The littoral zone of polar lakes: inshore–offshore contrasts in an ice-covered High Arctic lake

Abstract

In ice-covered polar lakes, a narrow ice-free moat opens up in spring or early summer, and then persists at the edge of the lake until complete ice loss or refreezing. In this study, we analyzed the horizontal gradients in Ward Hunt Lake, located in the High Arctic, and addressed the hypothesis that the transition from its nearshore open-water moat to offshore ice-covered waters is marked by discontinuous shifts in limnological properties. Consistent with this hypothesis, we observed an abrupt increase in below-ice concentrations of chlorophyll a beyond the ice margin, along with a sharp decrease in temperature and light availability and pronounced changes in benthic algal pigments and fatty acids. There were higher concentrations of rotifers and lower concentrations of viruses at the ice-free sampling sites, and contrasts in zooplankton fatty acid profiles that implied a greater importance of benthic phototrophs in their inshore diet. The observed patterns underscore the structuring role of ice cover in polar lakes. These ecosystems do not conform to the traditional definitions of littoral versus pelagic zones, but instead may have distinct moat, ice-margin and ice-covered zones. This zonation is likely to weaken with ongoing climate change.

Changes in the Community Structure of Under-Ice and Open-Water Microbiomes in Urban Lakes Exposed to Road Salts

“…There was also an increase in abundance of various chlorophytes during the ice-cover period, mainly small ovoid flagellates such as Chlamydomonas and Oophila . Chlamydomonas is frequently encountered during the ice-cover period, including in perennially ice covered systems (e.g., Bégin et al, 2020 ).…”

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

“…These groups have been previously identified by microscopy in the littoral zone of this lake 19 , 21 . Diatoms have been conspicuously sparse or absent from previous analyses of Ward Hunt Lake phytoplankton samples from beneath the ice, although they occur in the open waters of the moat 16 . These fast sinking taxa may be more likely to thrive under full water column mixing with the loss of summer ice, while such conditions may cause the breakup of delicate chrysophytes such as the large Uroglena colonies, resulting in a shift of species (Fig.…”

“…The presence of a moat (Supplementary Fig. S1 ) and the associated horizontal structure 16 may contribute to the water column features observed here, but these effects would be lost with mid-summer ice loss (Fig. 7 ).…”

“…This complete mixing would be favored by exposure to strong winds in the area, without the protection provided by ice-cover, and water temperatures below 4 °C that are conducive to cold monomixis. Other potential changes would include a cooler water column, ventilation of gases to the atmosphere, oxygenation of the sediments, loss of the moat and associated horizontal structure (described in Bégin et al 16 ), changes in phytoplankton composition, increased planktonic versus benthic production and increased turbidity (Fig. 7 ).…”

“…Lake water for seston absorbance measurements was filtered through 25 mm GF/F filters that were preserved at -80 °C until analysis. The optical density of the material collected on the filters (in vivo) was measured from 300 to 720 nm in a Varian Cary Bio 300 dual-beam spectrophotometer (Agilent, Santa Clara, California) equipped with an integrating sphere (Labsphere Inc., North Sutton, NH) and processed as described in Bégin et al 16 . Absorption coefficients for pure water were obtained from the IOCCG Protocol Series 48 .…”