Sea ice is a major feature of Antarctica and has considerable influence on ocean currents and marine ecology. In this episode I describe how sea ice forms and traps marine algae that stimulates the marine food web when the ice melts.  Thus, sea ice provides a direct link between the physical and biological worlds that is important for marine productivity all the way up the food chain to penguins, seals, and whales.  Sea ice extent varies each year in Antarctica and also is an important driver for ocean currents and mixing.  Polynyas are highly productive areas of open water surrounded by sea ice and are formed by katabatic winds or warm water upwelling. They determine the colony locations for most Adelie penguins as they are important foraging grounds for this species.