Collaborative Research: Mesoscale variability in nitrogen sources and food-web dynamics supporting larval southern bluefin tuna in the eastern Indian Ocean

东印度洋(IO)位于澳大利亚西北部和印度尼西亚之间的小区域是南蓝鳍金枪鱼(SBT)唯一已知的产卵场，SBT是一种极度濒危的顶级海洋捕食者。成体SBT每年从高纬度摄食区迁徙数千英里，在这些热带水域产卵，那里的食物浓度平均低于支持其幼虫最佳摄食和生长的水平。这种栖息地的许多关键方面知之甚少，例如维持系统生产力的氮营养的主要来源、浮游食物网如何运作以产生金枪鱼幼体喜欢的不寻常类型的浮游动物猎物、与海洋锋面和漩涡有关的栖息地质量的环境差异如何被产卵的成体金枪鱼利用以使其幼体有更大的快速生长和生存成功的机会。该项目在2021年初的一次为期38天的探险中调查了这些问题，当时正值SBT产卵的高峰期。这个项目是美国对第二次国际印度洋探险(IIOE-2)的贡献，该探险促进了对研究较少的东部IO的生物地球化学和生态动力学的了解。这是第一次对连接太平洋和IO水域的地区的氮和碳循环进行详细研究。SBT幼鱼与其他大型金枪鱼和比目鱼有共同的饮食偏好，这也可能使所获得的见解广泛适用于理解其他海洋生态系统中最大消费者的幼体补充问题。这项研究的新信息将加强对SBT的国际管理努力。大型金枪鱼和比目鱼的共同幼体饮食偏好也可能将获得的洞察力广泛推广到许多其他海洋最大消费者，包括在美国墨西哥湾水域产卵的大西洋蓝鳍金枪鱼。端到端研究方法强调了物理环境变异性、生物地球化学和浮游生物食物网之间的联系，这些联系导致了富有魅力和经济价值的鱼类生产，这是通过学校科学家？开发教育工具和模块的主题。通过佛罗里达州立大学海洋-大气预测研究中心的一项计划，通过一项为夏威夷人数不足的学生改进STEM学习途径的计划，以及通过在圣地亚哥白桦水族馆展示的公共推广产品。这项研究还旨在支持身临其境的实地体验，将有才华的高中生介绍到海洋研究中，目标是为佛罗里达州西北部农村地区未被充分代表的学生制定可持续的海洋相关教育计划。南部蓝鳍金枪鱼(SBT)从高纬度觅食地长途迁徙，专门在热带东印度洋(IO)的一个小的少营养区产卵，该区域拥有丰富的中尺度结构，受复杂的洋流和季节性逆转的季风风的推动。为了生存，SBT幼虫必须在环境条件下取食和快速生长，这些环境条件挑战了人们对恶劣公海系统中食物网结构和功能关系的传统理解。对于SBT幼虫的首选猎物--枝角类和Corycaeidae桡足类，研究很少，并且对幼虫的营养转移效率有很大不同的影响。维持该区域新生产的氮源的差异--固氮和源于太平洋的深层硝酸盐--也影响到可能选择猎物类型(特别是枝角类)的条件，以提高SBT幼虫的转移效率和生长速度。这些N源对IO生态系统的相对重要性可能会影响SBT对预计海洋层化增加的恢复能力。这项研究考察了新产量、食物网络结构和营养通量的中尺度变异性如何影响SBT幼虫的摄食和生长条件。跨中尺度特征的采样测试了SBT幼体摄食的变异性和猎物偏好(肠道内容物)、生长速度(耳石分析)和营养位置(TP)与成年产卵者选择的水域环境条件之间的假设关系。用化合物特定的氨基酸同位素分析(CSIA-AA)确定了幼虫及其猎物的营养位置。拉格朗日实验通过测量水柱14C生产力、氮气固定、15NO3吸收和硝化作用；群落生物量和组成(流式细胞仪、色素、显微镜、原位成像、遗传分析)；以及通过微型和中型浮游动物放牧、再矿化和出口的营养通量，调查潜在的过程速率和关系。通过CSIA-AA(氮源，TP)、15N约束的预算和建模，将研究的生物地球化学和食物网络元素联系在一起。项目内容包括一项端到端的生物地球化学-营养耦合研究，这是以前没有对任何远洋生态系统进行的研究。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The small area between NW Australia and Indonesia in the eastern Indian Ocean (IO) is the only known spawning ground of Southern Bluefin Tuna (SBT), a critically endangered top marine predator. Adult SBT migrate thousands of miles each year from high latitude feeding areas to lay their eggs in these tropical waters, where food concentrations on average are below levels that can support optimal feeding and growth of their larvae. Many critical aspects of this habitat are poorly known, such as the main source of nitrogen nutrient that sustains system productivity, how the planktonic food web operates to produce the unusual types of zooplankton prey that tuna larvae prefer, and how environmental differences in habitat quality associated with ocean fronts and eddies might be utilized by adult spawning tuna to give their larvae a greater chance for rapid growth and survival success. This project investigates these questions on a 38-day expedition in early 2021, during the peak time of SBT spawning. This project is a US contribution to the 2nd International Indian Ocean Expedition (IIOE-2) that advances understanding of biogeochemical and ecological dynamics in the poorly studied eastern IO. This is the first detailed study of nitrogen and carbon cycling in the region linking Pacific and IO waters. The shared dietary preferences of SBT larvae with those of other large tuna and billfish species may also make the insights gained broadly applicable to understanding larval recruitment issues for top consumers in other marine ecosystems. New information from the study will enhance international management efforts for SBT. The shared larval dietary preferences of large tuna and billfish species may also extend the insights gained broadly to many other marine top consumers, including Atlantic bluefin tuna that spawn in US waters of the Gulf of Mexico. The end-to-end study approach, highlights connections among physical environmental variability, biogeochemistry, and plankton food webs leading to charismatic and economically valuable fish production, is the theme for developing educational tools and modules through the ?scientists-in-the-schools? program of the Center for Ocean-Atmospheric Prediction Studies at Florida State University, through a program for enhancing STEM learning pathways for underrepresented students in Hawaii, and through public outreach products for display at the Birch Aquarium in San Diego. The study also aims to support an immersive field experience to introduce talented high school students to marine research, with the goal of developing a sustainable marine-related educational program for underrepresented students in rural northwestern Florida.Southern Bluefin Tuna (SBT) migrate long distances from high-latitude feeding grounds to spawn exclusively in a small oligotrophic area of the tropical eastern Indian Ocean (IO) that is rich in mesoscale structures, driven by complex currents and seasonally reversing monsoonal winds. To survive, SBT larvae must feed and grow rapidly under environmental conditions that challenge conventional understanding of food-web structure and functional relationships in poor open-ocean systems. The preferred prey of SBT larvae, cladocerans and Corycaeidae copepods, are poorly studied and have widely different implications for trophic transfer efficiencies to larvae. Differences in nitrogen sources - N fixation vs deep nitrate of Pacific origin - to sustain new production in the region also has implications for conditions that may select for prey types (notably cladocerans) that enhance transfer efficiency and growth rates of SBT larvae. The relative importance of these N sources for the IO ecosystem may affect SBT resiliency to projected increased ocean stratification. This research expedition investigates how mesoscale variability in new production, food-web structure and trophic fluxes affects feeding and growth conditions for SBT larvae. Sampling across mesoscale features tests hypothesized relationships linking variability in SBT larval feeding and prey preferences (gut contents), growth rates (otolith analyses) and trophic positions (TP) to the environmental conditions of waters selected by adult spawners. Trophic Positions of larvae and their prey are determined using Compound-Specific Isotope Analyses of Amino Acids (CSIA-AA). Lagrangian experiments investigate underlying process rates and relationships through measurements of water-column 14C productivity, N2 fixation, 15NO3- uptake and nitrification; community biomass and composition (flow cytometry, pigments, microscopy, in situ imaging, genetic analyses); and trophic fluxes through micro- and mesozooplankton grazing, remineralization and export. Biogeochemical and food web elements of the study are linked by CSIA-AA (N source, TP), 15N-constrained budgets and modeling. The project elements comprise an end-to-end coupled biogeochemistry-trophic study as has not been done previously for any pelagic ecosystem.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.