The impact of dissolved organic carbon and bacterial respiration on pCO 2 in experimental sea ice

• Published in: Progress in Oceanography
• Main Authors: Zhou, J, Kotovitch, M, Kaartokallio, H, Moreau, S, Tison, J-L, Kattner, G, Dieckmann, G, Thomas, DN, Delille, B
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon-Elsevier Science Ltd 2016
• Subjects: Earth Sciences; Oceanography; Biological Oceanography; Antarctic; Arctic; The Antarctic; Antarc*; Sea ice
• Online Access: https://doi.org/10.1016/j.pocean.2015.12.005; http://ecite.utas.edu.au/109568

Previous observations have shown that the partial pressure of carbon dioxide (pCO 2 ) in sea ice brines is generally higher in Arctic sea ice compared to those from the Antarctic sea ice, especially in winter and early spring. We hypothesized that these differences result from the higher dissolved organic carbon (DOC) content in Arctic seawater: Higher concentrations of DOC in seawater would be reflected in a greater DOC incorporation into sea ice, enhancing bacterial respiration, which in turn would increase the pCO 2 in the ice. To verify this hypothesis, we performed an experiment using two series of mesocosms: one was filled with seawater (SW) and the other one with seawater with an addition of filtered humic-rich river water (SWR). The addition of river water increased the DOC concentration of the water from a median of 142μmolL water − 1 in SW to 249μmolL water − 1 in SWR. Sea ice was grown in these mesocosms under the same physical conditions over 19days. Microalgae and protists were absent, and only bacterial activity has been detected. We measured the DOC concentration, bacterial respiration, total alkalinity and pCO 2 in sea ice and the underlying seawater, and we calculated the changes in dissolved inorganic carbon (DIC) in both media. We found that bacterial respiration in ice was higher in SWR: median bacterial respiration was 25nmolCL ice − 1 h −1 compared to 10nmolCL ice − 1 h −1 in SW. pCO 2 in ice was also higher in SWR with a median of 430ppm compared to 356ppm in SW. However, the differences in pCO 2 were larger within the ice interiors than at the surfaces or the bottom layers of the ice, where exchanges at the airice and icewater interfaces might have reduced the differences. In addition, we used a model to simulate the differences of pCO 2 and DIC based on bacterial respiration. The model simulations support the experimental findings and further suggest that bacterial growth efficiency in the ice might approach 0.15 and 0.2. It is thus credible that the higher pCO 2 in Arctic sea ice brines compared with those from the Antarctic sea ice were due to an elevated bacterial respiration, sustained by higher riverine DOC loads. These conclusions should hold for locations and time frames when bacterial activity is relatively dominant compared to algal activity, considering our experimental conditions.