Background & Rationale
Recent studies have demonstrated that glaciers, typically excluded from carbon cycle models, can both store and export organic carbon (1–3). Glaciers can store organic carbon derived from over-ridden vegetation (4), deposition of anthropogenic aerosols (5), microbial fixation (6), and microbial growth on substrates on and within the glacier (4). Due to the high presence of microbial life and lack of plant life on glaciers, glacial organic carbon pools are composed of low proportions of aromatic compounds (7,8), high molecular weight compounds (7) and a low proportion of humic components (8) (Table 1). In contrast, terrestrially sourced carbon pools are composed of proportions of aromatic compounds (9–11), high molecular weight compounds (9) and a low proportion of humic components (9) due to high proportions of terrestrial carbon being sourced from plant life and soils (in contrast to microbial communities) (Table 1). During the summer, organic carbon previously stored within glaciers is released via meltwaters into glacially fed streams, where it mixes with terrestrial carbon sources which dominant stream organic carbon pools. Due to the export of unique glacial organic carbon into downstream regions, stream organic carbon pools of glacially fed rivers may exhibit different characteristics compared to non-glacially fed stream.
Table 1: Summary of differences in dissolved carbon isotopic values, concentration, and absorbance characteristics between glacially derived and terrestrially derived organic carbon.
Within streams organic carbon can be processed by microbial communities either by being respired as carbon dioxide or becoming incorporated into biomass, which may be assimilated by higher trophic level organisms (12) (Figure 1). Carbon pools within glacially fed streams have been found to be more readily consumed (bioavailable) by microbial communities compared to carbon pools in non-glacially fed streams (12). The higher levels of carbon bioavailability in these glacially fed streams has been attributed to export of unique organic carbon properties from glaciers into these streams (12,13). This is thought to be due to the high proportion of microbial sourced organic carbon and low proportion of plant organic carbon sources in glacial carbon. Plant organic carbon contains the highly aromatic and high molecular weight compound lignin in high proportions (7). Lignin molecules are difficult for microbes to breakdown, and are common in terrestrial sourced carbon (7). If glacial carbon is more bioavailable to microbial communities then despite the low levels of organic carbon in glacial carbon pools (8,12,14) (Table 1), glacial carbon may be contributing more to food web structuring and carbon dioxide emissions in glacially fed streams, since microbial communities form the base of aquatic food webs and respire carbon dioxide, than terrestrial carbon. Despite these findings, it has not yet been experimentally determined whether microbial communities are preferentially utilizing ancient glacially derived organic carbon over the younger, terrestrially derived organic carbon that is commonly present in stream systems.
Figure 1: Simplified image showing the processing of glacially derived and terrestrially derived dissolved organic carbon by stream microbial communities.
My MSc research aims to increase our understanding of how glacially sourced organic carbon is being processed by microbial communities in glacially fed streams in the Canadian Rockies. My first data chapter of my MSc thesis will be focused on the characterization the organic carbon pool (through absorbance characteristics (Box 1) and isotope tracers (box 2, table 1)) and microbial communities in four glacially fed streams in Banff and Jasper Parks along a downstream transect and throughout a glacial melt season (May-October 2019). The purpose of this is to (1) assess whether the stream organic carbon pool displays more qualities of glacially derived organic carbon or terrestrially derived organic carbon (such as in a non-glacially fed stream), (2) investigate how carbon pool characteristics varies throughout the year (with changes in stream hydrological inputs) and with distance away from glacier and (3) explore relationships between organic carbon properties and microbial community structure. The results from my first data chapter will increase our knowledge of how organic carbon is being cycled in glacially fed streams in the Canadian Rockies, as well as be used to inform the experimental design of my second MSc data chapter. The second data chapter of my MSc thesis will investigate if glacially derived organic carbon is being preferentially utilized by stream microbial communities. This will be achieved through an incubation experiment measuring the change in glacial organic carbon pool characteristics from microbial community uptake throughout time. For the purpose of this class, my project will focus on the methods, data analysis, and interpretation of my first data chapter only.
Box 1: Absorbance characterization of organic carbon Absorbance spectroscopy is a technique that measures the amount of light absorbed by a sample at a known wavelength. Specific absorbance parameters (e.g A254, S275-295, SUVA254) can be calculated from the resulting absorbance spectra that can be utilized to characterize general features of organic carbon composition (such as proportion humic components, proportion of low molecular weight compounds and proportion of aromatic compounds) (15) |
Box 2: Isotopes of carbon Carbon has two stable isotopes (12C and 13C), due to the different weights of these isotopes abiotic and biotic process can alter the ratio of 12C:13C from the natural ratio. The ratio of 12C:13C can then be used as a tracer to identify the source of organic carbon (since certain processes have characteristic 12C:13 (delta13C) values (16). Additionally, carbon has one radioactive isotope (14C). The ratio of 12C:14C (normalized to 13C) in a sample can then be used to infer approximate age of the sample (17). |
Objectives & Hypotheses
The first data chapter of MSc thesis aims to better understand organic carbon cycling in the Canadian Rockies, and to address this I have developed 3 objectives:
Objective I: Investigate spatial and temporal trends in organic carbon pools (using absorbance characteristics and isotope tracers) in glacially fed streams in Banff and Jasper National Parks, and compare organic carbon pool characteristics to glacial meltwater and non-glacially fed stream carbon pools.
Hypothesis I: Samples closer to the glacier and at times of high glacier melt (late summer) will have more glacial characteristics rather than terrestrial characteristics.
Objective II: Investigate spatial and temporal trends in microbial communities in glacially fed streams in Banff and Jasper National Parks.
Hypothesis II: If there is variation in the carbon pool due to temporal and spatial differences then I hypothesize that the microbial communities will also vary due to temporal and spatial differences.
Objective III: Explore relationships between the organic carbon pool absorbance and isotopic values and microbial communities structuring.
Hypothesis III: Organic carbon properties will drive variation in microbial communities.
Expected Result
Generally, I expect that the organic carbon pool in glacially fed streams will be similar to those of a non-glacially fed stream (Table 1) with the exception being samples taken in close proximity to glaciers and at times with high glacial meltwater input into streams (August). I expect samples taken close to glaciers and in August to display characteristics common to glacial meltwater samples (low molecular weight compounds, low humic components, low aromatic components, low amounts of organic carbon, elevated delta13C, and depleted delta14C), and samples taken at further distances and at other times in the season to display characteristics common to terrestrial meltwater samples (high molecular weight compounds, high humic components, high aromatic components, high amounts of organic carbon, depleted delta13C, and elevated delta14C). If this trend is observed, then it would provide evidence that glacial organic carbon export is altering the characteristics of the stream organic carbon pool. Alternatively, if carbon pool characteristics still exhibit terrestrial characteristics at times during peak melt and at sites near the glacier this could indicate that glacial organic carbon export is not significantly altering the stream organic carbon pool, or that other mechanisms are impacting the stream organic pool (e.g flow rates).
Additionally, I expect that if the organic carbon pool varies temporally and spatially that the microbial community will also vary spatially and temporally. I expect that there will be variation in microbial communities, and that some of this variation is due to organic carbon characteristics. Specifically, I expect that if the stream organic carbon pool demonstrates glacial features then this will lead to the formation of a unique microbial communities compared to when the stream organic carbon pool demonstrates terrestrial features. If this trend is seen it could suggest that the type of organic carbon in the stream is an important structurer to microbial communities. If this trend is not observed it could be suggested that the composition of carbon pools (and therefore glacial organic carbon export) may not be that important to microbial communities, or less important than other factors (e.g stream nutrient composition).