Description: In Eastern Boundary Upwelling Systems nutrient-rich waters are transported to the ocean surface, fuelling high photoautotrophic primary production. Subsequent heterotrophic decomposition of the produced biomass increases the oxygen-depletion at intermediate water depths, which can result in the formation of oxygen minimum zones (OMZ). OMZs can sporadically accumulate hydrogen sulfide (H2S), which is toxic to most multicellular organisms and has been implicated in massive fish kills. During a cruise to the OMZ off Peru in January 2009 we found a sulfidic plume in continental shelf waters, covering an area 〉5500 km2, which contained ~2.2×104 tons of H2S. This was the first time that H2S was measured in the Peruvian OMZ and with ~440 km3 the largest plume ever reported for oceanic waters. We assessed the phylogenetic and functional diversity of the inhabiting microbial community by high-throughput sequencing of DNA and RNA, while its metabolic activity was determined with rate measurements of carbon fixation and nitrogen transformation processes. The waters were dominated by several distinct γ-, δ- and ε-proteobacterial taxa associated with either sulfur oxidation or sulfate reduction. Our results suggest that these chemolithoautotrophic bacteria utilized several oxidants (oxygen, nitrate, nitrite, nitric oxide and nitrous oxide) to detoxify the sulfidic waters well below the oxic surface. The chemolithoautotrophic activity at our sampling site led to high rates of dark carbon fixation. Assuming that these chemolithoautotrophic rates were maintained throughout the sulfidic waters, they could be representing as much as ~30% of the photoautotrophic carbon fixation. Postulated changes such as eutrophication and global warming, which lead to an expansion and intensification of OMZs, might also increase the frequency of sulfidic waters. We suggest that the chemolithoautotrophically fixed carbon may be involved in a negative feedback loop that could fuel further sulfate reduction and potentially stabilize the sulfidic OMZ waters

Description: We present our experiences in monitoring hypoxia and assessing oxygen-related phenomena in aquatic systems, resulting from the EU-FP7 project HYPOX (“In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and land-locked water bodies”, www.hypox.net). For this presentation, we selected approaches and technologies which could be relevant for monitoring fast fluctuations at the pelagic redoxcline, seasonal benthic hypoxia and decadal trends oxycline boundary shifts in the Black Sea. Using novel technologies like e.g., the profiling instrumentation platform GODESS, ARGO floats with oxygen optodes and long-term moorings equipped with ADCP and oxygen optodes, temporal and spatial patterns of water column oxygenation from hours to seasons, and from basin-scale to local-scale patterns were resolved. We present examples from study sites in the Baltic Sea and in the Black Sea. The time series recordings of GODESS and mooring arrays allowed a thorough characterization of oscillating redoxclines in the central Baltic Sea and in the Black Sea off southwestern Crimea as temporally dynamic, three-dimensional systems. For the first time, oxygen sensor equipped ARGO type profiling floats were deployed in the Black Sea and proved to be powerful tools to address seasonal changes in patterns of water column oxygenation on larger spatial scales and emphasize the importance of mesoscale processes for oxygen distribution in the Black Sea basin. A 3-month continuous time series recording of a stand-alone static mooring equipped with optical oxygen sensors, current meters, and turbidity sensors identified summer hypoxia to be a highly dynamic process and provided insights into the controls of hypoxia formation on the north-western Black Sea shelf. Existing multi-decadal time-series monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions and, hence, the importance of maintaining long-term commitments to oxygen monitoring programs. Such time series data allow separating out the effects that climatic forcing and eutrophication exert on oxygen depletion i.e., in the Black Sea. Standard CTD measurements in the central Black Sea over the last 90 years reflect the rising of the upper boundary of the suboxic zone in the 1970s and 1980s due to eutrophication, and again in the 1990s and 2000s due to NAO forcing, while eutrophication relaxed. Our comprehensive study within HYPOX was able to address many aspects of hypoxia in aquatic systems and revealed the vital need for dedicated oxygen monitoring at appropriate spatial and temporal scales with appropriate technologies. The variety of hypoxia characteristics and consequences emphasizes also the need for a careful selection of locations and periods of time for oxygen observations in order to adequately address the risk for hypoxia formation and ecosystem response.