Drake Passage Traverse

Cruising southward from the Chilean margin, we finished our oceanographic profile of the Cape Horn current located in the southeast Pacific.  To present little is known about the Cape Horn current even though it plays a key role in our understanding of the present day and paleoceanography in the Subantarctic Southeast Pacific as well as the exchange of water masses in the northern Drake Passage. Our oceanography group led by Wolfgang Schneider (University of Concepcion, Chile) and Harold Fenco (National Institute for Research and Development, Argentina) is being strongly supported by our Argentinean and Chilean observers on board. The main device used for the oceanographic work is the CTD-Rosette which measures water conductivity, temperature and depth in the entire water column. Additionally installed acoustic current meters provide information on ocean currents.

With the CDT-Rosette water samples from different depths can be obtained. Coordinated by Lester Lembke-Jene (AWI) and Hartmut Schulz (University of Tübingen), these water samples are used to improve, calibrate and validate microfossil proxy methods. For the Cape Horn current we had eight CTD stations and during our Drake Passage traverse we performed 16 CTD stations. The transect covers the major oceanographic currents of the southern Antarctic Ocean over a relatively short distance (from north to south: Sub-Antarctic, Polar and Southern Antarctic Circumpolar Current Front). For that reason oceanographic work during the past 25 years has extensively focussed on the Drake Passage. However, this work has mainly been located between Cape Horn on the Sub-Antarctic islands and hence not on the Pacific side.

The ProIron group (Fig. 2) from the Alfred Wegener Institute, University of Geneva, University of Bremen, University of Oldenburg and ETH Zurich, conducted its first test sampling on February 28th. The goals of the study are to elucidate the impact of iron limitation and trace metal cycling (iron, zinc, cobalt, manganese and vitamin B₁₂) in the Drake Passage and the West Antarctic Peninsula under current and future climate. The test sampling from the open ocean is necessary to ensure that the low trace metal containing seawater could be sampled without introducing any trace metal contamination from the Polarstern ship and equipment. To achieve this, a teflon membrane pump connected to a polythelene line was used to pump surface seawater from 25 meters depth (Fig. 3) directly into a trace metal clean van where the seawater could be sampled. Our first test run was carried out under very good weather conditions, the sun was shining and the sea state was calm, hence providing perfect sampling conditions. The test was successful as microscopic observations revealed that we were able to sample a diverse and healthy phytoplankton assemblage showing no sign of damage due to the pumping.

Since we were still not in waters south of the Polar Front, our target region in the Drake Passage, we had to wait to sample for our main experiment until March 1st at which point we had finally passed the front. Unfortunately this also coincided with a storm, not atypical for this region, causing wind speeds of up to 17 m s-1 and producing waves of up to 4 meters. This made it impossible to pump any seawater. Hence the ProIron group was forced to wait yet again for better weather conditions and to catch a few more hours of sleep. Finally, on March 2nd at 5:25 a.m. the polythelene line could be deployed for our first big BIO1 station. We then proceeded to pump water for the next 23h. This water contained a, for a HNLC (high nutrient low chlorophyll) area, very typical and diverse phytoplankton community (Fig. 4). We used this water to set up experiments aimed at shedding light on the role of iron limitation on Southern Ocean phytoplankton community structure and productivity, and to identify other potential co-limiting factors such as cobalt, vitamin B₁₂, zink, manganese and light. Given that most of the iron is associated with loosely defined organic compounds, which greatly affects the chemistry and bioavailability, thus potentially limiting the growth of phytoplankton, experiments were also designed to test phytoplankton sensitivity to different iron complexes. Here we focussed on compounds that are biologically produced such as siderophore, saccharides and exopolymeric substances.