Impact of lateral intrusions and mixing on the biogeochemistry and microbiology of pelagic redoxclines
Zusammenfassung der Projektergebnisse
Recent observations from numerous anoxic marine systems show clear indications for the importance of lateral intrusions of oxic waters into the redox transition zone, challenging the traditional view that the evolution of pelagic redoxclines is largely controlled by local element turnover and purely vertical transport mechanisms. Taking the Baltic Sea as an example, the purpose of this project was to improve our understanding about the properties of such intrusions, the factors controlling their generation and decay, and their impact on the biogeochemistry and microbiology of pelagic redoxclines. Using long-term data from a quasi-autonomous profiling station in the central Baltic Sea, we were able to show that intrusions are ubiquitous features, observed during all seasons throughout the entire deep-water volume. An oxygen budget of the hypoxic transition zone (HTZ), located between the halocline and the deeper redoxcline, showed that intrusions are critical for regulating the depth of the redoxcline, and provide the by far largest oxygen source in this ecologically particularly relevant depth range. Data from turbulence sensors mounted on the profiling platform resulted in two unexpected but important insights: (a) shear-induced turbulent mixing between intrusions and ambient waters is generally negligible except inside the strongly turbulent bottom boundary layers, and (b) double-diffusive mixing is a wide-spread process, in particular in the vicinity of intrusions, supporting previous speculations about the possible importance of this process in the Baltic Sea. The project greatly profited from an exceptional oceanographic event that was not anticipated during the project planning. In spring 2015, a large-scale intrusion of dense and oxic waters from the North Sea (a so-called Major Baltic Inflow) entered the central Baltic Sea, and ventilated a large fraction of the sulfidic bottom waters. We had the unique opportunity to directly investigate the interaction of small-scale physical and biogeochemical processes during the active phase of this exceptional event. Our detailed, direct observations were the first to simultaneously document mixing processes, microbiological transformations, and the distributions of nutrient and trace gases during the active inflow phase. As one important result, we could show that mixing between the intruding oxic and the ambient sulfidic waters increased the abundance and activities of methaneoxidizing bacteria within the turbulent redox interface, which in turn enhanced methane turnover rates. These findings are among the rare demonstrations of a direct connection between turbulent mixing, microbiological activity, and biogeochemical processes. We also obtained new understanding about how a Major Baltic Inflow affects the distribution and composition of the bacterioplankton. A series of shifts in Baltic Sea water masses among the basins and a successive dilution of inflowing North Sea water with ambient waters resulted in rather similar communities between inflowing and ambient waters in the central Baltic Sea. A more drastic effect occurred when the bottom-water inflow reached the anoxic, sulfidic deep basins, resulting in an uplifting of the formerly anoxic bacterial community to oxygenated higher water strata. A simulation of small-scale redoxcline mixing was conducted in a ship-board experiment, where water samples from an oxic intrusion and from the sulfidic ambient waters were mixed. A detailed analysis of the evolution of gene expression suggested that frequent, intermittent, small-scale intrusions and mixing across the associated redox interfaces stimulate bacterial activities involved in nitrogen and sulfur cycling. Further, a permanent upregulation of genes involved in nitrification, denitrification, and sulfur oxidation was observed in anoxic water layers, devoid of known electron acceptors. These zones might have been recently affected by small lateral intrusions evident from weak temperature anomalies. However, whether there exists a kind of “bacterial memory effect” for past intrusions in form of enhanced gene expression needs further investigations and comparative studies among basins that are differently affected by lateral intrusions.
Projektbezogene Publikationen (Auswahl)
- (2020) Interleaving of oxygenized intrusions into the Baltic Sea redoxcline. Limnol Oceanogr (Limnology and Oceanography) 65 (3) 482–503
Holtermann, Peter; Prien, Ralf; Naumann, Michael; Umlauf, Lars
(Siehe online unter https://doi.org/10.1002/lno.11317) - Dense bottom gravity currents and their impact on pelagic methanotrophy at oxic/anoxic transition zones. Geophys. Res. Lett.
Schmale, O., S. Krause, P. L. Holtermann, N. C. Power Guerra, and L. Umlauf
(Siehe online unter https://doi.org/10.1002/2016GL069032) - Deep-water dynamics and mixing processes during a major inflow event in the central Baltic Sea, J. Geophys. Res., 122, 6648–6667
Holtermann, P. L., R. Prien, M. Naumann, V. Mohrholz, and L. Umlauf
(Siehe online unter https://doi.org/10.1002/2017JC013050) - Diffusive convection under rapidly varying conditions. J. Phys. Oceanogr., 48, 1731–1747
Umlauf, L., P. L. Holtermann, C. A. Gillner, R. Prien, L. Merckelbach, J. R. Carpenter
(Siehe online unter https://doi.org/10.1175/JPO-D-18-0018.1) - Impact of a Major Inflow Event on the composition and distribution of bacterioplankton Communities in the Baltic Sea, Frontiers in Marine Science, 5, 383
Bergen, B., M. Naumann, D.P. Herlemann, U. Gräwe, M. Labrenz, and K. Jürgens
(Siehe online unter https://doi.org/10.3389/fmars.2018.00383) - A metatranscriptomics-based assessment of small-scale mixing of sulfidic and oxic waters on redoxcline prokaryotic communities. Environ. Microbiol., 21, 584-602
Beier, S., P. L. Holtermann, D. Numberger, T. Schott, L. Umlauf, and K. Jürgens
(Siehe online unter https://doi.org/10.1111/1462-2920.14499) - Effect of large magnetotactic bacteria with polyphosphate inclusions on the phosphate profile of the suboxic zone in the Black Sea. ISME J., 13, 1198-1208
Schulz-Vogt, H.N., F. Pollehne, K. Jürgens, H.W. Arz, S. Beier, R. Bahlo, O. Dellwig, J.V. Henkel, D.P.R. Herlemann, S. Krüger, T. Leipe, and T. Schott
(Siehe online unter https://doi.org/10.1038/s41396-018-0315-6)
