Changes in the sea-ice brine community during the spring-summer transition, McMurdo Sound, Antarctica. I. Photosynthetic protists

Dunng the austral spring, a characteristic microbial community develops in the subsurface brine pockets and channels of the annual land-fast sea-lce in Mch4urdo Sound. This community IS distinct from the d~atom-dominated commun~ty that develops in the channels at the base of the scaice, at the seawater/icc interface, and In the platelet layer The photosynthetic biomass In the brlne pockets is domlnated by athecate dinoflagellates Chrysophyte statocysts (sometimes known as archaeomonads) and <5 l m photosynthetic flagellates are also charactel-~stlcally found in thls assemblage. In December, chlorophyll a content and blornass peak, and photosynthetic gymnodinloid dinoflagellates can reach densities of over 10' ml-' of brine The photosynthetic d~noflagellates form cysts (hypnozygotes) during late December and early January, and chrysophyte statocysts also become abundant DUI-lng austral summer, total autotroph~c biomass In the upper Ice brine decreases d u e to dilutlon by melt water, flushlng of brine Into the water column, and grazlng. By late summer, the annual sea-ice in McMurdo Sound has broken out. The yearly decay and retreat of sed-ice introduces a characteristic set of brlne protists and thelr cysts into Mcblurdo Sound.