Coale, K. H.. Moss Landing Marine Laboratories, coale@mlml.calstate.edu
Fitzwater, S. H.. Moss Landing Marine Laboratories,
Gordon, R. M.. Moss Landing Marine Laboratories,
Tanner, S. A.. Moss Landing Marine Laboratories,
Johnson, K. S.. Moss Landing Marine Laboratories,

 
IRON LIMITATION OF PHYTOPLANKTON GROWTH AFFECTS NUTRIENT DRAWDOWN RATIOS IN THE SOUTHERN OCEAN
 
A series of water column measurements and deckboard incubation experiments indicate that iron limitation controls phytoplankton growth and biomass in the HNLC regions south of the Antarctic Polar Fronts Zone and the Ross Sea over much of the growing season. Dose/response experiments show that phtyoplankton populations are limited by iron concentrations below 0.5 nM but community growth rates saturate at concentrations between 0.5 and 0.75 nM. These values are similar to those found in the equatorial Pacific. A series of vertical profiles indicate that surface water dissolved iron concentrations are about 0.05 nM in the mixed layer, increasing to 0.3 nM by 500 m. These profiles are also consistent with previous MLML measurements. Nutrient uptake experiments show that iron limitation causes higher Si:N uptake ratios consistent with polar fronts and recent coastal findings. With the exception of waters north of the Polar Fronts Zone where low silicate may prevent rapid diatom growth, these results indicate that iron limitation is prevalent throughout large regions of the Southern Ocean and Ross Sea and existing phytoplankton populations are adapted to low concentrations of dissolved iron. The ratio of nutrient drawdown was observed to be sensitive to iron concentration. At low values of iron, the Si:N drawdown ratio was about 2. At iron concentrations of 0.5 nM and above, Si:N drawdown ratio is about 1. Diatoms grow heavier frustuals under iron limitation whereas under iron replete condition, the frustuals appear to be weakly silicified. We believe iron acts to control the rate of nitrate uptake and thereby growth rate, whereas silicification is less affected by iron. Iron limitation may, therefore, be partially responsible for the unequal ratio of silicate:nitrate drawdown observed in the APFZ. These results together with those of other potentially limiting metals, will be presented.
 
Day: Thursday, Feb. 4
Time: 02:00 - 02:15pm
Location: Sweeney Center
 
Code: SS31TH0200S