The dynamics and distribution of marine biota are strongly controlled by its physical surrounding, such as temperature, light, and circulation. In turn, phytoplankton can modify the optical and viscous properties of the seawater, thereby potentially feeding back on the distribution of heat and momentum in the ocean. Due to the close coupling of the upper ocean and the atmosphere, changes in ocean physics might also affect atmospheric processes, and hence the whole Earth system.

In order to study the effects of the feedbacks induced by the presence of phytoplankton on the Earth system, we implement several bio-physical coupling mechanisms into the MPI-ESM:

• Presence of marine organisms in sea water affects light absorbtion. Hence, the distribution of phytoplankton modifies the spatially and temporally varying penetration depth of incoming solar radiation.  As absorbed light is mostly converted into heat, this affects the sea water temperature distribution.

• Phytoplankton modifies the ocean surface color. Changes in the surface color imply changes in the albedo, and hence changes in the total amount of solar radiation that is reflected at the sea surface, and that enters the ocean.

• Surface accumulations of phytoplankton may increase the viscosity of the sea water, and thus reduce the surface drag coefficient. This affects the momentum transfer from the atmosphere to the ocean, leading to changes in the wind-driven circulation and turbulence.

Resulting changes in ocean physics influence the growth conditions of phytoplankton itself, closing the feedback loops between ocean biology and physics.

Phytoplankton species that are particularly prominent in changing the sea water properties are nitrogen fixing cyanobacteria (see nitrogen cycle), due to their capacity to form extensive surface mats. Therefore, special emphasize in our study is on the role of these organisms for the feedbacks (see Figure 1).

Contact: Hanna Paulsen