Changes in airborne high spectral resolution lidar (HSRL) measurements of scattering, depolarization, and attenuation coincided with a shift in phytoplankton community composition across an anticyclonic eddy in the North Atlantic. We normalized the total depolarization ratio (δ) by the particulate backscattering coefficient (bbp) to account for the covariance in δ and bbp that has been attributed to multiple scattering. A 15% increase in δ/bbp inside the eddy coincided with decreased phytoplankton biomass and a shift to smaller and more elongated phytoplankton cells. Taxonomic changes (reduced dinoflagellate relative abundance inside the eddy) were also observed. The δ signal is thus potentially most sensitive to changes in phytoplankton shape because neither the observed change in the particle size distribution (PSD) nor refractive index (assuming average refractive indices) are consistent with previous theoretical modeling results. We additionally calculated chlorophyll-a (Chl) concentrations from measurements of the diffuse light attenuation coefficient (Kd) and divided by bbp to evaluate another optical metric of phytoplankton community composition (Chl:bbp), which decreased by more than a factor of two inside the eddy. This case study demonstrates that the HSRL is able to detect changes in phytoplankton community composition. High spectral resolution lidar measurements reveal complex structures in both the vertical and horizontal distribution of phytoplankton in the mixed layer providing a valuable new tool to support other remote sensing techniques for studying mixed layer dynamics. Our results identify fronts at the periphery of mesoscale eddies as locations of abrupt changes in near-surface optical properties.

中文翻译：

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高光谱分辨率激光雷达散射测量揭示了反气旋涡流中浮游植物群落结构的变化

机载高光谱分辨率激光雷达 (HSRL) 散射、去极化和衰减测量值的变化与北大西洋反气旋涡流中浮游植物群落组成的变化相吻合。我们通过颗粒背向散射系数 (bbp) 对总去极化率 (δ) 进行归一化，以解释归因于多次散射的 δ 和 bbp 的协方差。涡流内 δ/bbp 增加 15% 与浮游植物生物量减少以及向更小和更细长的浮游植物细胞的转变相吻合。还观察到分类学变化（涡流内甲藻相对丰度降低）。因此，δ 信号可能对浮游植物形状的变化最敏感，因为观察到的粒度分布 (PSD) 和折射率（假设平均折射率）的变化都与之前的理论建模结果不一致。我们还根据漫射光衰减系数 (Kd) 的测量结果计算了叶绿素 a (Chl) 浓度，并除以 bbp 以评估浮游植物群落组成的另一个光学指标 (Chl:bbp)，其内部减少了两倍以上涡流。本案例研究表明 HSRL 能够检测浮游植物群落组成的变化。高光谱分辨率激光雷达测量揭示了混合层中浮游植物垂直和水平分布的复杂结构，为支持其他遥感技术研究混合层动力学提供了一种有价值的新工具。我们的结果将中尺度涡旋外围的前沿确定为近地表光学特性突然变化的位置。