We have identified a new mechanism that may be extremely important for vertical and horizontal transport in lakes. Our time series temperature data, taken at intervals of 30 seconds or less, indicates that high frequency, non-linear waves are ubiquitous in small to moderate sized lakes. Such waves have previously been noted in the ocean, large lakes, and fjords. We have found them in two tropical lakes, 8 temperate lakes, and one Arctic lake. These lakes range in size from 1 km2 to 350 km2. Our turbulence measurements in the Arctic lake indicate that these waves are breaking and cause mixing. Turbulence induced by these waves has not previously been documented. They occur whenever the thermocline rapidly upwells or downwells. Accumulations of organisms have been documented in the oceans because of the non-linearity of these waves. Hence, they are likely to be an important mechanism for aggregation of food for predators in lakes. Breaking of these waves is likely to be the dominant mechanism whereby heat is transported from the mixed layer into the thermocline during the spring heating period and for fluxes of gases and nutrients whenever lakes are stratified.

We have begun modeling these waves, taking into account the modal structure of the basin scale waves within the lakes, the extent of tilting of the thermocline, and the bathymetry of the lake basins. In addition, follow through studies summer 2000 will indicate the currents induced by these waves as well as the shears within them. With this additional data, we will better be able to predict the fluxes of biologically important materials in enclosed water bodies.