Topic 8.2 Notes – Human Impacts on Ecosystems

How Human Activities Stress Aquatic Ecosystems

Aquatic pollution is not just “poison in water.” Human activity can change physical conditions like turbidity or temperature, chemical conditions like pH or dissolved oxygen, and biological processes like decomposition and algal growth.

What happens depends on more than how much pollutant is present. Effects also depend on:

• Toxicity: how harmful the substance is
• Persistence: how long it stays in the environment
• Location: where it enters and where organisms are exposed
• Interactions: temperature, nutrients, sediment, and water movement can make effects worse

Organisms have a range of tolerance for factors like temperature, salinity, pH, dissolved oxygen, turbidity, and pollutant concentration.

• In the optimum range, they maintain homeostasis and can grow and reproduce.
• Outside that range, they show physiological stress. Growth and reproduction drop before death happens.
• Different species have different tolerance ranges, so pollution often changes the community structure first.

That is why indicator species matter. Sensitive species disappear first, while pollution-tolerant species remain.

A common AP chain is:

• human activity → pollutant or altered condition → mechanism → ecosystem response

Dissolved Oxygen, BOD, and Dead Zones

Dissolved oxygen and biological oxygen demand

Dissolved oxygen (DO) is oxygen available in water for aerobic respiration. Fish and many aquatic organisms need it.

DO increases through:

• Diffusion from the atmosphere
• Photosynthesis by aquatic producers

DO decreases through:

• Respiration
• Decomposition

Warm water holds less oxygen than cool water, which is why heat stress and oxygen stress often go together.

Biological oxygen demand (BOD) is the amount of oxygen aerobic decomposers need to break down organic matter.

$$\text{BOD} = \text{oxygen needed by decomposers}$$

High BOD means decomposers are using lots of oxygen, so DO drops.

• Hypoxia = very low oxygen
• Anoxia = no oxygen

Oxygen sag curves

An oxygen sag curve shows dissolved oxygen versus distance downstream from a pollution source. In the graph here, the blue line shows BOD dropping downstream, and the orange line shows DO falling after the discharge point and then recovering.

Oxygen sag curve

What you need to notice:

1. Upstream clean zone has relatively high DO.
2. Organic waste or nutrient input increases decomposition.
3. BOD rises and DO falls.
4. The lowest DO is downstream, not right at the discharge point.
5. Farther downstream, dilution, mixing, reaeration, and photosynthesis help DO recover.

If both are shown, DO sags downward while BOD gradually declines downstream.

Oceanic dead zones

Dead zones are low-oxygen areas, usually caused by nutrient pollution, especially nitrogen and phosphorus.

The main sequence is:

• nutrient input
• algal bloom
• decomposition
• higher BOD
• lower DO

Warm water and stratification make this worse because oxygen-rich surface water does not mix well with deeper water.

Illustrative example:

• Gulf of Mexico dead zone linked to Mississippi River nutrient runoff

Major Human Impacts on Aquatic Ecosystems

Coral reef damage

Coral reefs depend on coral animals and their symbiotic photosynthetic algae.

• Increasing ocean temperature causes bleaching because corals lose the algae
• Sediment runoff increases turbidity, blocks light, and can smother corals
• Destructive fishing can break reef structure or poison reef organisms

A common visual comparison is a bleached reef next to a healthy reef, which helps you connect temperature stress with the loss of color and living algal partners.

Coral bleaching compared with a healthy reef

Oil spills

Oil spills harm water, coastlines, wildlife, and local economies.

• Hydrocarbons in oil are toxic
• Surface oil coats bird feathers and marine mammal fur, reducing insulation and waterproofing
• Some oil sinks and harms bottom-dwelling organisms
• Oil on beaches damages habitat and is hard to clean up
• Fishing and tourism often lose money after spills

Illustrative examples:

• Exxon Valdez
• Deepwater Horizon

Heavy metals and mercury

Heavy metals such as lead, mercury, and cadmium can come from mining, industry, waste, and fossil-fuel combustion.

• They can contaminate groundwater and drinking water
• They persist because they do not biodegrade

Mercury has a key extra step:

• mercury release → aquatic entry → bacterial conversion → methylmercury → food chain

Litter and sediment

Litter can cause:

• intestinal blockage
• choking
• entanglement
• toxic exposure

Sediment causes two major problems:

• Suspended sediment increases turbidity and reduces light, hurting primary producers and visual predators
• Deposited sediment buries eggs, benthic organisms, plants, and corals

Sediment can also carry phosphorus, pesticides, and metals.

How to Explain Impacts and Match Solutions

On FRQs, clear cause-and-effect earns points. A strong explanation usually follows this order:

1. Identify the human activity
2. Name the pollutant or altered condition
3. Explain the mechanism
4. State the organism or ecosystem effect
5. Match the solution to that cause

Examples of matching solutions:

• Reduce nutrient runoff for dead zones
• Control erosion for sediment pollution
• Improve wastewater treatment for high BOD and nutrient input
• Prevent oil releases for spill damage
• Manage mine drainage and industrial discharge for heavy metals
• Improve waste collection for litter

The best final sentence explains why the solution works by saying which link in the chain it breaks.