Marine Pollution: Plastics, Oil, and Persistent Toxins

Core Idea

Marine pollution is the entry of harmful substances or energy into the ocean from human activity, and most of it starts on land. Plastic debris breaks into smaller pieces but does not readily biodegrade, so it builds up and is gathered by currents into the centers of subtropical gyres. Oil enters the sea from spills, routine discharges, runoff, and natural seeps, and it harms wildlife by coating and poisoning them. Persistent toxins such as mercury and some pesticides become more concentrated at each step up the food web, so top predators and the people who eat them carry the highest loads.

How It's Best Learned

Have students sort a list of pollution sources into land-based and ocean-based, then compare their sizes. Use a map of surface currents to predict where floating debris should collect, and compare it with maps of measured plastic concentrations in the gyres. Model biomagnification with a simple food chain, multiplying a toxin concentration at each step, and connect it to fish consumption advice.

Common Misconceptions

• The Great Pacific Garbage Patch is a solid island of trash you could walk on. (It is a wide region of the North Pacific gyre where plastic, mostly small fragments, is more concentrated than elsewhere; much of it is hard to see from a ship.)
• Most ocean pollution comes from ships and oil spills. (Most comes from land, through rivers, runoff, sewage, and the air; large spills are dramatic but supply a smaller share of oil than ongoing sources and natural seeps combined.)
• Plastic in the ocean dissolves away over time. (Sunlight and waves break plastic into smaller pieces called microplastics, but the material itself persists for a very long time.)

Explainer

The ocean receives what people release on land and at sea. You already know how surface currents form gyres and how energy and matter move through marine food webs. Those ideas explain where pollutants go and whom they harm.

Start with sources. Most marine pollution begins on land. Rivers carry trash, fertilizers, and chemicals to the coast. Rain washes oil and litter off streets. Sewage outfalls release nutrients and microbes. Some pollutants, such as mercury from burning coal, travel through the air and fall into the sea. Ships add waste, fuel, and occasional large spills.

Plastics are now found in every part of the ocean, from beaches to deep-sea sediments. A widely cited study led by Jenna Jambeck estimated that 4.8 to 12.7 million metric tons of plastic entered the ocean from land in 2010. Plastic does not readily biodegrade. Sunlight and waves break it into ever smaller pieces, and those smaller than 5 millimeters are called microplastics. Floating plastic is carried by currents toward the centers of subtropical gyres. The region of the North Pacific gyre often called the Great Pacific Garbage Patch is a broad area where small fragments are unusually common, and most pieces are too small to see from a ship. Animals are harmed by tangling in nets and lines and by swallowing plastic, which can block their guts.

Oil reaches the sea through tanker and pipeline spills, offshore well blowouts, routine discharges, runoff, and natural seeps. Once spilled, oil spreads, partly evaporates, mixes with water, and is slowly broken down by microbes. It coats birds and marine mammals, destroying the insulation of feathers and fur, and its toxic compounds harm fish eggs and larvae. Major events include the 1989 Exxon Valdez spill in Alaska and the 2010 Deepwater Horizon blowout in the Gulf of Mexico.

Persistent toxins such as mercury, PCBs, and DDT do not break down easily and are stored in fat or tissue. They increase in concentration at each level of the food web, a process called biomagnification. Top predators, such as tuna, seals, and seabirds, carry the most. People who eat a lot of large predatory fish can be exposed as well.

Nutrient pollution from farms and sewage adds another problem, feeding algal blooms and low-oxygen dead zones. Reducing marine pollution therefore depends mostly on actions on land: better waste handling, cleaner fuels, and less fertilizer runoff.