Phytoplankton growth and biological response to iron and zinc addition in the Ross Sea and Antarctic Circumpolar Current along 170°W

Deckboard enrichment experiments were conducted during the US JGOFS Antarctic Environment Southern Ocean Process Study to determine the community response to added iron and zinc and their effect on phytoplankton growth. Seawater was collected into acid-cleaned 20-l polycarbonate carboys and incubated with varying additions of iron and zinc. Cells representing individual taxa were counted before and after incubation. Chlorophyll a, particulate organic carbon, and particulate organic nitrogen were measured periodically throughout the experiments. Zinc is not a limiting factor on phytoplankton growth in both the Ross Sea and the Antarctic Circumpolar Current (ACC). The Ross Sea is less iron-stressed than the ACC, in particular at the southern site. However, iron addition largely increased phytoplankton growth and chlorophyll a in both regions. Community growth showed a stronger response to iron addition in high-silicate water than in low-silicate water. Dinoflagellates had the lowest abundance in both natural and iron-enriched seawaters. Prymnesiophytes and pennate diatoms were most sensitive to iron enrichment and were responsible for the bulk of the growth signal. In the high-silicate water of the ACC, Phaeocystis showed the strongest response to iron addition, increasing its abundance from <5% in natural seawater to ∼20% in 2.5 nM Fe-enriched water. Its maximum growth rate was 0.57 day−1 and the half-saturation constant was 0.27 nM, which were higher than most of the diatoms (0.2–0.4 day−1 and 0.05–0.13 nM, respectively).