Phytoplankton processes. Part 2: rates of primary production and factors controlling algal growth during the Southern Ocean Iron RElease Experiment (SOIREE)

The Southern Ocean Iron RElease Experiment (SOIREE) fertilised 200 km² of polar waters (mixed-layer depth, 65 m) south of Australia in February 1999. During the 13-d SOIREE, iron enrichment stimulated primary production, algal growth and biogenic silica production rates. A floristic shift from small to large cells resulted in a greater contribution to community production by diatoms, which contributed >60% to community production towards the end of SOIREE. Column-integrated rates of primary production increased by more than 10-fold, and community production rates (normalised to chlorophyll) had doubled by day 4, but then declined after day 6 to initial rates. Iron enrichment increased the growth rates of the algal community from 0.02 to 0.15 d⁻¹ (based on changes in in-situ chlorophyll concentrations), from 0.02 to 0.2 d⁻¹ (based on algal carbon), and more than doubled initial rates (based on ¹⁴C) to 0.2 d⁻¹ during SOIREE. However, these iron-elevated rates were less than the maximum rates (0.69 d⁻¹) predicted for waters at 2.5°C. Community biogenic silica production rates increased by 6-fold, i.e. to a lesser extent than primary production rates, resulting in a 2-fold reduction in the silicic-acid : carbon uptake ratio. Shipboard perturbation experiments indicated that iron, rather than both iron and light, limited algal growth in these waters. However, a significant increase in light attenuation in situ due to algal self-shading, suggested that light limitation of algal growth might become significant after day 13 at this locale. Limitation of algal growth by both iron (and silicic acid and/or zinc) within iron-fertilised waters after day 8 of SOIREE was not evident from the results of a shipboard experiment. A simple mathematical model of particulate organic carbon (POC) potential export during SOIREE predicted large increases in export, due to both elevated production rates and a floristic shift to large cells. However, no significant increase in POC export was observed during SOIREE.