Water column gradients beneath the summer ice of a High Arctic freshwater lake as indicators of sensitivity to climate change

Paschale N Bégin,Alexander I Culley,Warwick F Vincent,Isabelle Laurion,Masaki Uchida,Milla Rautio,Yukiko Tanabe,Maxime Wauthy

doi:10.1038/s41598-021-82234-z

Abstract

Ice cover persists throughout summer over many lakes at extreme polar latitudes but is likely to become increasingly rare with ongoing climate change. Here we addressed the question of how summer ice-cover affects the underlying water column of Ward Hunt Lake, a freshwater lake in the Canadian High Arctic, with attention to its vertical gradients in limnological properties that would be disrupted by ice loss. Profiling in the deepest part of the lake under thick mid-summer ice revealed a high degree of vertical structure, with gradients in temperature, conductivity and dissolved gases. Dissolved oxygen, nitrous oxide, carbon dioxide and methane rose with depth to concentrations well above air-equilibrium, with oxygen values at > 150% saturation in a mid-water column layer of potential convective mixing. Fatty acid signatures of the seston also varied with depth. Benthic microbial mats were the dominant phototrophs, growing under a dim green light regime controlled by the ice cover, water itself and weakly colored dissolved organic matter that was mostly autochthonous in origin. In this and other polar lakes, future loss of mid-summer ice will completely change many water column properties and benthic light conditions, resulting in a markedly different ecosystem regime.

Highlights

Winter ice cover is a key feature of north temperate and high latitude lakes and has a controlling effect on underwater light and gas exchange with the atmosphere
Inverse thermal stratification was observed in all mid-summer sampling profiles (Supplementary Fig. S2 for 2016 and 2017, with additional years shown in Supplementary Fig. S3a)
Temperatures measured in the Ward Hunt Lake water column were higher in 2016, reaching 6.5 °C (Supplementary Fig. S2)

Summary

Introduction

Winter ice cover is a key feature of north temperate and high latitude lakes and has a controlling effect on underwater light and gas exchange with the atmosphere. Given that the inputs of allochthonous carbon to the lake from the poorly developed soils in its barren polar desert catchment (Fig. 1; Supplementary Fig. S1) are likely to be small, we hypothesized that the underwater light regime would be controlled largely by the ice-cover, as well as by phytoplankton and water itself, with little contribution by colored dissolved organic matter (CDOM). To test this hypothesis, we characterized the DOM by spectral absorption and parallel factor fluorescence analysis (PARAFAC), and measured a set of apparent and inherent optical properties of the water column beneath the summer ice

Objectives

Methods

Results

Discussion

Conclusion