Volker Mohrholz - Fields of activity

Variability of estuarine circulation of the Baltic Sea and dynamics of water exchange

The Baltic Sea is one of the largest brackish water areas in the world. River discharge and the inflow of saline water from the North Sea drive a thermohaline circulation, which influence the Baltic Sea ecosystem in many ways. Due to the long residence time of the water of about 30 years, changes in the overall budget of salt, nutrients and pollutants occur only very slowly. On the other hand, due to its shallow depth, the Baltic Sea reacts much more strongly and quickly to climatic changes. An understanding of the dynamic processes is therefore of fundamental importance for the protection and sustainable use of the Baltic Sea. As part of the IOW long-term observation programme, our working group operates two long-term moorings in the eastern Gotland Basin. In addition, the exchange of water between the Baltic Sea and the North Sea is one focus of our work.

Upwelling, water mass dynamics and oxygen minimum in the Benguela system

The Benguela upwelling system off south-western Africa is among the most productive marine ecosystems on our planet. In contrast to other major upwelling systems the Benguela comprises large hypoxic and anoxic shelf areas, which have strong influences on the recruitment strategies of many species inhabiting these areas. Annual fish catches in Benguela are an order of magnitude smaller than that of the Humboldt system despite the similar high levels of primary production in both systems. The oxygen poor environment on the shelf has been sustained for millenniums. This requires balanced oxygen and nutrient budgets, which are generally believed to be controlled primarily by large-scale circulation. These basin scale dynamics is overlayed with the mesoscale upwelling dynamics, forced by the local wind fields. The upwelling process lifts large amounts of nutrients into the euphotic layer and feeds the primary production.

The focus of our work is on:

the interaction between remote and local forcing
the exchange between the shelf area and the adjacent ocean
and on small scale processes (turbulence, internal waves) that have a major impact on vertical fluxes.

Turbulent processes in shelf seas

Turbulent processes influence and control marine systems in many ways. At interfaces such as surface, seafloor or halocline, breaking waves or current shear cause the conversion of kinetic energy into turbulent kinetic energy, which can increase mixing and vertical transport of constituents and impulses by orders of magnitude. The measurement of turbulence parameters allows the quantification of turbulent mixing. Our group is specialized in the measurement of marine turbulence using free-falling microstructure probes and acoustic current meters, and uses this expertise in all ongoing projects.