■ High oxygen concentration
■ Populated with cold water fish such as trout, steelhead, whitefish, and
salmon ■ Example: Lake Superior- MESOTROPHIC (MIDDLE-AGED LAKE)—moderate nutrient content and
moderate amount of phytoplankton, reasonably productive. Characteristics
include: ■ Less steep shorelines
■ A mixture of conifer and deciduous trees, such as oak, maple, and ash
along the shore ■ Bottom is mostly sand, resulting from erosion and
weathering of rocks ■ Few aquatic weeds and more plants growing on
the shoreline ■ Less deep on average than oligotrophic lakes ■ Water is
still very clear
■ Supports both cold and warmer water fish, such as bass, perch, and
bluegill ■ Example: Lake Michigan - EUTROPHIC (OLD LAKE)—shallow, warm, large surface area relative to depth,
nutrient-rich, phytoplankton more plentiful and productive, waters often
murky, high organic matter content in benthos, which leads to high
decomposition rates and potentially low oxygen. Eutrophication occurs over
long periods of time as runoff brings in nutrients and silt. Pollution from
fertilizers often causes algae populations to dramatically increase (algal
bloom) causing a decrease in oxygen content of the water with detrimental
consequences for life in the lake. Characteristics include: ■ Gentle, mostly
flat shorelines
■ Mostly deciduous trees along the shore and an abundance of shoreline
plants ■ Usually quite shallow compared with sandy or rocky bottom
lakes ■ Very little oxygen in waters deeper than 30 feet ■ The water is
murky from organic material and single cell planktonic algae ■ Heavy
aquatic weed growth
■ Few, if any, cold water fish; bass, pike, and carp thrive in old lakes
■ Example: Lake Erie
Lake Stratification
The stratification or layering of water in lakes is due to density changes caused
by changes in temperature. The density of water increases as temperature
decreases until it reaches its maximum density at about 39°F (4°C), causing
thermal stratification—the tendency of deep lakes to form distinct layers in the
summer months. Deep water is insulated from the sun and stays cool and denser,