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 Canadian 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.
Highlights
Lakes are sentinels of environmental change and integrate the conditions of their surrounding watersheds (Williamson et al 2009)
Moat dynamics During our sampling in July 2015, the ice cover of Ward Hunt Lake had a mean thickness of 196 cm in the intermediate under-ice site (S3) and 218 cm in the central under-ice site (S4), and the moat was approximately 20 m wide
Our results suggest that under the ice, zooplankton diet is more likely to be composed exclusively of seston, whereas in the inshore moat site microbial mats or epibionts associated with mosses, with their higher concentrations of polyunsaturated fatty acids (PUFA) (Fig. 8), could contribute to a larger extent to zooplankton diet
Summary
Lakes are sentinels of environmental change and integrate the conditions of their surrounding watersheds (Williamson et al 2009). Dissolved and particulate nutrients enter the littoral zone by inflows from the watershed and influence primary production throughout the lake (Jones et al 1998). Ecosystem productivity is the result of autochthonous processes and heterotrophic production fuelled by allochthonous organic carbon entering the littoral zone from terrestrial sources (Ask et al 2009). These inputs of detrital organic matter are a potential food source for zoobenthos (Solomon et al 2011) and zooplankton (Rautio et al 2011; Harfmann et al 2019), and can stimulate bacterial production (Traving et al 2017). The abundance and composition of the biota in the littoral zone can be seasonally influenced by changing inflow conditions, and by the combined mechanical effects of suspended particles, wave action and ice that alter near-shore sediments and vegetation (Strayer and Findlay 2010), thereby affecting the whole lake ecosystem
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