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Winter is a harsh but essential period for freshwater organisms across the Northern Hemisphere. Not only does it influence the phenology of turnover periods that infuse and disperse oxygen throughout the water columns, it also allows the aquatic life to reduce their energy consumption because of the environmental changes that ice and snow provide. However, climate changes are causing worldwide major reductions in the ice thickness, coverage and duration. With these important changes steadily amplifying, ecologists find themselves with a lack of knowledge about the consequences on the aquatic ecosystem shifts. Indeed, very little information is available on the winter ecology of lakes. Only 2% of peer-reviewed freshwater literature includes under-ice lake processes. Nowadays, winter ecology is increasingly seen as an important component in limnology and recent studies contradict the preconceived idea of a dormant lifeless period. Considering that nearly 50% of the world’s lakes are seasonally ice covered, and that the period of ice cover on lakes and rivers around the world is decreasing each year, it is clear that there is an urgent need for research on ice ecosystems in order to be able to make predictions about the impacts of ice cover loss on the ecology and productivity of lakes.
If winter limnology in general is little known, even less is known about ecology in the interface between the ice and water, and inside the lake ice. While the presence of ice and changes in its duration, thickness and physical characteristics are known to drive many important biological activities in marine ecosystems, there is little information about the biogeochemistry and biology of freshwater ice and the underlying ice-water interface in northern lakes.
The goal of my MSc study is to characterize the algal and other microbial communities present in freshwater ice in comparison with those present in the water column to better understand the dynamics of freshwater algae in winter. In addition, I will measure the ice and underlying water for a suite of biogeochemical variables to study the role of freshwater ice as storage and a place for transformations in the biogeochemical carbon cycle in ice-covered lake ecosystems.
For my project, I will characterize the algal and other microbial communities’ variability at spatial and temporal scales. The project will take place along a latitudinal gradient from the southern coniferous forest in boreal Quebec (48°N, 71°W) where lakes are ice-covered for five months a year having a maximum ice thickness of about 70 cm, to the mid-Arctic on Victoria Island (69°N, 105°W) where lakes are ice-covered for 10 months a year with a maximum ice thickness of 2 m, and finally to the High Arctic on Northern Ellesmere Island and Ward Hunt Island (83°N, 74°W) where perennially ice covered lakes are found.
There were 11 boreal lakes sampled in February 2018, with the exception of Lake Simoncouche (48°13'57.2"N 71°15'02.6"W) that has been sampled 5 times, once a month, between December 2017 and April 2018, to provide a high seasonal resolution data of the biogeochemical and biological succession in the lake ice.
Six lakes in the vicinity of Cambridge Bay, Nunavut have been sampled between October 30th and November 4th 2017, with a second sampling in April 2018. The two lakes in Northern Ellesmere Island and Ward Hunt Island have been sampled in July 2018.
Four of the lakes (Boreal lakes Simoncouche, Allen and ELB, and Lake Greiner in Cambridge Bay) are employed with an automated logger (RBR Maestro, Ruskin) for the continuous measures of temperature, conductivity, oxygen, CDOM and light. The logger is located about 0.5 m below the annual maximum ice thickness. For this project, the main variable of interest is the penetration of light through the ice in different moments of winter.
Each lake was sampled at a point where the depth of the water was near its maximal depth to have a representative water column sample. Water was sampled from a hole made with a power ice auger of 10 inches diameter, using a Limnos water bottle to obtain an integrated sample i.e. a mixed sample representing water collected from different depths between the bottom and the surface.
The ice was sampled using a 9 cm diameter Mark II ice corer (Kovacs Enterprise, Lebandon, NH, USA). One to three ice cores, depending on the ice thickness, were sampled and combined to be analyzed on the total of ice cover biogeochemical and biological analyses. Additional three ice cores were sub-sectioned into two to five parts to study the distribution of algal biomass (chl-a) in the ice among the water-ice interface and the upper layers.
At each site, ice thickness, snow depth, light penetration (Li-Cor Li-192 underwater quantum sensor attached to a graduated pole) through ice and profiles of temperature, conductivity and/or pH (RBR Concerto or YSI 6820 V2) are taken.
Water and ice melted at 4ºC in a dark room were measured for pH, phytoplankton community composition and total nutrients as unfiltered total phosphorus (TP) and unfiltered total nitrogen (TN). Water was also GF/F filtered for chlorophyll-a and HPLC pigments as an indication of algal biomass and the characterization of algal pigments. samples were also filtered for seston fatty acid and a stable isotope signature of carbon (δ13C) to further characterize the composition and in-lake versus terrestrial origin of the particulate material in the ice. A subset of lakes will be further analysed for DNA/RNA marker genes to determine taxonomic composition and to estimate the microbial activity in the ice. The dissolved fraction in the samples was sampled for dissolved organic carbon (DOC) concentration, for optical measures of colored dissolved organic matter (CDOM) as a proxy of the source and aromaticy of carbon, soluble reactive phosphorus (SRP) and nitrate and nitrogen dioxide (NO3 + NO2).
We found that ice contained substantial concentrations of inorganic nutrients and carbon. Compared to the high biomass of ice algae present in sea ice, the lake ice algae were less abundant but taxonomically diverse. Our results also suggest that because of the large proportion of lake volume that ice occupies in late winter, the storage of nutrients and carbon, and the inoculum of algae it contains have an underestimated role in stimulating spring and summer production in lakes ecosystems. This study demonstrates that we currently underestimate the role of freshwater ice in aquatic ecosystems and that studying Arctic lake ice is necessary to predict the effects of global warming on these fragile ecosystems.