Biomass of nanoprotozooplankton in the Atlantic sector of the Southern Ocean

The dynamic of early spring nanoprotozoa was investigated in three characteristic water masses of the Southern Ocean: the Marginal Ice Zone, the intermediate waters of the Antarctic Circumpolar Current and the Polar Frontal Zone. Biomass and feeding activities of nanoprotozoa were measured, as well as the biomass of their potential prey-bacteria and phototrophic flagellates-on the 6°W meridian in the Southern Ocean along three repetitive transects between 47 and 60° South from October to November 1992. On average, nanoprotozooplankton biomass accounted for 77% of the combined biomass of bacteria and phototrophic flagellates, and was dominated by dinoflagellates and flagellates smaller than 5 µm. As a general trend, low protozoan biomass of 2 mg C/m**3 was typical of the ice covered area, while significantly higher biomasses culminating at 15 mg C/m**3 were recorded at the Polar Front. Biomasses of bacteria and total phytoplankton were distributed accordingly, with larger values at the Polar Front. Phototrophic flagellates did not show any geographical trend. No seasonal trend could be identified in the Marginal Ice Zone and in the intermediate waters of the Antarctic Circumpolar Current. On the other hand, at the Polar Front region a three-fold increase was observed within a 2-month period for nanoprotozooplankton biomass. Such a biomass increase was also detected for bacterioplankton and total phytoplankton biomass. Half-saturation constants and maximum specific ingestion of nanoprotozoan taxons feeding on bacteria and phototrophic flagellates were determined using the technique of fluorescent labelled bacteria (FLB) and algae (FLA) over a large range of prey concentrations. Maximum ingestion rates ranged between 0.002 and 0.015/h for bactivorous nanoprotozoa and heterotrophic flagellates larger than 5 µm feeding on phototrophic flagellates. The markedly high maximum ingestion rates of 0.4/h characterising nanophytoplankton ingestion by dinoflagellates evidenced the strong ability of dinoflagellates for feeding on nanophytoplankton. Daily ingestion rates were calculated from nanoprotozoan grazing parameters and carbon biomass of prey and predators. This indicated that nanoprotozoa ingestion of daily bacterioplankton and phytoplankton production in early spring ranged from 32 to 40%.本研究针对南大洋三类典型水团——边缘冰区（Marginal Ice Zone）、南极绕极流（Antarctic Circumpolar Current）中层水以及极锋区（Polar Frontal Zone）内早春时节纳米原生动物（nanoprotozoa）的动态变化展开调查。1992年10月至11月期间，研究团队沿南大洋6°W经线，在南纬47°至60°之间设置三条重复样带，测定了纳米原生动物的生物量与摄食活性，同时测定了其潜在猎物——细菌与光合鞭毛虫（phototrophic flagellates）的生物量。平均而言，纳米原生动物浮游生物（nanoprotozooplankton）的生物量占细菌与光合鞭毛虫总生物量的77%，其类群以甲藻（dinoflagellates）和粒径小于5μm的鞭毛虫为主。整体趋势显示，覆冰区域的原生动物生物量普遍较低，仅为2 mg C/m³；而极锋（Polar Front）区域的生物量显著更高，峰值可达15 mg C/m³。细菌与总浮游植物（phytoplankton）的生物量分布与此一致，极锋区域数值更高。光合鞭毛虫未表现出明显的地理分布趋势。在边缘冰区与南极绕极流中层水中，未观测到季节变化趋势；而在极锋区域，纳米原生动物浮游生物的生物量在两个月内增长了三倍。浮游细菌（bacterioplankton）与总浮游植物的生物量也呈现出类似的增长趋势。 研究团队通过荧光标记细菌（FLB, fluorescent labelled bacteria）与荧光标记藻类（FLA, fluorescent labelled algae）技术，在大范围猎物浓度梯度下，测定了以细菌和光合鞭毛虫为食的纳米原生动物类群的半饱和常数（Half-saturation constants）与最大比摄食率。食菌纳米原生动物（bactivorous nanoprotozoa）以及以光合鞭毛虫为食的粒径大于5μm的异养鞭毛虫（heterotrophic flagellates）的最大摄食率介于0.002/h至0.015/h之间。甲藻摄食微型浮游植物（nanophytoplankton）的最大摄食率可达0.4/h，这一显著偏高的摄食率证实了甲藻具备极强的微型浮游植物摄食能力。 研究基于纳米原生动物的摄食参数（grazing parameters）以及猎物与捕食者的碳生物量，计算得到日摄食率。结果表明，早春时节纳米原生动物对每日浮游细菌与浮游植物生产量的摄食占比为32%至40%。