Water World

Biological oceanographers on board determine 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.

Microplankton communities play a fundamental role within marine ecosystems by providing organic matter to oceanic food webs and by fostering major biogeochemical cycles. Currently, they are facing drastic alterations of their abiotic environment due to Climate Change, especially in the Arctic. To understand the interactive effects of rising temperatures and nitrogen limitation on the structural composition of arctic microplankton communities, we conduct ex situ experiments in the cooling chambers of the ship.

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 so-called 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.

Gelatinous zooplankton, i.e. fragile, soft-bodied organisms grouping together a number of phylogenetically unrelated taxa including ctenophores, scyphozoans, hydrozoans and tunicates, are generally understudied, especially in polar regions. Because of their fragile bodies, they are easily fragmented or destroyed with traditional net sampling, which is why they are often neglected in pelagic studies, or when considered, their biomass and diversity are greatly underestimated. A combination of various net sampling, in situ optical observations and eDNA (environmental DNA) methods is applied during our expedition to establish a comprehensive baseline knowledge of their diversity, abundance and distributional patterns and how these may link to oceanography and primary production.

Most of the organic matter produced by phytoplankton in the surface ocean is eaten and degraded by these organisms before it sinks to depths deeper than 100 m. The remaining material feeds life deep below the ocean’s surface, sustaining the biomass of seafloor organisms and deep-sea fish. To quantify the export of organic carbon flux through the water column we are using a special camera system during the expedition and the autonomous BioOptical Platform (BOP), attached to moorings for longer time-scales to assess seasonal variations in this settling transport. A BOP deployed during a previous Polarstern cruise two years ago was successfully recovered, with another redeployed by us to record another year of observations.

The study of the arctic nutrient and carbon budgets is of relevance in order to understand biogeochemical processes at the pan-Arctic scale, and the status of these at any given time, their role in ocean productivity and the climate system. Seawater samples were taken at all stations, for later analysis of dissolved inorganic nutrients back in the laboratory at the AWI. Moored autonomous water samplers which resolve seasonal variations in nutrient concentrations were recovered and redeployed.

Besides nutrients, light is another important factor for biological productivity, especially for photosynthetic organisms such as phytoplankton. The number of these 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 the determination of water constituents if in situ measurements for calibration are available. Therefore, we also measure optical parameters such as 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.

Apropos light: Highlight of the week was an 80% solar eclipse, which we could enjoy here at almost 80°N, while a female polar bear and its young passed by – well, sometimes everything fits nicely!

The next weekly report from RV Polarstern expedition PS126 will provide information on the work carried out by our benthic biogeochemists and biologists.

With best regards from all participants,

Thomas Soltwedel