| Summary: | In northern regions, large volumes of water are needed for activities such as winter road construction. Such withdrawals, particularly from small lakes, can reduce oxygen concentrations and water levels, potentially affecting aquatic organisms. Withdrawal limits have been developed by regulatory agencies, but are largely theoretical. Water withdrawal thresholds were tested in two small lakes by removing 10% and 20% of their respective under-ice volumes and comparing oxygen parameters, temperature, over-wintering habitat, and northern pike (Esox lucius) abundance to reference conditions. Because of a milder winter, oxygen parameters were elevated in reference lakes in the period following withdrawal compared to the prewithdrawal period. The 10% withdrawal resulted in a −0.2 m shift in the oxygen concentration profile at 4 mg/l in that lake, but had no effect on total volume-weighted oxygen, or volume of over-wintering habitat. In contrast, the 20% withdrawal caused 0.7 m reduction in the oxygen concentration profile at 4 mg/l compared to the previous year, a 26% decline in the volume-weighted oxygen concentration, and a 23% reduction in the volume of over-wintering habitat compared to prewithdrawal conditions. Water temperatures were slightly (≤ 10%) colder in the upper strata in the year following the withdrawal in both withdrawal and reference lakes. Northern pike abundance was not impacted by water withdrawals in either of the lakes. The results of this study show that the effects of water withdrawal on the parameters investigated reflected the characteristics of the lakes, and would therefore be expected to vary from lake to lake. Policy development to mitigate impacts must therefore reflect the site-specific nature of water withdrawal.
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