Phytoplankton community structure and stocks in the East Antarctic marginal ice zone (BROKE survey, January-March 1996) determined by CHEMTAX analysis of HPLC pigment signatures

The distribution and abundance of phytoplankton communities off east Antarc tica were surveyed using CHEMTAX analysis of HPLC pigment profiles, supplem ented by microscopy. Eight north-south transects were surveyed between 80 d egrees E and 150 degrees E, from approximately 63 degrees S to the sea-ice, during Jan.-Mar. 1996. Spatial resolution of 1-16 km along the shelf break allowed fine-scale resolution of features associated with the ice edge and the Antarctic Slope Front. The maximum concentration of chlorophyll a (Chl a) was 3.4 mu g 1(-1), although most transects had maxima less than or equ al to 1.0 mu g 1(-1). Five 'low chlorophyll' transects had average integrat ed abundances of chi a < 38 mg m(-2), while three 'high chlorophyll' transe cts had average abundances > 52 mg m(-2). CHEMTAX software was used to esti mate the contribution of the different algal classes to total chi a. Eight algal categories were operationally defined by their pigment content: Diato ms, Dinoflagellates, Cryptophytes, Prasinophytes, Chlorophytes, Cyanobacter ia, and two categories of haptophytes: Hapto3s (typified by coccolithophori ds) and Hapto4s (including Phaeocystis antarctica plus Parmales and other c hrysophytes). Regions with melting pack ice typically had an algal bloom th at was variable in composition and usually fairly uniform above a deep pycn ocline. Significant quantities of detrital matter sank from beneath the mel ting ice. At each ice edge, there was a local minimum in surface Chi a conc entration associated with krill and, in one case, salps. Most algal categor ies had concentration minima there, but Cryptophytes and often Dinoflagella tes and Cyanobacteria had local maxima, perhaps due to selective grazing. N orth of the ice edge, strong subsurface Chi a maxima were the norm, with co ncentrations on average 170% of surface values (495% max.) Chlorophyll conc entrations were lower in the eastern half of the survey area than the west, although the composition of communities was similar. The composition, conc entration and vertical distribution of algal stocks appeared related to the degree of thermal stratification of the mixed layer. Stratified waters had the highest concentrations of Chi a and were associated with high concentr ations of Diatoms, whereas well-mixed regions were associated with Hapto4s. Maximum concentrations of most algal groups were principally found on the seasonal pycnocline. However, in stations with well-mixed surface waters, a community dominated by Prasinophytes and Hapto4s was consistently found in the T-min layer whereas other algal groups were found on the pycnocline. S ubduction of communities from the T-min layer was apparent at the Antarctic Slope Front. Significant local grazing effects were noted, and it is likel y that regional differences in dominant zooplankton may be related to diffe rences in algal stocks. (C) 2000 Elsevier Science Ltd. All rights reserved.