Christian Katlein

• Light transmission through Arctic sea ice is an important process for the energy partitioning in the climate system. Thus understanding its spatial variability is important for a precise determination and prediction of energy fluxes across the atmosphere-ice-ocean interface. In this thesis the variables driving this variability – such as melt pond cover and ice thickness – as well as the length scales of this variability are investigated together with experimental and theoretical analysis of the geometry of optical properties of the ice cover. The work is based on optical measurements conducted during under ice surveys with remotely operated vehicles employing an interdisciplinary sensor package. It is found that the spatial scale of the light field variability under sea ice is driven by variations in ice albedo on scales of hundred meters and by ice thickness variability on larger scales. Also, the geometry of the light-field under sea ice is strongly influenced by the lateral inhomogeneity of the sea ice cover, further complicating the interpretation of standard ocean optics measurements. This thesis shows, that due to the lamellar crystal structure of sea ice, light propagation within is dependent on the direction of photon travel. The operations of under ice remotely operated vehicles also enabled an interdisciplinary study, showing that the spatial distribution of algal aggregates underneath sea ice is not governed by typical habitat properties such as light availability or temperature, but by a hydrodynamic interaction of the buoyant algal aggregates with the ice bottom topography. These results were applied in a new light parameterization allowing for the calculation of Arctic wide in and under ice primary production and will lead to a better ability to assess the impact of the spatial inhomogeneity in sea ice on the large scale energy budget of the melting Arctic sea ice.