Measuring rates of gross photosynthesis and net community production in cryoconite holes: a comparison of field methods

Abstract

AbstractPhotosynthesis by microbes on the surfaces of glaciers and ice sheets has the potential to fix carbon, alter the albedo of ice surfaces via the production of organic matter and so enhance ice melt. It could also be important for supplying labile organic matter and nutrients to in situ and downstream ecosystems. This study compares in situ 24 hour incubation methods for measuring rates of gross photosynthesis, respiration and net community production (NCP) in cryoconite holes on three Svalbard valley glaciers. Rates of gross photosynthesis and respiration measured by the ΔCO2 method were closely balanced, resulting in rates of NCP close to the detection limit (mean of –1.3 μg C g−1 d–1) consistent with previous measurements in Arctic cryoconite holes. This suggests that organic matter within cryoconite holes may be derived largely from allochthonous sources. The molar ratio of ΔO2 to ΔCO2 in incubations gave mean respiratory and photosynthetic quotients of 0.80 ± 0.17 (1 × SD) and 1.24 ± 0.20 (1 × SD), respectively. The 14C method typically underestimated rates of gross photosynthesis (ΔCO2 method) by more than one order of magnitude and measured a rate closer to NCP.