RAPID - Assessing the response of the Seychelles-Chagos Thermocline Ridge ecosystem to an Indian Ocean Dipole event

浮游植物构成了海洋食物网的基础。它们的多样性、生产力和丰度对渔业有着巨大的影响。例如，富含养分的深海沃茨涌入赤道印度洋上层阳光照射的水域，造成浮游植物生物量季节性增加，导致金枪鱼种群增加。然而，赤道印度洋东部的塞舌尔-查戈斯温跃层海脊区域海面温度不定期上升，这一现象被称为印度洋偶极子，削弱了深海水涌，似乎改变了浮游植物群落结构，降低了浮游植物的生产力，损害了金枪鱼种群。然而，这种不可预测的事件的影响尚未得到充分了解。该项目调查了正在进行的印度洋偶极子对浮游植物的影响，这是有记录以来最严重的偶极子之一。该项目更广泛的影响涉及其为该地区渔业管理计划提供信息的价值。从这项研究中获得的数据对于了解和预测一个正在承受全球变暖压力的地区的食物网的更广泛变化是有价值的。研究小组正在与韩国合作者合作，以了解浮游植物生物量和深层叶绿素最大值的变化如何与浮游植物生产力和生长率的变化联系起来，这是由于深水上升流减弱导致营养和/或铁限制造成的。该团队还在研究偶极子事件对主要浮游植物种群细胞大小的影响。最后，他们正在测试浮游植物群落是否主要依赖于回收的而不是新的含氮营养物质进行光合作用和偶极子事件期间的生长。通过广泛的生物、水化学和地球化学测量对假设进行了测试，包括：1）浮游植物生物量、群落组成和大小结构的基于显微镜、流式细胞术、FlowCAM和HPLC色素的分析，以及沿着航行轨迹和水柱中不同深度的浮游植物光合能力的基于快速重复率的测量，以及2）基于甲板培养的光合作用和养分吸收实验。作为韩国-美国印度洋科学研究方案（KUDOS）巡航的一部分，进行了现场测量和样本收集（2020年4月至5月）在韩国海洋科学技术研究所R/V Isabu船上，这是为塞舌尔群岛计划的唯一多学科海洋学巡航之一，该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的智力价值和更广泛的评估支持影响审查标准。

Phytoplankton constitute the base of the marine food web. Their diversity, productivity and abundance have a huge influence on fisheries. For example, seasonal increases in phytoplankton biomass caused by the surge of deep ocean nutrient rich waters into the upper sunlit layers in the Equatorial Indian Ocean lead to an increase in tuna stocks. However, non-periodic increases of sea surface temperatures in the Seychelles-Chagos Thermocline Ridge region in the eastern Equatorial Indian Ocean, a phenomenon known as the Indian Ocean Dipole, weakens deep ocean water surge, which seems to alter phytoplankton community structure and to reduce phytoplankton’s productivity to the detriment of tuna stocks. Yet, the impact from such unpredictable events is not fully understood. This project investigates the impacts of an ongoing Indian Ocean Dipole, one of the most severe on record, on phytoplankton. The broader impacts of the project relate to its value to inform fisheries management plans in that area. Data derived from this study is valuable to understand and predict wider changes in the food web in a region that is coming under increasing pressures of global warming. All the data is made publicly available.The research team is working with South Korean collaborators to understand how changes in phytoplankton biomass and in deep chlorophyll maxima may be linked to changes in phytoplankton productivity and growth rates due nutrient and/or iron limitation as a result of the weakening of deep-water upwelling. The team is also investigating the impact of the dipole event on cell size of dominating phytoplankton populations. Finally, they are testing if the phytoplankton community rely primarily on recycled as opposed to new nitrogenous nutrients for photosynthesis and growth during dipole events. The hypotheses are tested through extensive biological, hydro-chemical and biogeochemical measurements that include: 1) microscopic, Flow Cytometry, FlowCAM and HPLC pigment based analysis of phytoplankton biomass, community composition and size structure and Fast Repetition Rate based measurements of phytoplankton photosynthetic competency along a cruise track and different depths in the water column, and 2) on-deck incubation based photosynthesis and nutrient uptake experiments. In situ measurements and sample collection is carried out as part of the Korea-US inDian Ocean Scientific (KUDOS) Research Program cruise (April-May 2020) on board the Korea Institute of Ocean Sciences and Technology Ship R/V Isabu, one of the only multi-disciplinary oceanographic cruises planned for the Seychelles-Chagos Thermocline Ridge region during the dipole event.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.