Eutrophication

Ecological Succession and Aging of Lakes

Image and info on ecological succession provided by: https://news.uchicago.edu/explainer/what-is-ecological-succession

Succession of Terrestrial Systems and How it Links to Lake Aging

Humans age, all living things age, and environments also age. Ecological succession as depicted above, is a textbook example of how an ecosystem ages. This aging process is defined by the growth of plants as seen in the image above: in this example of primary succession, an ecosystem starts from "scratch" and slowly builds. Different plant communities replace one another over time and will continue to do so; this is how rocks with lichen on them may one day turn into a mature and diverse forest over many years. However, terrestrial systems are not the only ecosystems to age; lakes will also age through lake succession which is also commonly referred to as eutrophication. Similarly to terrestrial ecological succession, lake aging may also be tracked through plant communities, but other factors such as lake depth, nutrient input, and sediment also come into play. Before getting into this further, what does eutrophication and lake succession mean and how do lakes form?

Formation of a Lake and Eutrophication

Lakes form from mature rivers, catastrophic events moving sediment, ecosystem engineers like beavers, or depressions in the Earth. Lakes formed by depressions may arise from an array of sources like glaciers, volcanoes, and asteroids: many lakes in the United States, like Lake Lacawac are glacially formed. Large glaciers from the most previous ice age (estimated to be about 13000-18000 years ago) would slowly move across Earth's surface and scrape out land, creating holes called lake basins. Lake basins are then filled with water as glaciers melted over time. For a brief demonstration and summary on lake formation, check out the video above published by ECHO Leahy Center for Lake Champlain.

The image to the right shows succession of a body of water; in this case, that body of water is a pond or small lake. Some key points to know about succession and eutrophication and the aging process of lakes are:

Eutrophication is a natural process in which sediment and nutrients are inputs from the watershed around a lake that result in a shallower and more nutrient-rich lake. Typically, this process occurs over centuries and increases in nutrients correlate with increases in algae and plants–nutrients help feed these organisms resulting in a lot of growth!

Ponds and lakes become more shallow as sediment and organic matter from dead organisms builds up on the bottom, slowly filling the lake.

As lakes become shallower more aquatic and emergent plants can establish themselves as seen in the succession image below.

Succession is typically depicted as a linear process, but it is not always straightforward. Wetlands are a great example of the intermediate stages between life and death of a lake, or terrestrial and aquatic environment. The rate at which a lake ages heavily depends on environmental factors.

https://texasaquaticscience.org/lakes-and-ponds-aquatic-science-texas/

https://biogeography.weebly.com/succession.html

Trophic State

Trophic states comprise the trophic state index which categorizes lakes based on their productivity and nutrient content. This helps identify and class lakes when studying them and may aid in comparing them to other lakes as well as make the regulation process for the lake easier. The three main trophic states are; oligotrophic, mesotrophic, and eutrophic.

https://www.rmbel.info/primer/lake-trophic-states-2/

Nutrients, particularly phosphorus and nitrogen, are typically limiting and essential nutrients for plants, algae, and all life. The trophic states range from low to high nutrients and are as seen above and described below:

Oligotrophic lakes are very low in nutrients with blueish waters and are typically very deep with mineral bottoms. These lakes also have little algae and are colder due to greater depth, permitting for higher levels of dissolved oxygen.

Mesotrophic lakes are middle ground and have an intermediate amount of nutrients in the water. These lakes are typically clear, can experience algal blooms in late summer with warmer temperatures, and stratify, meaning they form distinguishable layers that do no mix with one another.
- The epilimnion is the top layer that is warmer due to solar radiation. This layer tends to have high oxygen levels due to the algae photosynthesizing in this layer (oxygen is produced in this process!).
- The metalimnion is in the middle and contains the thermocline. The thermocline is the steepest change in temperature. As depth increases, temperature decreases and will decrease more drastically with depth due to an inability for the sun to penetrate deeper waters.
- The bottom layer is the hypolimnion. This layer has the lowest temperatures in the lake that will gradually decrease or stop decreasing near the bottom. This layer is cold as it generally is not touched by the sun and decomposition of dead organisms occurs in this layer. Since decomposition requires oxygen, the hypolimnion has little to no oxygen and may become completely anoxic during the summer when lakes tend to be very productive.

Eutrophic lakes have the highest levels of nitrogen and phosphorus. As nutrients become more available as a lake ages, productivity increases, more organisms live, die, and then are decomposed. The lake fills with organic matter from those dead organisms as well as sediment, making these lakes significantly more shallow. Waters become murky in these lakes. Since these lakes have high nutrient levels, they can support large amounts of algae and aquatic life. However, these lakes are more likely to become anoxic in the summer.

Trophic State of Lacawac

Natural Eutrophication vs Human Accelerated Aging

Image from: https://www.rmbel.info/primer/lake-eutrophication/

Consequences of Cultural Eutrophication: Anoxic Zones

Anoxic zones may also be referred to as “dead zones” in bodies of water that result from full depletion of dissolved oxygen within water, making them inhabitable for many creatures.
One of the main causes for depletion of oxygen is eutrophication. Eutrophication is a natural process in which nutrients, particularly phosphorus and nitrogen, as well as sediment are received from its surrounding watershed. Naturally, this process typically occurs over centuries but deposits of nutrients can be accelerated by human disturbance such as through agriculture, land use, and usage of fertilizers that contain those nutrients.
Excessive nutrients result in extensive algal growth, particularly cyanobacteria (blue-green algae) that can result in algal blooms of potentially toxic organisms.
Notably, the growth of such algae results in higher decomposition rates once the algae die. Decomposition of the algal blooms caused by excess nutrients by bacteria utilizes all of the dissolved oxygen within an area, resulting in a “dead zone”.

References

https://education.nationalgeographic.org/resource/lake.
https://education.nationalgeographic.org/resource/dead-zone
https://www.rmbel.info/primer/lake-eutrophication/
https://www.rmbel.info/primer/lake-trophic-states-2/