Abundance and biomass of protozooplankton from the iron-induced phytoplankton bloom during the EisenEx experiment in 2000 in the Southern Ocean

The responses of larger (>50 µm in diameter) protozooplankton groups to a phytoplankton bloom induced by in situ iron fertilization (EisenEx) in the Polar Frontal Zone (PFZ) of the Southern Ocean in austral spring are presented. During the 21 days of the experiment, samples were collected from seven discrete depths in the upper 150 m inside and outside the fertilized patch for the enumeration of acantharia, foraminifera, radiolaria, heliozoa, tintinnid ciliates and aplastidic thecate dinoflagellates. Inside the patch, acantharian numbers increased twofold, but only negligibly in surrounding waters. This finding is of major interest, since acantharia are suggested to be involved in the formation of barite (BaSO_4 ) found in sediments and which is a palaeoindicator of both ancient and modern high productivity regimes. Foraminifera increased significantly in abundance inside and outside the fertilized patch. However the marked increase of juveniles after a full moon event suggests a lunar periodicity in the reproduction cycle of some foraminiferan species rather than a reproductive response to enhanced food availability. In contrast, adult radiolaria showed no clear trend during the experiment, but juveniles increased threefold indicating elevated reproduction. Aplastidic thecate dinoflagellates almost doubled in numbers and biomass, but also increased outside the patch. Tintinnid numbers decreased twofold, although biomass remained constant due to a shift in the size spectrum. Empty tintinnid loricae, however, increased by a factor of two indicating that grazing pressure on this group mainly by copepods intensified during EisenEx. The results show that iron-fertilization experiments can shed light on the biology and the role of these larger protists in pelagic ecosystem which will improve their use as proxies in palaeoceanography.

本研究报道了南半球春季，南大洋极锋带（Polar Frontal Zone, PFZ）内通过原位铁添加实验（in situ iron fertilization, EisenEx）诱发浮游植物水华后，直径大于50 μm的大型原生浮游动物类群的响应特征。实验周期共计21天，于施肥斑块内部及周边海域上层150米的7个离散深度采集海水样本，用于计数棘虫类（Acantharia）、有孔虫类（Foraminifera）、放射虫类（Radiolaria）、太阳虫类（Heliozoa）、丁丁虫类纤毛虫（Tintinnid Ciliates）以及无质体甲藻（Aplastidic Thecate Dinoflagellates）。施肥斑块内棘虫类的数量增长了一倍，但周边海域仅出现极微弱的数量变化。该发现具有重要科研价值，因为已有研究表明棘虫类参与了沉积物中重晶石（Barite, BaSO₄）的形成，而重晶石是古今高生产力生态系统的古环境代用指标。有孔虫类的丰度在施肥斑块内外均出现显著提升。但满月事件后幼体数量的显著增长表明，部分有孔虫物种的繁殖周期存在月相周期性，而非对食物可获得性提升的繁殖响应。与之相反，成年放射虫在实验期间未表现出明确的变化趋势，但幼体数量增长了三倍，表明其繁殖活动增强。无质体甲藻的数量与生物量几乎翻倍，且该类群在斑块外的丰度同样有所提升。丁丁虫类纤毛虫的数量下降至初始水平的一半，但由于其个体大小组成发生偏移，生物量保持稳定。不过，空的丁丁虫壳（Tintinnid Loricae）数量增加了一倍，表明在EisenEx实验期间，桡足类（Copepods）对该类群的牧食压力有所增强。本研究结果表明，铁添加实验有助于阐明这些大型原生生物在远洋生态系统中的生物学特性与生态功能，进而提升其作为古海洋学（Palaeoceanography）代用指标的应用价值。