By Ranger Steve Mueller
Chilly fall air settles upon lakes and cools surface water. As water cools, it becomes denser and the heavier water sinks. Water is unique in that after reaching 39 degrees F its molecules begin to expand again making them lighter. This is extremely important for wildlife survival for lake nature niches.
Water molecules can hold the greatest quantity of oxygen at 39 degrees F.
The weight of water molecules and its ability to hold oxygen is a life and death process for fish and other aquatic organisms. During warmer months, the sun warms the upper lake. Warm water allows fish to become more active and their oxygen consumption increases. Warm water with less oxygen passing over gills might cause fish to suffer from lack of oxygen.
Fortunately, algae and other aquatic plants replenish oxygen during the day and this aids fish survival. At night the plants consume oxygen and over abundant vegetation might deplete oxygen during the night killing fish or other gill feeding organisms.
In the depths of lakes below the sun lighted portion, water remains cold but oxygen becomes depleted during the summer months. Colder water at depths does not mix with the upper warm water and in effect two lakes are created in one. Deep-water oxygen gets consumed by bacteria and other organisms.
Fish find it necessary to move to the upper warm lake layer. Variation in oxygen content in the upper layer allows fish to move, creating interesting challenges for people trying to find them. The warmer upper lake is called epilimnion and the lower colder lake is called hypolimnion. Epi for above lake, and hypo for below lake. The dividing point between upper and lower lake is known as a thermocline, separating a lake into two bodies of water that do not mix easily during warmer months.
Fish can cool their bodies by entering the hypolimnion but might suffocate if oxygen has been depleted too much during the summer. They can swim high in the epilimnion but their warmed bodies might require more oxygen than is available because warm water holds less oxygen. Some fish can survive in lower oxygenated water and others like trout need more oxygen available.
By hanging out near the thermocline, they can cool their bodies and reduce oxygen needs.
In fall, cold air settling on the lake surfaces cools water that captures the greatest amount of oxygen before sinking to the bottom of the lake. The sinking water destroys the thermocline that separated upper and lower lake bodies during summer. It replenishes oxygen to a maximum level throughout the lake. It also creates a phenomenon known as lake overturn.
If you are fortunate, you might get to see a lake full of churned up gunk when overturn occurs. Lakes with mucky bottoms show this best. Visit Chrishaven Lake at the Howard Christensen Nature Center on Red Pine Drive between Kent City and Cedar Springs to witness the event. Invite friends to come promptly to see it because the murky water will only last a few days. Nutrients will have been stirred, spread throughout the lake and will provide floating plant nutrient needs. While visiting HCNC, visit the interpretive center to purchase a membership and to learn about the activities for all family members.
Growth of plants and animals will be slowed during winter months but life persists in wait of the next year’s warm season. When spring arrives, a spring overturn will occur after ice out and before a new epilimnion and hypolimnion are separated by a thermocline. Fish go where they want but it is determined in part by food availability, temperature, water oxygen content, and sheltered hiding locations.
Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at firstname.lastname@example.org – Ody Brook Nature Sanctuary, 13010 Northland Dr. Cedar Springs, MI 49319 or call 616-696-1753.