Dissolved O 2 /Ar and other methods reveal rapid changes in productivity during a Lagrangian experiment in the Southern Ocean

• Published in: Journal of Geophysical Research: Oceans
• Main Authors: Hamme, RC, Cassar, N, Lance, VP, Vaillancourt, RD, Bender, ML, Strutton, PG, Moore, TS, DeGrandpre, MD, Sabine, CL, Ho, DT, Hargreaves, BR
• Format: Article in Journal/Newspaper
• Language: English
• Published: Amer Geophysical Union 2012
• Subjects: Earth Sciences; Oceanography; Biological oceanography; Southern Ocean
• Online Access: https://doi.org/10.1029/2011JC007046; http://ecite.utas.edu.au/80215

We use continuous and discrete measurements of the dissolved O 2 /Ar ratio in themixed layer to investigate the dynamics of biological productivity during the SouthernOcean Gas Exchange Experiment in March and April 2008. Injections of SF 6 defined twowater masses (patches) that were followed for up to 2 weeks. In the first patch,dissolved O 2 /Ar was supersaturated, indicating net biological production of organiccarbon. In the second patch, rapidly decreasing O 2 /Ar could only be reasonablyexplained if the mixed layer was experiencing a period of net heterotrophy. Theobservations rule out dominant contributions from vertical mixing, lateral dilution, orrespiration in the ship's underway seawater supply lines. We also compare nine differentestimates of net community, new, primary, or gross production made during theexperiment. Net community and new production estimates agreed well in the first patchbut disagreed in the second patch, both during an initial net heterotrophic period but alsoduring the apparently autotrophic period at the end of the observations. Rapidly changingproductivity during the second patch complicated the comparison of methods thatintegrate over daily and several week timescales. Primary productivity values from on-deck24 h 14 C incubations and gross carbon production values from photosynthesis-irradianceexperiments were nearly identical even during highly dynamic periods of netheterotrophy, while gross oxygen production measurements were 3.5-4.2 times higherbut with uncertainties in that ratio near r+-2. These comparisons show that thephotosynthesis-irradiance experiments based on 1-2 h 14 C incubations underestimatedgross carbon production.