Posted on 05 December 2013.
By Ranger Steve Mueller
It’s been a cold week. Snow arrived and icy roads have challenged drivers. One driver lost control at Ody Brook and slammed into a large spruce tree. It knocked the tree to a 60-degree angle. This Thanksgiving the driver can be thankful he was not injured. The tree probably will not survive. Meanwhile ice has formed on the ponds and protects the water world of nature niche life underneath.
Have you wondered why lakes don’t freeze from the bottom up? If they did, fish would be killed because lakes would freeze solid. Instead they freeze at the top and form an insolating layer that provides safe haven of aquatic wildlife for the winter.
Beavers construct a lodge they enter and exit from under the ice. Branches stored on the lake bottom are brought indoors for bark dinners. The top of beaver lodges rise above the ice allowing air exchange for breathing. A cozy lodge is insolated from extreme winter temperatures.
When fall arrives, air temperature cools and heats more rapidly than water. When cold air-cools surface water, the water sinks at 39-degrees F. At that temperature, water becomes its most compact and heaviest. It also holds the most oxygen possible at 39-degrees F. Because it is most dense, it sinks carrying oxygen to the depths of the lake.
During summer when sun warms water, a layer called a thermocline forms separating the upper and lower lake. The layer prevents easy movement between the lower (hypolimnion) and upper (epilimnion) lake water. Most plant life is above the thermocline, where sunlight reaches allowing photosynthesis to add oxygen to water during the day. At night, plants need oxygen and consume it for their needs. If algae and other plants are too abundant, they consume the oxygen and suffocate fish. This is known as summer kill.
Below the hypolimnion oxygen is slowly depleted because it is not replenished by photosynthesis or water mixing. Plants are few in the dark water, so they do not consume all the oxygen. Fish will often hang out at the thermocline, where they can cool down and slow metabolism so they require less oxygen and require less food.
In fall, the cold dense water holding oxygen sinks to the bottom of the lake oxygenating the entire lake. The movement stirs bottom sediments. I have seen Chrishaven Lake at the Christensen Nature Center look like someone stirred the lake with a giant stick in fall. The lake becomes filled with nutrient rich sediments. The activity destroys the thermocline and the lake becomes one even temperature body until the following summer when a new thermocline forms.
As water-cools below 39-degrees F, it begins to expand and does not sink. At 32-degrees F, the cold water freezes at the surface forming an insolating blanket. If windy, the blanket will not form smoothly. One can see if air was active or still by how smooth the ice layer is at the surface. Sun can penetrate ice allowing algae photosynthesis to continue. This plant growth will add oxygen to the water during the winter.
Sometimes when the snow layer on lakes is thin, light enters allowing algae to become abundant. When too abundant, the algae might consume all the oxygen during the long winter nights causing what anglers know as winter kill. At ice out in spring, dead fish float at the surface from winter suffocation. If the lake has streams flowing in, oxygen might be replenished. Fish will be found at these oxygen rich areas of the lake. A heavy snow blanket can prevent too much winter sun from entering the lake.
Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at the firstname.lastname@example.org Ody Brook, 13010 Northland Dr, Cedar Springs, MI 49319-8433.