Denitrification under lake ice

Many lakes, ponds and reservoirs are subject to long and changing periods of ice cover. However, limited winter research has created key knowledge gaps. How does nitrogen cycling change under ice? And what does changing ice cover duration mean for water quality? Here we present the first measurements of denitrification rates under ice in temperate, polymictic waterbodies. Surprisingly, despite lower winter temperatures, winter and summer rates of denitrification did not differ. Experimental work suggests that denitrification rates are controlled hierarchically, first by nitrate concentrations, then by temperature. As a result, controls on nitrate inputs such as changing hydrology and nitrification, combined with physical controls on delivery of nitrate to sediments, may be more important to nitrate retention via denitrification than the duration of low temperature or ice cover. Nitrous oxide was typically supersaturated under-ice, suggesting an ice-out flux will occur, and this flux may be greatest in systems with elevated nitrate.