Award: OCE-1736375

Three-quarters of the primary productivity of thenorthern hemisphere oceans is supported by Subantarctic Mode Water (SAMW), which is formed in the Southern Ocean, around 50oS latitude and supplies nutrients to the north. We surveyed the Indian Ocean basin and the SW Pacific basin where SAMW is formed,with the goal to understand factors controlling the algal distributions in the formation region, to perform experiments on the limiting factors that affect the biogeochemistry of these waters and to better model the role of coccolithophores and other phytoplankton groups(diatoms, dinoflagellates nanoplankton and picoplankton) which condition SAMW before it is subducted northwards into the northern hemisphere. Intellectual merit-We confirmed the importance of the calcifying algae, coccolithophores (which produce vast quantities of calcium carbonate, aka particulate inorganic carbon, PIC, orlimestone) in the Great Calcite Belt, the largest such high-reflectance coccolithophore feature in the world ocean, found in the Subantarctic Waters around the globe. Moreover, we demonstrated the relative importance of another mineralizing algal class, diatoms with such high concentrations of biogenic silica, which produce high reflectance waters closer to Antarctica. These have confounded previous attempts to measure PIC in these polar waters. We documented the rate of conditioning of SAMW by microbial communities using CFC tracers. We mapped-out the distributions of the phytoplankton around these important frontal features using a video plankton recorder (VPR). This allowed us to better document the communities of phytoplankton and larger zooplankton that make up the Southern Ocean ecosystem. We also performed experiments on the ship that elucidate the factors affecting the fundamental biogeochemistry of the SAMW. Broader Impacts-Twelve students (undergraduates, graduate students, post-docs) were trained aboard ship on these cruises. The graduate students gained valuable research experience towards their doctoral dissertations. Moreover, we brought two educators on the cruises who translated the science to students ashore through regular blogs, social media and remote interactions. This work with students ashore interactively reached 100 students, pre-, during-, and post-cruise. The results of this work have elucidated the factors that limit productivity of a significant fraction of the world ocean. Last Modified: 11/29/2023 Submitted by: DennisJMcgillicuddy