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The role of bottom boundary turbulence for the transport of tracers in marine basins (ROBOTRACE)

Subject Area Oceanography
Term from 2016 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 322366882
 
Final Report Year 2023

Final Report Abstract

Anoxia (the lack of oxygen) in marine systems have been increased strongly over time, with the Baltic Sea being a prominent example of this worldwide trend. The reason for the lack of oxygen is an imbalance of the oxygen demand and the ability of the marine system to transport oxygen from the well oxygenated upper water column into the deep. While the general processes of this imbalance are well understood, the detailed processes are still to some degree unclear, this includes the role of turbulent transport processes of oxygen within the water column and at the watersediment interface. These questions were addressed during the duration of ROBOTRACE project by a seagoing part, sampling oxygen concentrations together with turbulence, within the water column and a modelling part, that aimed to formulate the sediment-water exchange processes based on turbulence theory. The seagoing part of the projected consisted of three expeditions into the central Baltic Sea in autumn 2017, winter 2018 and summer 2019. The main goals of the expeditions were a seasonal sampling of oxygen fluxes through the Baltic Sea halocline and into the sediment. To reliably sample the oxygen fluxes within the water column, a fast oxygen sensor is needed, due to the strong vertical gradients. This issue was was solved by using a newly developed oxygen logger, that was originally designed for eddy-covariance measurements in combination with a microstructure shear probe, measuring the dissipation rate of turbulent kinetic energy within the water column. Over the course of the project, the external device was, in collaboration with the manufacturer, integrated into the shear probe and is now a commercial product. The results have been published in a peer reviewed journal. Additional information was acquired by several moorings, equipped with velocity meters, temperature, salinity and oxygen loggers. The sediment water fluxes have been sampled by three different techniques: a microprofiling system, an eddy-covariance system and an chamber lander. Results of these devices are at the moment analysed within a master thesis and are not yet published. The second part, which involved the numerical modelling focussed on a implementation of the sediment-water fluxes in cohesive sediments. During the development of the model it turned out that an implementation, that fulfils the conservation of tracers as well as the requirements of a full biogeochemical model with arbitrarily complex reactions between the tracers, required much more time than initially planned. With some additional time and at the expense of some work tasks in the project plan, an extension of the General Turbulence Ocean Model (GOTM) was developed, tested and published. The extension allows GOTM to have a vertically resolved sediment underneath the watercolumn in combination with a biogeochemical model of a user-defined complexity.

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