Particle flux characterisation and sedimentation patterns of protistan plankton during the iron fertilisation experiment LOHAFEX in the Southern Ocean

The taxonomic composition and types of particles comprising the downward particle flux were examined during the mesoscale artificial iron fertilisation experiment LOHAFEX. The experiment was conducted in low-silicate waters of the Atlantic Sector of the Southern Ocean during austral summer (January-March 2009), and induced a bloom dominated by small flagellates. Downward particle flux was low throughout the experiment, and not enhanced by addition of iron; neutrally buoyant sediment traps contained mostly faecal pellets and faecal material apparently reprocessed by mesozooplankton. TEP fluxes were low, <5 mg GX eq/m**2/day, and a few phytodetrital aggregates were found in the sediment traps. Only a few per cent of the POC flux was found in the traps consisting of intact protist plankton, although remains of taxa with hard body parts (diatoms, tintinnids, thecate dinoflagellates and foraminifera) were numerous, far more so than intact specimens of these taxa. Nevertheless, many small flagellates and coccoid cells, belonging to the pico- and nanoplankton, were found in the traps, and these small, soft-bodied cells probably contributed the majority of downward POC flux via mesozooplankton grazing and faecal pellet export. TEP likely played an important role by aggregating these small cells, and making them more readily available to mesozooplankton grazers.

本研究针对中尺度人工铁施肥实验LOHAFEX期间的下行颗粒通量的分类组成与颗粒类型展开了分析。该实验于2009年南半球夏季（1月至3月）在南大洋大西洋扇区的低硅酸盐水体中开展，诱导形成了以小型鞭毛虫为优势类群的水华。实验全程下行颗粒通量水平较低，且未因铁肥投加而显著提升；中性浮力沉积物捕集器内主要收集到粪球粒以及明显经中型浮游动物再加工的粪便物质。透明胞外聚合物（Transparent Exopolymer Particles, TEP）通量偏低，不足5 mg GX 当量/（平方米·天），且沉积物捕集器中仅发现少量植物碎屑聚集体。完整原生生物浮游生物仅占颗粒有机碳（Particulate Organic Carbon, POC）通量的极小比例；尽管具有硬质躯体结构的类群（硅藻、砂壳纤毛虫、具壳甲甲藻及有孔虫）的残骸数量众多，远多于该类群的完整个体。尽管如此，捕集器中仍发现了大量隶属于皮微浮游生物（pico-plankton）与纳米浮游生物（nanoplankton）的小型鞭毛虫及球状细胞；这些小型软躯体细胞大概率通过中型浮游动物的摄食与粪球粒输出途径，贡献了下行POC通量的主体部分。TEP可能通过聚集这些小型细胞，使其更易被中型浮游动物摄食，从而发挥了重要作用。