Dissolved O₂/Ar and other methods reveal rapid changes in productivity during a Lagrangian experiment in the Southern Ocean

Abstract

[1] We use continuous and discrete measurements of the dissolved O 2 /Ar ratio in the mixed layer to investigate the dynamics of biological productivity during the Southern Ocean Gas Exchange Experiment in March and April 2008. Injections of SF 6 defined two water 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 organic carbon. In the second patch, rapidly decreasing O 2 /Ar could only be reasonably explained if the mixed layer was experiencing a period of net heterotrophy. The observations rule out dominant contributions from vertical mixing, lateral dilution, or respiration in the ship's underway seawater supply lines. We also compare nine different estimates of net community, new, primary, or gross production made during the experiment. Net community and new production estimates agreed well in the first patch but disagreed in the second patch, both during an initial net heterotrophic period but also during the apparently autotrophic period at the end of the observations. Rapidly changing productivity during the second patch complicated the comparison of methods that integrate over daily and several week timescales. Primary productivity values from on-deck 24 h 14 C incubations and gross carbon production values from photosynthesis-irradiance experiments were nearly identical even during highly dynamic periods of net heterotrophy, while gross oxygen production measurements were 3.5-4.2 times higher but with uncertainties in that ratio near AE2. These comparisons show that the photosynthesis-irradiance experiments based on 1-2 h 14 C incubations underestimated gross carbon production.Citation: Hamme, R. C., et al. (2012), Dissolved O 2 /Ar and other methods reveal rapid changes in productivity during a

Insight into chemical, biological, and physical processes in coastal waters from dissolved oxygen and inert gas tracers

“…For example, the GEMS could be used along with a second mass spectrometer measuring O 2 /Ar ratios (e.g., an equilibrator inlet mass spectrometer (Cassar et al, 2009), a membrane inlet mass spectrometer (Tortell, 2005;Virkki et al, 1995;Kana et al, 1994), or the GEMS system described above, with the getter chambers eliminated) and a well-calibrated sensor for O 2 concentration. The O 2 /Ar ratio and the O 2 concentration could be used to derive the Ar concentration (Eveleth et al, 2014;Hamme et al, 2012) and the other noble gas concentrations could be determined from the GEMS noble gas ratios and the Ar concentration. Another potential modification is changing the system to measure individual samples, instead of a continuous gas stream (Visser et al, 2013;Machler et al, 2012).…”

“…I compare these gas tracer results with estimates of net community production and net primary production from incubations (measuring 14 C uptake, 15 NO − 3 uptake, 15 NH + 4 uptake, and nitrification) and organic carbon export from sediment traps, to form a detailed picture of the ecosystem metabolism and carbon cycle state. To date, most published studies simultaneously comparing net community production from 15 NO − 3 uptake and from O 2 /Ar were conducted in high nitrate, low chlorophyll regions of the open ocean (Hamme et al, 2012;Giesbrecht et al, 2012) rather than nitrogen-limited, high productivity, coastal regions.…”

“…There is currently no single universally-accepted method of calculating GOP, and many authors have argued that the ideal choice of parameters and constants may depend on properties of the system (e.g., the microbial species of interest and isotopic composition of oxygen in water, which is the source of photosynthetic O 2 ) (Luz and Barkan, 2000;Juranek and Quay, 2005;Reuer et al, 2007;Luz and Barkan, 2009;Stanley et al, 2010;Kaiser, 2011;Luz and Barkan, 2011;Nicholson et al, 2011;Prokopenko et al, 2011;Hamme et al, 2012). However, there is strong scientific consensus that calculations should be performed using the individual isotope ratios rather than the empirically defined term 17 ∆ which is a combination of two isotope ratios (Kaiser, 2011;Luz and Barkan, 2011;Nicholson et al, 2011;Prokopenko et al, 2011 as a supplement to this paper (Data set 2) so that GOP can be re-calculated in the future using new formulas (Kaiser, 2011).…”

“…transect cruises) or sampling at the same location occurs very far apart in time (much longer than the residence time of O 2 in the mixed layer, which is typically on the order of two weeks), making it necessary to assume the gases are at steady state in order to calculate NCP and GPP (Stanley et al, 2010;Giesbrecht et al, 2012;Munro et al, 2013). When a higher-frequency time-series of measurements is obtained (as occurred during this cruise), investigators can quantify the change in [O 2 ] and 17 ∆ with time and include these terms in the productivity estimates when appropriate (Hamme et al, 2012;Tortell et al, 2014;Wilson et al, 2015). Because the calculations of NCP and GPP from O 2 /Ar and triple oxygen isotope data can vary substantially between studies, we outline our calculations in section 4.5.…”

“…Here we present one of the first published data sets where productivity estimates from were measured simultaneously at the same locations (Emerson et al, 1993;Giesbrecht et al, 2012;Hamme et al, 2012). Although theoretically these methods should be equivalent at steady state and/or when averaged over large spatiotemporal scales (Laws, 1991;Falkowski et al, 2003), this assumption has been tested at a limited number of locations, primarily high nitrate, low chlorophyll open ocean regions such as the Southern Ocean and subarctic North Pacific (Hendricks et al, 2005;Reuer et al, 2007;Giesbrecht et al, 2012).…”