(Tables 2) The bPSi:POC ratio of the total diatom community, and abundance of full, empty, and broken diatom cells compared to heavily silicified diatoms during the EIFEX experiment

During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms >10 µm were the main beneficiaries of iron fertilization, the growth of small diatoms (2-8 mm) was also enhanced, leading to a shift from a haptophyte- to a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton (>20 µm) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2-20 µm) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups.

本研究在南大洋开展的欧洲铁肥实验（European Iron Fertilisation Experiment, EIFEX）期间，针对不同浮游植物粒径类群对铁施肥的响应展开了系统性调查，采用分级粒径色素测量、颗粒有机质分析、显微镜观测及流式细胞术等多种实验手段。施肥后至第26天，20米水深处的叶绿素a（Chlorophyll a, Chl a）浓度增幅超过5倍；而颗粒有机碳（Particulate Organic Carbon, POC）、颗粒有机氮（Particulate Organic Nitrogen, PON）与颗粒有机磷（Particulate Organic Phosphorus, POP）的浓度则在至第29天时大致翻倍。实验后期，叶绿素a与颗粒有机质的浓度逐步下降，表明铁诱导形成的浮游植物水华已然消亡。尽管实验末期总硅藻生物量有所降低，但因重度硅化硅藻的相对占比提升，生物成因颗粒硅酸盐（biogenic particulate silicate, bPSi）的浓度持续攀升。虽然粒径大于10微米的硅藻是铁施肥的主要受益类群，但小型硅藻（2-8毫米）的生长也得到了促进，使得该粒径类群从定鞭金藻主导的群落转变为以硅藻为主的群落。总生物量的碳氮比（C:N）、氮磷比（N:P）及碳磷比（C:P）均低于雷德菲尔德比值（Redfield），但在铁施肥后未呈现显著变化趋势，这一现象掩盖了不同粒径类群元素组成的各类改变。微型浮游生物（粒径>20微米）的C:N比持续降低，而N:P比与C:P比则不断升高，该变化可能由氮吸收增强以及细胞内磷库的消耗所引发。纳米浮游生物（粒径2-20微米）的C:N比基本保持稳定，而N:P比与C:P比则呈下降趋势。本研究结果表明，后者的元素比例变化源于分类类群组成的转变。