Arctic sea-ice ridges: a major contributor to algal habitable space in spring

Arctic sea ice is highly heterogeneous and composed of a mosaic of different habitats. Our understanding of the impact of climate change on Arctic sea ice and especially on the ice-associated ecosystems is hindered by both a lack of data and a limited understanding of the processes associated with different sea-ice habitats. In particular sea-ice ridges are one of the most under-sampled and poorly understood components of the Arctic sea-ice system. During a spring campaign in the Arctic Ocean, we combined a number of sampling approaches to quantify: 1) the spatial variability of sea-ice algae at single floe and multiple floe scales; 2) the contribution of ridges to ice algal spatial variability; and 3) the role of ridges in shaping the sea ice as a habitat. For upscaling purposes, algal biomass retrieved from ice cores was compared with biomass estimates based on under-ice profiles covering a total of 36 km. Our results show that the level-ice spatial variability measured on a single ice floe can be representative of the larger scale variability. However, only when ridges are included in the analysis we are able to obtain a comprehensive picture of the large-scale ice algal biomass variability. In spring, ridges let more light pass through the ice due to their geometry and their effects on snow distribution, they thus offer a potentially favorable environment for algae to grow within, and they can act as funnels of light for pelagic organisms. On a large scale, ridges contribute more than 50% percent of the potential habitable space for ice algae for snow-covered Arctic sea ice in spring.