The latitudinal variation in the photosynthesis-irradiance (P-E) relationship of phytoplankton was investigated across a cyclonic eddy in the Antarctic Divergence zone along the 140°E longitude. Both the maximum photosynthetic rate (P*max) and maximum light utilization coefficient (α*) were lower at stations within the eddy as compared with those in the continental shelf-slope area and the offshore area. The low-density water mass of the eddy prevented the upward and horizontal supply of macronutrients and micronutrients, which likely resulted in a reduction of P*max and α*. In contrast, both P*max and α* were large and variable in the slope area, probably owing to a local stability resulting from the less-saline surface water. The present study indicates the possibility that longitudinal variations in the latitudinal pattern of meanders, eddies, seasonal sea-ice extent, and topographical conditions significantly influence the mesoscale variations in the P-E parameters and primary production in the Southern Ocean.