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The expeditions ANTARKTIS XVIII/1-2 of the research vessel Polarstern

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Smetacek, V. , Bathmann, U. and El Naggar, S. E. D. (2001): The expeditions ANTARKTIS XVIII/1-2 of the research vessel Polarstern , Berichte zur Polar- und Meeresforschung = Reports on polar and marine research / Alfred-Wegener-Institut für Polar- und Meeresforschung .
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The first leg of the 18th Antarctic cruise of RV "Polarstern" was started in Bremerhaven on 29.09.2000 and was completed in Cape Town on 23.10.2000. During this cruise different scientific instrumentations were tested and an atmospheric marine program was carried out. The ship was sailed on the shortest way to Cape Town (Fig. 1). The transfer time was about 24 days including 2 days for station works between Bremerahaven and Las Palmas for testing the winchs and the new machine control system. An other station of about 6 hours was carried out nearby Cape Blanc to recover one mooring at approx. 21°17 N; 20°43 W and redeploy it at the same position. A part of the testing crew (AWI, INTER, RFL, ROCHEM, MTU, STNH, WERUM) was disembark on 08.10.2000 in Gran Canaria (Las Palmas).During the third step of the Midlife Conversion (MC) of RV "Polarstern" between 29.8.2000 - 29.09.2000 in Bremerhaven, the main machine control system was replaced by the company "Motoren und Turbinen Union, (MTU)", Friedrichshafen. The winch's power supply and control system was also replaced by the company "STN-Atlas-Elektronik", Hamburg and Bremerhaven. Sea trials of the mentioned systems were carried out during this cruise.The old data acquisation and display system of RV "Polarstern" (PODEV) was replaced by a new (PODAS) on a real-time based data management system (RTDBMS). The company WERUM, Lüneburg has been developed this software package and did the sea trial during the cruise.The on VMS-based computers (Digital Equipment) were replaced by SUN-Server (UNIX-based). More than 30 Compaq computers (Windows 2000) are installed for data display and acquisation. Installation and tests of the hardware has been carried out.The UV-B-group of AWI has measured the UV-B-irradiance distributions (spectral and doses measure ments) as a function of latitude. The AWI-spectrometer (UV-B & UV-A) and the electronic UV-B-personal dosimeter (ELUV-14) were used. Calibration of instruments was done. In addition, ozone profile soundings were carried out during the cruise.A scientfic group from University of East Anglia, UK, has carried out different on-line measurements during the cruise:- Aerosol and rain samples were collected using Graseby-Anderson high-volume samplers and collection funnels to determine the deposition of iron to Atlantic surface waters.- Biogenic production of volatile organo-halogen and organo-nitrogen compounds in seawater were measured and analysed.- Quantifying the air-sea exchange of carbon dioxide was the third programme of this group during this cruise.The geoscience department of the University of Bremen, SFB 261, is monitoring the Sahara dust deposition to the Atlantic waters by using a mooring system located nearby Cape Blanc (21°17 N; 20°43 W ). The mooring was recovered and a new one was deployed at the same location.The institute of environmental Physics of the university of Heidelberg has carried out Differential Optical Absorption Spectroscopy (DOAS) measurements during the cruise to determine the distributions of different chemical tracers in the atmosphere.The trace metal group from NIOZ made continuous underway measurements of dissolved iron using a towed fish and trace metal clean pumping system. Intercalibration of measuring systems was carried out.ANT XVIII/2:CRUISE SUMMARYU. Bathmann und V. Smetacek (AWI)The in situ iron fertilization experiment EisenEx (Eisen-Experiment, ANT XVIII/2) was carried out from RV Polarstern in the Polar Frontal Zone (PFZ) due south of Africa (48° S; 21° E), between 25th Oct. and 5th Dec. 2000. The aim of the cruise was to fertilize, with iron-sulphate solution marked with an inert tracer, a patch of water in an eddy of the Antarctic Circumpolar Current and follow the physical, chemical and biological processes in the fertilized water till the end of the cruise. The 54 scientific personnel on board, representing a total of 15 nationalities, had been selected on the basis of expertise necessary to carry out the tasks required by the experiment. About half the scientists were biologists with physical and chemical oceanographers contributing about equal proportions to the other half.Funding for EisenEx was provided primarily by the German Bundesministerium für Bildung und Forschung with additional support from the European Union (CARUSO), the Netherlands-Bremen-Oceanography programme (NEBROC), the National Environmental Research Council (United Kingdom) and various other funding sources.Scientific background:Ice-core data show that changes in global temperature are closely linked to atmospheric CO2 concentrations. However, the sources and sinks of atmospheric CO2 between glacial and interglacial periods are not known as also the factors regulating its concentration during these periods. Since the oceans contain far more CO2 than the atmosphere, the balance of carbon at the ocean surface has a critical effect on atmospheric concentrations. This balance is influenced by phytoplankton, the single celled algae that grow in the surface, sunlit layer. By taking up and converting dissolved CO2 into biomass, the algae create a deficit in the surface layer that is compensated by CO2 drawdown from the atmosphere. Phytoplankton growth is dependent on light supply and the availability of dissolved nutrients, of which nitrate is the most important. However in three vast regions of the ocean, the so-called high-nitrate, low-chlorophyll (HNLC) regions (Equatorial and Subarctic Pacific and the entire Southern Ocean), phytoplankton production is low, despite favourable light conditions and high nitrate concentrations. A solution to this enigma is emerging from studies over the past decade. Iron appears to be the major limiting nutrient in these regions. It has been shown that the retention ability of sea water for iron is much less than that of nitrate and phosphate in terms of Redfield stoichiometry. In other words, iron is selectively lost to sinking processes relative to N and P. The iron retained in deeper water is sufficient to enhance phytoplankton biomass by a factor of 2 4 relative to surrounding water. Were all available nitrate utilised, the resulting biomass would be 2 orders of magnitude higher than the normal HNLC biomass.The most convincing evidence for iron limitation of HNLC water has come from three in situ iron fertilization experiments, two carried out in the Equatorial Pacific (IRONEX I and II) and one in the Southern Ocean (SOIREE) showing that phytoplankton blooms could be induced by fertilising a patch of water a few km2 in extent with a few tonnes of dissolved iron. These experiments showed that it is essential to choose a suitable hydrographically calm region in order to follow and monitor the fertilized patch for several weeks. The Southern Ocean Iron Release Experiment (SOIREE), carried out in waters south of Australia, resulted in a phytoplankton bloom of over a 100 square kilometres in size. Because of a shortage of ship time, the fate of the SOIREE bloom could not be ascertained during the experiment. This is crucial for the CO2 budget. If the biomass is broken down in the surface layer by bacteria and zooplankton, no net removal of CO2 occurs. If the organic matter sinks out of the surface layer, however, the equivalent amount of CO2 is removed from the atmosphere for tens to hundreds of years.The source of the high nutrients in HNLC regions is upwelling and admixture of sub-pycnocline water to the surface mixed layer. In the North and Equatorial Pacific this upwelled water remains in the surface so the nutrients are eventually converted into organic matter, the bulk of which is transferred to depth via the normal pathway of the biological pump. Because of its hydrography, the situation in the Southern Ocean is very different. Most of the Southern Ocean consists of a broad eastward flowing ring of water, the Antarctic Circumpolar Current (ACC), divided by the Antarctic Polar Front (APF) into the northern Polar Frontal Zone (PFZ) and the southern Antarctic Zone. Along stretches of the northern border of the PFZ, i.e. along the Subantarctic Front, surface water tends to subduct under the neighbouring subantarctic zone leading to formation of Antarctic Intermediate Water. Since these are HNLC waters, the process of downwelling results in the transfer of substantial amounts of unutilized nitrate and phosphate from the surface into the ocean interior. Were enough iron available, the algae would grow faster, take up more nutrients and fix more carbon, which would either sink out as a rain of particles or be carried down to the deep sea with the subducting PFZ water.John Martin (1990) proposed the hypothesis that iron, transported in airborne dust from the continents and deposited on the Southern Ocean, enhanced productivity during glacial periods and contributed to the lower concentrations of CO2 in the atmosphere that characterized these colder periods in the Earths history. Simple calculations indicate that about a third of the difference between glacial and interglacial CO2 levels (ca. 30 Gigatonnes) could be accounted for by this iron-fertilization mechanism, i.e. complete incorporation of N and P into organic matter.Prior to EisenEx a series of cruises dedicated to elucidating the relationship between mesoscale physics and plankton biology in the Antarctic Circumpolar Current (ACC) had been carried out from RV Polarstern. During cruise ANT X/6 performed in austral spring of 1992 high iron concentrations found north of the Polar Front (APF) led to build-up of a series of phytoplankton blooms there, whereas south of the Front, iron concentrations were lower and phytoplankton biomass remained an order of magnitude lower. An exceptional feature of this period was the large number of icebergs throughout the ACC (5 10 bergs in a radius of 10 nm around the ship) systematically recorded during all three transects carried out from end-October to mid-December (Smetacek et al. 1997). Although their role was discounted at the time (de Baar et al. 1995), we now suspect that the source of the high iron concentrations (>2 nmol/l-1) was more rapid melting of the bergs in the warmer water of the PFZ (2° C) as compared to AZ water (-1.5°C). During subsequent cruises in austral summer and autumn (ANT XIII/2 and ANT XVI/2) iron concentrations were low throughout and not a single berg was recorded. Higher biomass of phytoplankton (the maximum values were factor-of-four higher than in the surroundings) was restricted to the vicinity of the APF where some iron could have been supplied by local upwelling of deeper water in connection with meandering of the APF. Species composition and biomass of phytoplankton was clearly related to mesoscale water masses, indicating differences in pre-conditioning of the respective stocks in adjacent water masses. Apparently, iron availability played a key role in determining productivity and species composition of the ACC plankton.Although we were aware that ship-time allotted for EisenEx would not suffice to follow the eventual fate of the fertilized biomass, it was still worthwhile to carry out a short experiment to answer the following questions:1. Is phytoplankton of the ACC iron-limited also in spring, at the start of the growing season and in the presence of heavy grazing pressure? SOIREE was carried out south of the APF in late austral summer, a period when grazing pressure is low and the phytoplankton is most likely to be iron-limited.2. In what way would the spring pelagic ecosystem react differently to fertilization as compared to the experimental conditions tested during SOIREE.3. Would it be possible to fertilise a patch and follow it over time during stormy spring conditions and in the strong flow field the ACC in the Atlantic sector of the Southern Ocean? Earlier studies had shown that meandering of the APF resulted in formation of mesoscale eddies. We reasoned that the low current speeds recorded in the centre of such an eddy would provide a stable water mass that maintained its integrity over the course of the experiment.Description of cruise:The first week of the cruise was dedicated to selection of a suitable site for the experiment: the centre of a large eddy with low iron concentrations and phytoplankton biomass. In order to gain an overview of the position of Southern Ocean fronts we first carried out a meridional transect with the towed, undulating instrument package ScanFish that records temperature, salinity and chlorophyll fluorescence in the upper 250 metres. The 750 km transect along the 20E meridian commenced at the Subantarctic Front at 45S, covered the PFZ, crossed the APF and ended in the Antarctic Zone at 52S. Temperatures were lower, diatom stocks larger and silicate concentrations, used by diatoms to construct their shells, higher across a stretch of water around 48S. We identified this band of water as an eddy originating from the APF that had drifted 400 km northward. It exhibited similar current speeds as the water around it but in the opposite direction, i.e. towards the west. Where the currents shifted directions, speeds were at their lowest. Further support came from altimeter images indicating the core of an eddy.The second week of the cruise was devoted to a fine scale hydrographical survey of the region. The preconditions were good: very low iron concentrations throughout and a sparse but species rich plankton community combined with fairly shallow mixed layers. Measurements of the photosynthetic performance of individual cells showed that the algae were growing at only 30% of their potential rates. A cause for concern was the large numbers of copepods (millimetre sized zooplankton) in the area. Their grazing pressure could nip the bloom in the bud.Fertilization began on Nov. 7 in the centre of the eddy which we marked with a drifting buoy at approximately 48S, 21E. Four tonnes of iron sulphate dissolved in 30 cubic metres of acidified sea water was released through a hose around a spiral about 70 km in length and 7 km in diameter relative to the buoy trajectory.The previous iron enrichment experiments had shown that the iron added to the patch vanishes within a few days. Thus an inert tracer, sulphur hexafluoride (SF6), is added to the enriched water to identify the patch. Because SF6 is volatile and escapes into the atmosphere, the iron and tracer mixture was released at a depth of 15 metres in the wake of the ships propeller. About 40 gramms of SF6 were added to the initial fertilizer. Subsequent fertilizations were not marked with SF6.The first signs of a response in the phytoplankton was found two days after fertilization, a day or two earlier than expected. Data from a fast repetition rate fluorometer, which measures the photosynthetic efficiency of algal cells, showed that the algae in the patch had increased their efficiency, hence growth rate, significantly. Despite the favourable light climate for the phytoplankton during the first half of the cruise, chlorophyll levels throughout the region stayed more or less constant. Within the patch, they more than doubled over the first five days. The bloom had begun.The first severe storm of the cruise occured five days after fertilization. The water column was mixed down to 60 m. A few long transects after the storm showed strong signals of SF6 to the west of the buoy. The patch had stayed in the eddys eye and was now moving west after describing a semicircle. After carrying out a series of long stations within and outside the patch, another few tonnes of iron in solution was added in its centre. Although iron concentrations were still high, they had dropped in the week since the first fertilization. Another complete spiral was fertilised on Nov. 16. By Nov. 18, the highest chlorophyll levels within the patch had increased to bloom proportions, with values roughly four times those found in the surrounding waters.Preliminary results suggested that although the iron enriched plankton were growing at least twice as fast as those in the surrounding water, the accumulation of biomass was kept in check by the poor light conditions associated with intermittent stormy weather in combination with heavy grazing pressure exerted by the various organisms, ranging from protozoa to crustacean zooplankton.

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