Warmer winters in the lakes: the late appearance of ice favors the wintering of consumers and shapes planktonic food webs in spring

Importance

Freshwater ice is being lost at an accelerating rate with global warming. Yet the ecological implications of shortening ice-covered seasons remain largely unexplored due to a historical lack of freshwater research in winter. We provide an innovative experimental study that realistically reproduces the effects of warming on the timing of ice formation to address the consequences for lake food webs. We find that late application of ice maintains photosynthesis longer, allowing greater accumulation of high-quality fat stores in primary consumers and likely facilitating winter survival. In addition, we show that a greater prevalence of overwintering consumers shapes spring food webs through increased top-down control of algal blooms in the spring. Overall, this study demonstrates that warmer winters can have interseasonal cascading effects on ecological processes.

Abstract

Global warming is causing the loss of freshwater ice around the northern hemisphere. Although the timing and duration of ice cover is known to regulate ecological processes in seasonally ice covered ecosystems, the consequences of shorter winters for freshwater biota are poorly understood due to the scarcity of ice cover. looking under the ice. Here we present one of the first experiments in the lake to delay the onset of ice cover (by at least 21 days), thereby prolonging the availability of light (by at least 40 days) at the start of winter, and explicitly demonstrate cascading effects on pelagic food web processes and phenologies. The delay in ice formation caused a sequence of events from winter to spring: 1) relatively higher densities of algal resources and primary consumers at the start of winter; 2) an increased prevalence of active winter consumers (overwintering) throughout the ice-covered period, associated with increased storage of high quality fat probably due to longer access to algal resources at the onset of ice. Winter ; and 3) an altered trophic structure after ice melt, with higher initial spring densities of overwintering consumers resulting in stronger and earlier downregulation, effectively reducing algal blooms in the spring. An increasingly late appearance of ice can thus encourage overwintering by consumers, which can give a competitive advantage to taxa capable of surviving winters in the event of pack ice; a process that can decrease the availability of food in the spring for other consumers, potentially disrupting trophic links and energy flow pathways in the following open water season. By envisioning 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 interseasonal cascade on food web processes performed.

Footnotes

    • Accepted 20 October 2021.
  • Author Contributions: M.-PH conducted the research, analyzed the data, and wrote and edited the article; and M.-PH, BEB, MR and GFF discussed the study, contributed material or analysis tools, and approved the manuscript.

  • The authors declare no competing interests.

  • This article is a direct PNAS submission.

  • This article contains additional information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2114840118/-/DCSupplemental.

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