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Explain the environmental effects of excessive use of fertilizers and detergents on aquatic ecosystems.

Aquatic Nutrient Levels

The contrast between low-nutrient, clear water bodies (oligotrophic) and high-nutrient, algae-infested waters (eutrophic). — *Oligotrophic ecosystems are clear, indicating low nutrient levels*.

The amount and type of nutrients available to aquatic ecosystems determines the amount and type of living organisms living in that ecosystem.

Oligotrophic ecosystems are low in nutrients, usually clear and deep, with stable algae populations, and high in dissolved oxygen.

Eutrophic ecosystems, on the other hand, are high in nutrients, turbid and shallow, with high algae populations, and low in dissolved oxygen.

You may also hear about mesotrophic ecosystems. A mesotrophic ecosystem falls somewhere in between oligotrophic waterways and eutrophic waterways. The aging progression of a waterway starts with oligotrophic, moves to mesotrophic, and ends with eutrophic. Lakes and ponds may actually fill in during the process resulting in a terrestrial ecosystem such as a marsh, pond, swamp, and eventually, a forest climax community.

The ecological succession of a water body over time, depicted in five stages. — ***Believe it or not, there is an aquatic aging, or successional process, starting with lakes and resulting in a forest climax community.***

Essential Knowledge

Compared to eutrophic waterways, oligotrophic waterways have very low amounts of nutrients, stable algae populations, and high dissolved oxygen.

Cultural Eutrophication

Usually, nitrogen and phosphorous are limiting in aquatic ecosystems. The amount of these nutrients in the water limits how much phytoplankton can grow. If nitrogen and phosphorous are added to an aquatic ecosystem, however, it can cause far-reaching negative effects on the ecosystem.

You might think it would be a good thing to add a nutrient that is limiting to the environment because it stimulates growth. However, these ecosystems are adapted to phosphorous as a limiting nutrient. Adding too much of a good thing can be too much of a good thing. The addition of phosphorous to these environments can cause algae and cyanobacteria to grow unchecked. When algae and cyanobacteria grow unchecked, their populations get larger than what is expected for the environment.

an aerial gif of a eutrophic waterway - most of the surface of the water is covered with green algae. — ***Above is an aquatic ecosystem in the algal bloom phase of cultural eutrophication.***

You may have seen a pond that is located near lawns or agricultural areas.

These ponds are green because fertilizers from the lawns and/or agricultural areas have run off into the pond, increasing the amount of nitrogen and phosphorous in the water, and causing an explosion in the algal and cyanobacteria populations.

When the algal and cyanobacteria populations experience explosive growth in these waters, they frequently cover the surface of the pond. This allows them to get the sunlight they need, but they also block sunlight from reaching the bottom of the waterway, pond, or lake. This causes photosynthetic organisms attached to the bottom to die because they can no longer photosynthesize.

Once these plants die, they are decomposed by decomposers. The process of decomposition uses oxygen in the water, reducing the amount of oxygen in the waterway because there aren’t enough plants producing more oxygen. This causes more organisms in the water to die, which increases the amount of decomposition and oxygen usage, which further reduces the amount of oxygen available in the water. This positive feedback loop process continues until there is no more oxygen in the water.

Once an area no longer has oxygen, it is called a dead zone.

An illustrative diagram of cultural eutrophication, demonstrating the impact of nutrient runoff on aquatic ecosystems over time. — ***Too much of a good thing can be too much of a good thing, especially when an ecosystem is adapted to a limiting nutrient.***

The entire process, from explosive algal growth to dead zone, is called cultural eutrophication. Sometimes you will hear it as eutrophication and others will say “cultural eutrophication”. The word “cultural” refers to humans as the cause so you could substitute “human-caused eutrophication” in place of cultural eutrophication. Typically, fertilizer and animal waste runoff from yards and agricultural lands are the main culprits in supplying nitrogen and phosphorus. Being careful with fertilizer applications and managing animal waste carefully helps avoid cultural eutrophication.

This is what has happened in the Gulf of Mexico because the Mississippi River has dumped so many nutrients into the Gulf of Mexico.

Essential Knowledge

Eutrophication occurs when a body of water is enriched in nutrients.

The increase in nutrients in eutrophic aquatic environments causes an algal bloom. When the algal bloom dies, microbes digest the algae, along with the oxygen in the water, leading to a decrease in the dissolved oxygen levels in the water. The lack of dissolved oxygen can result in large die-offs of fish and other aquatic organisms.

Hypoxic waterways are those with bodies of water that are low in dissolved oxygen.

Anthropogenic causes of eutrophication are agricultural runoff and wastewater release.

AP Exam Tip

Too much of a good thing can be too much of a good thing when it comes to nutrients in aquatic environments.

You should know the process of cultural eutrophication from nutrient runoff to dead zone.

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