http://swrc.ontoware.org/ontology#Article Chlorophyll a, macronutrient concentrations, and photosynthetically active radiation measured during the Umitaka-maru cruise of the 61st Japanese Antarctic Research Expedition, January 2020 en Southern Ocean Chlorophyll a macronutrients The chlorophyll a concentration is the most commonly used indicator of phytoplankton biomass, which is regulated by macronutrients and light intensity. Long-term monitoring of chlorophyll a provides basic information on the ecosystem changes associated with climate change. As part of the monitoring program of the Japanese Antarctic Research Expedition (JARE), the chlorophyll a concentration and macronutrients (nitrate, nitrite, phosphate and silicic acid) have been measured by the icebreakers Fuji and Shirase since JARE-14 (1972/73) and JARE-7 (1965/66), respectively, and during the cruise of the training vessel (T/V) Umitaka-maru, of Tokyo University of Marine Science and Technology, since JARE-55 (2013/14). The Umitaka-maru follows almost the same course every year; after leaving Fremantle, it travels to the sea ice edge (ca. 65°S) along the 110°E meridian and then returns to Hobart. Its course covers waters in the Southern Ocean that range from sub-tropical to polar. During the cruise, water samples for determining chlorophyll a and macronutrient concentrations are obtained from an underway pump twice a day, and photosynthetically active radiation is measured continuously by a sensor mounted on the bridge of the ship. Vertical water sampling to determine the chlorophyll a concentration is conducted at six stations along the 110°E meridian, and the macronutrient concentrations at the same stations are determined by another monitoring program. This report includes the latest data collected during the T/V Umitaka-maru cruise in the 2019/20 season. In 2019/20, three vertical sampling stations (KC7-9) were added to enhance the bottom water monitoring performed under the “Physical and Chemical Oceanography” routine observation program of the JARE. Polar Data Journal 6 9-16 2022-04 National Institute of Polar Research 2432-6771 https://doi.org/10.17592/001.2021040702|Original Data Marine biology