I checked a couple of sources and growth rates are all over the board depending on species and the light, temperature, and nutrients conditions. A good ballpark figure would be a specific growth rate of 0.05-0.10 per day (so 10-20 days for a population to double) (McMinn et al., 1999, Botanica Marina), however specific growth rates from 0.02-0.45 per day can be found. Most of these ice associated “algae” however are not green algae. The word “algae” is troublesome as it is used to refer to some single celled photosynthesizers taxonomically, but is also used to describe any photosynthetic cell that is not planktonic (is not floating free in the water column). So all cells, regardless of taxonomy, that are living on the seafloor, sea ice, anchor chains, rocks, etc. are algae. I personally don’t like this but that’s the convention! Most of the sea ice “algae” are in fact diatoms (class Bacillariophyceae, the true green algae are Chlorophyceae). To complicate the picture you can find true green algae in the fresher portions of sea ice, such as in melt ponds on the surface of summertime ice.