Ice core and snow data from the Amundsen, Bellingshausen, and Weddell Seas, Antarctica show that the formation of superimposed ice and the development of seawater-filled gap layers with high algal standing stocks is typical of the perennial sea ice in summer. The coarse-grained and dense snow had salinities mostly below 0.1 per mil. A layer of fresh superimposed ice had a mean thickness ranging from 0.04-0.12 m. 0.04 to 0.08 m thick gap layers extended downwards from 0.02 to 0.14 m below the water level. These gaps were populated by diatom standing stocks up to 439 µg/l chlorophyll a. We propose a comprehensive heuristic model of summer processes, where warming and the reversal of temperature gradients cause major transformations in snow and ice properties. The warming also causes the re-opening of incompletely frozen slush layers caused by flood-freeze cycles during winter. Alternatively, superimposed ice forms at the cold interface between snow and slush in the case of flooding with negative freeboard. Combined, these explain the initial formation of gap layers by abiotic means alone. The upward growth of superimposed ice above the water level competes with a steady submergence of floes due to bottom and internal melting and accumulation of snow.