Abstract:

Three shipboard iron enrichment phytoplankton incubations were carried out in the Prydz Bay, Antarctica, from January to March of 2002. Waters for the incubations (Exp. 1, 2 and 3) were collected from 20 m depth in 3 stations (St. Ⅰ 1, Ⅶ 1 and Ⅶ 5), respectively. Although the nutrient concentrations in the surface waters of the three stations were consistently high, the chlorophyll a concentrations varied much. Chlorophyll a concentrations in the 20 m depth of St. Ⅰ 1 and Ⅶ 1 were 0.13-0.17 μg·L -1 and 0.20-0.26 μg·L -1 , respectively, while this figure was 2.35-2.65 for St. Ⅶ 5. There were six levels of enriched iron concentrations (control, 5, 10, 20, 40 and 80 nM) in Exp. 1 while three enriched iron levels (control, 10 and 40 nM) were arranged in Exp. 2 and 3. The iron enrichments stimulated the phytoplankton growth and nutrient drawdown in Exp. 1 (6-26th, January) and Exp. 2 (20th February-4th March). In Exp. 3, phytoplankton growth and nutrient drawdown were at nearly the same rate in the control and iron enriched bottles. In Exp. 1, the chlorophyll a concentrations in the bottles with 20, 40 and 80 nM iron enrichments grew exponentially to 40-43 μg·L -1 on the 17 th day, with a growth rate of 0.36-0.38 d -1 . Chlorophyll a concentration in the bottle enriched with 10 nM iron reached its peak (19.35 μg L -1 ) on the 23 rd day (growth rate 0.27 d -1 ). Phytoplankton in the control bottle and the bottle enriched with 5 nM iron grew at the rates of 0.13 and 0.16 d -1 , respectively. In Exp. 2, the chlorophyll a growth rates were 0.13, 0.32 and 0.40 d -1 in the control bottle and bottles with 10 and 40 nM iron enrichments, respectively. It seems that 10 nM iron enrichment was not enough to stimulate the phytoplankton to reach their maximum growth rate.