Diurnal mixed layer dynamics: Insights from high frequency sensor data

Background/Question/Methods

The definition of the surface mixed layer of a lake is highly dependent upon the data that is available to define it. Traditionally, lake temperature has been measured by manually lowering a temperature sensor at the center of a lake at some regular time interval. The resulting series of vertical temperature profiles are used to describe both physical and biological variation in lake ecosystems. With weekly to monthly profiles of temperature, the mixed layer, or epilimnion, is commonly defined by a temperature change on the order of 1ºC m^-1. This temperature criterion defines a layer that changes gradually in response to seasonal changes in thermal structure. However, with microstructure profilers (which allow direct measurement of turbulent mixing), the dynamics of the mixed layer have been shown to vary considerably on a diurnal timescale. How do diurnal mixing dynamics, unresolved by manual sampling at intervals of a week or more, influence pelagic communities? In the ocean, the mixed layer depth (MLD) defined by turbulent dissipation corresponded to the equivalent of a 0.02ºC change in temperature. This suggests that mixing is suppressed by stratification much weaker than what has been traditionally used to define the epilimnion.

Results/Conclusions

Under a wide range of atmospheric conditions, I found that the MLD defined by turbulent dissipation did indeed correspond to a 0.02ºC increase in temperature. In the open ocean, MLDs were several tens of meters deep. In small, sheltered, temperate lakes, the MLDs were very shallow: less than a half meter by day down to maximum depths of a few meters overnight. Verified by comparison with turbulent dissipation, high frequency temperature data allows extrapolation of the MLD through time. The resulting MLD was dynamic and generally a meter or more shallow than the traditionally defined epilimnion. Such shallow mixing depths can have important consequences for pelagic organisms with limited mobility. Evidence suggests that shallow mixing delays the onset of infection of Daphnia by a non-motile fungal spore that must be resuspended from bottom sediments. Shallow diurnal mixing also influences the oxygen dynamics and thus has implications for estimating lake metabolism.