Arctic sea ice is changing dramatically in the last decades and the consequences for the sea-ice-associated ecosystem are difficult to assess. Sea ice is observed to become thinner, younger and more pond-covered. It also allows more light to transmit into and under the ice. Intensive melting might impact the life of sea-ice algae within the brine channels. Algal aggregates underneath the sea ice of the central Arctic have been described sporadically, but the frequency and distribution of their occurrence as well as their role in the ecosystem remain unknown due to the lack of large-scale observations. During the TransArc and IceArc expedition of RV Polarstern in the late summer of 2011 and 2012, we observed ice algal aggregates with a remotely operated vehicle (ROV) underneath various ice types in the central basins. We observed different types of ice algal aggregates floating underneath, and attached to, the underside of the sea ice. Maps of the distribution of aggregate abundance could be obtained by complementing the upward-looking imagery with synchronously recorded ROV attitude and position data. Besides the analysis of spatial distribution patterns and estimates of total biomass of the algal assemblages, this data allowed for shape analysis and the extraction of size distributions. Aggregate distributions can be compared with physical properties of the habitat such as ice thickness, light availability, temperature, salinity and dissolved oxygen measured during the ROV surveys.