AP Environmental Science

## Learning Outcomes

- Explain how oil spills harm organisms through coating, ingestion, inhalation, and toxic exposure
- Distinguish between short-term mortality and long-term environment disruption after a spill
- Identify why wetlands, estuaries, shorelines, and surface waters are especially vulnerable
- Understand how oil weathering changes spill behavior and persistence
- Evaluate the advantages and drawbacks of dispersants, burning, mechanical recovery, bioremediation, and natural recovery
- Recall key facts about the **Deepwater Horizon** spill and apply them in AP-style responses

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand oil spill ecological effects, with a focus on the following syllabus points:

- Know that oil spills are a major form of aquatic and coastal pollution caused by transport, drilling, pipelines, and offshore extraction accidents
- Identify direct physical effects such as coating feathers and fur, smothering habitats, and reducing light penetration
- Explain toxic effects of petroleum compounds, especially **polycyclic aromatic hydrocarbons (PAHs)**
- Understand why eggs, larvae, juveniles, seabirds, marine mammals, sea turtles, and benthic organisms are especially vulnerable
- Explain how oil affects producers, food webs, wetlands, estuaries, marshes, mangroves, beaches, and seafloor habitats
- Understand the role of **weathering**, including evaporation, dissolution, oxidation, biodegradation, and emulsification
- Evaluate spill-response methods, including booms, skimmers, dispersants, burning, bioremediation, and natural recovery
- Recall the **Deepwater Horizon** case study: **April 20, 2010**, about **134 million gallons** released over **87 days**, and more than **1,300 miles of shoreline** damaged

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why are seabirds and marine mammals often among the first visible victims of an oil spill?
2. How do the physical effects of oil differ from the toxic effects of PAHs?
3. Why are wetlands and estuaries usually damaged for longer than open-ocean areas?
4. What do dispersants do, and why can they both help and harm natural habitatss?
5. Which facts from the Deepwater Horizon spill are useful to remember for the exam?

## Introduction

Oil spills are a high-yield AP Environmental Science topic because they combine several course ideas at once: aquatic pollution, habitat disruption, food web effects, toxicity, and environmental tradeoffs in human responses. Students often think of oil spills as mainly a “bird covered in oil” issue, but the exam expects a wider view. Oil can damage entire systems, from phytoplankton at the surface to marsh roots, fish larvae, shellfish beds, and deep-sea communities.

For APES, the strongest answers show cause and effect clearly. Do not stop at “oil is bad for wildlife.” Explain how oil moves, what it touches, what chemicals it contains, which organisms are exposed first, and why some habitats recover quickly while others remain damaged for years or even decades.

> **Key Term: oil spill**
> The release of petroleum into the environment, especially marine or coastal waters, where it can harm organisms, habitats, and food webs.

### How oil spills damage organisms and food webs

Oil harms living things in four main ways: **physical contact**, **ingestion**, **inhalation**, and **absorption** of toxic compounds. This is the first distinction students need to make.

Physical damage is often the most immediate. Oil coats feathers, fur, shells, gills, eggs, and plant surfaces. In seabirds, coating breaks down waterproofing and insulation. A bird that cannot stay dry loses body heat, becomes less buoyant, and may drown or die from hypothermia. Birds also ingest oil while preening, which can damage internal organs such as the liver and kidneys and can lead to dehydration because feeding becomes difficult.

Marine mammals can be harmed in similar but not identical ways. Fur-bearing mammals lose insulation when oil fouls their coats. Mammals and sea turtles can also inhale volatile compounds near the surface, especially soon after a spill when lighter fractions evaporate. Fish and invertebrates may suffer gill damage, smothering, or poisoning.

> **Key Term: polycyclic aromatic hydrocarbons (PAHs)**
> Toxic components of crude oil that can persist in sediments and organisms and cause developmental, reproductive, and chronic health effects.

Toxic effects are less visible than coating but often more persistent. PAHs are especially important. They can damage tissues, impair development, reduce reproduction, increase disease susceptibility, and lower survival even at low concentrations. This is why a shoreline that “looks cleaner” may still be biologically injured. Chronic PAH exposure can continue after the obvious slick disappears.

Primary producers are also affected. Because oil can float on the surface, it reduces light penetration into the water. Less light means less photosynthesis by phytoplankton, algae, and submerged plants. This matters because producers form the base of the food web. If producers decline, primary consumers lose food, then fish lose prey, and higher consumers such as birds and marine mammals are affected in turn.

This creates a chain of effects:

- oil reduces photosynthesis and contaminates small organisms
- plankton, larvae, and invertebrates are harmed
- fish recruitment falls because fewer young survive
- predators lose prey or ingest contaminated prey
- ecological productivity and food web stability decline
  > **Key Term: food web disruption**
  > A change in feeding relationships caused when pollution alters producers, prey, or predators, reducing stability in an food web.

Students also need one caution here: oil contaminants can move through food webs, but **biomagnification** is more strongly associated with pollutants such as mercury, DDT, and PCBs than with oil in general. It is safer in APES answers to say that oil compounds can **bioaccumulate** in organisms and disrupt food webs, rather than overstate biomagnification unless the question is specifically about persistent compounds.

> **Test Tip:** In FRQs, separate **physical effects** from **chemical toxicity**. Examiners reward this distinction because it shows more precise understanding.

### Why some habitats are hit harder than others

Oil spill damage is highly variable. The same volume of oil can have very different effects depending on habitat type, oil type, temperature, wave energy, season, and which organisms are present. This is why broad claims such as “all oil spills have the same effect” are inaccurate.

Surface waters often show the most acute visible damage because the slick sits where seabirds, turtles, marine mammals, and floating eggs or larvae may be exposed. But open-ocean areas can sometimes recover faster than sheltered coastal habitats because waves, sunlight, and mixing increase dilution and weathering.

Shorelines are much more vulnerable. Beaches, tidal flats, marshes, and mangroves can trap oil rather than disperse it. Once trapped, oil persists longer because oxygen levels are lower, movement is weaker, and microbial breakdown can be slower.

> **Key Term: weathering**
> The physical and chemical changes that oil undergoes after a spill, including evaporation, dissolution, oxidation, emulsification, sedimentation, and biodegradation.

Weathering changes how oil behaves. Lighter compounds may evaporate first, while heavier compounds remain. Some oil dissolves partially into water; some oxidizes; microbes can biodegrade some components. But weathering does not mean the oil has become harmless. In fact, some changes increase persistence.

One key example is **emulsification**, when oil mixes with water to form a thicker water-in-oil substance. This “mousse-like” material is more persistent and harder to remove. Oil can also bind with sediment and sink, which shifts damage from the surface to the seafloor and benthic habitats.

> **Key Term: emulsification**
> The formation of a thick water-in-oil mixture that increases oil persistence and can make cleanup more difficult.

Wetlands and estuaries deserve special attention in APES. They are among the most productive habitatss on Earth and act as nurseries for fish and shellfish. Oil can coat marsh grasses, smother roots, and reduce oxygen transfer in mangrove aerial roots. That weakens vegetation, and when plants die back, shorelines become more vulnerable to erosion. So the effect is not just “plants die.” The physical structure of the habitat begins to fail.

This matters because:

- marshes and mangroves protect coasts from erosion
- roots stabilize soil and trap sediment
- juvenile fish and shellfish use these habitats as nursery grounds
- if vegetation dies, both biodiversity and ecological function services decline

After the **Deepwater Horizon** spill, oiling of Gulf salt marshes was linked to increased shoreline erosion for at least two to three years in heavily affected areas. That is a powerful example of how oil can damage not just organisms but the structure of the habitat itself.

Deep-sea and benthic habitats can also be affected. If oil or oily particles sink, bottom-dwelling organisms and deep-sea corals may experience long-lasting contamination. Recovery is often slow because deep-sea growth rates are slow and conditions are stable, so disruption lasts longer.

Eggs, larvae, and juveniles are especially vulnerable in almost all habitats. They have limited mobility, thin membranes, and rapidly developing organs. Oil-derived PAHs can damage developing hearts and other organs in fish embryos and larvae, sharply lowering survival and future recruitment into the adult population.

> **Test Tip:** If a question asks why young organisms are vulnerable, mention **rapid development**, **thin tissues**, **limited mobility**, and **high sensitivity to PAHs**.

### Cleanup methods and why they involve tradeoffs

A common mistake is to assume that cleanup methods simply “remove oil.” In reality, many response methods redistribute oil or reduce one kind of harm while increasing another. APES questions often reward students who can evaluate these tradeoffs.

Mechanical recovery includes booms, skimmers, vacuums, and manual removal. Booms help contain surface oil; skimmers remove some oil from the surface. These methods are useful, but they are limited by wind, waves, currents, debris, and access. They also work best before oil becomes too dispersed or stranded.

> **Key Term: dispersants**
> Chemicals used to break surface oil into smaller droplets, moving more oil from the surface into the water column.

Dispersants are one of the clearest tradeoff topics. They can reduce surface slicks and shoreline oiling, which may protect birds, beaches, marshes, and coastal tourism. But they do not make oil disappear. They push more oil into the water column as tiny droplets, which can increase exposure for plankton, fish larvae, and other pelagic organisms.

That means the environmental logic is:

- less oil remains at the surface
- less direct coating of birds and shorelines may occur
- but more subsurface organisms may be exposed

This is why “dispersants remove oil” is wrong. They redistribute it.

In situ burning can remove large amounts of oil quickly from the surface, but it creates smoke, releases air pollutants, and leaves residue. Bioremediation uses or stimulates microorganisms that break down oil. It can be less physically destructive than aggressive cleanup, but it is slow and depends on oxygen, nutrients, temperature, and the type of oil.

> **Key Term: bioremediation**
> The use or stimulation of microorganisms to break down oil pollution.

Sometimes the best option is limited intervention. In marshes, mangroves, or fragile shorelines, aggressive cleanup can do more damage than the remaining oil. Heavy machinery, trampling, or sediment removal can destroy roots, compact soils, and worsen habitat loss. In those situations, **natural recovery** may be preferred.

> **Key Term: natural recovery**
> Allowing physical, chemical, and biological processes such as weathering, sunlight, wave action, and microbial breakdown to reduce oil over time.

This does not mean “do nothing because oil is harmless.” It means responders choose the option that minimizes **total environmental damage** overall. In some cases, leaving some oil to weather naturally is less harmful than tearing apart the habitat to remove it.

### Deepwater Horizon as the key case study

For AP Environmental Science, **Deepwater Horizon** is the strongest oil spill case study to know. The offshore drilling rig explosion began on **April 20, 2010** in the Gulf of Mexico. **Eleven workers were killed**. Oil flowed for **87 days**, and NOAA reports that about **134 million gallons** were released, making it the largest offshore oil spill in U.S. history.

More than **1,300 miles of shoreline** were damaged across Gulf states, including beaches, bays, estuaries, and marshes. Fishery closures peaked at **88,522 square miles**, nearly **37% of federal Gulf waters**, to protect public health and seafood safety.

These numbers matter because they let you show scale in exam answers. But link them to ecological effects:

- salt marsh vegetation died in heavily oiled areas
- shoreline erosion increased where roots were damaged
- sea turtles were exposed at the surface and in coastal habitats
- dolphins experienced long-term health and reproductive problems in some populations
- deep-sea corals showed tissue damage and slow recovery
- fisheries, tourism, and coastal communities were economically affected

Visible oil decreased long before full recovery occurred. That is one of the most useful lessons from this case. Environmental recovery is uneven. Long-lived species, deep-sea habitats, and heavily oiled marshes may take many years or decades to recover.

The best AP-style summary of this case is not just “a large spill caused damage.” It is: a major offshore drilling accident released a very large amount of oil over a long duration, contaminating both open-water and shoreline habitats, harming multiple trophic levels, disrupting nursery habitats, and requiring long-term restoration.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Oil spills harm organisms through coating, ingestion, inhalation, and absorption of toxic compounds
- Seabirds lose insulation and buoyancy when oil coats feathers, and may ingest oil while preening
- PAHs are major toxic components of crude oil and can cause developmental, reproductive, and chronic effects
- Oil reduces light penetration, lowering photosynthesis in phytoplankton, algae, and aquatic plants
- Eggs, larvae, and juveniles are especially vulnerable because they are sensitive and cannot easily escape exposure
- Wetlands, estuaries, marshes, mangroves, and shorelines often trap oil and recover more slowly than open water
- Weathering changes oil through evaporation, dissolution, oxidation, biodegradation, and emulsification
- Dispersants reduce surface slicks but increase oil exposure in the water column
- Natural recovery may be less damaging than aggressive cleanup in sensitive habitats
- Deepwater Horizon began on April 20, 2010 and released about 134 million gallons over 87 days
- More than 1,300 miles of shoreline were damaged in the Deepwater Horizon spill
- Oil spill effects can persist long after visible oil disappears

## Key Terms and Concepts

- oil spill
- polycyclic aromatic hydrocarbons (PAHs)
- food web disruption
- weathering
- emulsification
- dispersants
- bioremediation
- natural recovery


Human Impacts on Ecosystems - Oil spill ecological effects