Phytoplankton ice-edge blooms in the marginal ice zone at Princess Astrid Coast in Antarctica

Abstract

Daily variation in sea surface temperature at the ice edge zone in Antarctic polynya during summer from 25th December 1991 to 25th February 1992, showed that the values did not exceed 0 degrees C. Isotherms and isohalines showed vertically homogeneous profile during major part of the season. Nitrate, phosphate and silicate remained high. The ratio of SiO sub(3):NO sub(3):PO sub(4) was 5:12:1. The hydrographical features, the vertical profiles of nutrients and high SiO sub(3):PO sub(4) ratio suggest that the surface waters are continuously enriched by bottom silicates resulting out of vigorous mixing of different water masses in these coastal waters. Chlorophyll a showed frequent occurrence of blooms with upto 3.68 mg m sup(-3). The linear regression of chl a with NO sub(3), PO sub(4) and SiO sub(3) showed significant negative correlation suggesting active utilization of nutrients by phytoplankton. Size classification of phytoplankton showed that microplankton (more than 20 mu m) populations dominated the bloom conditions and nannoplankton (5 to 20 mu m) prevail the non-bloom periods while the picoplankton (less than 5 mu m) constituted a minor fraction during most of the period. Weekly changes in phytoplankton showed inverse relationship with zooplankton. Spiral and straight chains of Fragilaria striatulla, F. cylindrus, Nitzschia closterium and N. seriata were abundant during bloom. Copepods dominated the zooplankton. Increase in euphausiids on 31st December were responsible for reduction of the phytoplankton crop. Species distribution showed that herbivore zooplankters dominated the phytoplankton bloom periods while carnivores during non-bloom periods. The results suggest that while mixing processes are prominent the active growth of phytoplankton and zooplankton provide sufficient food for the proliferation of euphausiid crop