Coccolithophore size, abundance and calcification across Drake Passage (Southern Ocean), 2009@en

Although coccolithophores are not as numerically common or as diverse in the Southern Ocean as they are in subpolar waters of the North Atlantic, a few species, such as Emiliania huxleyi, are found during the summer months. Little is actually known about the calcite production (CP) of these communities or how their distribution and physiology relate to environmental variables in this region. In February 2009, we made observations across Drake Passage (between South America and the Antarctic Peninsula) of coccolithophore distribution, CP, primary production, chlorophyll a and macronutrient concentrations, irradiance and carbonate chemistry. Although CP represented less than 1 % of total carbon fixation, coccolithophores were widespread across Drake Passage. The B/C morphotype of E. huxleyi was the dominant coccolithophore, with low estimates of coccolith calcite (~0.01 pmol C coccolith-1) from biometric measurements. Both cell-normalised calcification (0.01-0.16 pmol C cell-1 d-1) and total CP (< 20 µmol C m-3 d-1) were much lower than those observed in the subpolar North Atlantic where E. huxleyi morphotype A is dominant. However, estimates of coccolith production rates were similar (0.1 1.2 coccoliths cell-1 h-1) to previous measurements made in the subpolar North Atlantic. A multivariate statistical approach found that temperature and irradiance together were best able to explain the observed variation in species distribution and abundance (Spearman's rank correlation p = 0.4, p < 0.01). Rates of calcification per cell and coccolith production, as well as community CP and E. huxleyi abundance, were all positively correlated (p < 0.05) to the strong latitudinal gradient in temperature, irradiance and calcite saturation states across Drake Passage. Broadly, our results lend support to recent suggestions that coccolithophores, especially E. huxleyi, are advancing polewards. However, our in situ observations indicate that this may owe more to sea-surface warming and increasing irradiance rather than increasing CO2 concentrations.尽管颗石藻（coccolithophores）在南大洋的数量丰度与物种多样性均不及北大西洋亚极地海域，但夏季仍可观测到赫氏颗石藻（Emiliania huxleyi）等少数物种的存在。对于该海域颗石藻群落的方解石生产（calcite production, CP），以及其分布与生理特征如何与该区域环境变量的关联机制，目前仍所知甚少。2009年2月，我们对南美洲与南极半岛之间的德雷克海峡（Drake Passage）开展了原位观测，测定了颗石藻的分布、方解石生产速率、初级生产力、叶绿素a（chlorophyll a）与大量营养盐浓度、辐照度（irradiance）及碳酸盐化学（carbonate chemistry）参数。尽管方解石生产仅占总碳固定（carbon fixation）总量的1%以下，但颗石藻在德雷克海峡全域广泛分布。赫氏颗石藻的B/C型形态（morphotype）为该区域的优势类群，通过生物计量测量（biometric measurements）估算得到的颗石藻方解石含量约为0.01 pmol C·颗石藻⁻¹。单位细胞钙化速率（cell-normalised calcification，0.01~0.16 pmol C·细胞⁻¹·d⁻¹）与总方解石生产速率（<20 µmol C·m⁻³·d⁻¹）均远低于以赫氏颗石藻A型形态为优势类群的北大西洋亚极地海域的观测值。不过，颗石藻生产速率的估算值（0.1~1.2 颗石藻·细胞⁻¹·h⁻¹）与此前在北大西洋亚极地海域获得的测量结果相近。多元统计方法（multivariate statistical approach）分析显示，温度与辐照度的组合能够最佳解释观测到的物种分布与丰度变化（斯皮尔曼等级相关Spearman's rank correlation，p=0.4，p<0.01）。单位细胞钙化速率、颗石藻生产速率、群落总方解石生产速率以及赫氏颗石藻的丰度，均与德雷克海峡沿线显著的温度、辐照度及方解石饱和状态（calcite saturation states）纬度梯度（latitudinal gradient）呈显著正相关（p<0.05）。总体而言，本研究结果支持近期提出的假说：颗石藻，尤其是赫氏颗石藻，正在向极地海域扩张。不过我们的原位观测结果表明，这一现象更可能由海表增温（sea-surface warming）和辐照度提升所驱动，而非二氧化碳浓度（CO2 concentrations）升高。