The Arctic also enjoys 24-h daylight through the summer months, and is blanketed by 24-h darkness for several winter months depending on latitude. Halfway between the Arctic Circle and the North Pole (78° N), the sun sets on October 27th and rises again on February 15th, resulting in quite low annual inputs of solar radiation. However, starting in mid-April the sun never sets for the next 4 months, and thus summer daily values of insolation received by lakes can be -800 W m"2 (-40 MJ day"1), which is equivalent to what lakes receive in the temperate zone. The extinction of light by dissolved and partic-ulate material in arctic lakes is similar to that found in temperate or tropical regions; extinction coefficients can range from quite low (~0.1m_1) in some ice-covered lakes up to 2.5 m_1 in lakes clouded by glacial flour. Although recent increases in UV radiation are small in the Arctic compared with the large increases in Antarctica driven by the ozone hole, shallow arctic lakes may receive more UV in the future, especially if there are reductions in winter snow cover, which blocks UV much more effectively than does ice.
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