Warming of the winter in lakes: The later setting of ice promotes the wintering of consumers and forms planktonic food webs in spring

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meaning

Freshwater ice is being lost faster and faster with global warming. However, the ecological effects of shortening the ice-covered seasons remain largely unexplored in winter due to a historical lack of freshwater research. We offer an innovative experimental study that realistically reproduces the warming effects on the time of ice onset in order to address the consequences for the food webs in lakes. We find that later ice maintains photosynthesis longer, which allows for greater accumulation of high quality fat reserves in primary consumers and likely facilitates winter survival. In addition, we show that a greater prevalence of wintering consumers shapes the food webs in the spring through increased top-down control of the spring algae bloom. By and large, this study shows that warmer winters can have cross-season cascading effects on ecological processes.

abstract

Global warming is causing the loss of freshwater ice around the northern hemisphere. Although it is known that the timing and duration of ice cover regulate ecological processes in seasonally ice-covered ecosystems, the consequences of shortening winters for freshwater biota are poorly understood due to the scarcity of under-ice research. Here we present one of the first in-lake experiments to shift the beginning of the ice cover (by ≤21 d) and thereby extend the light availability (by ≤40 d) in early winter, and we explicitly show cascade effects on pelagic food web processes and phenologies. The delay in ice formation triggered a number of events from winter to spring: 1) relatively greater density of algal resources and primary consumers in early winter; 2) an increased prevalence of winter-active (hibernating) consumers during the ice-covered period, combined with increased storage of high-quality fats, probably due to longer access to algae resources in early winter; and 3) an altered trophic structure after glaciation, with greater initial spring densities of overwintering consumers causing greater, earlier top-down regulation, thereby effectively reducing spring algae bloom. An increasingly late onset of ice can thus encourage consumers to hibernate, which can give taxa a competitive advantage that are able to survive winter after the ice has collapsed; a process that can reduce the availability of spring food to other consumers, potentially disrupting trophic connections and energy flow pathways in the ensuing open water season. Considering a future with warmer winters, these results provide empirical evidence that can help predict phenological responses to freshwater ice loss and generally represent a case of a climate-induced cross-season cascade on realized food web processes.

Footnotes

    • Accepted October 20, 2021.
  • Authors’ contributions: M.-PH carried out the research, analyzed the data, and wrote and revised the work; and M.-PH, BEB, MR and GFF discussed the study, contributed materials or analysis tools, and approved the manuscript.

  • The authors do not declare any competing interests.

  • This article is a PNAS direct submission.

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


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