Nutrients, algal biomass and communities in land-fast ice and seawater offAdelie Land (Antarctica)

Land-fast ice in the vicinity of Adelie Land was sampled during spring 1995 , The ice was annual, thin, with no consistent snow cover. and exposed to o ceanic conditions. Temporal and spatial variations of the vertical pigment distribution were studied in relation to environmental factors, during the break up of the ice. Different levels were sampled in the congelation ice a nd the platelet ice-like layer (PLI). Under-ice water and open water masses were also sampled. The algal biomass was greater in the PLI (24 +/- 14 mu g ch1 a l(-1) offshore and up to 9 mg chi a l(-1) near-shore). than in the under-ice water, and fell to 0.9 +/- 0.64 mu g ch1 a l(-1) open water masse s. Homogenous ion pigment concentrations were detected in the upper levels of congelation ice. A gradient was identified along a 7 km seaward transect , sampled in November with the lowest biomass offshore. The integrated pigm ent concentrations in fast ice reached very high levels (>500 mg ch1 a m(-2 ) near the coast and 0.8 mg m(-2) offshore), with apparently no relationshi p with either the ice thickness or snow cover. In the congelation ice nutri ent concentrations were lon and their distribution homogenous. whereas in t he PLI high concentrations of nitrate (up to 100-300 mu M NO3-) and silicic acid [30-100 mu M Si(OH)(4)] were detected, often related to high pigment concentrations and proximity to islands. The sea ice algae communities were diverse, but mostly composed of chain-forming and tube-dwelling pennate di atoms (Amphiprora, Berkeleya, Nitzschia and Navicula). Cell densities in PL I reached up to 10(10) cells l(-1). At very low biomass and cell densities (2 10(4) cells l(-1)) the phytoplankton also had a low diversity; some spec ies were similar to those of the PLI. such as Navicula glaciei but other we re typically planktonic (Chaetoceros), At sea ice break-up it is estimated that a significant proportion of particulate matter (up to 0.5 g ch1 a m(-2 ) near-shore) was transferred to the underlying water masses (on an average 15 t POC km(-1) shoreline).