Table_1_Phytoplankton Community Structure Is Driven by Stratification in the Oligotrophic Mediterranean Sea.docx

The phytoplankton community composition, structure, and biomass were investigated under stratified and oligotrophic conditions during summer for three consecutive years in the Mediterranean Sea. Our results reveal that the phytoplankton community structure was strongly influenced by vertical stratification. The thermocline separated two different phytoplankton communities in the two layers of the euphotic zone, characterized by different nutrient and light availability. Picoplankton dominated in terms of abundance and biomass at all the stations sampled and throughout the photic zone. However, the structure of the picoplanktonic community changed with depth, with Synechococcus and heterotrophic prokaryotes dominating in surface waters down to the base of the thermocline, and Prochlorococcus and picoeukaryotes contributing relatively more to the community in the deep chlorophyll maximum (DCM). Light and nutrient availability also influenced the communities at the DCM layer. Prochlorococcus prevailed in deeper DCM waters characterized by lower light intensities and higher picophytoplankton abundance was related to lower nutrient concentrations at the DCM. Picoeukaryotes were the major phytoplankton contributors to carbon biomass at surface (up to 80%) and at DCM (more than 40%). Besides, contrarily to the other phytoplankton groups, picoeukaryotes cell size progressively decreased with depth. Our research shows that stratification is a major factor determining the phytoplankton community structure; and underlines the role that picoeukaryotes might play in the carbon flux through the marine food web, with implications for the community metabolism and carbon fate in the ecosystem.

本研究对地中海夏季连续三年中，分层及寡营养条件下浮游植物群落组成、结构和生物量进行了调查研究。研究发现，浮游植物群落结构深受垂直分层的影响。热跃层将表光层分为两个不同的浮游植物群落，这两个群落因营养和光照条件的差异而呈现出不同的特征。在所有采样站点及整个光区中，微型浮游植物在数量和生物量上均占主导地位。然而，微型浮游植物群落的结构随深度变化而变化，其中蓝藻和异养原核生物在热跃层以下的水体中占据主导地位，而拟杆菌和微型真核生物在深层叶绿素最大值层（DCM）对群落贡献相对较大。光和营养的可用性也影响了DCM层的群落。在光强度较低、微型浮游植物丰度较高的深层DCM水域中，拟杆菌占主导地位；而微型真核生物在DCM层对碳生物量的贡献尤为显著（表面可达80%，DCM层超过40%）。此外，与其它浮游植物群体不同，微型真核生物的细胞大小随深度逐渐减小。本研究揭示了分层是决定浮游植物群落结构的主要因素；并强调了微型真核生物可能在海洋食物网碳通量中发挥的作用，这对群落代谢和生态系统中的碳命运具有重要意义。