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

Abstract

Land-fast ice in the vicinity of Adélie Land was sampled during spring 1995. The ice was annual, thin, with no consistent snow cover, and exposed to oceanic 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 and 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 μg chl a l−1 offshore and up to 9 mg chl a l−1 near-shore), than in the under-ice water, and fell to 0.9 ± 0.64 μg chl al−1 in open water masses. Homogenous low 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 pigment concentrations in fast ice reached very high levels 500 mg chl a m−2 near the coast and 0.8 mg m−2 offshore), with apparently no relationship with either the ice thickness or snow cover. In the congelation ice nutrient concentrations were low and their distribution homogenous, whereas in the PLI high concentrations of nitrate (up to 100–300 μM NO3) and silicic acid [30–100 μM 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 diatoms (Amphiprora, Berkeleya, Nitzschia and Navicula). Cell densities in PLI reached up to 1010 cells l−1. At very low biomass and cell densities 2 104 cells l−1) the phytoplankton also had a low diversity; some species were similar to those of the PLI, such as Navicula glaciei, but other were typically planktonic (Chaetoceros). At sea ice break-up it is estimated that a significant proportion of particulate matter (up to 0.5 g chl a m−2 near-shore) was transferred to the underlying water masses (on an average 15 t POC km−1 shoreline).