Different reactions of Southern Ocean phytoplankton size classes to iron fertilization

During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms .20 mm were the main beneficiaries of iron fertilization, the growth of small diatoms (2–8 mm) was also enhanced, leading to a shift from a haptophyteto a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton (.20 mm) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2–20 mm) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups. The Southern Ocean is one of the three High Nutrient Low Chlorophyll (HNLC) regions of the world’s oceans, where macronutrients abound but phytoplankton growth is limited by the availability of iron (Martin et al. 1990). In situ iron fertilization experiments have been performed in all HNLC regions and all of them have in common that they induced strong phytoplankton blooms (see review in de Baar et al. 2005). The massive increase in chlorophyll a (Chl a) concentrations was mainly due to large or chain-forming diatoms, which benefit most from iron fertilization. Because iron uptake is dependent on cell surface area, smaller cells are favored under low iron concentrations due to their higher surface-to-volume ratios (Hudson and Morel 1990; Sunda and Huntsman 1997). The natural phytoplankton community of the iron-depleted Southern Ocean is therefore dominated by picoand nanophytoplankton (Gervais et al. 2002). The small diatom Chaetoceros brevis did not change its growth rates under different iron concentrations in laboratory experiments, leading to the assumption that this species is not iron limited under natural conditions (Timmermans et al. 2001). 1 Corresponding author ([email protected]). Acknowledgments We thank Captain U. Pahl and the crew of RV Polarstern for their support. Without the help of V. Smetacek (chief scientist), H. Leach, V. Strass, H. Prandke, B. Cisewski (CTD sampling), and the other cruise participants, our work would have been impossible. Special thanks to K. Nachtigall, C. Krieger, S. Krug, and A. Kähler for their help with HPLC measurements. We also thank K. Nachtigall and P. Fritsche for POC/PON, POP, and bPSi measurements. The manuscript was greatly improved by the comments of two anonymous reviewers. This research was funded by the German Research Foundation (DFG) grant PE_565_5 awarded to Ilka Peeken. May 2006