The summer of 1997 was characterized by unusually large amounts of pack ice in the southeastern
Weddell Sea, and less than 10% of the area that is commonly ice-free in summer was open. A modest
phytoplankton bloom developed in the upper mixed layer in the northernmost area (72°S). The bloom
peaked in mid-February with max chlorophyll concentrations of 1.5 μg l−1, and integrated stocks of
55–60 mg m−2. Autotrophic flagellates dominated the biomass (80–90% of the chlorophyll) at first, while
diatoms increased relative to flagellates during the bloom. Nutrient deficits, however, indicated that a much
larger biomass was produced than was observed. Freezing starting after mid-February probably terminated
the bloom, resulting in a pelagic growth season limited in time (less than two months) and space. The sea ice
had a distinct brown layer of algae, usually at 1–2 m depth, with average chlorophyll biomass of
10.3 mg m−2. The ice cover exhibited a substantial amount of ridges, with ice algae growing in cavities and
other structures, but with lower biomass than in the bands. Ice algae were also found growing on the lower
2 m of the ice shelf (visible at low tide). The overall growth season in the ice lasted several months, and ice
algal production may have exceeded pelagic production in the Weddell Sea during the growth season of
1997. Pennate diatoms, like Fragilariopsis curta and F. cylindrus, dominated both in ice and in open water
above the pycnocline, while Phaeocystis antarctica dominated in deeper layers and in crack pools.
Euphausiids, particularly young stages, were frequently observed grazing on ice algae in ridges and on all
sides of the floes, (confirmed by the gut content). Ice algae would thus have served as an ample food supply
for the krill in the summer of 1997.
ICESat observations of seasonal and interannual variations of sea-ice freeboard and estimated thickness in the Weddell Sea, Antarctica (2003–2009)