Organic matter processing in arctic lake sediments

Lakes are a common landscape feature in the arctic and interact with carbon cycling through the sequestration of organic matter in their sediments. My research assessed the relative importance of landscape- and within-lake-scale factors on organic matter cycling in arctic lake sediments. Sediment organic matter mineralization (measured as sediment oxygen demand) varied between -8 and 40 mmol O2 m-2 d-1 and was proportional to water temperature and oxygen concentration. There was greater variation in organic matter mineralization within lakes than between lakes, suggesting that variation in mineralization occurs primarily at the within-lake scale. At the both the landscape- and within-lake-scale, sediment slurries with greater percent organic matter had a higher rate of dissolved inorganic carbon (DIC) production and the effect of oxygen exposure on DIC production was greatest in those sediments with the lowest percent organic matter. Despite the variation in mineralization, organic matter content of the sediments (17 to 69%) varied primarily among lakes (i.e., at the landscape-scale) and was driven by organic matter inputs via benthic primary production. My results suggest that the attenuation of light by dissolved organic carbon in the water indirectly influences organic matter storage in arctic lake sediments. In addition to the direct limitations of organic matter input via benthic photosynthesis, the amount of light attenuation indirectly alters sediment organic matter cycling via changes to the distribution of temperature and oxygen within the lake. Light attenuation was inversely proportional to the depth of thermal stratification, which determines the distribution of temperature and oxygen in stratified lakes. I estimated that a doubling of the light attenuation would result in a 30% decrease in the area of the sediments exposed to the relatively warm and oxygenated waters of the epilimnion. The interconnection between these factors provides a potential climate change feedback to arctic carbon cycling. Changes in terrestrial organic matter inputs to lakes due to climate change will alter transparency and the depth of the thermocline, changing the distribution of light, temperature and oxygen in the lake and thus the factors limiting the production and decomposition of organic matter in the lake sediments.