Freeboard layers

‘Freeboard’ layers (continuous or semi-continuous horizontal gap layers within the ice) develop over flood-freeze cycles during the spring and summer in the sea ice floes, creating a habitat for large algal assemblages (Figure 5) (Garrison & Buck 1991, Fritsen et al. 1998, 2001, Haas et al. 2001).

Water ingress and nutrient replenishment occurs when ice floes are broken and rafted together, causing stress fractures and fissures to be produced (Thomas et al. 2008). The horizontal nature of the layers enables water ingress to be enhanced by swell and wave action. This produces ideal conditions for algal growth as the high light intensity and replenishment of nutrients not found in the brine channels further down the ice, promote growth. The bottom layer above the seawater gap is the most productive with the greatest algal abundance as the algae have the easiest access to fresh nutrients.  The upper surface ice layers are less abundant as they are nutrient limited.  Algal abundance is likely to be correlated with the amount of flushing that occurs through these ice layers (Kattner et al. 2004).

Figure 5. Vertical cross sections of 2 ice floes. Photograph shows a 14cm section removed from a re-frozen (autumn) ice floe. The bottom layer colouration is due to the high algal biomass (reproduced (from a modified diagram from Haas et al): Kattner et al 2004).

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