The Thores Lake proglacial system: remnant stability in the rapidly changing Canadian High Arctic

We describe limnological data sets from Thores Lake, a large ice-contact proglacial lake in northern Ellesmere Island, Nunavut (82.65 degrees N), including longitudinal and cross transects (vertical resolution 0.03 m, horizontal resolution 100-200 m). The lake is formed due to damming by Thores Glacier at its northwest margin, has multi-year ice cover and a cold (<1.54 degrees C) fresh water column with a bottom layer of <0 degrees C, high-conductivity water in the deepest basin. Thores Lake is ultraoligotrophic, with low nutrient and phytoplankton stocks. Accessory pigment data and metagenomics were used to describe the eukaryotic microbial community. Diversity and taxonomic composition in the water column were homogeneous down to a depth of 40m, consistent with density profiles. Surface water at the glacier interface was characterized by high turbidity and total phosphorus concentrations, and a distinct phytoplankton community dominated by chlorophytes, whereas the lake water column had higher relative abundances of chrysophytes and photosynthetic dinoflagellates. Thores Lake has a contracted pelagic food web, with the highest trophic level occupied by phytoplankton-feeding rotifers, and no crustacean zooplankton; profiles showed that omega-3 fatty acids (FAs) ranged from <1% (glacier interface) to 3.6% (central lake) of total seston FAs. Given the stability of the Thores Glacier ice dam and the persistence of cold water capped by perennial ice, Thores Lake provides a baseline to assess the impact of climate change on far northern lakes.