Lakes: An Alien World Just Below The Surface

What Is A Lake?

The most striking thing about lakes is that two lakes that are very close to each other can be very, very different. They might not look very different to us but to the creatures trying to make a living in their waters, their lake might be as different from their neighbour’s as another planet is to us. It is often the case that the fish in one lake would quickly die in a lake less than a mile away. Generally speaking every lake has an autobiography written in the deposits of sediment wasting away at the bottom of the lake.

So what is a lake? Let’s just say it is a water filled depression in the Earth’s surface. Some might call certain such bodies of water a pond, a reservoir or even a pool. Most lakes are larger than any of the afore mentioned three bodies of water, but, science does not have a definitive answer for the question; exactly how large must a pond, reservoir or pool be to be called a lake. It is not a burning scientific question. Some people might talk about such factors as the size of the body of water, how it was formed, whether it was made by man or nature, the source of its water, etc. But mostly scientists are content with the name given the body of water by some explorer or pioneer.

Distinctive Layers of Lake Water Seldom Mix

If you have ever dived into a lake you may have noticed, with some alarm, that the deeper you let yourself go down, the less comfortable the water temperature gets. If you get deep enough it is downright cold; very cold; really, really cold. In fact, a column of water that is 18 degrees Centigrade (64◦ F) at the surface can cool to 4 degrees centigrade (39◦ F) at the bottom the lake. Nice, if you’re a trout.

The water at the top 26ft cools quite gradually the deeper one goes. At about eight metres (26 ft.) there is a transition zone about one metre thick in which the decrease in temperature accelerates much faster and below which the much colder water again cools at a slower rate as one descends. The transition zone is at a depth where the sun’s rays, which are chiefly responsible for heating water, can no longer penetrate. 

This transition zone serves not only as insulation between the warm upper layer of the lake and the cold lower layer; it is also a physical barrier between the water of these two layers. In other words, during the summer months, the water of these zones seldom mixes.
A second part of this temperature difference phenomenon is that the warm water at the top of a lake may have a different chemical makeup than the cold water at the bottom, notably it contains a lot more oxygen. Since, in the summertime, these warm and cold zones’ waters do not mix, the plentiful oxygen at the top never gets to the lake bottom.

Layering Is Important to Aquatic Life

As the summer progresses and fish (and rotting plant and animal life in the bottom of the lake) use up the small amount of summertime oxygen at the bottom, life can get pretty desperate for the poor trout trying to “breath”. They may have to resort to living a little higher in the warmer part of the cold zone where they are otherwise less comfortable.
Food also becomes an issue since the trout like to eat plankton (almost microscopic animal and plant life) which, along with oxygen, is more plentiful in the warmer regions of the lake. By late summer bottom plankton becomes more and more scarce. Trout are forced to make quick sorties into the dangerous warmer zones to obtain food.

Surrounding Land Forms Determined Lake Chemistry

Beyond water temperature there are many other factors that determine what life will be able to flourish in a lake. Chief among the determinants of a lake’s chemistry is the chemistry of the surrounding landscape. Rugged, steep hills of granite, such as exist in the Canadian Shield and in mountainous areas, carry rain water away very quickly to nearby lakes. Rain water flowing at a fast rate over the very hard granite or marble erodes away very little of the rock’s nutrients. Nor does the surrounding soil contain very many eroded nutrients. The water in the lakes fed by these waters is very clear and lacking in nutrition, hence, it cannot support a huge population of fish.
On the other hand, a lake surrounded by less rugged limestone hills, such as the Texas Hill Country, will receive water rich in calcium and other nutrients that can support a good population of fresh water crustaceans and other plankton that many fish thrive on.

The Shape of The Lake Basin Makes A Difference in What It Can Support

Also of great importance to the health of a lake, and important in determining the type of life it can support, is the shape of the basin that contains the lake. Lake life needs some shallow water in which to grow vegetation that can harbour many kinds of amphibious and water animals. These shallows along the shore line are the bread basket of the lake and they serve as nurseries for the spawning fish. Shoreline shallows that are too deep for vegetation still offer spawning grounds for such cold water species as trout.

Turnover Refreshes Lower Zones

The next factor that is important to the health of the lake and its inhabitants is what is known as the turnover. During the spring and fall when temperatures are cooler but there is no ice, the lake gets a chance to get a good mixing. As top zone spring temperatures more evenly match bottom temperatures, the zone barriers disappear, further facilitating the mixing of the lake’s water. Further, in the spring, the lake is flushed with melting snow and ice, and spring rain runoff. The lake’s health depends upon large quantities of water (to a point) from a clean and low acid content, input stream, or clean melted ice. The more completely flushed a lake is and the better mixed its water becomes during this period, the healthier the like will be during the coming summer. During this spring mix the cold water fish species can be found at the top of the lake, in a feeding frenzy, preparing for the scarcity of mid-summer. The shape of a lake basin might not be conducive to good turnover, in which case the lake will seem a bit more stagnant than a better flushed lake. Because the turnover occurs during the fish spawning season the spawning fish depend upon it to help keep the spawning nest clean and cool. A flush with high concentrations of acid rain, however, can destroy the spawn.

Human Activity Plays An Important Role

The last consideration regarding lake health and the lake’s life supporting ability is the human activity within the watershed that feeds water to the lake. Pollution of waterways takes many forms and all are preventable. The most dangerous activity to lakes is the release of poisons into the watershed. Heavy metals are particularly insidious because they are difficult or impossible to get rid of once they are there. Mercury and lead are stable elements that remain in the environment forever. One major source of lead in lakes is lead sinkers from broken fishing lines. Sinkers made of tin and other less noxious metals are readily available now and should be used instead of lead sinkers. Loons are the chief victims of lead fishing weights because they dive deep to the bottom of lakes in search of heavy stones to aid their gizzard in grinding food. Lead affects the brain, causing the loon to become confused and to neglect their brood, ultimately killing the loon and her chicks.

This description of lakes has been necessarily sketchy and incomplete. I urge you to find more information on the internet and in your library. Lakes are fascinating and play such an important role here in any environment. I encourage young people to take the science courses offered in your high school, even if you are not interested in a career in science. Certainly we need good research scientists and good science teachers, but we also need citizens well informed in science even if they become taxi drivers, business owners, social workers or mechinists.

Click <a href="http://expertscolumn.com/content/history-shield-example-all-geologic-shields">here</a>to read another artice by this author titled: A History of The Canadian Shield as an Example of All Geologic Shields.

Click <a href="http://expertscolumn.com/content/earth-s-magnetic-flip-how-and-why-it-happens">here</a>to read another article by this author titled: Earth's Magnetic Flip: How And Why It Happens.