The Variability of chlorophyll-specific phytoplankton light absorption [a*_ph (λ)] was examined over depth and time in stratified offshore waters of the North-Western Mediterranean Sea. Coherent water patches were tracked with Lagrangian drifters during two oceanographic cruises in September (late summer) and May (post-spring bloom phase). By simultaneously analysing the phytoplankton absorption and pigment measurements, we explicitly separated the impact of pigment composition from that of pigment packaging on a*_ph (λ). We further partitioned the packaging effect by comparing the variation of the packaging index [Q_a*(440)], the phytoplankton community size structure (derived from diagnostic pigment analysis), and the chlorophyll-specific beam attenuation by particles [c_p* (660)] as an optical index of phytoplankton photophysiology. In the ensemble of cruises, around 50% of the a*_ph (440) vertical variation was explained by changes in the pigment composition (ruled by the decrease of photoprotective pigments with depth). The remaining vertical and inter-cruise variation of a*_ph (440) was attributed to the packaging effect. We found that differences in the c_p*(660) index (most likely indicating changes in the intracellular pigment concentration due to photoacclimation) mainly explained the observed variation in the packaging effect. Differences in c_p* (660) were coincident with either the vertical gradient of light availability or the lower mean daily PAR irradiance in the euphotic layer of the September cruise. These explained the stronger packaging and lower a*_ph (440) values observed with increasing depth in both cruises, and in September relative to May. On the other hand, differences in the phytoplankton community size structure did not explain the observed patterns in the packing effect. Our results highlight the importance of phytoplankton short-term acclimation to the prevailing light conditions, determining the vertical and temporal variability of a*_ph (λ). A better understanding of the a*_ph (λ) variability and its main drivers are key to improve different bio-optical applications.

在西北地中海的分层离岸水域中，随着深度和时间的变化，研究了叶绿素特异性浮游植物光吸收[ a * _ph（λ）]的变化。在9月（夏季末）和5月（春季后开花期）两次海洋航行中，用拉格朗日漂流器跟踪了连贯的水斑。通过同时分析所述浮游植物吸收和颜料的测量，我们明确地分开上颜料组合物的从该颜料包装的影响一个* _pH值（λ）。通过比较包装指数的变化[ Q _a*（440）]，浮游植物群落大小结构（根据诊断性色素分析得出）以及颗粒引起的叶绿素特异性束衰减[ c _p *（660）]作为浮游植物光生理学的光学指标。在巡游合奏中，a * _ph（440）垂直变化的大约50％是由颜料成分的变化（由光防护性颜料随深度的减少而决定的）解释的。的其余的垂直和帧间巡航变异一个* _pH值（440）的原因是在包装的效果。我们发现c _p的差异*（660）指数（最有可能表明由于光驯化引起的细胞内色素浓度变化）主要解释了观察到的包装效果变化。在差异Ç _p *（660）与任一的光可用性垂直梯度，或在九月巡航的透光层的较低的平均每日PAR辐照度一致。这些解释了更强的包装和降低一个* _pH值（440）值在两次巡航中以及9月相对于5月都随着深度增加而观察到。另一方面，浮游植物群落大小结构的差异不能解释堆积效应中观察到的模式。我们的结果强调了浮游植物短期适应主要光照条件的重要性，确定了* _ph（λ）的垂直和时间变异性。更好地理解a * _ph（λ）变异性及其主要驱动因素是改善不同生物光学应用的关键。