Bender
Michael L.
Bender
Michael L.
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ArticleEvaluating Southern Ocean biological production in two ocean biogeochemical models on daily to seasonal timescales using satellite chlorophyll and O2 / Ar observations(Copernicus Publications on behalf of the European Geosciences Union, 2015-02-04) Jonsson, Bror F. ; Doney, Scott C. ; Dunne, John P. ; Bender, Michael L.We assess the ability of ocean biogeochemical models to represent seasonal structures in biomass and net community production (NCP) in the Southern Ocean. Two models are compared to observations on daily to seasonal timescales in four different sections of the region. We use daily satellite fields of chlorophyll (Chl) as a proxy for biomass and in situ observations of O2 and Ar supersaturation (ΔO2 / Ar) to estimate NCP. ΔO2 / Ar is converted to the flux of biologically generated O2 from sea to air (O2 bioflux). All data are aggregated to a climatological year with a daily resolution. To account for potential regional differences within the Southern Ocean, we conduct separate analyses of sections south of South Africa, around the Drake Passage, south of Australia, and south of New Zealand. We find that the models simulate the upper range of Chl concentrations well, underestimate spring levels significantly, and show differences in skill between early and late parts of the growing season. While there is a great deal of scatter in the bioflux observations in general, the four sectors each have distinct patterns that the models pick up. Neither model exhibits a significant distinction between the Australian and New Zealand sectors and between the Drake Passage and African sectors. South of 60° S, the models fail to predict the observed extent of biological O2 undersaturation. We suggest that this shortcoming may be due either to problems with the ecosystem dynamics or problems with the vertical transport of oxygen.
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PreprintGross and net production during the spring bloom along the Western Antarctic Peninsula( 2014-10) Goldman, Johanna A. L. ; Kranz, Sven A. ; Young, Jodi N. ; Tortell, Philippe D. ; Stanley, Rachel H. R. ; Bender, Michael L. ; Morel, Francois M. M.This study explores some of the physiological mechanisms responsible for high productivity near the shelf in the Western Antarctic Peninsula despite a short growing season and cold temperature. We measured gross and net primary production at Palmer Station during the summer 2012/2013 via three different techniques: 1) incubation with H218O; 2) incubation with 14CO2; and 3) in situ measurements of O2/Ar and triple oxygen isotope. Additional laboratory experiments were performed with the psychrophilic diatom Fragilariopsis cylindrus. During the spring bloom, which accounted for more than half of the seasonal gross production at Palmer Station, the ratio of net to gross production reached a maximum greater than ~60%, among the highest ever reported. The use of multiple-techniques showed that these high ratios resulted from low heterotrophic respiration and very low daylight autotrophic respiration. Laboratory experiments revealed a similar ratio of net to gross O2 production in F.cylindrus and provided the first experimental evidence for an important level of cyclic electron flow (CEF) in this organism. The low ratio of community respiration to gross primary production observed during the bloom at Palmer Station may be characteristic of high latitude coastal ecosystems and partially supported by a very active CEF in psychrophilic phytoplankton.
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ArticleThe δ18O of dissolved O2 as a tracer of mixing and respiration in the mesopelagic ocean(American Geophysical Union, 2009-02-18) Levine, Naomi M. ; Bender, Michael L. ; Doney, Scott C.The isotopic composition of dissolved oxygen in the mesopelagic ocean is a unique tracer of respiration and transport. New δ 18O of O2 data from the tropical South Atlantic oxygen minimum zone are presented and compared to global δ 18O data. The δ 18O variability in oxygen poor waters is attributed to differences in physical and biogeochemical processes. Simple respiration-transport models show that both isopycnal diffusion and advection must be properly considered when interpreting oxygen isotope signatures along an isopycnal surface. We estimate rates of respiration and oxygen isotope fractionation for the study region using a two-dimensional (2-D) isopycnal and 1-D diapycnal model. Estimated respiration rates are consistent with previous studies. However, to account for observed δ 18O values at low [O2], model solutions need to invoke either very low [O2] that have not been observed in the South Atlantic or an isotope effect that is lower than values measured in the laboratory or euphotic zone.
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ArticleEvaluation of the Southern Ocean O2/Ar-based NCP estimates in a model framework(John Wiley & Sons, 2013-04-02) Jonsson, Bror F. ; Doney, Scott C. ; Dunne, John P. ; Bender, Michael L.The sea-air biological O2 flux assessed from measurements of surface O2 supersaturation in excess of Ar supersaturation (“O2 bioflux”) is increasingly being used to constrain net community production (NCP) in the upper ocean mixed layer. In making these calculations, one generally assumes that NCP is at steady state, mixed layer depth is constant, and there is no O2 exchange across the base of the mixed layer. The object of this paper is to evaluate the magnitude of errors introduced by violations of these assumptions. Therefore, we examine the differences between the sea-air biological O2 flux and NCP in the Southern Ocean mixed layer as calculated using two ocean biogeochemistry general circulation models. In this approach, NCP is considered a known entity in the prognostic model, whereas O2 bioflux is estimated using the model-predicted O2/Ar ratio to compute the mixed layer biological O2 saturation and the gas transfer velocity to calculate flux. We find that the simulated biological O2 flux gives an accurate picture of the regional-scale patterns and trends in model NCP. However, on local scales, violations of the assumptions behind the O2/Ar method lead to significant, non-uniform differences between model NCP and biological O2 flux. These errors arise from two main sources. First, venting of biological O2 to the atmosphere can be misaligned from NCP in both time and space. Second, vertical fluxes of oxygen across the base of the mixed layer complicate the relationship between NCP and the biological O2 flux. Our calculations show that low values of O2 bioflux correctly register that NCP is also low (<10 mmol m−2 day−1), but fractional errors are large when rates are this low. Values between 10 and 40 mmol m−2 day−1 in areas with intermediate mixed layer depths of 30 to 50 m have the smallest absolute and relative errors. Areas with O2 bioflux higher than 30 mmol m−2 day−1 and mixed layers deeper than 40 m tend to underestimate NCP by up to 20 mmol m−2 day−1. Excluding time periods when mixed layer biological O2 is undersaturated, O2 bioflux underestimates time-averaged NCP by 5%–15%. If these time periods are included, O2 bioflux underestimates mixed layer NCP by 20%–35% in the Southern Ocean. The higher error estimate is relevant if one wants to estimate seasonal NCP since a significant amount of biological production takes place when mixed layer biological O2 is undersaturated.
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ArticleExport production and its regulating factors in the West Antarctica Peninsula region of the Southern Ocean(American Geophysical Union, 2012-04-24) Huang, Kuan ; Ducklow, Hugh W. ; Vernet, Maria ; Cassar, Nicolas ; Bender, Michael L.In connection with the Palmer LTER program, mixed layer water samples were collected during the cruise of the L.M. Gould in Jan., 2008 at 49 stations on a 20 × 100 km grid in the West Antarctica Peninsula (WAP) region of the Southern Ocean. In this study, [O2]/[Ar] ratios and the triple isotope composition of dissolved O2 were measured, and were used to estimate net community O2 production (NCP) and gross primary O2 production (GPP), respectively. These estimates are further converted to carbon export production, primary production and the f-ratio. Our measurements give NCP ranging from −3 to 76 mmol O2 m−2 day−1 (−25 to 650 mg C m−2 day−1), and GPP from 40 to 220 mmol O2 m−2 day−1 (180 to 1010 mg C m−2 day−1). The O2 NCP/GPP ratios range from −0.04 to 0.43, corresponding to f-ratios of −0.08 to 0.83. NCP and the NCP/GPP ratio are highest in the northern coastal areas, and decrease to lower values toward the southern coastal area and the open ocean. The inshore-offshore gradient appears to be regulated primarily by iron availability, as supported by the positive correlation between NCP and Fv/Fm ratios (r2 = 0.22, p < 0.05). Mixed layer depth (MLD) is inversely correlated with NCP (r2 = 0.21, p < 0.002) and NCP/GPP (r2 = 0.21, p < 0.02), and highest NCP occurred in the fresh water lenses probably formed from melted coastal glaciers. These results suggest that export production and the f-ratio increase where water stratification is intensified by input of fresh meltwater, and that mixed layer stratification is the major factor regulating NCP in the inner-shelf and coastal regions. Along-shelf variability of phytoplankton community composition is highly correlated with NCP, i.e., NCP increases when the diatom-dominated community in the south transitions to the cryptophyte-dominated one in the north. A high correlation is also observed between NCP and the logarithm of the surface chlorophyll concentration (r2 = 0.72, p < 0.0001) , which makes it possible to estimate carbon export as a function of Chl a concentration in this region.
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ArticleNet community production and gross primary production rates in the western equatorial Pacific(American Geophysical Union, 2010-10-12) Stanley, Rachel H. R. ; Kirkpatrick, John B. ; Cassar, Nicolas ; Barnett, Bruce A. ; Bender, Michael L.Net community production (NCP) and gross primary production (GPP) are two key metrics for quantifying the biological carbon cycle. In this study, we present a detailed characterization of NCP and GPP in the western equatorial Pacific during August and September 2006. We use continuous measurements of dissolved gases (O2 and Ar) in the surface water in order to quantify NCP at subkilometer scale resolution. We constrain GPP in discrete samples using the triple isotopic composition of O2. We find the average NCP in the western equatorial Pacific is 5.9 ± 0.9 mmol O2 m−2 d−1 (equivalent to 1.5 ± 0.2 mol C m−2 yr−1 with error estimates reflecting 1σ confidence levels) and the average GPP is 121 ± 34 mmol O2 m−2 d−1 (equivalent to 32 ± 9 mol C m−2 yr−1). The measurements reveal significant spatial variability on length scales as small as 50 km. The NCP/GPP ratio is 5.7% ± 1.8%. We also present results for NCP and GPP in the coastal area off Papua New Guinea and for GPP in the central Pacific along the equator.