Patterns of Lake Mixing

Energy inputs and the cover of ice control the stratification and mixing patterns in arctic lakes. Ice cover isolates the lake from the mixing energy of the wind, which in lakes that lack an ice-free period tends to result in permanent inverse stratification. Because freshwater is most dense at °C the temperature profile in these lakes is inverted, with the coldest and least dense water near 0 °C at the surface just under the ice; temperature and density increase toward the bottom. Although inputs of solar radiation through the ice may warm surface water and create convection currents and mixing, these energy inputs tend to be weak and in general the vertical mixing rates of solutes under ice are very slow, often on the order of molecular diffusion. In winter the sediments of arctic lakes may release heat accumulated during the ice-free summer, warming the adjacent water toward 4 °C and creating density currents that slowly move oxygen-reduced water toward the deep basins. A second category of lakes that are permanently stratified (meromictic - never fully mix top to bottom) also is represented in the Arctic, although as in other parts of the world they are rare. These lakes may or may not have permanent ice cover, but have salty, high-density water in the bottom layers, which requires too much energy to be lifted and mixed with surface waters. The result is a continuous separation and varied trajectories of evolution of water layers and their chemical and biological contents.

In contrast to these poorly mixed lakes, ice-free shallow lakes that mix continuously throughout (polymictic) are most common in the Arctic. In slightly deeper lakes (>4-5 m maximum depth) a typical stratification pattern occurs where during the ice-free summer period a mixing and warmer upper layer (epilimnion) overlies a middle transition zone where temperature drops rapidly with depth (meta-limnion), which is in turn underlain by a poorly mixed bottom layer (hypolimnion), where salts or particulate materials accumulate. When ice-covered, these lakes exhibit the inverse stratification described earlier. In the cold temperatures of the Arctic, even when lakes are ice-free the balance between stratification and mixing is adjusted by energy inputs.

In the coldest lakes during the ice-free period, if solar heating cannot raise temperatures to the density maximum of 4.0 °C, then the lakes will have no summer stratification.

Once lakes stratify, the isolation of water masses strongly impacts chemical and biological processes. Various mechanisms of mixing such as convection, wind stirring, density currents, or breaking of internal waves may increase the rate and extent of vertical mixing of oxygen, nutrients, and organisms. In addition, stream water inputs during storms, especially to smaller lakes, can strongly modify water column structure and mixing rates, again impacting chemistry and biology. Determining the relative importance of these various mixing processes and their impacts on lake ecosystem function is a current puzzle and theme of research in the physical limnology of arctic lakes.

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