Human Activities on Aquatic Ecosystems (College Board AP® Environmental Science): Study Guide

Ecosystem tolerance range

• Organisms have an optimum range for each factor where they can maintain homeostasis

• Organisms, therefore, have a range of tolerance for various pollutants

• Outside of this range, organisms may experience physiological stress, limited growth, reduced reproduction, and, in extreme cases, death

• The bell graph above shows the optimal range for a species of fish to thrive; however, this curve can also be used to indicate species tolerance to various pollutants

Human impact on coral reefs

• Coral reefs have been suffering damage due to a variety of factors

  • Sediments and fertilizers from agriculture, sewage, and urban development runoff into the ocean, suffocating corals and promoting algae growth

  • Destructive fishing practices like

    • Blast fishing or cyanide to stun and remove live fish

    • Overfishing harms coral ecosystems by removing key reef species

    • Bottom trawling can break reef structures and stir up sediment

  • Climate change causes:

    • Coral bleaching due to rising ocean temperatures, releasing the symbiotic algae that feed corals, leading to reef death

    • Acidification through increased ocean CO₂ lowers the pH, making it hard for corals to form calcium carbonate skeletons

  • Over-harvesting live sponges and corals for aquariums can destroy reef ecosystems

The aquatic consequences of oil spills

• Marine life like seabirds, fish, and marine mammals can be directly harmed through oil spills

  • Depending on its chemical composition, direct contact with oil can cause skin irritation, physiological disruption, and poisoning in marine species

• Oil that floats on the surface of water can coat the feathers of birds and fur of marine mammals who will ingest the oil when cleaning themselves, leading to death

  • Birds may lose their buoyancy and ability to fly if their feathers are coated with oil

  • In sea otters, oil disrupts air insulation, making them vulnerable to hypothermia

  • The eggs of turtles, fish, and shellfish are contaminated by oil spills

  • Adult fish can suffer a lower growth rate, enlarged livers, changes in heart and respiration rates, fin erosion, and reproduction issues

  • Oil spills can make fish and shellfish dangerous to eat

• Oil spills harm primary producers like phytoplankton and zooplankton, disrupting the food web and affecting larger predators

• Oil spills reduce sunlight for aquatic photosynthetic organisms

• Decomposing oil in water depletes dissolved oxygen, causing hypoxia and stressing aquatic life

• Oil can suffocate coral reefs and seagrass beds, while some elements of oil sink to the ocean floor, harming sea-floor organisms

• Degradation of habitats, nursery and feeding grounds leads to loss of biodiversity

• Increased risk of coastal erosion as oil can settle deep in the root systems of mangrove and salt marshes, effectively choking the roots and limiting oxygen and nutrient uptake and killing the plant

• The severity of these effects depends on the type of oil spill, its size, and the conditions in the environment

  • If an environment is unhealthy to start with, then the effects will be more devastating

• Exposure to oil toxins can have long-term effects on reproduction and population numbers

Economic impact of oil spills

• Although the economic impacts of spills are mostly negative, there are a few beneficial impacts on the local economy

• Positive economic impacts are short-term and include:

  • Increase in job opportunities during the cleanup

  • Local revenue increases as money is spent in shops, restaurants, and hotels

  • Financial aid is brought into the local economy through grants, settlements and disaster relief

• Negative economic impacts are long-term and include:

  • The cleanup needs expensive equipment, staff, and specialized techniques, which significantly impacts the company responsible for the oil spill

  • For the company, there are financial losses of not only the spilt oil but also legal settlement costs and substantial fines

  • Loss of tourist revenue due to concerns about oil-contaminated beaches and waters, which can lead to job losses

  • Loss of revenue for commercial fishing owing to polluted fish population and seafood safety concerns costing fishermen and related businesses money

  • Oil spill contamination can lower coastal property values, affecting local real estate markets

Oceanic dead zones

• Where there is no life to be found in bodies of water, these are called 'dead zones'

• Caused by very low or absent levels of dissolved oxygen

• Usually due to eutrophication, oil spills, dumping of toxic chemicals, and other human activities that lead to algal blooms that remove oxygen from the air when they break down

• These zone usually occur in coastal regions where nutrient runoff is high

• For example, the Gulf of Mexico experiences one of the largest dead zones, largely caused by nutrient pollution from agricultural runoff in the Mississippi River

• The Baltic Sea is another region plagued by dead zones, largely due to nutrient runoff from agriculture and sewage

• Dead zones can lead to:

  • Severe disruptions in marine food chains

  • Collapse of local fisheries

The oxygen sag curve

• An oxygen sag curve is a plot of dissolved oxygen levels versus the distance from a point-source pollution, usually excess nutrients and biological waste or refuse

• Scientists assess the health of aquatic ecosystems by measuring dissolved oxygen, nutritional levels, and the presence of disease-causing organisms or toxins

• The curve can also show how much PPM (parts per million) of oxygen different fish species need to survive

Dissolved oxygen

• Water contains dissolved oxygen that organisms can use for aerobic respiration, but this also removes oxygen from aquatic habitats

• The cooler the water, the more dissolved gases like oxygen it can hold

• Dissolved oxygen is introduced to aquatic environments through air-water interactions such as waves and oxygen from photosynthesizing organisms

• Low dissolved oxygen can lead to hypoxia, suffocating or killing aquatic life

Biochemical oxygen demand

• Biochemical oxygen demand (BOD) is a measure of the amount of dissolved oxygen required to break down the organic material in a given volume of water through aerobic biological activity

• When there is a higher abundance of organisms or an increased rate of respiration, more oxygen is consumed

• The ecosystem is considered to be hypoxic if BOD exceeds the systems dissolved oxygen

• If there is no dissolved oxygen in the water, then it is anoxic

Awaiting image of dissolved oxygen sag curve

The graph shows the stages of oxygen depletion in water from the point of pollution over time

Heavy metals & groundwater

• Heavy metals enter water due to human activities like mining, industrial waste disposal, and agricultural practices

• Heavy metals are highly toxic when present in high concentrations and pose a serious threat to drinking water supplies and ecosystems

• These elements do not degrade in the environment and can build up in living tissue (a process known as bioaccumulation), disrupting the food chain

• Key heavy metals include:

  • Lead (Pb) is used in ceramic glazes and crystal glassware, batteries, pipes, roofing, scientific electronic equipment, military tracking systems, medical devices, and products to shield X-rays and nuclear radiation

  • Mercury (Hg) is a naturally occurring metal that is liquid at room temperature and produces a toxic, odorless vapor. It is found in button-cell batteries and fluorescent lights

  • Arsenic (As) can naturally occur in some places, which contaminates groundwater

  • Cadmium (Cd) is used in batteries and industrial processes

  • Chromium (Cr) has varying toxicity due to existing in different forms

Aquatic ecosystems & litter

• Litter, particularly plastic, and other pollutants can harm aquatic ecosystems by introducing excess nutrients and toxins into the water from human activities like construction, agriculture, and waste disposal

• Besides being unsightly, litter in any form can cause internal damage, digestive tract blockage, and a false impression of fullness, leading to malnutrition and death

• Fishing lines, plastic rings, and nets can entangle sea turtles, marine mammals, and birds, limiting their movement, injuring them, and possibly killing them by starvation or drowning

• Litter can choke sensitive ecosystems like coral reefs and seagrass beds

• Plankton and other species can ingest microplastics, which can disrupt the food chain and bioaccumulate

• Plastics absorb pollutants from the water, releasing harmful chemicals into the environment when ingested by animals

Sediment in waterways

• Sediment blocks sunlight from reaching aquatic plants, which are essential aquatic primary producers, affecting photosynthesis and food availability in the ecosystem

• Fine sediment can reduce visibility for predators as well as block the gills of fish, reducing their ability to exchange oxygen

• Sediment can also settle, disrupting habitats and burying fish eggs (which then die), impacting population dynamics

• Sediment can bind to and transport pollutants such as heavy metals and pesticides over long distances, impacting aquatic life when ingested in other oceans

Mercury in the aquatic environment

• When sources of mercury enter aquatic environments, bacteria in the water convert it to highly toxic methylmercury, which accumulate in the bodies of fish

• As larger fish eat smaller fish, the concentration of methylmercury within the tissues of these fish increases, leading a build up in humans who eat large predatory fish such as tuna or swordfish; this is known as biomagnification