Long-term changes in phytoplankton biomass and community composition are important in the ecosystem and biogeochemical cycle in the Southern Ocean. We aim to ultimately evaluate changes in phytoplankton assemblages in this region on a decadal scale. However, yearly continuous data are lacking, and long-term datasets often include seasonal variability. We evaluated the seasonal changes in phytoplankton abundance/composition across latitudes in the Indian Ocean sector of the Southern Ocean via multi-ship observations along the 110 degrees E meridian from 2011 to 2013. The chlorophyll a concentration was 0.3-0.5 mg m(-3) in the Subantarctic Zone (40-50 degrees S) and 0.4-0.6 mg m(-3) in the Polar Frontal Zone (50-60 degrees S); pico-sized phytoplankton (<10 mu m), mainly haptophytes, were dominant in both zones. In the Antarctic Divergence area (60-65 degrees S), the chlorophyll a concentration was 0.6-0.8 mg m(-3), and nano-sized phytoplankton (>10 mm), mainly diatoms, dominated. Chlorophyll a concentrations and phytoplankton community compositions were the same within a latitudinal zone at different times, except during a small but distinct spring bloom that occurred north of 45 degrees S and south of 60 degrees S. This small seasonal variation means that this part of the Southern Ocean is an ideal site to monitor the long-term effects of climate change. (C) 2014 Elsevier B.V. and NIPR. All rights reserved.