The thermocline occurs deeper in large lakes because wind energy is transmitted to greater depths Wind energy increases with fetch In small lakes convection also plays a role in determining thermocline depth In deep lakes only the surface layers are well mixed and quite warm, whereas the deeper parts remain cold. Lakes partition themselves into temperature zones Thermal stratification in lakes z t = thermocline depth (m ) and A = lake Area (km 2 )

To estimate F, most people use either the max length of the lake, or the square root of lake Area Empirical formulae used to estimate wave height and wave length.

The seasonal pattern of thermal stratification in a deep temperate zone lake Depth-time graph of isotherms Vertical thermal profiles During spring turnover the entire Water column is 4 o C—why 4 o C Same thing happens again in the fall

Fig in text Heat diffuses much faster down the water column in large lakes—wind mixing Hence the thermocline is deeper in large lakes

In small lakes mixing is more determined by convection currents driven by solar heating and is determined by how deep light penetrates Top of the thermocline Middle of thermocline

In very large lakes horizontal thermal shear zones occurs at river mouths A thermal bar Important habitat feature for many fish species in spring.

Waves- the gravitational response to wind disturbance The bigger the wind fetch the bigger the wave oscillation Wave energy and slope together determine the depositional zone boundary The velocity in these oscillations attenuates sharply with depth At depths > depostional Boundary depth fine mud accumulates after Rasmussen and Rowan (1997) Log DBD(m)= ─ Log F (km) slope (%)