As already announced in the last weekly report, today we will provide details about the work of the biogeochemists and biologists during “Polarstern” expedition PS99.2. Their goal is to investigate organisms and processes at the deep seafloor.
To better understand the global carbon cycle it is critical to know what role the oceans play in carbon sequestration. Open ocean deep-sea ecosystems have been studied intensively; however, information on benthic carbon mineralization rates in the Arctic Ocean is scarce. During this cruise we deployed benthic lander systems at three HAUSGARTEN stations, each equipped with three benthic chambers and a microprofiler. The chambers incubate a sediment volume of about 6000 cm2 to measure the total benthic oxygen consumption, whereas the high-resolution oxygen profiles measured by the microprofiler can be used to determine the diffusive oxygen consumption mainly driven by microbial respiration. Preliminary results from our in situ measurements indicate that the Arctic is quite oligotroph with oxygen penetration depths of several centimetres and consequently low diffusive oxygen consumption rates. Our time-series data from flux measurements will help us to identify temporal variations in the benthic mineralization activity.
Within the long-term observations at HAUSGARTEN we also regularly take sediment samples for microbial analyses. Especially the bacterial communities in the sediment surface layers are of interest to us and the question whether these communities change over time. Bacteria are very susceptible to environmental changes like changes in food availability or in temperature and can therefore be used as first indicators of changes in the ecosystem.
Furthermore we selected a few dedicated stations for a more thorough look into the degree of community overlap between individual compartments from the sea surface to the deep seafloor. At these stations, in addition to sediment sampling, we also sampled sea ice, different water depths and sinking particles from the water column, all of which will be used for community analyses back in the home lab.
For the sampling of smallest sediment-inhabiting organisms (bacteria, meiofauna) at the deep seafloor, we are using the multiple corer (MUC), which is lowered by a cable to the seabed. The fibre optic cable on “Polarstern” allows following the seafloor sediment-sampling online on a video screen. The sediments are partly processed, e.g. to assess how much organic matter from primary production at the sea surface settled down to the deep-sea floor. For this we analyse the concentration of plant pigments (chlorophyll and its degradation products) in the sediments.
Another parameter immediately analysed on board is the activity of bacteria living in the sediment. For this we are feeding the bacterial community with a special substrate and measure how much of it is cleaved by the bacteria over a certain time span. All remaining sediments are either deep frozen or fixed with formalin for later investigations in Bremerhaven.
To assess the larger benthic fauna we take sediment samples with a box corer and use a special camera system, the so-called Ocean Floor Observation System (OFOS). This OFOS is towed by the ship one and a half meters above the seafloor for four hours along the same tracks every year. Changes in the bottom-dwelling fauna can be deduced through comparison with images from previous years. This method also enabled us to (regrettably) show that the amount of man-made litter has increased markedly at HAUSGARTEN.
Within the scope of the FRAM infrastructure programme, we intend to expand the use of camera-bound systems. During this expedition, we successfully tested a time-lapse camera in deployments of our benthic lander systems (see above). On both occasions, they were frequently visited by demersal fish (eelpout Lycodes frigidus). Now it is getting serious: the time-lapse camera will be fitted to our long-term lander and will film the bottom ground and its inhabitants on a daily basis. This allows us to investigate changes on the seafloor over the course of a year.
On Tuesday, July 12th we finished our scientific work and set sail for Tromsø, where this expedition will come to an end on July 15th in the late evening. During the cruise, we collected numerous samples and obtained a vast amount of physical, geochemical and biological data, which will extend our time-series at HAUSGARTEN observatory and finally help to understand the effects of Global Change induced environmental changes in the Arctic Ocean.
We would like to thank Captain Thomas Wunderlich and his crew for their hospitality, the great team work and the pleasant atmosphere on board, and gratefully acknowledge the support by the HeliService team during our expedition.
We are now looking forward to see our families and beloved friends at home, and we hope that there are some warm summer nights left after our return.
With best wishes from board “Polarstern”,
Thomas Soltwedel
(This weekly report was mainly written by Melanie Bergmann, Christiane Hasemann, Josephine Rapp, Ingo Schewe and Frank Wenzhöfer.)
