Phytoplankton communities and production in Arctic fjords undergo strong seasonal variations. Phytoplankton blooms are periods with high primary production, leading to elevated algal biomass fueling higher trophic levels. Blooms are typically driven bottom-up by light and nutrient availability but may also be top-down controlled by grazing. While phytoplankton spring blooms are common across all Arctic systems, summer and autumn blooms and their drivers are less predictable. Here we compare the long-term (≥4 years) bloom phenology and protist community composition in three Arctic fjords: Nuup Kangerlua in western Greenland, Ramfjorden in northern Norway, and Adventfjorden in western Svalbard. While Nuup Kangerlua is impacted by tidewater glaciers, Ramfjorden and Adventfjorden are impacted by river-runoff. We discuss and contrast the presence and predictability of spring, summer, and autumn blooms in these fjords and the main physical, chemical, and biological drivers. Spring blooms occurred in all three fjords in April/May as soon as sufficient sunlight was available and typically terminated when nutrients were depleted. Chain-forming diatoms together with the haptophyte were key spring bloom taxa in all three fjords. Summer blooms were found in Nuup Kangerlua and Ramfjorden but were not common in Adventfjorden. In Nuup Kangerlua nutrient supply via subglacial upwelling was the key driver of a diatom-dominated summer bloom. This summer bloom extended far into autumn with strong winds resupplying nutrients to the surface later in the season. In Ramfjorden runoff from a vegetated catchment provided organic nutrients for a flagellate-dominated summer bloom in 2019. A late autumn bloom dominated by S. and other chain-forming diatoms was present after nutrients were resupplied by wind mixing. In Adventfjorden, we observed only minor summer blooms in 2 of the 8 years, while autumn blooms were never observed. With global warming, we suggest that summer blooms will be negatively impacted in fjords where tidewater glaciers retreat and become land terminating. In fjords with rich vegetated catchments, harmful algal blooms may occur more frequently as summers and autumns become warmer and wetter. However, for fjords in high-Arctic latitudes (>78 N), the day length will continue to restrict the potential for autumn blooms.

中文翻译：

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跨气候梯度的三个北极峡湾浮游植物群落的季节性和生产


北极峡湾的浮游植物群落和生产经历强烈的季节性变化。浮游植物大量繁殖是初级生产力较高的时期，导致藻类生物量增加，从而推动更高的营养水平。水华通常是由光和养分可用性自下而上驱动的，但也可能是由放牧自上而下控制的。虽然浮游植物春季大量繁殖在所有北极系统中都很常见，但夏季和秋季浮游植物的大量繁殖及其驱动因素却难以预测。在这里，我们比较了三个北极峡湾的长期（≥4年）水华物候和原生生物群落组成：格陵兰岛西部的Nuup Kangerlua、挪威北部的Ramfjorden和斯瓦尔巴群岛西部的Adventfjorden。努普康格鲁阿 (Nuup Kangerlua) 受到潮汐冰川的影响，而拉姆峡湾 (Ramfjorden) 和安德文特峡湾 (Adventfjorden) 则受到河流径流的影响。我们讨论并对比了这些峡湾春季、夏季和秋季水华的存在和可预测性，以及主要的物理、化学和生物驱动因素。一旦有足够的阳光，所有三个峡湾都会在四月/五月出现春季水华，并且通常在养分耗尽时终止。链状硅藻与固定藻一起是所有三个峡湾春季开花的关键类群。努普康厄鲁阿 (Nuup Kangerlua) 和拉姆峡湾 (Ramfjorden) 盛开夏季花朵，但在安德文特峡湾 (Adventfjorden) 并不常见。在努普康格鲁亚，通过冰下上升流提供的养分是硅藻主导的夏季开花的关键驱动因素。夏季的花期一直持续到秋季，强风在季节后期向地表重新提供养分。在 Ramfjorden，来自植被流域的径流为 2019 年以鞭毛虫为主的夏季水华提供了有机养分。深秋的水华以 S. 而其他成链硅藻则是在风混合重新供给养分后出现的。在 Adventfjorden，我们在 8 年中的 2 年仅观察到少量夏季开花，而从未观察到秋季开花。随着全球变暖，我们认为峡湾的夏季水花将会受到负面影响，因为峡湾中的潮水冰川会退缩并成为陆地终端。在植被丰富的集水区的峡湾，随着夏季和秋季变得温暖和湿润，有害藻华可能会更频繁地发生。然而，对于北极高纬度地区（>78 N）的峡湾来说，白天的长度将继续限制秋季开花的潜力。