May 10 – May 16, 2008

The 4th and final weekly report is centred on the „analytics“, e.g. all in-situ samplings and analysis of the biological and chemical composition of the surface-near ocean. The studies are performed by groups from the University Bremen, the GKSS Research Centre, and the IFM-GEOMAR. The responsible persons at sea – who also provided the following work descriptions - are Bettina Schmitt, Marc Taylor, Laila Bentama, Martina Gehrung, Tobias Steinhoff, and Imke Grefe:

The “Phyto-Optics“ Group from the AWI and the GKSS research institute in Geesthacht, Germany have been continuously taking water samples since the beginning of the cruise to measure phytoplankton, the planktonic primary producers in the surface waters, which form the basis of all oceanic foodwebs. Although microscopic in size (between 1µm and 1mm), phytoplankton are responsible for about 50% of the total primary production worldwide.

During this cruise, the group wants to investigate the distribution and abundance of different phytoplankton groups in the Southern and Northern Atlantic. Back in Germany the data will also be used to validate satellite pictures taken of the study area each day. As the optical properties of the surface waters depend on the phytoplankton within, measurements of light absorption, reflectance and fluorescence can give important information about the phytoplankton community.

In the mornings at 6:00 and the evenings at 22:00, a water sample of approximately 80 l is taken and filtered for the pigment analyses, whereas absorption is measured directly on board (Fig 1a). Additionally, daily samples are taken around midday with a special water sampler from as deep as 200 m. Approximately 5.200 l of water have been pumped through the filters since we left Punta Arenas. As expected, the phytoplankton density is low in the surface waters of the open ocean; however, deeper layers often show a fluorescence maximum due to a higher density of these unicellular organisms.

Water samples were also are taken and filtered for the genetic examination of marine nitrogen fixers (diazotrophs). Only prokaryots have the ability to convert atmospheric nitrogen into bioavailable forms. The study of nitrogen fixers in the ocean is at an early stage and much is unknown about their diversity, distribution and role in the global nitrogen cycle.

The cyanobacterium Trichodesmium is one of the most prominent nitrogen fixers and builds characteristic blooms (“sawdust of the sea”) in surface waters with a temperature of 23-25°C. Slight variations in sea surface temperatures outside of this range will prevent the formation of these blooms.

Water is pumped from the bow of the ship (8 m depth) every 8 hours (in the morning, afternoon and at midnight) for filtering. Back in Kiel the samples will be screened for the genetical sequence of the enzyme nitrogenase. This enzyme mediates the reduction of molecular nitrogen to ammonia.

In the wet lab we installed numerous instruments for continuous measurement of chemical and biological parameters of the surface water. During the entire cruise approximately 900 m³ were pumped through the instruments by the ship’s pump.

The FerryBox (Fig. 2) from GKSS Research Centre measures standard oceanographic parameters like temperature and salinity, but also parameters that provide information about biological processes in the ocean: a turbidity sensor, a Chlorophyll-a-fluorescence probe, an oxygen optode and a pH-electrode. The data is stored along with time and position in one-minute intervals. Additionally, two nutrient analyzers are also connected to the underway system, which measure dissolved nitrate and phosphate. For testing purposes, a new sensor for variable Chlorophyll-a measurement was installed, which can determine the quantum efficiency of phytoplankton as affected by their productivity.

The IFM-GEOMAR also measures dissolved oxygen and the total gas pressure of all gases dissolved in the water, allowing for the calculation of air-sea gas exchange. Even with the high absolute oxygen values associated with warm waters, the variability of the data shows clear biological activity in the surface waters.

In addition, the partial pressure of CO₂ (pCO₂) in the surface water and the overlying atmosphere is measured to determine the CO₂-flux between the atmosphere and the ocean. The preliminary data shows the South Atlantic to be a source for CO₂ in this time of year, as the seawater pCO₂ is much higher than in the atmosphere (Fig. 3).

One goal of OCEANET is to develop autonomous systems for the determination of the parameters mentioned above. The oxygen optode and the gas tension device are handy and reliable instruments if they are well calibrated. The standard instruments for pCO₂ measurements are still bulky and one has to provide a set of standard gases. Therefore, we wanted to use this cruise to compare several newly developed and smaller pCO₂ instruments. Unfortunately, not all instruments arrived in time for departure from Punta Arenas, and thus we have only one of the additional sensors during this cruise.

In addition to the continuous sampling, discrete water samples are also taken every six hours. These samples will be analyzed later in Kiel for additional parameters of the CO₂ system to determine the processes that drive the CO₂-flux between the ocean and the atmosphere.

At the beginning of the fourth week we have still been under subtropical influence with few clouds. Later on, frontal clouds – mostly cirrus clouds – dominated the sky. On May 16, the last station was performed in the Biscay. From now on, we are mainly dismounting, cleaning, packing equipment. On Saturday, May 17, the crew is invited by the scientists for the farewell party.

ANT-XXIV/4, and with this the first Atlantic transect of POLARSTERN in the framework of the OCEANET project, comes to the end. It was a successful cruise. All measurement devices worked well. The scientific discussions and seminars on board have shown that the individual working groups complemented one another very well, and that the exchange of scientific results will continue for some time.

Weather and sea conditions have been very favourable, especially during the radiation measurements.

Many thanks to the crew for the excellent collaboration and the nice atmosphere on board. The participating working groups will meet again in varying combinations during future Atlantic transects, which makes the farewell a bit easier for us.
 
Best regards on behalf of all,

Andreas Macke