Modeling the bloom evolution and carbon flows during SOIREE: Implications for future in situ iron-enrichments in the Southern Ocean

The impact of a mesoscale in situ iron-enrichment experiment (SOIREE) on th e planktonic ecosystem and biological pump in the Australasian-Pacific sect or 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., 200 0), which describes the cycling of C, N, P, Si, Fe through aggregated chemi cal and biological components of the planktonic ecosystem in the high nitra te low chlorophyll (HNLC) waters of the Southern Ocean. Model runs were con ducted for both the iron-fertilized waters and the surrounding HNLC waters, using in situ meteorological forcing. Validation was performed by comparin g model predictions with observations recorded during the 13-d site occupat ion of SOIREE. Considerable agreement was found for the magnitude and tempo ral trends in most chemical and biological variables (the microbial food we b 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 m onitoring of the SOIREE bloom. The model results indicate that after the ve ssel departed the SOIREE site there were further iron-mediated increases in properties such as phytoplankton biomass, production, export production, a nd uptake of atmospheric CO2, which peaked 20-30 days after the initial iro n infusion. Based on model simulations, the increase in net carbon producti on at the scale of the fertilized patch (assuming an area of 150 km(2)) was estimated to 9725 t C by day 60. Much of this production accumulated in th e 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 se dimentation (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. Fi nally, 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. p ulsed additions) was conducted. The outcomes of this analysis offer insight s in the design and location of future in situ iron-enrichments. (C) 2001 P ublished by Elsevier Science Ltd.