Chlorophyll a, macronutrient concentrations, and photosynthetically active radiation measured during the Umitaka-maru cruise of the 61st Japanese Antarctic Research Expedition, January 2020

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. 65S) along the 110E 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 110E 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.