The topography of the ice-water interface - its influence on the colonization of sea ice by algae

The ice-water interface constitutes an important habitat for polar organisms, characterized by extreme variability in physical and biological properties, which can range over an order of magnitude on decimeter scales. The porous nature of sea ice allows vertical fluid exchange within the ice and across the ice-water interface in much the same way as the sediment-water interface. The present paper reports on experiments examining the effect of irregular undersides of sea ice on the variability of algal biomass, using flume tanks with constant unidirectional flow. Bulges and depressions at the ice-water interface altered the interfacial pore water flux and affected the spatial distribution and abundance of ice-associated algae substantially. Dye tracer experiments demonstrated that interfacial water fluxes around ice bulges are by a factor of 10-100 higher than published indirect estimates based on algal nutrient demands in flat fast ice. Further, underside relief structures migrated downstream, illustrating their dynamic and transient nature. The presence of a relief fostered algal accumulation compared to a flat underside. Algal growth occurred at bulge sites facing the current. while particles accumulated in the wake further downstream. We infer from our experiments that sea ice is not only a source for algae production. but can also serve as sink for organic material from the water column. We propose that local formation and ablation of ice around under-side features is an important process which induces high variations in the sea-ice habitat structure and in growth of sympagic organisms, and could partially explain the high natural variability observed in the abundance and colonization patterns of sea-ice organisms.