8.2 Human Impacts on Ecosystems

Each species has a range of tolerance for factors such as temperature, salinity, pH, dissolved oxygen and pollutant levels (topic 2.4). Near the middle of that range is its optimal zone, where it thrives. Toward the edges are zones of physiological stress, where it survives but grows and reproduces poorly. Beyond the limits it dies.

Pollution works by pushing conditions toward or past those limits. A pollutant may not kill an organism outright but may stress it enough to reduce reproduction, which over time shrinks the population.

Coral reefs cover well under 1 percent of the ocean floor but shelter roughly a quarter of all marine species. They are very sensitive because they tolerate only a narrow range of temperature, light and water clarity. The main threats are:

• Warmer water, which causes coral bleaching (topic 9.6).
• Sediment runoff from logging, farming and construction, which clouds the water so less light reaches the corals' symbiotic algae and can smother the coral.
• Nutrient runoff, which feeds algae that overgrow reefs.
• Destructive fishing, such as blast fishing with explosives and poisoning fish with cyanide to stun them for capture, which shatters or kills the reef.
• Ocean acidification, which makes it harder for corals to build skeletons (topic 9.7).

Oil spills come from tanker accidents, offshore drilling blowouts, pipeline leaks and runoff. Major examples are the Exxon Valdez tanker spill in Alaska's Prince William Sound (1989) and the Deepwater Horizon drilling-rig explosion in the Gulf of Mexico (2010), which leaked oil for 87 days.

Oil floating on the surface coats the feathers of birds and the fur of sea otters and seals, so they lose insulation and buoyancy and die of cold or drowning. Animals that swallow oil while cleaning themselves are poisoned by its hydrocarbons. Heavier parts of the oil sink and smother organisms on the seafloor, and oil washing ashore kills marsh plants and shellfish. Spills also hurt fishing and tourism economies. Cleanup uses floating booms to contain the oil, skimmers to collect it, chemical dispersants and controlled burning.

Plastic waste reaches the ocean from rivers, beaches, fishing boats and storm drains. Because plastic breaks into smaller pieces instead of decomposing, it lasts a very long time. Animals such as sea turtles, seabirds and whales swallow it, filling their stomachs so they starve. Others, such as seals, get entangled in lost fishing nets and plastic rings and drown or are injured. Tiny microplastics are eaten by plankton and small fish and can carry toxic chemicals up the food chain. Currents gather floating debris into areas such as the Great Pacific Garbage Patch.

When sewage, manure or other organic waste enters a river, bacteria decompose it and use up dissolved oxygen. If you plot dissolved oxygen against distance downstream from the source, you get an oxygen sag curve: oxygen is high upstream, drops to a low point some distance below the outfall where decomposition is fastest, then slowly climbs back as the waste is used up and air mixes into the water. Pollution-sensitive species such as trout and mayfly larvae disappear in the sag zone, while tolerant species such as sludge worms take over. Large amounts of nutrients reaching the coast cause the same oxygen loss on a bigger scale, creating ocean dead zones (topic 8.5).

Heavy metals such as mercury, lead, arsenic and cadmium come mainly from mining and from burning fossil fuels. They can seep into groundwater and contaminate wells and drinking water supplies.

Sediment washed in from farms, clear-cut slopes, construction sites and dredging clouds the water. Less light reaches algae and underwater plants, so photosynthesis drops, and predators that hunt by sight have a harder time finding prey. Settling sediment also buries fish eggs and bottom-dwelling animals.