Table_2_Tidewater glaciers as “climate refugia” for zooplankton-dependent food web in Kongsfjorden, Svalbard.docx

With climate warming, many tidewater glaciers are retreating. Fresh, sediment-rich sub-glacial meltwater is discharged at the glacier grounding line, where it mixes with deep marine water resulting in an upwelling of a plume visible in front of the glacial wall. Zooplankton may suffer increased mortality within the plume due to osmotic shock when brought in contact with the rising meltwater. The constant replenishment of zooplankton and juvenile fish to the surface areas attracts surface-foraging seabirds. Because access to other feeding areas, such as the marginal ice zone, has become energetically costly due to reduced sea-ice extent, glacial plumes may become increasingly important as “climate refugia” providing enhanced prey availability. Here, we investigated zooplankton concentrations within the plume and adjacent waters of four tidewater glaciers in Kongsfjorden, Svalbard, in early August 2016 and late July 2017. Our aim was to compare the zooplankton composition, abundance, and isotopic signatures within the plumes to those in adjacent fjord and shelf waters. Our hypothesis was that the plumes resulted in increased zooplankton mortality through osmotic shock and increased prey availability to predators. The mortality due to osmotic shock in the glacial plume was low (<5% dead organisms in samples), although slightly higher than in surrounding waters. This indicates that plumes are inefficient “death traps” for zooplankton. However, the high abundance and biomass of zooplankton within plume areas suggest that the “elevator effect” of rising glacial water supplies zooplankton to the sea surface, thereby enhancing prey availability for surface-feeding seabirds. Thus, our study provides evidence that glacial plumes are important as “climate refugia” for foraging seabirds. Stable isotope signatures showed that the glacial bay zooplankton and fish community represent a distinct isotopic niche. Additionally, zooplankton mortality associated with the plume estimated over 100-days of melt season supports a flux of 12.8 tonnes of organic carbon to benthic communities in the glacial bays. Benthic scavengers, such as Onisimus caricus and Anonyx nugax, were abundant in the glacial bay, where they feed on sinking organic matter.

随着气候变暖，众多潮间带冰川（tidewater glaciers）正逐渐退缩。富含沉积物的新鲜冰下融水于冰川接地带（glacier grounding line）排放，与深层海水混合后形成上升羽流（plume），可在冰壁前方清晰观测到。浮游动物（zooplankton）接触上升融水后，可能因渗透休克（osmotic shock）导致死亡率升高。浮游动物与幼鱼持续向表层水域汇聚，吸引了表层觅食海鸟（surface-foraging seabirds）。由于海冰范围缩减，前往边缘冰区（marginal ice zone）等其他觅食区域的能耗大幅升高，冰源羽流作为“气候避难所（climate refugia）”，可为捕食者提供更充足的猎物，其重要性或日益凸显。本研究于2016年8月初与2017年7月末，在斯瓦尔巴群岛（Svalbard）的孔斯峡湾（Kongsfjorden）对4处潮间带冰川的羽流及邻近水域的浮游动物浓度展开调查。研究目的为对比羽流内与峡湾及陆架（shelf）邻近水域的浮游动物组成、丰度与同位素特征（isotopic signatures）。本研究假设：冰源羽流会通过渗透休克提升浮游动物死亡率，并为捕食者提供更充足的猎物。冰源羽流中由渗透休克导致的死亡率较低（样本中死亡个体占比<5%），但仍略高于周边水域。这表明羽流并非浮游动物的高效“死亡陷阱”。但羽流区域内浮游动物的高丰度与生物量表明，冰川融水上升的“电梯效应”可将浮游动物输送至海面，进而提升表层觅食海鸟的猎物可获得性。因此，本研究证实冰源羽流可作为觅食海鸟的重要“气候避难所”。稳定同位素特征分析显示，冰川海湾内的浮游动物与鱼类群落拥有独特的同位素生态位（isotopic niche）。此外，通过对融冰季100余天内与羽流相关的浮游动物死亡率进行估算，结果表明冰川海湾内的底栖（benthic）群落可获得12.8吨有机碳通量。底栖食腐动物（benthic scavengers）如等足类Onisimus caricus与Anonyx nugax在冰川海湾内数量丰富，以沉降有机物质为食。