Floating Ice-Algal Aggregates below Melting Arctic Sea Ice

Abstract

During two consecutive cruises to the Eastern Central Arctic in late summer 2012, we observed floating algalaggregates in the melt-water layer below and between melting ice floes of first-year pack ice. The macroscopic (1-15cm in diameter) aggregates had a mucous consistency and were dominated by typical ice-associated pennatediatoms embedded within the mucous matrix. Aggregates maintained buoyancy and accumulated just above a strongpycnocline that separated meltwater and seawater layers. We were able, for the first time, to obtain quantitativeabundance and biomass estimates of these aggregates. Although their biomass and production on a square metrebasis was small compared to ice-algal blooms, the floating ice-algal aggregates supported high levels of biologicalactivity on the scale of the individual aggregate. In addition they constituted a food source for the ice-associatedfauna as revealed by pigments indicative of zooplankton grazing, high abundance of naked ciliates, and iceamphipods associated with them. During the Arctic melt season, these floating aggregates likely play an importantecological role in an otherwise impoverished near-surface sea ice environment. Our findings provide importantobservations and measurements of a unique aggregate-based habitat during the 2012 record sea ice minimum year.