As valley dwellers, we are aware of our lakes and how they dramatically change from summer’s warm recreational reservoirs to cold, and sometimes frozen, winter expanses.
But how often do we stop to ponder the lakes’ seasons?
I can remember a winter where the temperature dropped so low in November and stayed cold long enough for bays in the south end of Kalamalka Lake to freeze. Bitter winds swept off the snow leaving it as clear as glass. Skating on the ice was eerie; weeds, bubbles and even some fish were suspended in the glassy lake surface. It was fascinating! I wondered what was happening below that winter?
Water temperatures don’t fluctuate as rapidly as air temperatures. It’s the water’s chemistry that’s interesting and creates change. Water molecules contract and become denser (tightly packed) as temperatures lower. But at 4 C something unusual happens; water molecules start binding together, expand and become lighter, solid ice by 0 C. They can expand up to nine per cent. That’s why ice floats on the denser liquid water molecules.
Our smaller to mid-size valley and mountain lakes, such as Swan Lake, Otter Lake, Wood Lake and Duck Lake have interesting seasonal water cycles causing temperature layering and “turn overs” of oxygen and nutrients. Once the ice thaws from these lakes, the water temperature is an even 4 C. Winds push the surface water, pushing and stirring oxygen rich surface water to the lake’s bottom. This circulating current then rises plankton and other nutrients up to the top.
In summer, the upper water layer is warmed by the sun. It becomes less dense, floats and doesn’t mix with the lower, colder water. The temperature differences increase causing less mixing of the water. The warm, well-oxygenated surface is a paradise rich in life. This is where the summer action is—the algae blooms, the zooplankton (microscopic animals that feed on phytoplankton) graze and the insects and fish feed.
The lakes colder bottom gets depleted of oxygen from decomposition of lake life waste and by oxygen consumption from its deep water organisms—mostly bottom feeding scavengers. Autumn’s colder nights cool the surface again bringing the water to a uniform 4 C and the breezes result in re-circulation or turn-over, redistributing oxygen throughout.
This is critical as winter’s ice surface cuts off the air-oxygen exchange and reduces oxygen-producing photosynthesis by aquatic plants. Animals such as fish and hibernating frogs, toads and turtles wintering beneath the ice have to survive on the depleted oxygen.
Our larger valley lakes, however, such as Okanagan and Kalamalka Lakes, have massive, well-oxygenated deep water zones and enough water mass to maintain warm rising water through our moderate winters to prevent total freeze over. In past years, though, during long cold spells, certain shallow bays froze up. Stories are recounted of Kelowna and Vernon bays freezing paddlewheelers in solid.
It takes many years for our large lakes to turn over its nutrients.
Loons and grebes leave their smaller lake summer homes to winter on our larger lakes as they need open water to feed and take flight. Watch for them this month as they start to pair up.
Roseanne Van Ee enthusiastically shares her knowledge of the outdoors to help readers experience and enjoy nature. Discover exciting and adventurous natural events, best trails, and wild places. Follow her on Facebook for more.