Biogeochemistry of marine dissolved organic matter

The global total amount of dissolved organic carbon (DOC) stored in marine DOM is similar to the amount of carbon stored in atmospheric carbon dioxide and living land plants and 30-times larger than all carbon in marine animals, plants, bacteria and organic particles combined. DOM serves as a buffer in the organic carbon cycle, forms complexes with trace metals, and changes their solubility, distribution and toxicity. The aim of our studies is the molecular chemical characterization in order to identify specific sources of DOM (e.g. marine, terrestrial, sea ice), age and transformation processes (e.g. microbial and photo degradation).

The application of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) yielded major advances to achieve these aims. Each analysis provides thousands of molecular formulas - a chemical fingerprint.

Characterization of trophic interactions and carbon transfer within the polar food web

Seasonally changing environmental factors (sea ice cover, water mass characteristics, light and nutrient availability) affect not only the timing, but also the physiology and biochemistry in sea ice algae and phytoplankton. This has implications for the efficiency of trophic transfer, as well as for the use of trophic markers (stable isotopes and fatty acid composition, and highly branched isoprenoids (IP25) to characterize the transport of carbon through the food web. These biomarkers also include the signal of food items that are usually not recovered in diet analysis, such as fast-digesting prey or diffuse organic matter.

Particularly, fatty acid composition and stable isotope composition (BSIA, CSIA) can provide an integrated picture of the food sources of organisms over weeks to months, thereby also integrating the spatial distribution of foraging areas during this period. Furthermore, we are using algal cultures labeled with enriched 13C as a food source, which allows us to follow the pathway and transfer of carbon and individual compounds into copepods and higher trophic levels.

Structure and Function of Marine Lipids

Zooplankton organisms play a key role in the pelagic food web due to the transfer of energy from unicellular algae to higher trophic levels. In polar regions zooplankton has to cope with winter ice cover and darkness and hence with a relatively short spring/summer period of food plenty. In general, a variety of zooplankton groups has adapted to these variations by developing biochemical pathways which enable them to accumulate large lipid stores during summer phytoplankton blooms. The importance of lipids is connected to their capability to store energy in a very efficient way. The lipids of copepods, krill, amphipods and other zooplankton species serve as energy for e.g. herring, capelin, cod, seabirds and baleen whales.

Current studies are performed for understanding lipid metabolism and related processes of zooplankton key species (copepods, amphipods, pteropods and ctenophores) in the polar food web in general and in relation to different preys during overwintering situation. Feeding and starvation experiments are performed with the pteropod Clione limacina and on the transfer of lipids from calanoid copepods to the ctenophore Mertensia ovum, the re-conversion of wax esters and the fate of long-chain fatty alcohols of zooplankton origin.

Monitoring of nutrient dynamics

Investigations of the nutrient dynamics in the polar oceans are closely related to biological and hydrographical studies. These basic data are important to study phytoplankton blooms and to characterize water masses due to different nutrient concentrations in the oceans.

An example is the relationship between nitrate and phosphate which allows to distinguish between water of Pacific and Atlantic origin in the Arctic. Nutrient data from 1984 to 1997 in Fram Strait showed that a surface layer of about 50 m of pure Pacific Water is present above the slope. After 1997 this Pacific Water almost completely disappeared. It is now important to monitor how long it takes before Pacific Water again exits Fram Strait and to compare the Pacific Water outflow through Fram Strait and the Canadian Archipelago.