The impact of a mesoscale in situ iron-enrichment experiment (SOIREE) on the planktonic ecosystem and biological pump in the Australasian-Pacific sector of the Southern Ocean was investigated through model simulations over a period of 60-d following an initial iron infusion. For this purpose we used a revised version of the biogeochemical SWAMCO model (Lancelot et al., 2000), which describes the cycling of C, N, P, Si, Fe through aggregated chemical and biological components of the planktonic ecosystem in the high nitrate low chlorophyll (HNLC) waters of the Southern Ocean. Model runs were conducted for both the iron-fertilized waters and the surrounding HNLC waters, using in situ meteorological forcing. Validation was performed by comparing model predictions with observations recorded during the 13-d site occupation of SOIREE. Considerable agreement was found for the magnitude and temporal trends in most chemical and biological variables (the microbial food web excepted). Comparison of simulations run for 13- and 60-d showed that the effects of iron fertilization on the biota were incomplete over the 13-d monitoring of the SOIREE bloom. The model results indicate that after the vessel departed the SOIREE site there were further iron-mediated increases in properties such as phytoplankton biomass, production, export production, and uptake of atmospheric CO2, which peaked 20-30 days after the initial iron infusion. Based on model simulations, the increase in net carbon production at the scale of the fertilized patch (assuming an area of 150 km2) was estimated to 9725 t C by day 60. Much of this production accumulated in the upper ocean, so that the predicted downward export of particulate organic carbon (POC) only represented 22% of the accumulated C in the upper ocean. Further model runs that implemented improved parameterization of diatom sedimentation (i.e. including iron-mediated diatom sinking rate, diatom chain-forming and aggregation) suggested that the downward POC flux predicted by the standard run might have been underestimated by a factor of up to 3. Finally, a sensitivity analysis of the biological response to iron-enrichment at locales with different initial oceanographic conditions (such as mixed-layer depth) or using different iron fertilization strategies (single vs. pulsed additions) was conducted. The outcomes of this analysis offer insights in the design and location of future in situ iron-enrichments. |
