During the winter, a lake has a temperature profile. That is, coldest water (0 degrees)lies at the surface, just beneath
ice. Warm water sinks to the bottom to as much as 4 degrees C.
In spring, the water is warmed by the sun until surface temperature excees 4 degrees, this water sinks and the deep
water rises, and that is vertical mixing.
This is called spring over turn, brings nutrients above and takes O2 to the bottom!
In summer, sun rises higher each day, surface water heats faster, , this depth where temp. changes most is
The warmer, less dense water just floats on top of the dense, cool water below. This condition is known as
stratification ( lakes must be more than 5 m deep to experience this!)
May be located anywhere between 5 and 20m below surface.
Upper layer of warm water above thermocline is called epilimnion, and deeper cold water below it is called
Epilimnion most production happens here, oxygen is most found here, so this are is suited for animal life.
However, planst and alge deplete the supply fo minerals here. So, most animals and bacteria remain BELOW
thermocline, where there is little photosynthesis, & deplete oxygen there.
In late summer, productivity of lakes decreases, and oxygen + nutrients deplete
In fall, surface water cools of lakes, and sinks as it becomes dense. This vertical mixing is known as the fall
overturn. Fall overturn speeds O2 below, and nutrients above. This causes an explosion oh phytoplankton (aka fall
Vertical mixing of lakes is less dramatic in lakes that are not exposed to continental climates.
Lake temperatures that don’t fall below 4 degrees, only have ONE mixing event each year.
In tropical lakes, vertical mxing occurs ALOT. In temperate zones with deep lakes, vertical mxing hardly occurs,
and so they have no productivity!
Concern: tropical region lakes increase temp. Of surface water, and create thermocline at shallow depths
impairing vertical mixing and so reducing lake production!
Climate and Weather Undergo Irregular and Unpredictable Changes
- El nino- warm countercurrent, moves down the coast toward peru. Flows stongly and far enough to force the Preu
Current offshore and shutdown fishing indrusty
During normal years, without el nino, peru current warms up as the current moves westward across equatorial
El nin is triggered by Southern Osicllation: reversal of pressure areas