As already announced in the last weekly report, today we will provide details about the work of the phyto-opticians, biogeochemists, planktologists and sedimentologist during RV Polarstern expedition PS99.2. Their goal is to investigate the organisms and processes in the water column.
Plankton comprises a very diverse group of organisms, which float in the water column and cannot swim against currents. It includes (among others) bacteria, unicellular autotrophic algae, which are the basis of the arctic food web, and unicellular organisms which feed on particulate organic matter (heterotrophic). Among the larger organisms, the crustaceans are most abundant and they are therefore an important food source for higher trophic levels. Climate change may strongly influence these communities.
To investigate how algal and bacterial communities respond to changing environmental conditions, we collect water from different depths with a rosette water sampler. These samples are filtrated and deep-frozen. In addition, surface water samples are collected by the automated filtration device AUTOFIM, which is coupled to the seawater circulation system of the ship. Later, in the laboratories of the AWI and the GEOMAR we will measure a large set of parameters such as the nutrient contents, chlorophyll a concentrations, carbon and nitrogen contents, and the genetic fingerprints of the algae and bacteria.
Further to our classical water sampling program, this year we are attempting to implement a range of analytical equipment to measure various biological and chemical properties of surface waters. These devices include an autonomous underway nutrient sampler, a mass spectrometer that can measure the concentrations of argon and oxygen to calculate the amount of production in surface waters, and a device to measure CO2 in the water. The data will be coupled with those from “Ferrybox” measurements documenting changes in temperature, salinity, chlorophyll, and oxygen. We also applied numerous new biogeochemical sensors to test their performance and accuracy. The information we get from this work will allow us to make choices about which kinds of technology our best suited to our long/term mooring program and help us establish robust infrastructure for sustained observations of Arctic ecosystems into the future.
Ammonia-oxidizing archaea (AOA) are a special group of bacteria that oxidize ammonia into nitrite as their source of electrons and energy and fix carbon dioxide as their main carbon source. Organisms closely related to cultivated AOA account for a large fraction of the microbial community in marine waters, particularly in the deep sea (up to 25% of the bacterioplankton), suggesting that there is a significant ammonia oxidation flux in deep waters. Other bacteria fixate the nitrogen in the water column and thus provide an organic nitrogen source for other organisms, which are not capable of nitrogen fixation. On board we measure the rate at which they photosynthesize and utilize different forms of nitrogen.
Another working group on board determines the primary production of phytoplankton and biomass production of bacteria in seawater. The turnover of substances by phytoplankton and bacteria is reflected in the chemical composition of organic carbon in seawater. Therefore, we analyse carbohydrates and proteins, which provides a deeper insight into production and degradation processes driven by microorganisms.
To study bacteria-particle interactions in the water column, we use specific gear, i.e. the Marine Snow Catcher (MSC). The MSC is a large cylindrical device that is capable of sampling 100 litres of water. The MSC is lowered to the desired depth and then closed with a brass weight and messenger attached to the winch cable. Once on board the sample is left for 2-4 hours to allow large and fast sinking marine aggregates and faecal pellets to settle into a bottom chamber that can be removed. We use these particles for a range of studies including microscopic imaging, respiration measurements, and DNA sequencing to characterize the eukaryotic, bacterial and archaeal populations associated with these particles. In addition these particles are used for detailed experimental incubations to look at nutrient cycling on particles to learn something about the activity of the bacteria process the organic matter constituting these particles.
Larger organisms such as copepods, amphipods and jellyfish are collected with plankton nets. These samples have been preserved and will be analysed later in the laboratory. In addition, we study the depth distribution of the dominant species with the newly developed LOKI (Light-frame On-sight Key species Investigation). LOKI consists of a high-resolution digital camera, which continuously takes 20 pictures per second while being towed from 1000 m depth to the surface. Salinity, temperature and fluorescence are measured synchronously. This allows directly relating the abundance of different species to hydrography. During this expedition, we attached additional equipment to the LOKI frame – the Aquascat. This acoustical device measures the backscatter of plankton organisms and allows determining the abundance and size of the organisms in the water column.
Besides nutrients, light is another important factor for biological productivity, especially for photosynthetic organisms like phytoplankton. The number of those organisms, other organic and inorganic particles, and dissolved organic matter limit the solar irradiation that is available at different depths in the ocean. The reflected light is used for investigations from satellites (remote sensing) and allows determining the water constituents if in situ measurements for calibration are available. Therefore we also measure optical parameters like absorption and scattering of ocean water. These measurements allow a detailed determination of phytoplankton types, chlorophyll concentration and the amount of dissolved organic carbon in the Arctic Ocean.
The next and last weekly report from RV Polarstern expedition PS99.2 will provide information on the work carried out by our benthic biogeochemists and biologists.
With best regards from all participants,
Thomas Soltwedel
(This weekly report was mainly written by Barbara Niehoff, Ian Salter, Tania Klüver, Stephan Wietkamp, Allison Fong, Sebastian Hellmann and Pierre Offre)
