Temporal changes in chlorophyll a and nitrate concentrations under fast ice near Syowa Station, Antarctica, in austral summer

Abstract

Seawater samples beneath fast ice were collected from several depths using a Niskin bottle at four sites near Syowa Station, Antarctica, on 30 December 1996, and 12 and 21 January 1997. Vertical distributions of chlorophyll a (Chl a) and nitrate (NO3) concentrations were determined. Substantial temporal changes in Chl a and NO3 concentrations occurred in the surface stratified layer (<20m). There was a significant negative correlation between both the concentrations (n=40, P<0.01). This relationship suggests that an increase of 1μg l-1 of Chl a consumes 0.89μM of NO3 under fast ice. However, light limitation is severe for phytoplankton growing under thick sea ice (2.0-3.6m), as observed in the present study. It is unlikely that the observed increases of Chl a result from phytoplankton growth under heavy fast ice. Seawater, in which Chl a has increased and NO3 has been consumed under favorable light conditions, may be advected into the study area. This advection may be responsible for the apparent increase of Chl a and decrease of NO3 concentrations under the heavy fast ice.