Description: Recent modeling results suggest that oceanic oxygen levels will decrease significantly over the next decades to centuries in response to climate change and altered ocean circulation. Hence the future ocean may experience major shifts in nutrient cycling triggered by the expansion and intensification of tropical oxygen minimum zones (OMZs). There are numerous feedbacks between oxygen concentrations, nutrient cycling and biological productivity; however, existing knowledge is insufficient to understand physical, chemical and biological interactions in order to adequately assess past and potential future changes. We investigated the pelagic biogeochemistry of OMZs in the eastern tropical North Atlantic and eastern tropical South Pacific during a series of cruise expeditions and mesocosm studies. The following summarizes the current state of research on the influence of low environmental oxygen conditions on marine biota, viruses, organic matter formation and remineralization with a particular focus on the nitrogen cycle in OMZ regions. The impact of sulfidic events on water column biogeochemistry, originating from a specific microbial community capable of highly efficient carbon fixation, nitrogen turnover and N2O production is further discussed. Based on our findings, an important role of sinking particulate organic matter in controlling the nutrient stochiometry of the water column is suggested. These particles can enhance degradation processes in OMZ waters by acting as microniches, with sharp gradients enabling different processes to happen in close vicinity, thus altering the interpretation of oxic and anoxic environments.

Description: Fixed nitrogen (N) loss to biogenic N2 in intense oceanic O2 minimum zones (OMZ) accounts for a large fraction of the global N sink and is an essential control on the ocean's N budget. However, major uncertainties exist regarding microbial pathways as well as net impact on the magnitude of N-loss and the ocean's overall N budget. Here we report the discovery of a N-loss hotspot in the Peru OMZ associated with a coastally trapped mesoscale eddy that is marked by an extreme N deficit matched by biogenic N2 production, high NO2− levels, and the highest isotope enrichments observed so far in OMZ's for the residual NO3−. High sea surface chlorophyll (SSC) in seaward flowing streamers provides evidence for offshore eddy transport of highly productive, inshore water. Resulting pulses in the downward flux of particles likely stimulated heterotrophic dissimilatory NO3− reduction and subsequent production of biogenic N2. The associated temporal/spatial heterogeneity of N-loss, mediated by a local succession of microbial processes, may explain inconsistencies observed among prior studies. Similar transient enhancements of N-loss likely occur within all other major OMZ's exerting a major influence on global ocean N and N isotope budgets.

Description: Biological nitrogen fixation is the largest input of fixed nitrogen into the oceans and thus a key parameter in controlling primary productivity. Despite the importance of nitrogen fixation there is major controversy about its magnitude on a global scale, due to a gap in the marine nitrogen cycle on the input side. While this gap suggests that the nitrogen cycle is currently not in balance and the oceans are losing more nitrogen than they gain, stable isotope measurements from sediment cores suggest that the nitrogen cycle has been in balance over the last 3000 years. To resolve this paradox it has been suggested that marine nitrogen fixation is currently underestimated. We used a revised method to measure nitrogen fixation and compared it with the prior, widely applied method. Our study reveals that over the whole Atlantic Ocean the prior method underestimated nitrogen fixation rates. In certain areas the mean fixationrate increased over six fold when measured with the revised protocol. The magnitude of the difference is not stable but rather highly variable on a coarse geographic scale. We suspected that species composition has a great influence on the magnitude of underestimation of nitrogen fixation rates by the prior method, a theory we could confirm with a laboratory experiment. Taken together, our results imply that there is an urgent need to agree on a common protocol for nitrogen fixation rate measurements to assess the true potential of this nitrogen input process and be able to model the future development, given man-made climate changes

Description: FS Meteor Fahrt 93 Wochenbericht 2 (11.02-17.02. 2013)

Description: FS Meteor Fahrt 93 Wochenbericht 3 (18.02.-24.02.2013)

Description: FS Meteor Fahrt 93 Wochenbericht 4 (25.02.-03.03.2013)

Description: The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20 % of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO2−) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani Shelf where little NO2− accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO2−-producing and consuming processes, and examining the relationship between the overall NO2− balance and active N-loss in the Arabian Sea. Based on a combination of 15N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO2− accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO3−) reduction and to a certain extent ammonia oxidation. Meanwhile, NO2− consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH4+) were hardly detectable in this region, though some loss to NO2− oxidation was predicted from modeled NO3− changes. No significant correlation was found between NO2− and N-loss rates (p〉0.05). This discrepancy between NO2− accumulation and lack of active N-loss in the central-NE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO2− reduction to N2O, N2 and NH4+, and indirectly for the remineralized NH4+ required by anammox. Altogether, our data do not support the long-held view that NO2− accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO2− accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities.