Phytol and phytyldiol concentrations at DYFAMED time series station and sediment trap

Particulate samples from the water column were collected monthly from depths of 5-150 m, between May 1996 and March 1997, in the northwestern Mediterranean Sea (Ligurian Sea) as part of the DYFAMED project within the French JGOFS program. These samples were analyzed by gas chromatography-electron impact mass spectrometry for their phytol and 3-methylidene-3,7,11-trimethylhexadecan-1,2-diol (phytyldiol) content. The corresponding Chlorophyll Phytyl side chain Photodegradation Index, molar ratio of phytyldiol to phytol, was calculated and the mean amount of chlorophyll photodegraded within the euphotic zone estimated. Seasonal differences in the chlorophyll photodegradation process appear in the one-year study. The chlorophyll appeared more photodegraded in the surface water (generally more than 40% photodegraded at 5-10 m) than at the deep chlorophyll maximum (DCM) (40-50 m) observed in the summer stratified waters (about 20% photodegraded). This difference was attributed to the healthy state of the phytoplankton community (coincidence with the highest primary production levels) and to the lower intensity of irradiance at the DCM level. On the other hand, the bulk of the detrital chlorophyll (chlorophyll associated with phytodetritus, phaeopigments) undergoes photodegradation before it sinks out of the photic zone. However, in January (winter mixed water) the pigments exported towards the sea floor were less photodegraded. This is thought to result from a shorter period of residence of the pigments in the photic zone due to vertical convection and grazing activity of macrozooplankton (salps), which are producers of rapid sinking fecal pellets.

1996年5月至1997年3月期间，作为法国全球海洋通量联合研究（JGOFS）计划旗下DYFAMED项目的组成部分，研究团队于地中海西北部利古里亚海海域，按月采集了5至150米水深的水柱颗粒样品。采用气相色谱-电子轰击质谱法（gas chromatography-electron impact mass spectrometry）对样品中的植醇（phytol）与3-亚甲基-3,7,11-三甲基十六烷-1,2-二醇（phytyldiol，植烷二醇）含量进行分析。据此计算得到叶绿素植醇侧链光降解指数（即植烷二醇与植醇的摩尔比），并估算了真光层内发生光降解的叶绿素平均总量。

为期一年的研究显示，叶绿素光降解过程存在显著季节差异：表层水体（通常在5-10米水深处光降解率超过40%）中的叶绿素光降解程度，高于夏季层化水体中观测到的深层叶绿素最大值层（Deep Chlorophyll Maximum, DCM，40-50米水深，光降解率约20%）。该差异被归因于浮游植物群落的健康状态（与最高初级生产力水平相契合），以及深层叶绿素最大值层处的辐照强度更低。

此外，大部分碎屑态叶绿素（即附着于浮游碎屑的叶绿素及脱镁色素）会在从透光层沉降前发生光降解。但在1月的冬季混合水期，向海底输出的色素光降解程度相对更低。这一现象被认为是由于垂直对流活动与大型浮游动物（被囊动物）的摄食活动缩短了色素在透光层中的停留时间——被囊动物可产生快速沉降的粪球颗粒。