8.2 Human Impacts on Ecosystems

Human activities damage aquatic ecosystems through pollution like excess nutrients, oil spills, heavy metals, sediment, and litter, all of which can push organisms outside their range of tolerance. When pollutants overwhelm a system, you see stress, dead zones with low dissolved oxygen, and harm that moves up the food chain.

Why This Matters for the AP Environmental Science Exam

This topic is your foundation for the whole pollution unit. Understanding how pollutants stress aquatic life and lower dissolved oxygen connects directly to later topics like eutrophication, thermal pollution, and biomagnification.

On the exam, you may need to:

• Describe how a specific human activity harms an aquatic ecosystem and explain the cause-and-effect chain.
• Read and interpret data like an oxygen sag curve or dissolved oxygen measurements.
• Propose and evaluate solutions to a pollution problem, naming benefits and drawbacks.

Data analysis matters here. Practicing how to read graphs and explain relationships between variables (like dissolved oxygen versus distance from a pollution source) builds skills you will reuse across the free-response and multiple-choice sections.

Key Takeaways

• Every organism has a range of tolerance. Outside its optimum range, an organism faces physiological stress, slower growth, less reproduction, and possibly death.
• Coral reefs are damaged by rising ocean temperature, sediment runoff, and destructive fishing practices.
• Oil spills harm organisms through hydrocarbons, coat the feathers of birds and the fur of marine mammals, and sink to harm bottom-dwelling life, with added economic costs to fishing and tourism.
• Excess nutrient pollution causes oceanic dead zones, areas of dangerously low oxygen.
• An oxygen sag curve plots dissolved oxygen against distance from a pollution source, so dissolved oxygen drops near the source and recovers downstream.
• Heavy metals can reach groundwater, and bacteria convert elemental mercury in water into highly toxic methylmercury.

Aquatic Pollution and Range of Tolerance

Every aquatic organism, plant or animal, has preferred conditions where it functions best. When those conditions get disturbed, organisms experience stress. Stress can mean impaired function, slower growth, reduced reproduction, and in extreme cases, death.

The range of tolerance describes how abiotic factors affect whether an organism can survive in an ecosystem. These factors range from sunlight to the concentration of a pollutant in the water. Picture a bell-shaped curve: the optimum range sits in the middle, and survival drops off toward both ends.

Some organisms tolerate a wider range of conditions than others. The more sensitive species are often used as indicator species, because they are the first to disappear when an ecosystem is under stress. Scientists track these species and use their presence or absence to judge the health of an ecosystem.

Monitoring Water Quality

Scientists measure several factors to judge the health of an aquatic ecosystem, including dissolved oxygen, nutrient levels, and the presence of specific disease-causing organisms or toxins. These parameters are interconnected, and bodies of water used for recreation, drinking water, and fishing get monitored closely to protect human health.

Dissolved Oxygen

Dissolved oxygen (DO) is the amount of oxygen in the water that is available to organisms for aerobic respiration. Cooler water holds more dissolved gas, including oxygen, than warmer water. That temperature relationship is why thermal pollution later in the unit becomes such a big deal.

DO enters aquatic ecosystems two main ways:

• Moving water interacting with air, like waves or a flowing river.
• Oxygen released by photosynthetic organisms in the water.

DO is removed when organisms consume it during aerobic respiration.

Biological Oxygen Demand

Biological oxygen demand (BOD) measures how much dissolved oxygen the organisms in a system need. When a lot of organic waste enters the water, microbes decompose it using aerobic respiration, which raises BOD. If BOD exceeds the available DO, the water becomes hypoxic, meaning it has an oxygen deficit. With no dissolved oxygen at all, the water is anoxic.

Water Quality Trends to Know

Good water quality goes with low BOD, high DO, and cooler temperatures. Poor water quality goes with high BOD, low DO, and warmer temperatures.

The Oxygen Sag Curve

The oxygen sag curve plots dissolved oxygen versus distance from a pollution source, usually excess nutrients or biological waste. Near the source, microbes decomposing the waste use up oxygen, so DO drops. As you move downstream and away from the source, DO gradually recovers. The curve can also help you compare how much oxygen, often measured in parts per million (ppm), different fish species need to survive.

When you read one of these graphs, identify the lowest point of DO and connect it back to where the pollutant entered. That cause-and-effect reasoning is exactly the kind of thinking the exam rewards.

Other Major Aquatic Pollutants

Beyond nutrients and low oxygen, several other human-caused pollutants damage aquatic ecosystems.

• Coral reef damage: Rising ocean temperature, sediment runoff, and destructive fishing practices all harm reefs. Warmer water stresses corals, while sediment can smother them and block light.
• Sediment: Increased sediment in waterways reduces light infiltration, which affects primary producers and visual predators. Settling sediment also disrupts habitats on the bottom.
• Heavy metals: Metals from industry, especially mining and burning fossil fuels, can reach groundwater and contaminate drinking water supplies.
• Mercury: When elemental mercury enters aquatic environments, bacteria convert it into highly toxic methylmercury, which can move into the food chain.
• Litter: Trash that reaches water can cause intestinal blockage and choking hazards for wildlife, and it can introduce toxic substances into the food chain.

Oil Spills as Major Pollution Events

Some pollution happens gradually, but oil spills release huge amounts of pollutant at once. Oil spills harm marine life in several ways:

• Hydrocarbons in oil are toxic and can cause organism mortality directly.
• Oil floating on the surface coats the feathers of birds and the fur of marine mammals, which harms their insulation and ability to move.
• Some components sink to the ocean floor and harm bottom-dwelling organisms.
• Oil that washes ashore causes economic damage to fishing and tourism industries.

The following are real-world examples of these impacts, not required AP content, but they make the cause-and-effect chain concrete.

Example: Deepwater Horizon Oil Spill

On April 20, 2010, a drilling rig in the Gulf of Mexico failed catastrophically, causing the largest oil spill on record. Over 210 million gallons of oil flowed into the ocean before the well was sealed that September. The spill killed wildlife and hurt fisheries and tourism. Cleanup used millions of gallons of chemical dispersants along with controlled burns, and burning that oil released large amounts of atmospheric carbon.

Example: Exxon Valdez Oil Spill

On March 24, 1989, the oil tanker Exxon Valdez struck Bligh Reef in Prince William Sound off the coast of Alaska. The collision spilled about 10.8 million gallons of oil, which spread hundreds of miles and stained thousands of miles of coastline. The spill killed well over 100,000 birds, fish, seals, otters, and other animals, sharply reducing the area's biodiversity, and it cut into local tourism.

How to Use This on the AP Environmental Science Exam

MCQ

Expect questions that test cause and effect. If a passage describes excess nutrients entering a lake, trace the chain: more nutrients lead to more microbial activity, which raises BOD, which lowers DO, which stresses fish or causes fish mortality. Watch for the temperature and DO relationship, since warmer water holds less oxygen.

Free Response

You may be asked to describe a human impact on an aquatic ecosystem and explain the mechanism, not just name it. Use precise language: say that hydrocarbons are toxic, that oil coats feathers and fur, or that microbes decomposing waste lower dissolved oxygen. When a prompt asks for a solution, give a specific action and state a benefit and a drawback.

Data Analysis

Practice reading an oxygen sag curve and dissolved oxygen data. Identify where DO is lowest, explain why it drops near the pollution source, and describe how it recovers with distance. For more practice, the 2015 AP Environmental Science Free-Response Questions include a relevant problem.

AP Environmental Science 2015 Free-Response Questions

Common Misconceptions

• Range of tolerance is not the same as a hard cutoff. Organisms gradually experience more stress as conditions move away from their optimum. Death is the extreme end, not the only outcome.
• High BOD does not add oxygen. BOD measures oxygen demand. High BOD means microbes are using up oxygen, which lowers DO.
• Warmer water holds less dissolved oxygen, not more. This trips up many students. Heat reduces the water's capacity to hold oxygen.
• Dead zones are caused by nutrient pollution, not by toxins directly poisoning everything. Excess nutrients fuel growth and decomposition that strips oxygen out of the water.
• Methylmercury forms in the water. Bacteria convert elemental mercury into methylmercury once it enters aquatic environments. The mercury becomes far more toxic after that conversion.
• Oil harms wildlife in more than one way. It is not only the toxic hydrocarbons. Coating of feathers and fur and the smothering of bottom dwellers all matter.

Related AP Environmental Science Guides

• 8.1 Sources of Pollution
• Unit 8 Overview: Aquatic & Terrestrial Pollution
• 8.3 Endocrine Disruptors
• 8.4 Human Impacts on Wetlands and Mangroves
• 8.5 Eutrophication
• 8.8 Bioaccumulation and Biomagnification