Spatial and vertical distribution of phytoplankton pigments in the eastern Atlantic sector of the Southern Ocean

Abstract

Abstract The spatial and vertical distribution of phytoplankton pigments was analysed during the Scandinavian/South African Southern Ocean JGOFS expedition of the S.A. Agulhas from 31 December 1997 to 26 January 1998. In total, seawater from 12 different stations along a transect at 6°E from 60.38 to 49.82°S were sampled at water depths of approximately 2, 10, 20, 30, 50, 75 and 100 m. Pigment concentrations were analysed using high-performance liquid chromatography. The highest chlorophyll a (chl a ) concentrations were found within the Spring Ice Edge (SIE) at water depths of 2–30 m (1.5–2.0 μg l −1 ). The lowest phytoplankton biomass (chl a ) was found within the Winter Ice Edge (WIE) with typical values around 0.3 μg l −1 . While the chl a concentrations decreased with depth within the SIE, no apparent difference between depths was found within WIE. The same trend with similar chl a concentrations (0.8–1.2 μg l −1 ) at all depths was found for the northernmost part of the Antarctic Polar Front (APF) except for samples from 100 m. Principal component analysis (PCA) and the matrix factorisation program, CHEMTAX, were used to interpret the pigment data. The PCA indicated small differences between the samples in WIE and APF, while the SIE samples were separated into three groups. At the southernmost stations a clear separation was found between surface samples and deeper samples; this was not seen at the two northernmost stations within SIE. Along the whole transect, CHEMTAX suggested a dominance of diatoms and haptophytes. In general, the haptophytes contributed more to total chl a further south and the diatom contribution increased from south to north. Within WIE, CHEMTAX indicated that cyanobacteria were contributing approximately 3% to the total autotrophic biomass. In APF, a more diverse phytoplankton community was found compared with WIE and SIE. The pigment patterns interpreted by CHEMTAX showed that different phytoplankton assemblages were associated with distinct water masses along the transect.