DATA for Hébert et al 2021b PNAS

Global climate warming is causing the loss of freshwater ice around the Northern Hemisphere. Although the timing and duration of ice covers are known to regulate ecological processes in seasonally ice-covered ecosystems, the consequences of shortening winters for freshwater biota are poorly understood owing to the scarcity of under-ice research. Here, we present one of the first in-lake experiments to postpone ice-cover onset (by ≤21 d), thereby extending light availability (by ≤40 d) in early winter, and explicitly demonstrate cascading effects on pelagic food web processes and phenologies. Delaying ice-on elicited a sequence of events from winter to spring: 1) relatively greater densities of algal resources and primary consumers in early winter; 2) an enhanced prevalence of winter-active (overwintering) consumers throughout the ice-covered period, associated with augmented storage of high-quality fats likely due to a longer access to algal resources in early winter; and 3) an altered trophic structure after ice-off, with greater initial springtime densities of overwintering consumers driving stronger, earlier top-down regulation, effectively reducing the spring algal bloom. Increasingly later ice onset may thus promote consumer overwintering, which can confer a competitive advantage on taxa capable of surviving winters upon ice-off; a process that may diminish spring food availability for other consumers, potentially disrupting trophic linkages and energy flow pathways over the subsequent open-water season. In considering a future with warmer winters, these results provide empirical evidence that may help anticipate phenological responses to freshwater ice loss and, more broadly, constitute a case of climate-induced cross-seasonal cascade on realized food web processes.

全球气候变暖正在导致北半球周边的淡水冰体（freshwater ice）消融。尽管已知冰盖（ice cover）的形成时间与持续时长会调控季节性封冻淡水生态系统（seasonally ice-covered ecosystems）的生态过程，但由于冰下研究（under-ice research）的匮乏，我们对冬季缩短给淡水生物群落（freshwater biota）带来的影响仍知之甚少。本研究开展了首批原位湖泊实验之一，将冰盖形成起始时间（ice-cover onset）推迟≤21天，从而将初冬的光照可获得时长延长≤40天，并明确揭示了其对浮游食物网（pelagic food web）过程与物候（phenologies）的级联效应。推迟冰盖形成引发了从冬季到春季的一系列连锁反应：1）初冬时节藻类资源与初级消费者的种群密度相对更高；2）在整个封冰期内，越冬活跃消费者（winter-active (overwintering) consumers）的占比有所提升，这可能与初冬时期藻类资源可获取时长更长，使得此类消费者优质脂肪的储存量增加有关；3）冰消（ice-off）后营养结构（trophic structure）发生改变：越冬消费者在春季初期的种群密度更高，由此驱动了更强且更早的下行调控（top-down regulation），有效抑制了春季藻类水华（spring algal bloom）的发生。因此，冰盖形成起始时间愈发延后可能会促进消费者的越冬存活，这将赋予那些能在冰消后顺利越冬的类群（taxa）竞争优势；该过程会降低其他消费者在春季的食物可获得性，进而可能在后续的敞水期（open-water season）干扰营养联系（trophic linkages）与能量流动路径。在冬季愈发温暖的未来背景下，本研究结果提供了实证依据，有助于我们预测淡水冰体消融带来的物候响应；更广泛而言，本研究也为气候驱动的跨季节级联效应如何作用于实际食物网过程提供了一个典型案例。