Surface water properties, phytoplankton composition and photosynthesis rates of Amundsen Sea sites

The phytoplankton community composition and productivity in waters of the Amundsen Sea and surrounding sea ice zone were characterized with respect to iron (Fe) input from melting glaciers. High Fe input from glaciers such as the Pine Island Glacier, and the Dotson and Crosson ice shelves resulted in dense phytoplankton blooms in surface waters of Pine Island Bay, Pine Island Polynya, and Amundsen Polynya. Phytoplankton biomass distribution was the opposite of the distribution of dissolved Fe (DFe), confirming the uptake of glacial DFe in surface waters by phytoplankton. Phytoplankton biomass in the polynyas ranged from 0.6 to 14 µg Chl a / L, with lower biomass at glacier sites where strong upwelling of Modified Circumpolar Deep Water from beneath glacier tongues was observed. Phytoplankton blooms in the polynyas were dominated by the haptophyte Phaeocystis antarctica, whereas the phytoplankton community in the sea ice zone was a mix of P. antarctica and diatoms, resembling the species distribution in the Ross Sea. Water column productivity based on photosynthesis versus irradiance characteristics averaged 3.00 g C /m**2/d in polynya sites, which was approximately twice as high as in the sea ice zone. The highest water column productivity was observed in the Pine Island Polynya, where both thermally and salinity stratified waters resulted in a shallow surface mixed layer with high phytoplankton biomass. In contrast, new production based on NO3 uptake was similar between different polynya sites, where a deeper UML in the weakly, thermally stratified Pine Island Bay resulted in deeper NO3 removal, thereby offsetting the lower productivity at the surface. These are the first in situ observations that confirm satellite observations of high phytoplankton biomass and productivity in the Amundsen Sea. Moreover, the high phytoplankton productivity as a result of glacial input of DFe is the first evidence that melting glaciers have the potential to increase phytoplankton productivity and thereby CO2 uptake, resulting in a small negative feedback to anthropogenic CO2 emissions.

本研究针对冰川融解输入的铁（Fe），对阿蒙森海（Amundsen Sea）及其周边海冰区的浮游植物群落组成与生产力开展了特征表征。派恩岛冰川、多森冰架与克罗松冰架等冰川的高铁输入，使得派恩岛湾、派恩岛冰间湖以及阿蒙森冰间湖的表层水域形成了高密度浮游植物水华。浮游植物生物量的分布与溶解态铁（DFe，dissolved Fe）的分布呈显著负相关，证实了浮游植物在表层水域摄取了冰川来源的溶解态铁。冰间湖内的浮游植物生物量范围为0.6~14 µg 叶绿素a（Chl a）/L，而在冰川点位处生物量较低——此处观测到冰川舌下方的改性绕极深层水（Modified Circumpolar Deep Water）发生强烈上升流。冰间湖内的浮游植物水华以定鞭金藻门（haptophyte）的南极海囊藻（Phaeocystis antarctica）为优势类群，而海冰区的浮游植物群落则由南极海囊藻与硅藻（diatoms）混合组成，其物种分布特征与罗斯海（Ross Sea）相似。基于光合-辐照度特性计算的水柱生产力，在冰间湖点位的平均值为3.00 g C /m²/d，约为海冰区的两倍。派恩岛冰间湖的水柱生产力最高，该处水体兼具温度与盐度分层，形成了表层混合层较浅且浮游植物生物量较高的环境。与之相对，基于硝酸盐（NO3）摄取的新生产在不同冰间湖点位间差异不大：在热分层较弱的派恩岛湾，更深的上层混合层（UML）使得硝酸盐被移除的深度更深，从而抵消了表层较低的生产力。本研究是首次通过原位观测证实卫星观测到的阿蒙森海高浮游植物生物量与生产力的成果。此外，由冰川输入溶解态铁所带来的高浮游植物生产力，首次证明了冰川融解有潜力提升浮游植物生产力并进而增强二氧化碳吸收，由此对人为二氧化碳排放形成微弱的负反馈效应。