While most of the denitrifaction bacteria poop-out in the cold water, other bacteria like anaerobic bacteria can and do continue to digest wastes and release both methane and hydrogen sulphide gasses which can be very toxic to life. That's the reason the sludge we scoop out in the spring stinks so bad.
denitrifcating bacteria tend to follow these kinetics;
P = 0.25T^2,
P = Percent of denitrifying bacteria growth rate at 20°C,
T = Temperature, °C
so at minimum water temperature at the bottom, 4°C the bacteria would, be working at 1%, I don't buy the idea the bacteria "die" at any particular low temperature, they just go into near dormancy; some reactivate faster than others and have different kinetics.