Matter fluxes across the sediment-water interface of Potter Cove

1. Sub-project: Measuring in situ oxygen fluxes of benthic communities

A so called profiler was installed on sedimentary seafloor. The profiler is equipped with sensors for oxygen, hydrogen sulfide, conductivity, temperature and a pH. By pushing the sensors into the sediment in very small steps (100 µm) the concentration profile of each mentioned parameter is measured from the sediment-water interface down to 6 cm. From the oxygen profiles, the diffusive oxygen flux caused by benthic microbial processes can be calculated. By using in situ chambers (picture No. 1) the total oxygen flux of the entire benthic community is measured, i.e. fauna respiration as well as oxygen production of the algae community during daytime. The chambers are equipped with a stirring motor and an oxygen sensor. Additional Eddy correlation flux measurements are performed. From this non-invasive method oxygen net production or consumption can be determined for larger areas.

While profiler and chamber measurements can only be used for soft sediments, the Eddy correlation measurements are also applicable to hard bottom substrates. Additionally, recently developed oxygen, pH and CO₂ optodes are used for a first long term testing in cold Antarctic waters. This test is conducted within SenseOCEAN, an EU-project for the development of sensors, which is part of the 7th Framework Programme of the European Union - just like IMCONet.

2. Sub-project: Determination of the potential primary production of the microphytobenthos

For this project, sediment cores from the different shallow water sites are transferred to the laboratory, kept at in situ temperature and exposed to different light intensities. At each light intensity oxygen concentration profiles across the sediment-water interface are conducted to resolve the zone of oxygen net production and to determine the diffusive oxygen flux. Furthermore, closed sediment cores are incubated at the different light intensities to measure the oxygen flux of the bulk sediment. Finally, water samples are taken during the incubation to determine nutrient and DIC fluxes along with the oxygen fluxes.

3. Sub-project: Nitrogen fluxes of different nitrogen degradation pathways

Again, sediment cores from the different sites were transferred to the laboratory and sliced into different depth layers. To each depth layer, bottom water and isotopically-labelled nitrogen substrates (nitrate, ammonium) were added and afterwards incubated over 24 hours to study the activity of the microbial nitrogen cycling. By this approach nitrification, denitrification and anammox fluxes within the different horizons will be determined.

4. Sub-project:

Last but not least, sediment cores from the different sites were used to measure general parameters, providing the other sub-projects with necessary background data of the three stations. These general parameters include sediment samples for RNA (active bacterial community) and DNA (entire bacterial community) analyses, samples to determine abundance and biomass of the bacteria, meiofauna and macrofauna community from different depth layers, samples for pigment and grain size analyses and porewater profiles of the most prominent nutrients, DIC, sulfate and hydrogen sulfide.

Our expedition team includes Elisa Merz (Masterstudent from the University of Rostock), Dr. Moritz Holtappels (senior scientist at AWI, Group for Bentho Pelagic Processes), Christopher Nowak (student assistant) and myself, Ralf Hoffmann (Ph.D. candidate at the AWI). Elisa, Christopher and I are trained scientific divers. In principle, this allows us to conduct in situ measurements and samplings on our own. We nevertheless received great support by the argentine divers of the Carlini Station. Elisa Merz is responsible for the conduction and measurements of the nitrogen flux sub-project and as well involved in all other sub-projects. Dr. Moritz Holtappels is the expert in Eddy correlation measurements and as well very experienced with the method of the nitrogen flux sub-project. Additionally, as he is the most experienced scientist within the team, he is supporting all other sub-projects of the expedition. Christopher Nowak is a well-trained and highly experienced scientific diver and as well involved in the processing of samples within the sub-projects. I am, besides being the expedition leader, the Dallmann scientific leader and the dive leader (in cooperation with the argentine dive leader) responsible for the in situ profiling and chamber incubations, the experiment on the potential primary production and for the general sampling. While we will determine the matter flux data, our IMCONET partners will process the taken samples regarding microbial community (by Dr. Walter MacCormack and Dr. Susana Vasquez, University of Buenos Aires), meiofauna community (Dr. Francesca Pasotti, Gent University), macrofauna community (Dr. Ulrike Braeckman, Gent University) and microphytobenthic community (Prof. Dr. Angela Wulff, University Gothenburg) with the aim to connect the measured fluxes with the benthic community. Christopher and myself are financed by IMCONET, while Elisa is financed via SenseOCEAN and Moritz directly via AWI.

For me, it is the second stay at the Dallmann Laboratory/Carlini Station, while for the others it is the first time. The chief of the station, the entire staff and especially the divers of the station are doing a great job, are more than supportive and it is a pleasure to work with them to reveal the unknown knowledge of matter fluxes in the Potter Cove.