A physical mechanism for establishing algal populations in frazil ice

Abstract

In polar regions ice algal communities are not only conspicuous but may also be important production sites and sources of seed populations for pelagic communities1–3. Except for some studies near land-based stations1,4–7, there are few long-term observations of ice algal populations, and few studies have considered how they form and develop. Most researchers suggest that ice algae are derived from sparse water column assemblages that become trapped during ice formation; ice algal populations then develop through in situ growth1. Some previous observations have suggested that ice crystals forming in the water column concentrate algal cells6–8. Dense algal populations in young sea ice may have been recognized as early as the mid-nineteenth century, since Hooker (cited by Alexander2) described discoloured brash and pancake ice (“ …a pale ochreous colour”). But until now, neither the mechanism for this harvesting nor the effects on the composition of the ice community has been clearly demonstrated. In the Weddell Sea, we have sampled young sea ice discoloured by algae, and we present evidence that the algae were concentrated by a physical mechanism. We explain how such a process may accumulate planktonic forms in ice communities.