Marine Science Institute and the Department of Marine Science, The University of Texas at Austin, Port Aransas, Texas, United States of America.
PLoS One. 2012;7(4):e33109. doi: 10.1371/journal.pone.0033109. Epub 2012 Apr 6.
The summertime North Pacific subtropical gyre has widespread phytoplankton blooms between Hawaii and the subtropical front (∼30°N) that appear as chlorophyll (chl) increases in satellite ocean color data. Nitrogen-fixing diatom symbioses (diatom-diazotroph associations: DDAs) often increase 10(2)-10(3) fold in these blooms and contribute to elevated export flux. In 2008 and 2009, two cruises targeted satellite chlorophyll blooms to examine DDA species abundance, chlorophyll concentration, biogenic silica concentration, and hydrography. Generalized observations that DDA blooms occur when the mixed layer depth is < 70 m are supported, but there is no consistent relationship between mixed layer depth, bloom intensity, or composition; regional blooms between 22-34°N occur within a broader temperature range (21-26°C) than previously reported. In both years, the Hemiaulus-Richelia and Rhizosolenia-Richelia DDAs increased 10(2)-10(3) over background concentrations within satellite-defined bloom features. The two years share a common trend of Hemiaulus dominance of the DDAs and substantial increases in the >10 µm chl a fraction (∼40-90+% of total chl a). Integrated diatom abundance varied 10-fold over <10 km. Biogenic silica concentration tracked diatom abundance, was dominated by the >10 µm size fraction, and increased up to 5-fold in the blooms. The two years differed in the magnitude of the surface chl a increase (2009>2008), the abundance of pennate diatoms within the bloom (2009>2008), and the substantially greater mixed layer depth in 2009. Only the 2009 bloom had sufficient chl a in the >10 µm fraction to produce the observed ocean color chl increase. Blooms had high spatial variability; ocean color images likely average over numerous small events over time and space scales that exceed the individual event scale. Summertime DDA export flux noted at the Hawaii time-series Sta. ALOHA is probably a generalized feature of the eastern N. Pacific north to the subtropical front.
夏季北太平洋副热带环流带在夏威夷和亚热带锋（约 30°N）之间有广泛的浮游植物水华，卫星海洋颜色数据中叶绿素（chl）的增加表明了这一点。固氮硅藻共生体（硅藻-固氮菌共生体：DDAs）在这些水华期间通常增加 10（2）-10（3）倍，并促进了更高的输出通量。在 2008 年和 2009 年，两次巡航以卫星叶绿素水华为目标，以检查 DDA 物种丰度、叶绿素浓度、生物硅浓度和水文学。支持 DDA 水华发生时混合层深度<70 m 的普遍观察结果，但混合层深度、水华强度或组成之间没有一致的关系；22-34°N 之间的区域水华发生在更宽的温度范围内（21-26°C），而不是以前报道的。在这两年中，Hemiaulus-Richelia 和 Rhizosolenia-Richelia DDAs 在卫星定义的水华特征内，比背景浓度增加了 10（2）-10（3）倍。这两年有一个共同的趋势，即 Hemiaulus 主导 DDAs，并且>10 µm 的 chl a 分数（约 40-90+%的总 chl a）大幅增加。硅藻丰度在<10 km 范围内变化了 10 倍。生物硅浓度与硅藻丰度相关，主要由>10 µm 大小的部分主导，在水华期间增加了 5 倍。这两年在表面 chl a 增加的幅度（2009>2008）、水华内的羽纹硅藻丰度（2009>2008）以及 2009 年混合层深度显著增加方面有所不同。只有 2009 年的水华中有足够的>10 µm 部分的 chl a 产生了所观察到的海洋颜色 chl 增加。水华具有很高的空间变异性；海洋颜色图像可能平均了随着时间和空间尺度的增加，超过了单个事件的规模。夏威夷时间序列 Sta. ALOHA 记录的夏季 DDA 输出通量可能是北太平洋东部到亚热带锋的普遍特征。
