The ice cover of the Arctic Ocean has been changing dramatically in the last decades and the consequences for the sea-ice associated ecosystem remain difficult to assess. Algal aggregates underneath sea ice have been described sporadically but the frequency and distribution of their occurrence is not well quantified. We used upward looking images obtained by a remotely operated vehicle (ROV) to derive estimates of ice algal aggregate biomass and to investigate their spatial distribution. During the IceArc expedition (ARK-XXVII/3) of RV Polarstern in late summer 2012, different types of algal aggregates were observed floating underneath various ice types in the Central Arctic basins. Our results show that the floe scale distribution of algal aggregates in late summer is very patchy and determined by the topography of the ice underside, with aggregates collecting in dome shaped structures and at the edges of pressure ridges. The buoyancy of the aggregates was also evident from analysis of the aggregate size distribution. Different approaches used to estimate aggregate biomass yield a wide range of results. This highlights that special care must be taken when upscaling observations and comparing results from surveys conducted using different methods or on different spatial scales.

Measurements of solar radiation over and under sea ice as well as surveys of the distribution of algal aggregates under sea ice have been performed on various stations in the Arctic Ocean during the Polarstern cruise ARK-XXVII/3 (IceArc) between 10 August and 29 September 2012. All radiation measurements have been performed with Ramses spectral radiometers (Trios, Rastede, Germany). All data are given in full spectral resolution interpolated to 1.0 nm and integrated over the entire wavelength range (broadband, total: 320 to 950 nm). Two sensors were mounted on a Remotely Operated Vehicle (ROV) and one radiometer was installed on the sea ice for surface reference measurements (solar irradiance). On the ROV, one irradiance sensor (cos-collector) for energy budget calculations and one radiance sensor (7° opening angle) to obtain high resolution spatial variability were installed. Along with the radiation measurements, sea-ice draft, roughness as well as under-ice aggregate coverage were recorded. Videos were recorded by a camera mounted in an upward looking position onboard a ROV. Still images were extracted each 5 seconds and aggregates detected in the images. Final processed data are provided gridded on a 3x3m grid in local floe coordinates.

Supplement to: Katlein, Christian; Fernández-Méndez, Mar; Wenzhöfer, Frank; Nicolaus, Marcel (2014): Distribution of algal aggregates under summer sea ice in the Central Arctic. Polar Biology, Polar Biology, 38(5), 719-731