Beschreibung: This Master of Science project report focuses on the benthic respiration processes and the degradation of organic matter in Arctic deep-ocean sediments. The purpose is to estimate the rate by which organic and biogenic matter is degraded and recycled in these sediments. The study was performed during the Arctic expedition ARKXXII/1c onboard the German research vessel Polarstern in July 2007 in the Fram Strait located between Spitsbergen and Greenland. The expedition ARKXXII was a part of the HERMES project, Hotspot Ecosystems Research on the Margins of European Seas, which started in April 2005 with the purpose to get a better understanding of the complex, variable, and in some cases unique ecosystems and environments along the European deep-ocean margins. This is crucial to maintain a sustainable development by reason of increasing exploitation and economical interest of these sites. The measurements were carried out using an autonomous bottom lander and both in-situ measurements as well as water sampling for laboratory work and analysis were performed. The parameters measured in-situ was oxygen concentration (allowing calculation of consumption), conductivity, temperature, salinity and particle content. The water samples were analysed for dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), alkalinity and nutrients. Sediment cores were also taken from four stations, and were used to measure the C and N-content (not discussed in this report) as well as the porosity. Also pore water was extracted from the sediment in which DOC and nutrients were analysed. The three stations on a depth gradient (1200, 2550, 5550 m) occupied with the lander showed low fluxes and degradation rates generally. The highest degradation rates were found at the shallowest station. Also the deepest station showed high rates of organic carbon oxidation. The intermediate depth station is believed to be more oligotrophic with low input of organic matter, resulting in low rates of organic carbon oxidation and fluxes of DIC and oxygen.

Beschreibung: At the end of 2014, a Major Baltic Inflow (MBI) brought oxygenated, salty water into the Baltic proper and reached the long-term anoxic Eastern Gotland Basin (EGB) by March 2015. In July 2015, we measured benthic fluxes of phosphorus (P), nitrogen (N) and silicon (Si) nutrients and dissolved inorganic carbon (DIC) in situ using an autonomous benthic lander at deep sites (170-210 m) in the EGB, where the bottom water oxygen concentration was 30-45 μM. The same in situ methodology was used to measure benthic fluxes at the same sites in 2008-2010, but then under anoxic conditions. The high efflux of phosphate under anoxic conditions became lower upon oxygenation, and turned into an influx in about 50% of the flux measurements. The C:P and N:P ratios of the benthic solute flux changed from clearly below the Redfield ratio (on average about 70 and 3-4, respectively) under anoxia to approaching or being well above the Redfield ratio upon oxygenation. These observations demonstrate retention of P in newly oxygenated sediments. We found no significant effect of oxygenation on the benthic ammonium, silicate and DIC flux. We also measured benthic denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) rates at the same sites using isotope-pairing techniques. The bottom water of the long-term anoxic EGB contained less than 0.5 μM nitrate in 2008-2010, but the oxygenation event created bottom water nitrate concentrations of about 10 μM in July 2015 and the benthic flux of nitrate was consistently directed into the sediment. Nitrate reduction to both dinitrogen gas (denitrification) and ammonium (DNRA) was initiated in the newly oxygenated sediments, while anammox activity was negligible. We estimated the influence of this oxygenation event on the magnitudes of the integrated benthic P flux (the internal P load) and the fixed N removal through benthic and pelagic denitrification by comparing with a hypothetical scenario without the MBI. Our calculations suggest that the oxygenation triggered by the MBI in July 2015, extrapolated to the basin-wide scale of the Baltic proper, decreased the internal P load by 23% and increased the total (benthic plus pelagic) denitrification by 18%.

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.

Beschreibung: Journal of Medicinal Chemistry DOI: 10.1021/jm301511h