The effect of in situ iron addition on the sinking rates and export flux of Southern Ocean diatoms

During the Southern Ocean Iron RElease Experiment (SOIREE), conducted in February 1999 at 61°S, 141°E in high nutrient, low chlorophyll waters south of the Polar Front, we measured the intrinsic sinking rates of diatoms at two depths inside and outside of an iron-fertilised patch. Overall, the sinking rates of the diatoms estimated by the SETCOL method (Bienfang, Canadian Journal of Fisheries and Aquatic Sciences 38 (1981) 1289–1294 were significantly lower inside than outside the patch, and a time series of sinking rates (chlorophyll-based) indicated that the rates of cells >22 μm reduced to a minimum (0.47 m d −1) 8 days after the first in situ iron addition. A subsequent increase in sinking rates (chlorophyll-based) of >22 μm cells coincided with an increase in algal iron stress (based on diatom flavodoxin levels). The primary bloom species, Fragilariopsis kerguelensis and Nitzschia and Navicula sp. reduced their sinking rates most markedly within the patch, showing a decrease of up to 87% of initial rates, over the 13 days of the time series. In contrast, the very largest cells (>1 mm, Trichotoxon and Thalassiothrix) showed little change in sinking rate inside or outside the patch. Sinking rates of resident cells associated with a deep chlorophyll maximum (40–75 m) in and out of the patch also showed no significant sinking rate change upon iron addition. Given these results and the known longevity of the SOIREE bloom (at least 60 d), we infer that sinking rate reduction is an integral part of a species-specific bloom response to elevated Fe supply. We calculate that sinking losses of iron-saturated, unaggregated cells would have been ∼1% d −1. A 1% daily loss would have halved cell accumulation at the surface over 60 d, primarily via a reduction in growth potential.