Temperature fluctuations within the ice become increasingly greater towards the surface. The temperature at the sea/ice interface will always be around -20C as this is the temperature at which sea water freezes. The temperature at the surface of the ice is far more variable as it is dependent on atmospheric conditions and snow cover and can vary significantly over the course of a day. Temperatures at the air/ice interface can be as low as -200C (Meiners et al. 2009).
There is also a steep salinity gradient within the ice, with decreasing ice temperature increasing the brine salinity. Productivity within the ice is greatest at low salinities during the ice melt and at higher salinities during colder seasons, due to the adaptations of the algal species found in this environment (Thomas & Dieckmann 2003). Salinity levels vary widely from freshwater in the Arctic melt pools during the summer season to salinities greater than 100 (Brinkmeyer et al. 2004, Meiners et al. 2009).
Although the conditions may be more stable lower in the ice pack, the amount of photosynthetically active radiation (PAR) available to the algae decreases with increasing depth in the ice. Polar regions have periods of 24hr sunlight and therefore receive more radiation around midsummer than the tropics, however the low angle of the sun and the long periods of darkness in the winter mean that total radiation is about 43% less than at the equator over the year (Thomas et al. 2008). Ice over 1m thick, with a snow cover, receives less than 1% of the light at the snow surface. This means that the algae living here are adapted to extremely low light conditions (Thomas 2004).
Each algal species will have an optimum position within the ice structure, where it can withstand the temperature and salinity changes but still receive sufficient PAR to meet its metabolic needs.