Article about the photo-oxidation of Arctic dissolved organic carbon generates CO2, influencing local carbon cycles and potentially global climate. Written for an assignment.

— Based on: Sunlight controls water column processing of carbon in arctic fresh waters

Arctic streams, rivers and lakes account for a small percentage of the landscape, but have disproportionately large effects on the regulation of carbon flux between land, ocean and atmosphere. They are responsible for processing organic carbon as part of carbon cycling, but the mechanisms that govern it are not well elucidated. In an article published by Science, Cory et al. investigates the degree sunlight influences the processing of organic carbon in the Kupruk Basin of Alaska. It is found that sunlight is the dominating factor controlling the processing of the carbon, accounting for up to 95% of processed carbon regardless of the type of inland water body. This gives crucial insight to carbon exchange in the Arctic and how it might affect the global climate.

The Arctic is a vast carbon reservoir, with twice as much carbon in its frozen soils than the atmosphere. Thawing of the soil releases organic carbon into inland surface waters such as streams, lakes and rivers. The dissolved organic carbon (DOC) in these waters are naturally processed in four ways to yield CO2. Biologically, they can be fully oxidised by bacteria into CO2 via bacterial respiration. Photo-chemically, DOC can be fully photo-oxidised into CO2 (“photo-mineralisation”), or partially photo-oxidised into a substrate derivative that is more manageable for bacteria to process under sunlight. Sunlight can also induce bacterial respiration through photostimulation. Residual DOC, whether partially photo-oxidised, clumped, or unreacted, will be discharged into the ocean.

Cory et al. conducted a series of tests to quantify the rate and amount of DOC processed by the four aforementioned methods and compared them. He sampled the water across several sites within and adjacent to the Kupruk Basin of Alaska. The samples were subjected to various light conditions and the photo-products were measured to establish the degree bacteria and sunlight govern DOC processing. The depth of water column, amount and wavelength of penetrating sunlight were factored in as well. The results were then extrapolated and combined with the optical properties of the other smaller streams, larger lakes and large rivers to measure DOC processing in the entire Kupruk Basin.

It is shown that photo-chemical means, especially photo-mineralisation, is the dominant mechanism for DOC processing in Arctic water columns. Photo-mineralisation and the partial photo-oxidation account for most (75 - 95%) of the DOC processed and also at a much greater rate (3 - 19 times higher) than bacterial respiration. This is evidential that sunlight is the dominant driver for DOC processing in Arctic inland surface waters.

Three possible reasons are given for this phenomenon. Firstly, the rate of DOC processed through bacterial respiration is considered low, relative to other reported values based on inland surface waters outside the Arctic. Hence, this quantitatively promotes the dominance of sunlight-driven mechanisms in the Arctic waters (Vähätalo et al., 2003). Secondly, the total amount of DOC processed in the inland surface waters is comparatively higher than reported values from inland surface waters outside the Arctic (Koehler et al., 2014). This indicates that DOC in Arctic waters are more susceptible towards being processed, especially via photo-oxidation. Lastly, and perhaps the most importantly, Arctic inland surface waters are shallower and devoid of vegetative shade as compared to their woodland counterparts in other continents (Amor & Benner, 1996). This enables sunlight to permeate majority of the water column, subjecting a large proportion of DOC to react with sunlight. These reasons distinguish and reinforce the unique traits of the Arctic and its irreplaceable role in global carbon cycles.

Fig 1. Bacterial and photo-chemical processing of DOC in (A) small order streams (B) larger order rivers and (C) lakes in Kupruk Basin from mid to end 2012. Solid lines represent the average carbon processing rates while the similarly-hued areas represent the upper and lower 95% confidence interval of the carbon processing rates.

Although photo-oxidative DOC processing is principally more widespread, the upscaling of results to encompass the entire Kupruk Basin presented a disparity between the extent of outcomes by various sunlight-driven processing methods. Of the DOC processed in the surface water bodies of Kupruk Basin, 55% is fully oxidised into CO2 and 45% is partially oxidised. However, the dominating DOC processing method changes based on the position of the channel, therefore depending on the type of water body (Fig. 1). Photo-mineralisation is the dominant DOC processing mechanism in small streams, partial photo-oxidation is the dominant mechanism in larger rivers, and lakes are an intermediate of both.

This is attributed to smaller streams being located more upstream and DOC having less prior exposure to sunlight. When they do, they become photo-reactive, undergoing complete photo-mineralisation to become CO2 and removed from the system by escaping into the atmosphere or reacting with other compounds. Downstream towards larger rivers and lakes, but the photo-reactivity of the DOC as a whole would have already decreased, thus transitioning to partial photo-oxidation instead (Reader & Miller, 2014).