Mixing depths in New Zealand lakes

The seasonal mixing depth (depth of the epilimnion) in 24 stratified New Zealand lakes has been examined, mostly using published thermal data. Midsummer mixing depth (depth of the epilimnion, E in m) increases with lake size (fetch, f in km, defined as square root of surface area) according to a power law expression: E = 7.69 f 0.463 (r 2 = 0.940, N = 22) The exponent in this equation, and thus the fetch dependence of mixing depth, is similar to that reported by other workers for Northern Hemisphere lakes. However the coefficient is considerably larger, showing that in New Zealand's windy, maritime climate, lakes mix more deeply than lakes in continental areas characterised by relatively hot, calm summers. The ratio of epilimnetic depth, É (predicted from the above expression) to maximum depth, z m , is a useful index of the thermal character of a lake and can be used to classify New Zealand lakes into different mixing categories.