity and composition dynamics. DOC is a main part of DOM (dissolved organic matter), which drives processes central to carbon cycling, influences chemical and biological characteristics, for example bacterial production (BP) and bacterial respiration (BR), and lake DOC is usually comprised of allochthonous (terrestrial) and autochthonous (algal) sources. Within DOM, CDOM (coloured DOM) and FDOM (fluorescent DOM) are key components, which can impact productivity and act as indicators of DOC composition.

The remote area of Churchill Canada is an arctic region with an abundance of lakes, and this study aimed to investigate the relationships between microbial DOC reactivity and DOC composition in these lakes, and to improve our understanding of the influence of environmental properties, and potential CO2 emissions. In this study, we used documented methods to investigate DOC in 54 lake samples, primarily including dark laboratory incubations over a 28-day period. BP and BR were measured using leucine uptake and dissolved oxygen concentrations respectively, while DOC composition was investigated using fluorescent spectroscopy and PARAFAC. Three fluorescent components were identified: C1 (Terrestrial humic), C2 (Marine/microbial humic), and C3 (algal protein-like).

Weak relationships between BP, BR, and the components were found, while lake area proved to be a control on DOC amount and variations in reactivity and composition. As expected CDOM and FDOM showed net production, although this varied, as C3 had the most production especially in low CDOM lakes. pCO2 (partial pressure of CO2) and potential CO2 emissions were linked partially to BR, and DOC, but potentially photoreactivity is more important. The results from this study show that the role of Arctic lakes remains highly variable and is closely linked to site specific conditions. Further analysis of these and other lakes, across different environmental and hydrological conditions are suggested, to form a clearer view of the role of Arctic lakes in the carbon cycle, and the complex relationship between DOC reactivity and composition.

Popular Abstract

Lakes are significant parts of the global carbon cycle and emit large amounts of carbon dioxide, for example when Dissolved Organic Carbon (DOC) is degraded by bacteria in water. However, lakes in the arctic are understudied. There is a need to improve our understanding of how bacterial processes, such as biomass production and respiration, are related to the composition of DOC, as this impacts ecosystem functioning and carbon dioxide emissions. For example, some arctic lakes contain DOC which comes from the nearby land, which usually is less reactive (degradable), has a more brownish colour, and is a poorer source of bacterial production, compared to DOC which is made within lakes by algae. In this study, these relationships were tested within 53 arctic lakes located in Churchill, Canada, which is a geographical area where no similar study has been done in the past.

The composition and origin of DOC, including amounts of CDOM, were analysed by using fluorescent spectroscopy, in which three fluorescent DOC components were found: C1, which is linked to input from land; C2, which is linked to microbial sources, and; C3, which is linked to algal sources within the lakes. In addition, standard laboratory DOC degradation experiments were performed, which gave amounts of bacterial production and bacterial respiration over a 28-day dark incubation period.

Lake area proved to be a very important factor for these lakes, having a large control of not only the amount of DOC, but also the range in magnitude of bacterial processes. The results suggested that small arctic lakes play an especially important role in the carbon cycle. Expected relationships between bacterial processes and our 3 components were partially found, proving that DOC composition affects bacterial and lake processes. For example, bacterial production correlated positively with C3, while C3 had no correlation with DOC lost, suggesting this component does not act as a good indicator of the reactive DOC consumed. Bacterial respiration and DOC contributed to potential carbon dioxide emissions in indirect ways, while further emissions could be enhanced by sun-light induced processes. Overall, small arctic lakes have an increasing importance in the carbon cycle as the impacts of climate change continue, facing increased DOC input, and hydrological changes with extreme precipitation events.

• author: Sparkes, Bradley
• supervisor: Martin Berggren; Geert Hensgens
• organization: Dept of Physical Geography and Ecosystem Science
• alternative title: Dissolved organic carbon dynamics of arctic Canadian lakes
• course: NGEM01 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Physical Geography and Ecosystem Science, Dissolved Organic Carbon, Arctic, Lake, Fluorescence, Area, pCO2
• publication/series: Student thesis series INES
• report number: 613
• language: English
• additional info: Co-supervisor Geert Hensgens, VU research associate at Faculty of Science, Earth and Climate. Affiliated with the department of Physical Geography and Ecosystem Science.
• id: 9130169
• date added to LUP: 2023-06-27 13:35:09
• date last changed: 2024-06-26 03:42:33