Vertical distribution of phytoplankton pigments at EBENE cruise stations

In situ diel variations of extracted chlorophyllous pigments, beam attenuation by particles (c p), and in vivo chlorophyll fluorescence (F iv) were investigated during a 5-day time series in high-nutrient, low-chlorophyll waters of the equatorial Pacific (date line = 180°). Samples were taken hourly at 10 depths in the upper 100 m during the first 48 hours, then sampling frequency decreased to 3 hours. In the 30-70 m layer the integrated chlorophyll concentrations, c p, and F iv increased during the light period, but the minima and, especially, maxima were not fully synchronized. The lowest values of total chlorophyll a (Tchl a = chlorophyll a + divinyl-chlorophyll a) occurred around 5-6 hours, slightly (0-2 hours) before that of c p and F iv. Tchl a reached a maximum around 1500 hours ± 1 hour, clearly before c p (1700 hours) and F iv (1900 hours). In the 0-30 m layer, diel variations of the integrated chlorophyll concentrations, c p, and F iv were clearly out of phase. They showed a nocturnal increase in Tchl a, starting around midnight and peaking in early morning (0900 hours). In contrast, c p increased only during the light period in the upper 30 m, and variations of F iv were largely opposite to those of extracted Tchl a. Specific phytoplankton growth (µ0) and grazing loss (g) rates were estimated from diel variations in the 30-70 m layer and compared to independent rate estimates from experimental incubations. These results are discussed in the context of physical processes and physiological responses of the cells to the daily photocycle.

本研究于赤道太平洋国际日期变更线为180°的高营养盐低叶绿素（high-nutrient, low-chlorophyll）海域开展了为期5天的时间序列观测，对原位（in situ）提取的叶绿素类色素、颗粒光束衰减系数（beam attenuation by particles, c_p）以及活体叶绿素荧光（in vivo chlorophyll fluorescence, F_iv）的日周期变化进行了系统调查。前48小时内，研究人员在表层100米海域的10个深度层每小时采集一次样品，后续采样频率降至每3小时一次。在30~70米水层中，积分叶绿素浓度、c_p与F_iv均在光照时段出现上升，但各指标的最小值（尤其最大值）并未完全同步。总叶绿素a（total chlorophyll a, Tchl a = 叶绿素a + 二乙烯基叶绿素a）的最低值出现在5~6时左右，较c_p与F_iv的最低值略微提前0~2小时；Tchl a的峰值则出现在1500时±1小时，明显早于c_p（1700时）与F_iv（1900时）的峰值。在0~30米水层中，积分叶绿素浓度、c_p与F_iv的日周期变化明显不同步：该水层的Tchl a呈现夜间上升趋势，于午夜前后开始升高，并在清晨0900时达到峰值；与之相反，上层30米海域的c_p仅在光照时段出现上升，而F_iv的变化趋势与提取得到的Tchl a大致相反。研究人员基于30~70米水层的日周期变化估算了浮游植物比生长速率（specific phytoplankton growth, μ₀）与牧食损失速率（grazing loss, g），并将其与实验培养得到的独立速率估算结果进行了对比。本研究结合物理海洋过程以及浮游植物细胞对每日光周期的生理响应，对上述观测结果展开了讨论。