G4: Influence of salinity dynamics on the release, chemical structure and biological transformations of dissolved organic matter in a coastal peatland

supervised by Leinweber, Karsten


We want to develop a general understanding, how the salinity dynamics in a coastal peatland (NSG „Heiligensee und Hütelmoor“ as a well characterized and intensively studied, joint “playground” of the Research Training Group) affect the DOM-release, –composition and –dynamics (objective 1). Based on this, joint research with the other projects shall give us a deeper insight into the effects of the salinity-affected DOM dynamics on subsequent biogeochemical (e.g., gaseous emissions) and ecological processes (e.g., growth and fitness of microphytobenthos) at the land/sea-interface (objectives 2 +).

State of the art

Dissolved organic matter (DOM) is formed in coastal fens by partial dissolution of peat and by the release of organic compounds in the rhizosphere of the plants that form the fen vegetation community. This DOM likely affects all biogeochemical transformations in the fen such as the formation and release of greenhouse gases (GHG), dynamics of plant nutrients etc. Furthermore, from the geomorphological setting of the investigation site, it can be hypothesized, that fen-borne DOM reaches shallow coastal waters by subsurface transport (submarine groundwater discharge; SGD). Here, this DOM probably acts as a nutrient source for photosynthetically active biofilms. It is not understood how these processes are affected by fluctuations in salinity. Such salinity fluctuations are highly relevant, because subsurface saltwater intrusion into the fen may occur as well as over-flooding of the dune, each under certain weather and hydraulic conditions in the Baltic Sea. Literature evaluations reveal an inconsistent picture on possible effects of salinity on DOM. There are reports of decreasing DOM concentrations in the rhizosphere with increasing salinity but other studies highlighted a strong soil-dependency of Na-saturation and DOM-C and N-concentrations in soil solution. Several publications point out, that variables describing DOM composition, like for instance C:N-ratio in DOM, proportion of chromophore and fluorescent molecules and the microbial decomposition may be generally affected by the salinity. However, fragmentary evidence from reports worldwide cannot be applied to the Baltic Sea coast in general, and the joint study area NSG „Heiligensee und Hütelmoor“ of the Graduate School in detail. Thus, in summary, data on DOM concentrations and –composition in coastal fens are very scarce, and systematic studies on the effects of salinity on coastal fens in the Baltic Sea region are not available.

Work program

The work program builds on the experience of a successful multidisciplinary team in Soil Science and will start with the extraction and characterization of DOM from Hütelmoor fen peat that was equilibrated with water of increasing salinity. The kinetics of DOM release and the DOM composition will be studied. Chemical-analytical methods to be applied will involve various mass spectrometric methods that are established in the group (Pyrolysis-Field Ionization Mass Spectrometry) or done in co-operation with the Chair of Analytical Chemistry (FT-ICR MS) and with other institutes in Rostock (13C-, 31P-NMR). Samples will be also investigated in the Canadian Light Source Synchrotron in Saskatoon/Canada in the frame of concerted measurement campaigns (samples from various projects studied at different beamlines). Work in this phase requires an addiction to instrumental analytics. Nevertheless, the results must be put into the bigger picture of transport processes, gaseous formation and release and ecological effects on the relevant communities of microphytobenthos and biofilms, which requires close interaction with these projects and workgroups.