Abundance of microzooplankton determined during the EisenEx cruise ANT-XVIII/2 to the South Atlantic

The dynamics, composition and grazing impact of microzooplankton were studied during the in situ iron fertilisation experiment EisenEx in the Antarctic Polar Frontal Zone in austral spring (November 2000). During the 21 day experiment, protozooplankton and small metazooplankton were sampled from the mixed layer inside and outside the patch using Niskin bottles. Aplastidic dinoflagellates increased threefold in abundance and biomass in the first 10 d of the experiment, but decreased thereafter to values twofold higher than pre-fertilisation values. The decline after day 10 is attributed to increasing grazing pressure by copepods. They also constrained ciliate abundances and biomass which were higher inside the fertilised patch than outside but highly variable. Copepod nauplii abundance also remained stable whereas biomass doubled. Numbers of copepodites and adults of small copepod species increased threefold inside the patch, but doubled in surrounding waters. Grazing rates estimated using the dilution method suggest that microzooplankton grazing constrained pico- and nanoplankton populations, but species capable of feeding on large diatoms (dinoflagellates and small copepods including possibly nauplii) were selectively predated by the metazoan community. Thus, iron fertilisation of a developing spring phytoplankton assemblage resulted in a trophic cascade which favoured dominance of the bloom by large diatoms.

本研究于2000年11月（南半球春季），在南极极锋区（Antarctic Polar Frontal Zone）开展的原位铁肥实验EisenEx期间，针对微型浮游动物（microzooplankton）的种群动态、群落组成及其牧食效应展开了调查。实验周期为21天，研究人员采用尼斯金采水器（Niskin bottles）在施肥斑块内外的混合层中，采集了原生浮游动物（protozooplankton）与小型后生浮游动物（metazooplankton）样品。无质体甲藻（Aplastidic dinoflagellates）的丰度与生物量在实验前10天增长至初始值的3倍，但后续下降至施肥前水平的2倍。实验第10天后的种群衰减被归因于桡足类（copepods）牧食压力的升高；桡足类同时也对纤毛虫（ciliate）的丰度与生物量形成了约束——施肥斑块内的纤毛虫种群丰度与生物量高于斑块外，但波动幅度极大。桡足类无节幼体（copepod nauplii）的丰度保持稳定，但其生物量增长至初始值的2倍。小型桡足类物种的桡足类幼体与成体数量在施肥斑块内增长至初始值的3倍，而在周边水域增长至初始值的2倍。采用稀释法（dilution method）估算的牧食速率表明，微型浮游动物的牧食作用对皮微浮游生物（pico-）与纳米浮游生物（nanoplankton）种群形成了约束，但能够摄食大型硅藻（diatoms）的类群（如无质体甲藻，以及包括可能的无节幼体在内的小型桡足类）则被后生浮游动物群落选择性捕食。综上，针对正在发育的春季浮游植物群落进行铁肥添加，引发了营养级联效应（trophic cascade），最终使得大型硅藻成为藻华的优势类群。