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The role of intrusions

Sketch of small-scale interactions between intrusions and ambient waters with different biogeochemical composition. Arrows indicate solute exchange due to mixing of intrusions and adjacent water.

In an interdisciplinary project, conducted in collaboration with colleages from IOW's departments of marine biology and marine chemistry, we are currently investigating the role of intermittent intrusions and mixing on the microbiology and biogeochemistry of pelagic redoxclines (oxic-anoxic interfaces) based on a purely experimental approach. One key component of the project is a moored autonomous profiling system in the central Baltic Sea, developed in collaboration with IOW’s chemistry department, in order to study the evolution of the Baltic Sea redoxcline over longer time scales. For the purpose of this project, this platform was extended by an autonomous turbulence microstructure package and a high-resolution current profiler (ADCP) for the observation of mixing rates and mixing processes in the vicinity of intrusions.

The evaluation of long-term records by Holtermann et al. (2019) showed that intrusions are ubiquitous features in the central Baltic Sea. Intrusions determine the depth of the redoxcline, and import, over decadal time scales, considerably more oxygen into the deep layers of the Baltic Sea than the more well-known Major Baltic Inflows (MBIs). The turbulence sensors mounted on the autonomous profiling platform provided a number of especially suprising results. Different from previous assumptions, they did not show any indications of enhanced shear-induced turbulence in the vicinity of intrusions or even during an MBI that occured during the measurements (Holtermann et al., 2017), supporting the view that most of the deep-water mixing occurs in the turbulent bottom boundary layers. Secondly, our moored turbulence observations also provided the first direct evidence for the important of diffusive convection (Holtermann et al., 2017; Umlauf et al., 2018). It has long been speculated that this particular mixing process, relevant especially in the Arctic Ocean, might also be relevant in the Baltic Sea. Our study has also shown that the mixing of intruding and ambient waters has an important effect on the abundance, activity, and gene expressions of microorganisms in the vicinity of redox interfaces (Schmale et al., 2016; Beier et al., 2019).