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 Fe bruary 1999 at 61 degreesS, 141 degreesE 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. Over all, the sinking rates of the diatoms estimated by the SETCOL method (Bienf ang, 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 > 2 2 mum reduced to a minimum (0.47 m d(-1)) s days after the first in situ ir on addition. A subsequent increase in sinking rates (chlorophyll-based) of > 22-mum cells coincided with an increase in algal iron stress (based on di atom flavodoxin levels). The primary bloom species, Fragilariopsis kerguele nsis and Nitzschia and Navicula sp. reduced their sinking rates most marked ly 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 m m, Trichotoxon and Thalassiothrix) showed little change in sinking rate ins ide 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 60d), we infer that sink ing 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 similar to 1% d(-1). A 1% daily loss wo uld have halved cell accumulation at the surface over 60d, primarily via a reduction in growth potential. (C) 2001 Elsevier Science Ltd. Ah rights res erved.