ATP: Lesson - Human Impacts on Ecosystems (Topic 8.2) š
ā³ 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.
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 speciesā 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.
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.
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:
š”ļø 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):
šŖØ 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.
š¢ļø 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.
š¤ļø 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.
š 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.
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.
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.
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.
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 ābreatheā 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 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.
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.
ā ļø 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 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.
ā ļø 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.
ā ļø Acid Mine Drainage (click to reveal)
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.
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.
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.
Increased sediment in waterways can reduce light infiltration, which can affect primary producers and visual predators. Sediment can also settle, disrupting habitats.
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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