Mock T, Dieckmann GS, Haas C, Krell A, Tison JL, Belem AL, Papadimitriou S, Thomas DN
Fragilariopsis cylindrus, Oxygen, Methods, Micro-optodes, Sea ice, Biogeochemistry, mesocosm, 4 m3, HSVA test basin, Hamburg, Germany
Oxygen micro-optodes were used to measure oxygen dynamics directly within themicrostructure of sea ice by freezing the sensors into the ice during its formation. The experiment wasconducted in a 4 m3 mesocosm filled with artificial seawater and inoculated with a unialgal culture ofthe common Antarctic ice diatom Fragilariopsis cylindrus (Bacillariophyceae) to a final chlorophyll a(chl a) concentration of 11 ?g l–1. Ice growth was initiated 7 d after inoculation by reducing the airtemperature to –10 ± 2°C and terminated 17 d later. The final ice thickness was 27 cm. One optodewas frozen into grease ice and 2 others into the skeletal layer of the growing ice sheet. Increasingoxygen concentrations during ice crystal formation at the water surface and the ice-water interfacerevealed a strong inclusion of oxygen, which was either physically trapped and/or the result ofphotosynthesising diatoms. The major portion of oxygen was present as gas bubbles due to supersaturationas a result of increasing salinity and oxygen production by diatoms. An increase in salinitydue to a concurrent decrease in ice temperatures during subsequent sea ice development reducedthe maximum concentration of dissolved oxygen within brine. Thus, dissolved oxygen concentrationsdecreased over time, whereas gaseous oxygen was released to the atmosphere and seawater. Thesensors are a significant advance on more conventional microelectrodes, because the recordings canbe temperature and salinity compensated in order to obtain precise measurements of oxygen dynamicswith regard to total (dissolved and gaseous) and dissolved oxygen in sea ice. Optodes do notconsume oxygen during measurement over a long period under extreme conditions, which is anotheradvantage for long-term deployment in the field.