Award: OCE-2019983

This study was planned for the Seychelles-Chagos Thermocline Ridge (SCTR), a large and persistent open ocean upwelling feature in the southwestern Equatorial Indian Ocean Region (5-15oS, 54-70oE). This region characterized by elevated phytoplankton biomass, is a large foraging ground for tuna fisheries in the Indian Ocean. However, not much is known about the structure of SCTRs phytoplankton communities, on account of the lack of dedicated biological oceanographic cruises to the region. Most measurements in the region recent past focused on ocean circulation responses and air-sea interactions, because of their importance for water mass circulation, moisture transport across the Equatorial Indian Ocean, and regional rainfall patterns over SE Asia. In May of 2019, the SCTR began showing signs of unusual warming, and by the beginning of July 2019, it became evident that the Equatorial Indian Ocean was experiencing an IOD event. By Sept of 2019, the IOD began to intensify at a rapid pace, we requested for funding for a natural event based RAPID grant to study the implications of the IOD on native phytoplankton phytoplankton of the SCTR. Our study planned from the 1st of April to the 8th of May 2020, on board the South Korean research ship, R/V Isabu, included measurements and deck based incubation experiments, aimed at filling this significant knowledge gap; more specifically to understand the impact of the IOD event on phytoplankton photo-physiology, productivity and biomass accumulation as it relates to these 4 hypotheses: H1- During an IOD event the SCTR experiences a sharp decline in phytoplankton biomass including significant weakening of its deep chlorophyll maxima. H2 - The reduction in phytoplankton biomass in the SCTR during an IOD is due to acute nutrient and/or iron limitation of phytoplankton productivity and growth rates. H3 - The reduction in phytoplankton biomass in the SCTR is accompanied by a significant alteration in phytoplankton community structure towards populations dominated by smaller phytoplankton. H4 - During an IOD event, phytoplankton communities in the SCTR rely primarily on recycled versus new nitrogenous nutrients for photosynthesis and growth. In Feb. 2020, when preparing for shipment of equipment to Busan, South Korea, the home port of R/V Isabu, we were informed of potential delays in the departure date of the cruise because of Covid-19 in South Korea. By early March 2020 when the virus had reached pandemic proportions, we were informed that the cruise would be postponed indefinitely. We have been assured us that the cruise will take place despite the delay. On account of this setback, we turned to satellite data as our only resource to monitor the evolution of the IOD, while simultaneously monitoring changes in phytoplankton biomass. and changes in the SCTR using. This report presented here is based primarily on information garnered from satellite datasets. It was supplemented with data collected from our voluntary participation in the GO-SHIP IO7 of May-June 2018 cruise in the Indian Ocean led by NOAA PMEL These combined datasets allowed us to address H1, the results revealing that the IOD event 2019 (Fig. 1A-D) had a significant impact on phytoplankton biomass in the SCTR. Additionally, we report that although the IOD of 2019 was strong, biological response both in the SCTR and off the coast of Sumatra, Indonesia was muted as compared to the equally intense IOD event of 1997/1998. These findings are being written up into two research manuscripts. During this study, we participated in the publication of 2 research manuscripts, one fully supported by this study and 2 others supported partially by this study. Challenges and pathway forward: The delay of the cruise planned for the months of May-June of 2020 has been a significant setback for our planned studies. Our colleagues from KIOST have assured us that they do not plan to cancel the R/V Isabu cruise to the western Indian Ocean, especially because they have instruments on moorings in the water, which have to be retrieved and replaced with a fresh set of instruments. The biggest uncertainty for us now is the continuation of the pandemic, which has precluded us from obtaining a firm date for the cruise. If an opportunity arises, we would like to participate in this cruise and will seek an extension of our RAPID grant to fulfill our objectives. A request for an extension will be submitted as per NSF guidelines. Manuscript Published: Rixen T., Cowie G., Gaye B., Goes J., Gomes H. do R., Hood R. R, Lachkar Z. Schmidt H., Segschneider J., Singh A. (2020). Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean. Biogeosciences,17(23):6051-6080. Goes, J. I.., H. do R. Gomes, S. de Rada, D. Kiefer, K. Al-Hashmi (in press) Physical and biogeochemical characteristics of the Indian Ocean Marginal Seas - Sea of Oman; In Oceanography of the Indian Ocean Last Modified: 11/06/2024 Submitted by: JoaquimIGoes