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A solid knowledge of the mechanisms that influence successional patterns and species dominance in phytoplankton blooms and the subsequent fate of biomass after bloom termination is a prerequisite to predict the response of plankton communities to environmental changes and the associated past and future changes in geochemical cycles and pCO2. Process studies like iron fertilization experiments are a new and powerful approach to elucidate the in-situ mechanisms that determine dominance and succession patterns in phytoplankton blooms.All iron fertilization studies conducted so far have shown that iron is a limiting factor for phytoplankton growth in HNLC areas. Model studies and a recent comparison of all iron fertilization experiments have suggested that the deep mixed layers occurring in the Southern Ocean, might constrain the built-up of phytoplankton biomass and iron utilization below the levels found in other oceanic areas with stratified regimes. Here we report growth rates of phytoplankton species and biomass (in chlorophyll a) accumulation from two fertilization experiments carried out in the Southern Ocean (EISENEX and EIFEX). We show that despite deep mixed layers (> 40 to about 100 m depth) dominant species showed high growth rates that argue against the importance of light limitation in controlling biomass accumulation in the Southern Ocean.

Fossil fuel combustion causes annual emissions of 6 billion tons of carbon in the form of carbon dioxide (CO2) which significantly contributes to the greenhouse effect. Different strategies have been proposed in order to reduce anthropogenic input of CO2 into the atmosphere. Microalgae in the oceans (phytoplankton) take up large amounts of CO2 via photosynthesis and influence the carbon budget of our planet by transferring carbon from the surface to the deep ocean through sinking. This process called the biological pump leads to a long-term withdrawal of carbon from the atmosphere. An enhancement of the biological pump of the ocean through iron fertilization of the phytoplankton has been proposed as away to reduce atmospheric CO2. To test the feasibility of this approach, the Alfred Wegener Institute conducted an European iron fertilization experiment (EIFEX) on board R.V. Polastern in collaboration with several national and international partners.Phytoplankton constitutes the first link of the food chain in the global ocean the largest ecosystem in our planet. Although phytoplankton constitutes only a fraction (0.5 %) of the total plant biomass of the planet, it contributes about 30 % to the annual carbon-uptake via photosynthesis equivalent to approximately 150 billion tons. Consequently high turnover rates occur within the plankton, where growth, mortality and sinking rates are nearly balanced. In three major oceanic regions, the equatorial and subarctic Pacific as well as the Southern Ocean the growth of the phytoplankton is limited low iron concentrations. Previous iron fertilization experiments in these region led to a rapid increase of phytoplankton biomass and a clear decrease in dissolved inorganic carbon concentrations (including CO2) in the water column. However, the fate of these phytoplankton blooms had not been studied yet. The aim of EIFEX was to follow the fate of the iron-induced phytoplankton bloom (growth and decay) and determine the influence of the bloom on the carbon budget of the fertlized patch. During EIFEX we fertilized an area of over 150 km2 with 13 tons of iron sulphate dissolved in seawater. The temporal development of the phytoplankton bloom was followed over a period of more than five weeks. In the final week of the experiment a significant increase in turbidity of the deeper water layers down to the ocean floor could be observed as a result of the dying and sinking phytoplankton bloom.Microscopic analysis of water samples revealed the presence of aggregates composed of phytoplankton from the bloom together with zooplankton faecal material that sank out to deeper water layers. Geochemical measurements at the sediment/water boundary layer as well as the analysis of sediment cores indicated that a five millimeter thick layer of fresh plankton material had sedimented in the deep ocean. A detailed budget of the amount of carbon exported into the deep ocean is in preparation.The EIFEX results indicate that iron fertilization can lead to transport of organic material to the deep ocean. However, the amount of CO2 which could be biologically removed by fertilization of the entire Southern Ocean amounts to only a minor fraction of the annual CO2emissions. The overriding aim for a sustainable energy policy should be efficient energy use in combination with reduced CO2 emissions.

Dynamics of metazooplankton populations and the impact of grazing on their potential food supply was studied in an in situ iron fertilization experiment (EisenEx) in the Antarctic Circumpolar Current. Copepod nauplii, copepodites and the adults of small copepods (< 1.5 mm) were sampled with Niskin bottles (12 L). The exceptionally high concentrations we recorded (average 42 nauplii L-1 and 24 copepodites and adults L-1) suggest that vertical net hauls seriously undersample these organisms. During the experiment copepod nauplii numbers stayed more or less constant. Abundances of copepodites and adults of small copepods increased threefold indicating either enhanced recruitment from naupliar and copepodite stages respectively, and/or congregation within the patch due to migration of individuals from below the mixed layer. In either case the copepod population appeared to be food-limited outside the patch. Copepod grazing apparently suppressed protozooplankton stocks: Heterotrophic dinoflagellates and aloricate ciliates increased with food supply but decreased subsequently. Tintinnids decreased but empty loricae increased twofold indicating heavy grazing mortality. Thus interactions between grazer populations facilitated population growth of diatoms which dominated the iron-induced diatom bloom.

Phytoplankton blooms play a major role in global elemental cycles and marine food chains but different species impact these in different ways. Hence a mechanistic understanding of the factors regulating species dominance is a prerequisite to predictive modelling. Results from EisenEx revealed 6 distinct response behaviours to iron fertilization. Five species of the over 50 recorded, contributed 48% of total diatom biomass. Of these, two species (a pennate and a centric) reacted immediately with accumulation rates about double those of other species and built up population size exponentially. In contrast species with initially highest abundances responded after a week with a brief period of rapid accumulation which levelled off later. The ratio of live cells to intact empty and broken frustules (LC:EBF), used as a measure of mortality, increased steadily inside the fertilized patch indicating that the grazer assemblage could not keep pace with growth of the dominant species. Species-specific differences in the LC:EBF ratio and the ratio of intact empty to broken frustules indicated that selective grazing by proto- and metazooplankton played an important role in structuring the diatom assemblage.

Mesoscale in situ iron fertilisation experiments have resulted in the build-up of phytoplankton biomass and established beyond doubt that iron availability is the key factor limiting growth rates of oceanic phytoplankton in high-nutrient, low-chlorophyll (HNLC) regimes (see poster Assmy et al.). The response of important groups (microproto-, mesoproto- and small metazooplankton) of the pelagic community and the processes within the food web were studied in detail and compared with processes in the surrounding water during two iron fertilisation experiments in austral spring (EisenEx) and summer (EIFEX)

Biological response to iron fertilization in the Polar Frontal Zone of the Southern Ocean (EisenEx)Philipp Assmy1, Joachim Henjes1, Christine Klaas2 & Victor Smetacek11Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany2Max-Planck-Institute for Biogeochemistry, Carl-Zeiss-Promenade 10, 07745 Jena, GermanyDuring EisenEx - the second in situ iron fertilization experiment conducted in HNLC (High Nutrient Low Chlorophyll) waters of the Southern Ocean - an iron-enriched ocean eddy was followed over a period of three weeks in austral spring 2000. The aim of this experiment was to assess the impact of iron fertilization on the pelagic community and the processes within the food web driven by changes in plankton distribution and abundance. The diatom, large microzooplankton (in particular acantharians and tintinnids) and small mesozooplankton (copepodids and nauplii) community was quantified inside and outside the iron enriched patch.Inside the fertilized patch the diatom assemblage increased six-fold in abundance and four-fold in biomass. Outside the patch (control stations) diatom abundance remained constant whereas biomass doubled. Empty intact and broken diatom frustules were quantified as indicators of grazing pressure (i.e. mortality). Diatom debris increased three-fold inside and doubled outside the patch. Empty diatom frustules doubled inside and remained constant outside the patch. Acantharian abundance increased by a factor of 3 in the fertilized patch but only slightly outside the patch. This is of major interest, because acantharians are indirectly responsible for the formation of barite which is found in sediments and is suggested to be a paleo-indicator of high productivity regimes. The total abundance of the tintinnids ciliates (i.e. full and empty loricae) doubled inside the iron- enriched patch compared to no change outside the patch. Moreover the numbers of empty loricae, an indicator of grazing pressure on tintinnids, increased about 2.2-fold inside the fertilized patch with no change outside the patch. This suggests that particularly inside the iron enriched eddy, grazing pressure intensified highly on tintinnid ciliates and completely compensated tintinnid population growth in the course of the experiment. Copepodid numbers increased significantly (by a factor of ~ 3.6) inside the patch with a ~ 2.1-fold increase in the adjacent water mass. Nauplii numbers slightly increased in patch (by a factor of 1.7) in contrast to no changes out patch.Our results confirm the stimulation of diatom growth by iron addition in the Southern Ocean. Our results also indicate that a significant seasonal increase in diatom biomass occurs in the Polar Frontal Zone (PFZ) despite low iron conditions, consistent with satellite observations. Grazing pressure intensified more heavily inside the patch compared to out patch values due to greater food availability in the fertilized patch. Furthermore grazing pressure seems to be an important factor determining diatom productivity and assemblage composition in this area of the Southern Ocean.Key words: Southern Ocean, iron fertilization, diatoms, micro- and mesozooplankton, grazing

During the EisenEx experiment, conducted in HNLC (High Nutrients Low Chlorophyll) waters of the Southern Ocean, the centre of an eddy was fertilized with iron sulphate and followed over a period of 3 weeks in November 2000. The aim of this experiment was to assess the impact on the pelagic community and the processes within the food web driven by changes in plankton distribution and abundance. The large microzooplankton (in particular acantharians and tintinnid) and small mesozooplankton (copepodids and nauplii) community was surveyed inside and outside the iron enriched patch. Species abundance and taxonomic composition were quantified by microscopic techniques in concentrated water samples taken from seven discrete depth between 10 and 150 mBy the end of the experiment phytoplankton biomass had increased fourfold and the microzooplankton groups showed different trends in abundance. The abundance of acantharians increased by a factor of ~ 2.7 in the fertilized patch, but only slightly outside the patch. This is of major interest, because acantharians are suggested to be indirectly responsible for the formation of barite found in sediments and a paleo-indicator of high productivity regimes.The total abundance of the tintinnid ciliates (i.e. full and empty loricae) doubled inside the iron enriched patch compared to no change outside the patch. Moreover the numbers of empty loricae, an indicator of grazing pressure on tintinnids, increased about 2.2-fold inside with no change outside. This indicates that tintinnids increased growth rates with higher food availability but grazing pressure on them also intensified so that no net growth was recorded in the course of the experiment. With regard to small mesozooplankton, copepodid numbers increased significantly (by a factor of ~ 3.6) inside the patch with a ~ 2.1-fold increase in the adjacent water mass. Nauplii numbers slightly increased in patch (by a factor of 1.7) in contrast to no changes out patch.These results indicate that at least some of the large micro- and small mesozooplankton responded to the rapidly increasing food supply due to iron addition in this Southern Ocean HNLC region. In unfertilized waters, the slight increase in numbers of small grazers matched the seasonal increase in diatom biomass.Key words: Southern Ocean, iron fertilisation, grazing, micro-, mesozooplankton, abundance

Berichte zur Polarforschung = Reports on Polar Research