Many people wonder what causes lakes to shrink and eventually disappear. This process, called succession, is natural and it occurs in all lakes. Thankfully, the basics of how and why this occurs isn’t difficult to explain.
To understand succession, it is often best to explain the steps, at least in a general way. It should also be noted that while succession occurs in all lakes, as a rule the shallower the lake, the less time it takes the process to occur. Climate also plays a role, as will be seen.
The first step is plant colonization. Plants depend on sunlight and warmth, usually, for growth. Therefore, if the lake is in an area that gets plenty of sunshine and warm temperatures for long periods, the colonization happens more rapidly. At first, the colonizers of major importance are the algae. Duckweed is common on the surface of the water.
Each year, as they die out, they sink to the bottom and put down layer after layer of plant matter. Animals in the water add to this. Normally there are also plants growing on the shore that contribute leaves, branches, and so forth, to the growing layer of sediment. At times, the amount of debris is so great that it doesn’t even have the chance to decompose before it is covered by more sediment.
Streams and runoff entering the lake add to this by washing dirt and more plant material into the water.
Layers of sediment over plant material cause the plant debris to decompose slowly if at all, because of the lack of oxygen. In fact, this is how coal formed, over millennia.
The vegetative matter can still decompose, but at a much slower rate. This means that the top most layer of debris also leaches oxygen out of the water, in order to break down. Eventually, as the lake becomes shallower, the oxygen level can drop to a point where animal life in the water can’t be supported.
At the same time that the lake is filling up from the materials falling or washing into it, the plants at the edge of the lake are also changing. Water lilies and reeds appear, which are capable of living in shallow water. These add quickly and substantially to the plant material being laid down in the lake.
As the shoreline moves toward the middle of the lake, other plants move in behind the partly submerged plants, such as cattails and reeds. These provide even more fiber and plant material, and create a semi-solid mat of plant material. This is nearly perfect for plants like willows to grow in, so the willow-like plants move in next.
These plants are even higher in cellulose and greatly increase the amount of debris that is deposited, yearly. Also remember that all of this time, the shoreline is moving inward as the shallowest areas fill in.
Next in the succession come the trees, like alders, that can survive in water soaked soil. At this point, the ground that was formerly shallowly covered by water is now a boggy mat of dead vegetation, though there will still be living plants that thrive in this sort of soil. The vegetation and the alders add enormously to the ground, making it sturdier and drier.
This is perfect for oaks, maples, pines, and firs to take root and grow in the rich soil. Each of these trees also adds to the dirt, while drinking up the water, figuratively.
It is easy to see that the lake shrinks in stages, by a succession of plants, and this is in fact why it is called succession. Eventually, there won’t be a lake and the forest will dominate. Large, deep lakes can take quite some time for this to happen, but all lakes are prone to succession. Lakes have been disappearing in this way since long before man ever took his first step on this world. There is little doubt that it will continue long after man isn’t even a memory.
