Simulating the Southern Ocean Iron Experiment (SOFeX) using a marine ecosystem model

Abstract The Southern Ocean Iron Experiment (SOFeX) was conducted to better understand the role of iron in driving marine ecosystems by fertilizing two patches north and south of the Antarctic Polar Front Zone (APFZ). A one-dimensional marine ecosystem model was used to simulate the fertilized patches and surrounding unfertilized waters. Nutrient drawdown at the south patch compared well with observations but the north patch overestimated the nutrient drawdown. Model results for chlorophyll, total Particulate Organic Carbon (POC) and primary productivity compared well with those obtained during the experiment. Simulated sinking Particulate Organic Carbon fluxes were higher than observations, possibly due to not enough carbon being routed to Dissolved Organic Carbon (DOC) relative to sinking POC.

Aparna Krishnamurthy & J. Keith Moore, Email: aparnak@uci.edu, University of California, Irvine, Dept. of Earth System Science, Irvine, Ca.

IntroductionThe Southern Ocean Iron fertilization Experiment was conducted during austral summer of 2002. It involved fertilizing two patches north and south of the Antarctic Polar Front Zone (APFZ). The regions north of APFZ are characterized by low-silicate, high nitrate waters (North Patch) and regions south of APFZ are characterized by high-silicate, high nitrate waters (South Patch). The “North Patch” was created on 12th January, 2002 at 56.23S, 172W and “South Patch” was created on 24 January, 2002 at 66.45S, 171.8W. Successive iron additions were done by injecting acidified iron sulfate into ship’s wake such that mixed layer concentrations were 1.2 nM, 1.2 nM, 1.5 nM at the north patch and four additions were done at the south patch such that concentrations were 0.7 nM after each addition. Sulfur hexafluoride was added along with iron during the first infusion to track the patches. The enriched patches were studied by three research vessels, the Revelle, Melville and Polar Star for a period of 40 and 28 days for north and south sites respectively (Coale et al., 2004 supplemental material). Patch dilution was estimated from satellite images as well as from loss of Sulfur hexafluoride, with estimated values of 0.08 per day at the south patch and 0.11 per day at the north patch respectively (Coale et al., 2004). During the experiment diatoms dominated the south patch whereas they accounted for about half of the bloom at the north patch (Brzezinski et al., 2005). Coale et al. (2004) suggested that the blooms may have become light-limited during the experiment due to self-shading. Significant carbon export was observed at both patches during the experiment but the blooms were slow to develop because of low ambient temperatures.

Ecosystem model The ecosystem model that we used is from the global-scale Biogeochemistry-Ecosystem-Circulation (BEC) model of Moore et al., (2004). The BEC ecosystem has been implemented in a 1-D column model. It has 150 horizontal levels that includes the euphotic zone and shallow aphotic zones. Model includes limitation by multiple nutrients, iron, nitrogen, phosphorus and silicon. It also includes multiple plankton classes, small phytoplankton, diatoms, diazotrophs, coccolithophores and zooplankton (diazotroph biomass is always negligible in the simulations presented here). It is robust enough to shift the phytoplankton community structure from diatoms, with high production rates and high export to a small phytoplankton dominated low production, low export domain based on nutrient availability and physical forcing onto the ecosystem.Phytoplankton growth is parameterized to include multiple nutrient limitations. Growth can be limited by nitrogen (nitrate, ammonia), phosphate, iron, silicon (for diatoms), and/or ambient light. As the light and nutrient levels change, the model incorporates variable elemental ratios for C/Fe, C/Si, and C/Chlorophyll (Moore et al., 2004; photoadaptation after Geider et al., 1998). The losses of the four phytoplankton species occurs via natural mortality, zooplankton grazing and phytoplankton aggregation. Surface forcing values such as wind stress, transmittance, atmospheric pressure, sea surface salinity, and net fresh water input were obtained from NCEP reanalysis data and Photosynthetically Available Radiation (PAR) data were obtained from SeaWiFS. Dust/iron deposition from atmosphere data were from Luo et al., (2003).Model was run at each site without fertilization initially and results were used to allow for dilution from surrounding waters during the patch simulations using the Coale et al., (2004) estimates of dilution. Model outputs were averaged over upper 20m at both patches and compared with mixed layer observations from SOFeX.

Results Model Simulations for north patch started on 7th January, 2002, 7 days before the initial fertilization of the north patch during the SOFeX and ended 60 days later on 7th March, 2002. The south patch simulations started on 21st January, 2002, 5 days before the initial fertilization of the south patch, and ended 60 days later on 21st March, 2002. At the north patch, simulated phosphate, nitrate and silicate are lower than the observations (Hiscock et al., 2005). Model predictions of chlorophyll and POC are in general agreement with the observations (Coale et al., 2004; POC data from Altabet & Hunter). The integrated primary productivity over the euphotic zone was measured during the SOFeX for both the control and fertilized patches and model simulations compared well these values ( Lance et al., submitted). The simulations overestimate sinking POC in both patches compared to observations which were estimated by Bishop et al., (2004) at the north patch and by Buesseler et al., (2005) at the south patch. Experimental observations of siliceous biomass and silica production rates were estimated by Brzezinski et al., (2005). Simulated values of these do not compare well with the observations, lagging observations at the south patch. The south patch bloom was terminated in our simulation when iron returned to background levels shortly after the field campaign ended. At the north patch chlorophyll and POC remain elevated throughout our simulations, at higher than observed concentrations. At both patches diatom and small phytoplankton growth rate were iron limited. In the unfertilized out patches at both sites iron was limiting growth throughout the simulation whereas in the fertilized patches at both sites iron infusions relived iron stress but as bloom developed small phytoplankton and diatoms went back to being iron stressed. At the north patch diatoms were iron and silicon limited to about the same degree in the latter part of the north patch simulation. This was in part because our north patch bloom was almost all diatoms, whereas observations indicate approximately half of the real bloom was diatoms, leading to stronger depletion of silica and macronutrients than in observations. Light limitation played only a small role in bloom development in our simulations, reducing diatom growth rates by about 10% at the south patch and by less than 10% at the north patch.

North Patch:- Square : Model out patch; Cross: Model in patch

Triangle: SOFeX In patch; Star: SOFeX Out patch

References 1. James K.

B. Bishop, Todd J. Wood, Russ E. Davis, and Jeffrey T. Sherman (2000), Robotic Observations of Enhanced Carbon Biomass and Export at 55°S During SOFeX, Science Vol. 304. no. 5669, pp. 417 – 420, DOI: 10.1126/science.1087717 2. Mark A. Brzezinski, Janice L.Jones, and Mark S. Demarest, (2005), Control of silica production by iron and silicic acid during the SOFeX, Limnology and Oceanography, 50(3), 810-824 3. Buesseler, K. O., J. E. Andrews, S. Pike, M. A. Charette, L. E. Goldson, M. A. Brzezinski, and V. P. Lance. 2005.Particle export during the SOFeX. Limnology and Oceanography, 50(1), 311-327. 4. Coale, K. H. et al. 2004. Southern ocean iron enrichment experiment: Carbon cycling in high- and low-Si waters. Science 304, 408-414. 5. Geider, R.J., MacIntyre, H.L., Kana, T.M., 1998. A dynamic regulatory model of phytoplankton acclimation to light, nutrients, and temperature. Limnology and Oceanography 43, 679–694.6. Hiscock, W.T., Millero, F.J., Nutrient and carbon parameters during the SOFeX, Deep-Sea Research I 52 (2005) 2086-2108.7. Luo, C., N. Mahowald and J. del Corral, Sensitivity study of meteorological parameters on mineral aerosol mobilization, transport and distribution, J. Geophys. Res., 108, D15, 4447, 10.1029/2003JD0003483, 2003.8. Moore, J.K., S.C. Doney, Lindsay Keith, 2004, Upper ocean ecosystem dynamics and iron cycling in a global three-dimensional model, Global Biogeochemical cycles, 18, GB4028, doi: 10.1029/2004GB002220.

South Patch:- Square : Model out patch; Cross: Model in patch

Triangle: SOFeX In patch; Star: SOFeX Out patch

Sensitivity Runs In order to evaluate the effects of dilution on the bloom we performed simulations with 0, 0.5 and 1.5 times the dilution rates estimated by Coale et al., (2004). Dilution acts to dilute phytoplankton biomass and the elevated iron concentrations within the patches, and to mix in macronutrients from outside the patches. Peak bloom magnitude at each site was inversely correlated with dilution rate. At the highest dilution rate the north patch bloom was largely prevented (max. chl. < 1 mg/m3). At decreased dilution rates, the peak of the bloom was earlier and the macronutrient drawdown was stronger than in our standard simulations, particularly in the no dilution case. The no dilution case is indicative of the response of large-scale fertilization.

Diamond: 1.5 * Coale et al., (2004) dilution; Square: Coale et al., (2004) dilution (as in top plots);

Star: 0.5 * Coale et al., (2004) dilution ; Cross: 0 dilution; Triangle: Experimental observations

Acknowledgements We would like to thank Kenneth Coale, Ken Johnson, Ken Buesseler, Jodi Brewster, Veronica Lance, William T. Hiscock, Mark Altabet, Craig Hunter for their prompt response and timely help during the course of this work. This work was supported by NASA grant NAG5-12520 from the NASA Ocean Biogeochemistry Program.

Discussion Our simulations reproduce the SOFeX observations to a large extent. The bloom in our simulations was dominated by diatoms in both the patches hence, silicate drawdown was excessive at the north patch. The sinking POC fluxes during simulations were also higher than those observed during the experiment, in part due to diatom dominance at the north patch. Another reason for this could be that model was partitioning too much carbon into sinking POC rather than DOC. The simulated bloom developed later than the observed bloom, hence there is a lag between observed and simulated silicification rate increase and biogenic silica concentrations. Diatoms were silica limited at the north patch because of the lower ambient concentrations of silicic acid. The no dilution simulation had a maximum nutrient drawdown resulting in higher net primary productivity and chlorophyll peak because the nutrients remained within patch for phytoplankton to flourish.