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Summer diatom blooms in the eastern North Pacific gyre investigated with a long-endurance autonomous surface vehicle

机译：使用持久型自主地面车辆对北太平洋回旋区的夏季硅藻绽放进行了研究

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Satellite chlorophyll a (chl a) observations have repeatedly noted summertime phytoplankton blooms in the North Pacific subtropical gyre (NPSG), a region of open ocean that is far removed from any land-derived or Ekman upwelling nutrient sources. These blooms are dominated by N2-fixing diatom-cyanobacteria associations of the diatom genera Rhizosolenia Brightwell and Hemiaulus Ehrenberg. Their nitrogen fixing endosymbiont, Richelia intracellularis J.A. Schmidt, is hypothesized to be critical to the development of blooms in this nitrogen limited region. However, due to the remote location and unpredictable duration of the summer blooms, prolonged in situ observations are rare outside of the Station ALOHA time-series off of Hawai’i. In summer, 2015, a proof-of-concept mission using the autonomous vehicle, Honey Badger (Wave Glider SV2; Liquid Robotics, a Boeing company, Sunnyvale, CA, USA), collected near-surface (<20 m) observations in the NPSG using hydrographic, meteorological, optical, and imaging sensors designed to focus on phytoplankton abundance, distribution, and physiology of this bloom-forming region. Hemiaulus and Rhizosolenia cell abundance was determined using digital holography for the entire June–November mission. Honey Badger was not able to reach the 30°N subtropical front region where most of the satellite chl a blooms have been observed, but near-real time navigational control allowed it to transect two blooms near 25°N. The two taxa did not co-occur in large numbers, rather the blooms were dominated by either Hemiaulus or Rhizosolenia. The August 2–4, 2015 bloom was comprised of 96% Hemiaulus and the second bloom, August 15–17, 2015, was dominated by Rhizosolenia (75%). The holograms also imaged undisturbed, fragile Hemiaulus aggregates throughout the sampled area at ∼10 L−1. Aggregated Hemiaulus represented the entire observed population at times and had a widespread distribution independent of the summer export pulse, a dominant annual event suggested to be mediated by aggregate fluxes. Aggregate occurrence was not consistent with a density dependent formation mechanism and may represent a natural growth form in undisturbed conditions. The photosynthetic potential index (Fv:Fm) increased from ∼0.4 to ∼0.6 during both blooms indicating a robust, active phytoplankton community in the blooms. The diel pattern of Fv:Fm (nocturnal maximum; diurnal minimum) was consistent with macronutrient limitation throughout the mission with no evidence of Fe-limitation despite the presence of nitrogen fixing diatom-diazotroph assemblages. During the 5-month mission, Honey Badger covered ∼5,690 km (3,070 nautical miles), acquired 9,336 holograms, and reliably transmitted data onshore in near real-time. Software issues developed with the active fluorescence sensor that terminated measurements in early September. Although images were still useful at the end of the mission, fouling of the LISST-Holo optics was considerable, and appeared to be the most significant issue facing deployments of this duration.
机译：卫星叶绿素a（chl a）观测一再指出北太平洋亚热带回旋区（NPSG）夏季的浮游植物开花，这是一个远离任何陆源或埃克曼上升营养源的大洋地区。这些开花主要由硅藻属Rhizosolenia Brightwell和Hemiaulus Ehrenberg的N2固着硅藻-蓝细菌协会主导。他们的固氮内共生菌Richelia intracellularis J.A.据推测，施密特（Schmidt）对于在此氮有限区域内的水华发展至关重要。但是，由于地处偏僻，而且夏季花期难以预测，因此在夏威夷以外的ALOHA站时间序列之外，很少能进行长时间的原位观测。 2015年夏天，使用自动驾驶工具Honey Badger（Wave Glider SV2；波音公司Liquid Liquidics，波音公司，美国加利福尼亚州桑尼维尔）进行概念验证任务，收集了近地表观测（<20 m）。 NPSG使用水文，气象，光学和影像传感器设计成专注于浮游形成区域的浮游植物的丰度，分布和生理。在整个6月至11月的任务中，使用数字全息术确定了希俄罗斯和根瘤菌的细胞丰度。霍尼·ger格（Honey Badger）未能到达30°N的亚热带锋面区域，在该区域中已观察到大多数卫星chl a绽放，但近实时导航控制使其能够横切25°N附近的两次绽放。这两个分类单元并没有大量共生，相反，花朵主要由希米乌尔或根瘤菌主导。 2015年8月2日至4日的花朵由96％的希米欧里斯组成，而第二波花朵（2015年8月15日至17日）则以根瘤菌（75％）为主。全息图还以大约10 L -1 的图像在整个采样区域中成像了未受干扰的，脆弱的Hemiaulus聚集体。聚集的希米乌尔有时代表整个观察到的种群，并且分布不依赖于夏季的出口脉动，这是一个主要的年度事件，被认为是由聚集通量介导的。聚集体的出现与密度依赖性形成机制不一致，并且可能代表不受干扰条件下的自然生长形式。在两次开花期间，光合潜力指数（Fv：Fm）从约0.4增至约0.6，表明该开花中有健壮，活跃的浮游植物群落。 Fv：Fm的diel模式（夜间最大；昼夜最小）与整个任务中的常量营养素限制相一致，尽管存在固氮硅藻-氮营养盐组合，但没有Fe限制的迹象。在为期5个月的任务中， Honey Badger 覆盖了约5,690公里（3,070海里），获取了9,336幅全息图，并可靠地近岸实时传输了数据。有源荧光传感器产生了软件问题，该问题在9月初终止了测量。尽管图像在任务结束时仍然有用，但LISST-Holo光学器件的结垢相当可观，并且似乎是在此期间部署所面临的最重要问题。

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