⏳ Estimated Reading Time: 12 - 14 minutes

Describe the impacts of human activities on aquatic ecosystems.

Ecological Tolerance

Ecological tolerance describes the ability of a living organism to deal with the biotic and abiotic factors they encounter in an ecosystem. Different species thrive with different levels of biotic and abiotic resources. The optimum level for each resource is the level at which the organisms grow or survive the best.

A graph depicts organism abundance across environmental conditions with five zones: two ‘zones of intolerance’ with no organisms, two ‘zones of stress’ with fewer organisms, and a central ‘optimum range’ with the highest abundance — *The optimum range of tolerance for a species will have the most individuals because it is* easy for the population to live in this area.

It is rare for every resource to actually be at the optimum level in nature. Fortunately, all species have a range of tolerance, or a span that allows growth, and an optimum range in which they grow best. The limits of a species’ tolerance are points at the high and low ends of the range of tolerance. There are also zones of stress, which are between the optimal range and the high or low limit of tolerance. In these zones, the organism can grow, but not as well as in the optimal range.

The concept of ecological tolerance is particularly relevant when studying how organisms respond to changes in their habitat or face challenges posed by human activities, such as pollution or climate change.

Essential Knowledge

Organisms have a range of tolerance for various pollutants. Organisms have an optimum range for each factor where they can maintain homeostasis. Outside of this range, organisms may experience physiological stress, limited growth, reduced reproduction, and in extreme cases, death.

Impacts on Coral Reefs

Coral reefs face significant challenges that threaten their existence. Corals have a symbiotic relationship with a type of algae known as zooxanthellae. When corals are stressed, they often expel their zooxanthellae and turn white. This is known as coral bleaching.

The process of coral bleaching in three stages. The first stage shows a ‘HEALTHY’ coral, The second stage shows a ‘STRESSED’ coral with fewer algae The final stage, ‘BLEACHED,’ shows a white, barren coral without algae — *The optimum range of tolerance for a species will have the most individuals because it is* easy for the population to live in this area.

Bleached corals are more susceptible to disease because they cannot obtain food from their algal symbionts. If the stressors persist, the corals may die. There are several factors that can stress corals and cause coral bleaching. The most common causes of coral bleaching are:

🌡️ Changes in Ocean Temperature (click to reveal)

Increased oceanic temperatures caused by climate change are the leading cause of coral bleaching.

In 2005, the U.S. lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event. The warm waters centered around the northern Antilles near the Virgin Islands and Puerto Rico expanded southward. Comparison of satellite data from the previous 20 years confirmed that thermal stress from the 2005 event was greater than the previous 20 years combined (NOAA).

NOAA publishes Four-Month Coral Bleaching Outlook maps. To get this data, NOAA compiled data from the Coral Reef Watch members. The graphic below shows the lowest stress level predicted by the 90% highest-ranking ensemble members (at least one of the ensemble members in the highest ranking 90% predicted the stress level shown):

A global map from NOAA Coral Reef Watch displays the probability of coral bleaching heat stress levels for the week of March 24, 2024. — *The 4-month Coral Bleaching Heat Stress Outlook for March-November 2024*.

🪨 Sediment Runoff (click to reveal)

When soil erodes from land due to activities like deforestation or construction, it is carried into the ocean where it can smother coral reefs, blocking sunlight from reaching the corals. When this happens, the algae in the coral tissues cannot photosynthesize and grow. As a result, corals bleach and become more susceptible to disease.

🛢️ Pollution (click to reveal)

The same runoff that carries sediments into the ocean can also carry pollution from land-based sources into the ocean. These pollutants can stress corals, disrupt delicate coral reef ecosystems, and ultimately, cause coral bleaching.

🌤️ Over/Underexposure to Sunlight (click to reveal)

The algae in corals have a sunlight tolerance - they do not want too much or too little. If the level of sunlight is beyond the optimum range, corals can bleach.

Typically, corals bleach when sunlight in shallow waters is too intense, but if the symbiotic algae in corals do not receive enough sunlight, they can struggle to photosynthesize enough to support themselves and their coral symbiont.

🌊 Extreme Low Tides (click to reveal)

Exposure to air during extreme low tides can bleach corals.

Destructive fishing practices can also contribute to the damage inflicted upon coral reefs. Practices such as dynamite fishing, where explosives are used to stun or kill fish, can have devastating impacts on coral ecosystems. The explosions not only kill marine life directly but also destroy the physical structure of the reefs, leaving behind rubble instead of vibrant, healthy coral habitats.

Threats to coral reefs from overfishing and ways individuals can help. harmful fishing practices such as fishing in nurseries, indiscriminate fishing, targeting spawning aggregations, and catching too many large fish and advice on educating oneself on local fishing regulations, making sustainable seafood choices, and practicing catch-and-release are shown — ***Individuals can help coral reefs by making sustainable seafood choices, only taking fish for eating, and not releasing unwanted aquarium fish into the wild.***

Overfishing, as we learned in an earlier module, can disrupt the ecological balance of the reef. Fishing gear can inflict physical damage on coral reefs. For example, throwing an anchor overboard to fish at a coral reef can harm the reef if the anchor hits the reef. Lost fishing traps, known as ghost traps, fishing line, lost nets, and other abandoned fishing gear can physically damage coral reefs and entangle marine life.

Essential Knowledge

Coral Reefs have been suffering damage due to a variety of factors, including increasing ocean temperature, sediment runoff, and destructive fishing practices.

Oil Spills

Oil spills in marine waters have devastating consequences on the delicate ecosystem. When an oil spill occurs, organisms in the water are exposed to harmful hydrocarbons found in oil. These hydrocarbons can be toxic to marine life, leading to illness and even death of various species.

Oil that floats to the surface of the water forms a thick layer that sticks to the bodies of water birds and mammals. This coating disrupts the insulating properties of feathers and fur, making it difficult for the animals to regulate their body temperature. As a result, many birds and marine mammals suffer from hypothermia and other health issues.

Heavier components of oil sink to the bottom and impact bottom-dwelling organisms that are crucial to the marine food chain. The presence of oil on the ocean floor can smother these organisms, disrupting the balance of the ecosystem and leading to a cascading effect on the entire marine environment.

Oil that washes up on the beach impacts the economics of fishing operations as well as the tourism industry.

Explore the tabs below to learn about two famous oil spills: the Exxon Valdez spill in Alaska and the BP Deep Horizon spill in the Gulf of Mexico.

Exxon Valdez
Deepwater Horizon

Exxon Valdez

Where and When?

The Exxon Valdez oil spill occurred on March 24, 1989 in Prince William Sound, Alaska when the oil tanker struck a reef, running aground.

How much oil spilled?

Approximately 11 million gallons of oil (or about 260,000 barrels) spilled into the ocean.

What were the ecological impacts?

The spill covered about 1300 miles of coastline, resulting in the deaths of hundreds of thousands of seabirds, otters, seals, and whales.

Nearly 30 years later, pockets of oil remain in some locations.

What were the economic impacts?

Exxon paid about $2 billion in cleanup costs and another $1.8 billiion for habitat restoration and personal damages.

BP Deepwater Horizon

Where and When?

The BP Deepwater Horizon oil spill occurred on April 20, 2010 off the coast of Louisiana in the Gulf of Mexico when an oil rig suffered a series of explosions. For 87 days, oil flowed into the ocean before the well was finally capped.

How much oil spilled?

Approximately 210 million gallons (4.9 million barrels) of oil were released into the ocean, resulting in the largest marine oil spill in history.

What were the ecological impacts?

The spill covered about 1300 miles of coastline, resulting in the deaths of hundreds of thousands of seabirds, otters, seals, and whales.

Nearly 30 years later, pockets of oil remain in some locations.

What were the economic impacts?

The local fishing and tourism industry was deeply affected, especially since the spill affected the beaches during the busy tourism season on the Gulf Coast.

BP paid over $65 billion for the disaster, including $5.5 billion to the US government in a civil suit, $4 billion in a criminal suit, cleanup costs, fines, and compensation for damages.

By prioritizing prevention and adopting efficient remediation strategies, we can mitigate the adverse effects of oil spills and safeguard our oceans and their inhabitants. Together, we can work towards a cleaner and healthier marine ecosystem for future generations.

Essential Knowledge

Oil spills in marine waters cause organisms to die from the hydrocarbons in oil. Oil that floats to the surface of water can coat the feathers of birds and fur of marine mammals. Some components of oil sink to the ocean floor, killing some bottom-dwelling organisms.

Oil that washes up on the beach can have economic consequences on the fishing and tourism industry.

Dead Zones

Aquatic organisms need oxygen to survive. They “breathe” dissolved oxygen (DO). Dead zones are places in water where there is no oxygen, making it difficult or impossible for life to survive. Dissolved oxygen can be reduced when sediments, especially clay, are present in runoff or if fertilizers are present in runoff, which can catalyze cultural eutrophication, which reduces the amount of oxygen in the water.

A satellite image of the United States, with agricultural areas highlighted bright green and major cities highlighted in red. All of these flow into the Mississippi river, which flows into the Gulf of Mexico, creating a dead zone, shown in yellow and blue. — Green areas indicate agricultural land currently in production and the red areas are large cities. Each of these entities contributes to the formation of the “dead zone” in the Gulf of Mexico (yellow/blue area).

A huge, recurring dead zone exists in the Gulf of Mexico. This dead zone is the result of enormous amounts of nutrients and other contaminants washing into the Gulf from the Mississippi River.

When pollutants enter an aquatic ecosystem, they are high near the source, and gradually decrease in concentration the farther you get from the source. This means that eutrophication and biological oxygen demand (BOD) are highest near the source and dissolved oxygen is lowest near the source.

When you graph this phenomenon of low DO and high BOD near the source of the pollutant and low BOD and high DO the farther you get from the source, it is known as an Oxygen Sag Curve.

The changes in pollutant concentration and dissolved oxygen levels in a water body, conveying the impact of pollution on aquatic life and water quality over time or distance downstream. — ***This graph is called a “sag curve” due to the “sagging” of the DO level just downstream from the pollutant source.***

Essential Knowledge

Oceanic dead zones are areas of low oxygen in the world's oceans caused by increased nutrient production.

An oxygen sag curve is a plot of dissolved oxygen levels versus the distance from a source of pollution, usually excess nutrients and biological refuse.

Heavy Metal Pollution

Heavy metals can enter waterways, ending up in groundwater, making it unusable for human consumption or irrigation. They can also bioaccumulate and biomagnify in the food chain, coming back to haunt us in our food.

☠️ Mercury (click to reveal)

Mercury can enter waterways as runoff from industrial and mining operations as well as from the use of metal-containing fossil fuels, such as coal.

When mercury enters water, bacteria in the water convert it to highly toxic methylmercury.

A map of Japan, highlighting the location of Minamata and the Chisso factory — **The red area in the image above is the general area of the Minamata Disease outbreak.**

A famous case study of mercury poisoning was the town of Minamata in Japan. Between 1932 and 1968, Chisso, a fertilizer manufacturing company, released between 210 and 45,245 tons of mercury/year into the waterways near the fishing village of Minamata. The methylmercury bioaccumulated in fish and other marine life, which was then consumed by the residents of Minamata, who relied heavily on fishing for food. Over 2200 residents of Minamata developed what is now known as Minamata Disease. Afflicted individuals developed skeletomuscular deformities, lost the ability to control their motor functions, in addition to vision, hearing, and speech loss. Over 1700 individuals afflicted ended up dying as a result of their exposure.

The Chisso corporation has paid over $86 million in damages to families.

☠️ Lead (click to reveal)

Lead from pre-1986 gasoline, incinerators, and paints can contaminate waterways. Lead is very toxic to humans in small quantities, so it is highly regulated and no longer allowed in gasoline or paint. It can bioaccumulate in the food chain, leading to chronic and acute toxicity, which may cause neurological damage, liver and lung diseases, kidney damage, and even cancer.

☠️ Acid Mine Drainage (click to reveal)

A gif of a scientist in gloves testing the pH of running water — **A scientist tests acidic runoff.**

Acid mine drainage usually contains acidic water with heavy metals and sulfates. This occurs through the oxidation of sulfide minerals like pyrite (iron sulfide) when exposed to water and air. The process generates sulfuric acid and dissolved iron, which can lead to the formation of red, orange, or yellow sediments in streams.

Acid mine drainage from coal and other mines can also pollute waterways, making them too acidic for organisms to survive and making the water supply unfit for humans to drink.

Essential Knowledge

Heavy metals used for industry, especially mining and burning of fossil fuels, can reach the groundwater, impacting the drinking water supply.

When elemental sources of mercury enter aquatic environments, bacteria in the water convert it to highly toxic methylmercury.

Litter

Litter that reaches aquatic ecosystems poses serious threats to wildlife and the environment. Not only is it unsightly, but it can also have harmful effects on the delicate balance of these habitats. For example, marine animals such as turtles and seabirds may mistake plastic bags for food, leading to intestinal blockages and eventual death. Similarly, small aquatic organisms can get entangled in discarded fishing nets, leading to injuries or drowning.

Sea turtles eat jellyfish as part of their normal diet. Unfortunately, plastic bags in water can look a lot like jellyfish! Try the quiz below! We call this, "Are you smarter than a sea turtle?"

In addition to serving as a physical hazard, litter in aquatic ecosystems can also introduce toxic substances into the food chain. For instance, chemicals from plastic debris can leach into the water, affecting the health of fish and other marine animals. This contamination can have far-reaching consequences, impacting not only the aquatic life directly exposed to the litter but also organisms higher up in the food chain, including humans who consume seafood.

Essential Knowledge

Litter that reaches aquatic ecosystems, besides being unsightly, can create intestinal blockage and choking hazards for wildlife and introduce toxic substances to the food chain.

Sediment Pollution

Industrialized agriculture, deforestation, channelized waterways, overgrazed rangelands, construction sites, strip-mine areas, and other human activities erode topsoil along our rivers. Before agriculture disrupted the natural flow of sediments, rivers deposited sediments in their floodplains and near the mouth of the river.

Read through the presentation below to learn more about the cascade of effects caused by sediment pollution in aquatic ecosystems, eventually resulting in the collapse of the food chain:

Sediment pollution can also smother aquatic organisms and deposit toxins, making food fish hazardous to humans. Sediments can also clog waterways, which can interfere with transportation and commerce.

Essential Knowledge

Increased sediment in waterways can reduce light infiltration, which can affect primary producers and visual predators. Sediment can also settle, disrupting habitats.

AP Exam Tip

It is important to know the sources of aquatic pollution.

You should know the general ideas behind Exxon Valdez, Deepwater Horizon, and Minamata disasters - the College Board has asked about these disasters before!

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