Reconstructing past oxygen dynamics in boreal lakes in response to long-term changes in climate, ice phenology and DOC inputs
By Dr. Raoul-Marie Couture, Norwegian Institute for Water Research, Section for Catchment Processes & University of Waterloo, Ecohydrology Group.
Boreal lakes are impacted by the simultaneous effects of climate warming, reduced acid deposition, and changing loads of dissolved organic carbon (DOC) from the catchment. We describe how these forcing, in particular the measured increase in DOC by 0.1 mg C yr-1 over the past 40 years, interact to modulate the ice phenology and dissolved oxygen (DO) dynamics of a small boreal humic lake. We calibrated the physico-chemical lake model MyLake against time-series on ice phenology and daily observations of water-column temperature and oxygen. The model included new modules for sediment-water interactions, decomposition of DOC, and associated DO consumption. Results suggest that microbial respiration in the water-column, and not in the lake sediments, drives lake anoxia. We used the model to hindcast ice freezing and break-up dates back to 1970 and found that ice break-up occurs 8 days earlier now than it did in 1975. This early ice break-up enhances water column ventilation, resulting in higher oxygen concentrations in early spring. In late summer, however, warmer temperatures cause faster microbial DOC turnover which leads to bottom-water anoxia. Superimposed on these long-term trends is the effect of increased DOC loads from the catchment, which causes significant decline in DO, leading to 15% more hypoxic days (< 3 mg L-1) and 10% more anoxic days (< 15 mg L-1) in 2014 than in 1974. We conclude that, with the exception of enhanced spring ventilation, climate warming and increasing DOC loads act in synergy to lower DO availability, substantially reducing habitable water volume for fish populations and for aquatic biota that depend on oxygen.
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