Under-ice availability of phytoplankton lipids is key to freshwater zooplankton winter survival.

Guillaume Grosbois,Milla Rautio,Heather Mariash,Tobias Schneider

doi:10.1038/s41598-017-10956-0

Guillaume Grosbois, Milla Rautio + Show 2 more

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https://doi.org/10.1038/s41598-017-10956-0

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Abstract

Shortening winter ice-cover duration in lakes highlights an urgent need for research focused on under-ice ecosystem dynamics and their contributions to whole-ecosystem processes. Low temperature, reduced light and consequent changes in autotrophic and heterotrophic resources alter the diet for long-lived consumers, with consequences on their metabolism in winter. We show in a survival experiment that the copepod Leptodiaptomus minutus in a boreal lake does not survive five months under the ice without food. We then report seasonal changes in phytoplankton, terrestrial and bacterial fatty acid (FA) biomarkers in seston and in four zooplankton species for an entire year. Phytoplankton FA were highly available in seston (2.6 µg L−1) throughout the first month under the ice. Copepods accumulated them in high quantities (44.8 µg mg dry weight−1), building lipid reserves that comprised up to 76% of body mass. Terrestrial and bacterial FA were accumulated only in low quantities (<2.5 µg mg dry weight−1). The results highlight the importance of algal FA reserve accumulation for winter survival as a key ecological process in the annual life cycle of the freshwater plankton community with likely consequences to the overall annual production of aquatic FA for higher trophic levels and ultimately for human consumption.

Highlights

Winter is the most unexplored season in ecology and has often been portrayed as a dormant period for aquatic organisms especially if the ecosystem is ice-covered[1]
By use of fatty acid biomarkers (FAB), we showed that during times of low primary production in winter, contrary to our hypotheses, the prevalence of terrestrial organic matter and/or bacteria did not increase in copepod or cladoceran lipid reserves
Early winter accumulation of phytoplankton-derived polyunsaturated fatty acids (PUFA) and their progressive decline in copepod lipid reserves in midand late-winter suggests that phytoplankton FA are critical for several species of zooplankton to survive and remain in an active stage until spring

Summary

Introduction

Winter is the most unexplored season in ecology and has often been portrayed as a dormant period for aquatic organisms especially if the ecosystem is ice-covered[1]. The highest copepod lipid content in lakes has been measured during winter[18] precisely when phytoplankton resources are the scarcest in the environment During this time, dissolved organic carbon (DOC) of terrestrial origin is abundant in boreal lakes[20]. It cannot be directly consumed by zooplankton[21], DOC may be assimilated in the microbial loop and trophically upgraded by heterotrophic protists[22], constituting a potential resource for zooplankton to build lipid reserves or fuel day-to-day metabolism in an environment with low primary production and phytoplankton abundance. This information would contribute to our understanding of the roles, and relative importance of, autochthonous and allochthonous carbon sources in sustaining aquatic food webs

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