Beschreibung: The sediment-water interface is an important site for material exchange in marine systems and harbor unique microbial habitats. The flux of nutrients, metals, and greenhouse gases at this interface may be severely dampened by the activity of microorganisms and abiotic redox processes, leading to the “benthic filter” concept. In this study, we investigate the spatial variability, mechanisms and quantitative importance of a microbially-dominated benthic filter for dissolved sulfide in the Eastern Gotland Basin (Baltic Sea) that is located along a dynamic redox gradient between 65 and 173 m water depth. In August-September 2013, high resolution (0.25 mm minimum) vertical microprofiles of redox-sensitive species were measured in surface sediments with solid-state gold-amalgam voltammetric microelectrodes. The highest sulfide consumption (2.73–3.38 mmol m−2 day−1) occurred within the top 5 mm in sediments beneath a pelagic hypoxic transition zone (HTZ, 80–120 m water depth) covered by conspicuous white bacterial mats of genus Beggiatoa. A distinct voltammetric signal for polysulfides, a transient sulfur oxidation intermediate, was consistently observed within the mats. In sediments under anoxic waters (〉140 m depth), signals for Fe(II) and aqueous FeS appeared below a subsurface maximum in dissolved sulfide, indicating a Fe(II) flux originating from older sediments presumably deposited during the freshwater Ancylus Lake that preceded the modern Baltic Sea. Our results point to a dynamic benthic sulfur cycling in Gotland Basin where benthic sulfide accumulation is moderated by microbial sulfide oxidation at the sediment surface and FeS precipitation in deeper sediment layers. Upscaling our fluxes to the Baltic Proper; we find that up to 70% of the sulfide flux (2281 kton yr−1) toward the sediment-seawater interface in the entire basin can be consumed at the microbial mats under the HTZ (80–120 m water depth) while only about 30% the sulfide flux effuses to the bottom waters (〉120 m depth). This newly described benthic filter for the Gotland Basin must play a major role in limiting the accumulation of sulfide in and around the deep basins of the Baltic Sea.

Beschreibung: Redox-sensitive mobilization of nutrients from sediments strongly affects the eutrophic state of the central Baltic Sea; a region associated with the spread of hypoxia and almost permanently anoxic and sulfidic conditions in the deeper basins. Ventilation of these basins depends on renewal by inflow of water enriched in oxygen (O2) from the North Sea, occurring roughly once per decade. Benthic fluxes and water column distributions of dissolved inorganic nitrogen species, phosphate (PO43-), dissolved inorganic carbon (DIC), sulfide (HS-), and total oxygen uptake (TOU) were measured along a depth gradient in the Eastern Gotland Basin (EGB). Campaigns were conducted during euxinic conditions of the deep basin in Aug./Sept. 2013 and after two inflow events in July/Aug. 2015 and March 2016 when O2 concentrations in deep waters reached 60 μM. The intrusion of O2-rich North Sea water into the EGB led to an approximate 33 and 10% reduction of the seabed PO43- and ammonium (NH4+) release from deep basin sediments. Post-inflow, the deep basin sediment was rapidly colonized by HS- oxidizing bacteria tentatively assigned to the family Beggiatoaceae, and HS- release was completely suppressed. The presence of a hypoxic transition zone (HTZ) between 80 and 120 m water depth was confirmed not only for euxinic deep-water conditions during 2013 but also for post-inflow conditions. Because deep-water renewal did not ventilate the HTZ, where PO43- and NH4+ fluxes were highest, high seabed nutrient release there was relatively unchanged. Extrapolation of the in situ nutrient fluxes indicated that, overall, the reduction in PO43- and NH4+ release in response to deep-water renewal can be considered as minor, reducing the internal nutrient load by 2 and 12% only, respectively. Infrequent inflow events thus have a limited capacity to sustainably reduce internal nutrient loading in the EGB and mitigate eutrophication.

Beschreibung: Die Aufgaben des Fraunhofer IGD im Rahmen des Projektes FLEXMOT waren die Entwicklung von Modulen für den Datenempfang, die Datenverwaltung und die Datenvisualisierung. Seit der gemeinsamen Festlegung durch alle Projektpartner im Konsortialmeeting am 29.10.2013 zeichnete das Fraunhofer IGD auch für das Gesamtprojektmanagement verantwortlich. Dies beinhaltete die organisatorischen und koordinierenden Aufgaben innerhalb des Konsortiums. Das Vorhaben wurde vor dem Hintergrund gestartet, dass durch die wachsende Expansion der Offshore-Exploration der Öl & Gas-Industrie, dem wachsenden gesellschaftlichen Umweltbewusstsein und Großunfällen wie dem Macondo-Blowout 2010 im Golf von Mexiko (Deep Horizon Unglück) offenbar wurde, wie groß der Bedarf für Umweltmonitoring-Technik ist, die besser und schneller einsetzbar, rekonfigurierbar und benutzbar sind (modulares Grundkonzept, geringe Rüstzeiten, lange Standzeiten) und zugleich größere Bereiche abdecken und eine schnelle Datenverfügbarkeit bieten. Daher initiierte der Hersteller von Gas-Sensorik Contros GmbH aus Kiel, der über entsprechendes Branchen-KnowHow und Kontakte im Öl&Gas-Bereich verfügte, zusammen mit den Unternehmen Oktopus GmbH und LEONI Special Cables GmbH und den Wissenschaftspartnern GEOMAR (Kiel) und Fraunhofer IGD (Rostock) das Projekt FlexMoT. Durch dieses Konsortium war sichergestellt, dass alle Bereiche (Mechanik, Sensorik, Prozess-KnowHow, Software für Datenmanagement und -auswertung und wissenschaftliche Unterwasser-Expertise) im Projekt vorhanden waren. Zudem war der anvisierte industrielle Markt für funktionierende und langzeittaugliche Monitoringinstrumente aufgrund der hohen Ölpreise sehr vielversprechend. Ziel war es, ein entsprechendes flexibles und modulares System zu entwickeln, welches die entscheidenden Schwächen bisheriger System umgeht und auch für den Einsatz durch nichtwissenschaftliche Anwender (Industriekunden) geeignet ist. Im Projekt sollte zum einen eine Erprobung der Entwicklungen auf ihre Unterwasser- und Einsatztauglichkeit hin stattfinden. Zum andere sollte die Funktionstüchtigkeit des Gesamtsystems nachgewiesen werden. Für diese Feldtests war es notwendig, mit einem geeigneten Schiff im angestrebten Einsatzumfeld, der Nordsee, und in typischen Wassertiefen der Kontinentalschelf-Offshore-Förderung zu testen. Das GEOMAR organisierte und stellte dafür das Forschungsschiff FS ALKOR und Schiffszeiten sowie wissenschaftliches Equipment für die Referenzmessungen zur Verfügung.

Beschreibung: Cold-water coral reefs occur at various sites along the European continental margin, like in the Mediterranean Sea, on carbonate mounds West off Ireland, or at shallower depths between 100 and 350 m on the Norwegian shelf. Their occurrence is related to different physical parameters like temperature, salinity, seawater density, dissolved oxygen, and to other environmental parameters such as internal wave activity, nutrient supply, strong currents, which keep sediment input low, etc. Here, we present first results from a long-term observation in one of the nortnermost cold-water coral reefs at 70.5°N - the Stjernsund in northern Norway. The Stjernsund is a 30 km long and up to 3.5 km wide sound connecting the open North Atlantic with a fjord system. A deep-seated SW-NE oriented morainic sill with varying depths (203-236 m) splits the more than 400 m deep sound into two troughs. Living Lophelia pertusa dominated reef complexes occur on the NW slope between 235 and 305 m water depths and on the SE slope between 245 and 280 m. To investigate the dominating physical and biogeochemical boundary conditions a new modular seafloor observatory, MoLab, consisting of five sea-floor observatories and two moorings was deployed for 100 days during the summer of 2012. The various lander systems and moorimgs were equipped with sensors to measure current velocities and directions, temperature, salinity, pressure, pH, turbidity, fluorescence, oxygen concentration and saturation. Results showed that near-bottom salinities, temperature and current velocities are dominated by a semi-diurnal tidal forcing (pronounced M2 constituent), which cause vertical water mass movements of up to 100 m. These influence large parts of the living reef. Closer examination revealed overturning cells on the south-eastern slope of the sill during high tide, when Atlantic Water flows over the sill. The appearance of living cold-water corals is limited to a density envelope of sigma-theta=27.25-27.50 kg/m-3, which marks the boundary between Norwegian Coastal Water and Atlantic Water. Globally, Lophelia pertusa lives in waters covering a wide range of physical and biogeochemical parameters. This new data sets indicates parameter ranges, of e.g. current velocities (15-30 cm/s), temperature (6.0-6.8°C) and salinity (34.1-34.8), pH (8.22-7.39), turbidity (0.1-0.9 NTU), and oxygen concentration (300-339 μM) that are in agreement with other cold-water coral reefs in the NE Atlantic. The overall circulation depicts a complex dynamic system with pronounced differences not only vertically, but also important horizontal changes on top of the sill.

Beschreibung: Highlights • The largest Baltic dataset of in situ measured benthic DIC fluxes is presented. • 96% of the POC delivered to Baltic sediments is recycled back to the water column. • OC recycling rates are much higher and burial rates lower than previously reported. • C budgets for the Baltic Sea should be revised taking into account these new rates. In situ measured benthic fluxes of dissolved inorganic carbon (DIC), a proxy for organic carbon (OC) oxidation or recycling rates, are used together with burial rates based on measured sediment accumulation rates (SAR) and vertical distribution of OC in the sediment solid phase to construct a benthic OC budget for the Baltic Sea system. The large variability in recycling rates (4.3 ± 0.87–33 ± 17 mmol C m−2 d−1) and burial rates (1.2 ± 0.8–5.9 ± 1.8 mmol C m−2 d−1) between different sub-basins and between different depositional areas within the basins is accounted for in the budget. Our results indicate that sediments in the Baltic Sea have much higher recycling rates and lower burial rates of OC than previously found. The sediment budget calculations show that 22 ± 7.8 Tg C yr−1 of OC is recycled to the water column due to organic matter oxidation, while long term burial amounts to 1.0 ± 0.3 Tg C yr−1. For the Baltic Sea as a whole, 96% of the particulate OC (POC) deposited on the sea floor (23 ± 7.8 Tg C yr−1; the sum of recycling and burial) is recycled back to the water column. However, the burial efficiency (i.e. the fraction buried of the total deposition) shows large variability between the different basins (2.5–16%). The total benthic POC deposition is approximately 20% higher than the estimated POC source originating from primary production in the water column and riverine input. This difference is likely within the uncertainty range of our budget calculations, however it indicates that the POC sources might be underestimated. The results from this study enhance the understanding of OC delivery, deposition and cycling in the Baltic Sea, and help improving existing Baltic OC budgets.