Summer Diatom Blooms in the North Pacific Subtropical Gyre: 2008–2009

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 102–103 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 102–103 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.

北太平洋亚热带环流的夏季时段内，夏威夷与亚热带锋面（约30°N）之间存在广泛的浮游植物水华，这类水华会在卫星海洋色遥感数据中表现为叶绿素（chlorophyll, chl）浓度升高。固氮硅藻共生体（硅藻-固氮菌共生体，diatom-diazotroph associations，缩写DDAs）在这类水华中的丰度通常会提升10²~10³倍，并会推动输出通量上升。2008年与2009年，两项针对卫星叶绿素水华的科考航次，旨在调查DDAs的物种丰度、叶绿素浓度、生物硅（biogenic silica）浓度与海洋水文状况。学界普遍认为，当混合层深度（mixed layer depth）小于70米时会出现DDAs水华，本研究结果支持这一普遍观测结论，但混合层深度与水华强度、群落组成之间并不存在稳定的关联；北纬22°至34°之间的区域性水华，其发生的适宜温度区间（21~26℃）较此前报道的范围更广。在这两年的卫星划定的水华区域内，半管藻-列氏藻（Hemiaulus-Richelia）与根管藻-列氏藻（Rhizosolenia-Richelia）这两类DDAs的丰度较背景值提升了10²~10³倍。两年的观测均呈现出共同趋势：DDAs群落以半管藻属为主导，且粒径大于10微米的叶绿素a（chlorophyll a, chl a）组分占比大幅提升（约占总叶绿素a的40%~90%以上）。在小于10公里的空间尺度内，硅藻总丰度的变异幅度可达10倍。生物硅浓度与硅藻丰度的变化趋势高度一致，且主要由粒径大于10微米的组分构成，在水华区域内生物硅浓度最高可提升5倍。两年的观测结果存在显著差异：2009年的表层叶绿素a升高幅度大于2008年、水华区域内羽纹硅藻（pennate diatoms）的丰度高于2008年，且2009年的混合层深度显著更大。仅2009年的水华，其粒径大于10微米的叶绿素a组分浓度足够高，足以产生卫星海洋色遥感观测到的叶绿素升高信号。水华具有显著的空间异质性；卫星海洋色遥感图像实际上是对多个小型水华事件在超过单个水华事件尺度的时空范围内的平均结果。夏威夷长期观测站ALOHA（Sta. ALOHA）记录的夏季DDAs输出通量，可能是北太平洋东部海域向北延伸至亚热带锋面的普遍特征。