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 200km(2) of polar waters (mixed-layer depth, 65 m) south of Australia in February 1999. During the 13-d SOIREE, iron enrichment stimulated primary production, alg al growth and biogenic silica production rates. A floristic shift from smal l to large cells resulted in a greater contribution to community production by diatoms, which contributed > 60% to community production towards the en d of SOIREE. Column-integrated rates of primary production increased by mor e 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 .15d(-1) (based on changes in in-situ chlorophyll concentrations), from 0.0 2 to 0.2 d(-1) (based on algal carbon), and more than doubled initial rates (based on C-14) to 0.2 d(-1) during SOIREE. However, these iron-elevated r ates were less than the maximum rates (0.69 d(-1)) predicted for waters at 2.5 degreesC, Community biogenic silica production rates increased by 6-fol d, i.e. to a lesser extent than primary production rates, resulting in a 2- fold reduction in the silicic-acid:carbon uptake ratio. Shipboard perturbat ion experiments indicated that iron, rather than both iron and light, limit ed algal growth in these waters. However, a significant increase in light a ttenuation in situ due to algal self-shading, suggested that light limitati on of algal growth might become significant after day 13 at this locale. Li mitation of algal growth by both iron land silicic acid and/or zinc) within iron-fertilised waters after day 8 of SOIREE was not evident from the resu lts of a shipboard experiment. A simple mathematical model of particulate o rganic carbon (POC) potential export during SOIREE predicted large increase s in export, due to both elevated production rates and a floristic shift to large cells. However, no significant increase in POC export was observed d uring SOIREE. (C) 2001 Elsevier Science Ltd. All rights reserved.