The Southern Annular Mode (SAM) influences phytoplankton communities in the seasonal ice zone of the Southern Ocean

Abstract. Ozone depletion and climate change are causing the Southern Annular Mode (SAM) to become increasingly positive, driving stronger winds southward in the Southern Ocean (SO), with likely effects on phytoplankton habitat due to changes in ocean mixing, nutrient upwelling, and sea ice. This study examined the effect of the SAM and other environmental variables on the abundance of siliceous and calcareous phytoplankton in the seasonal ice zone (SIZ) of the SO. Samples were collected during repeat transects between Hobart, Australia, and Dumont d'Urville, Antarctica, centred around longitude 142° E, over 11 consecutive austral spring-summers (2002–2012). Twenty-two taxa, comprised of species, genera or higher taxonomic groups, were analysed using CAP analysis, cluster analysis and correlation. The SAM significantly affected phytoplankton community composition, with the greatest influence exerted by a SAM index averaged across 57 days centred on 11th March in the preceding autumn, explaining 13.3 % of the variance of taxa composition during the following spring–summer, and showing correlation with the relative abundance of 12 of the 22 taxa resolved. The day through the spring-summer that a sample was collected exerted the greatest influence on phytoplankton community structure (15.4 % of variance explained), reflecting the extreme seasonal variation in the physical environment in the SIZ that drives phytoplankton community succession. The response of different species of Fragilariopsis spp. and Chaetoceros spp. differed over the spring–summer and with the SAM, indicating the importance of species-level observation in detecting subtle changes in pelagic ecosystems. This study indicated that higher SAM favoured increases in the relative-abundance of large Chaetoceros spp. that predominated later in the spring–summer and reductions in small diatom taxa and siliceous and calcareous flagellates that predominated earlier in the spring–summer. Such changes in the taxonomic composition of phytoplankton, the pasture of the SO and principal energy source for Antarctic life, may alter both carbon sequestration and composition of higher tropic levels of the SIZ region of the SO.