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

Abstract

The distribution and abundance of phytoplankton communities off east Antarctica were surveyed using CHEMTAX analysis of HPLC pigment profiles, supplemented by microscopy. Eight north–south transects were surveyed between 80°E and 150°E, from approximately 63°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 μg l −1, although most transects had maxima ⩽1.0 μg l −1. Five `low chlorophyll’ transects had average integrated abundances of chl a <38 mg m −2, while three `high chlorophyll’ transects had average abundances >52 mg m −2. CHEMTAX software was used to estimate the contribution of the different algal classes to total chl a. Eight algal categories were operationally defined by their pigment content: Diatoms, Dinoflagellates, Cryptophytes, Prasinophytes, Chlorophytes, Cyanobacteria, and two categories of haptophytes: Hapto3s (typified by coccolithophorids) and Hapto4s (including Phaeocystis antarctica plus Parmales and other chrysophytes). Regions with melting pack ice typically had an algal bloom that was variable in composition and usually fairly uniform above a deep pycnocline. Significant quantities of detrital matter sank from beneath the melting ice. At each ice edge, there was a local minimum in surface Chl a concentration associated with krill and, in one case, salps. Most algal categories had concentration minima there, but Cryptophytes and often Dinoflagellates and Cyanobacteria had local maxima, perhaps due to selective grazing. North of the ice edge, strong subsurface Chl a maxima were the norm, with concentrations on average 170% of surface values (495% max.) Chlorophyll concentrations were lower in the eastern half of the survey area than the west, although the composition of communities was similar. The composition, concentration 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 Chl a and were associated with high concentrations 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. Subduction of communities from the T min layer was apparent at the Antarctic Slope Front. Significant local grazing effects were noted, and it is likely that regional differences in dominant zooplankton may be related to differences in algal stocks.