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BIO130 · Human Impacts Module · Pre-Lab

Microplastics: Sources, Pathways & Pollution

Explore how plastic pollution enters marine ecosystems — and why size is the most deceptive thing about it.

Human Impacts

Pre-Lab Reading

Learning Objectives

Define microplastics and distinguish primary from secondary sources

Identify major pathways microplastics use to enter marine systems

Recognize common plastic polymer types and their environmental persistence

Connect microplastic ingestion to food web dynamics

Definition & Classification

What Exactly Is a Microplastic?

The term "microplastic" was coined by marine biologist Richard Thompson in 2004. It refers to plastic particles smaller than 5 millimeters — roughly the size of a sesame seed at maximum. But they can get much, much smaller. The range is enormous, and size determines where they go and what they do.

Plastic Particle Size Spectrum

Nanoplastics <1μm → Increasingly small → Macroplastics >5mm

Nanoplastics

Nano

< 1 μm

Can enter individual cells; cross blood-brain barrier

Microplastics

Small Micro

1 μm – 1 mm

Ingested by zooplankton; found in blood & organs

Microplastics

Large Micro

1 mm – 5 mm

Ingested by fish, seabirds; mistaken for fish eggs

Macroplastics

Macro

> 5 mm

Entanglement hazard; breaks down into micro/nano over time

Key insight: There is no lower size limit for microplastics. As UV radiation and wave action break down larger pieces, particles continue to fragment — potentially indefinitely. A single plastic bag can eventually become billions of nanoplastic particles.

Origin Types

Primary vs. Secondary Microplastics

Not all microplastics start out as macroplastics. Some are manufactured small on purpose.

⚙️ Primary Microplastics

Intentionally manufactured at microscopic sizes for industrial or consumer use. They enter waterways directly — never needing to break down first.

Microbeads in exfoliants & toothpastes
Nurdles (pre-production pellets)
Microfibers from synthetic fabrics
Industrial abrasive blasting media
Cosmetic glitter particles

~35% of ocean microplastics

🌊 Secondary Microplastics

Formed by the physical, chemical, or biological breakdown of larger plastic items. UV degradation, wave action, and abrasion all contribute.

Fragmented plastic bottles & bags
Degraded fishing nets (ghost gear)
Tire rubber particles from roads
Painted surface degradation
Agricultural plastic film breakdown

~65% of ocean microplastics

Polymer Types

Common Plastic Polymers in the Ocean

Different plastic types have different densities, degradation rates, and ecological effects. Density determines whether they float, sink, or stay suspended mid-water.

🧴 Polyethylene (PE) — most abundant in ocean +

Low density — floats. Makes up the bulk of surface microplastics. Highly resistant to biodegradation (estimated 450–1000 years). Absorbs persistent organic pollutants (POPs) from surrounding water, concentrating toxins on particle surfaces.

Plastic bagsBottlesPackaging filmFood containers

🧵 Polypropylene (PP) +

Low density — floats. Very common in marine debris. Second most prevalent plastic type found in ocean samples. Like PE, it acts as a sponge for hydrophobic pollutants such as PCBs and DDT.

Bottle capsFood packagingRope & twineStraws

🧪 Polystyrene (PS) +

Moderate density — can float or sink depending on form. Expanded polystyrene (foam) breaks into tiny white beads that are readily ingested. Contains styrene monomers that leach from the plastic matrix, acting as endocrine disruptors.

Foam cupsFood containersPackagingDisposable cutlery

🧶 Polyester / Nylon (synthetic fibers) +

Microfibers shed from synthetic textiles during washing — up to 700,000 fibers per laundry load — pass through wastewater treatment and enter waterways. Extremely abundant in deep sea sediments and Arctic ice. Fibers are especially problematic because they resemble zooplankton food sources and tangle in gills and digestive tracts.

Fleece jacketsSportswearCarpetsFishing nets

Interactive Explorer

Microplastic Sources

Plastics enter marine environments from hundreds of different source categories. Use the filters below to explore by source type. Click any card to learn more.

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Urban Stormwater Runoff

Land-Based

Rain washes plastic debris, tire particles, and litter from roads and parking lots directly into storm drains, which typically discharge untreated into waterways.

Major pathway in coastal cities

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Agricultural Runoff

Land-Based

Plastic mulch films, irrigation tubing, and pesticide containers degrade in fields. Fragments are carried by irrigation water and rainfall into rivers and coastal zones.

Under-studied but widespread

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Landfill Leaching

Land-Based

Open or poorly managed landfills allow plastic debris to be windblown and leached into groundwater. Especially prevalent near coastlines in developing nations.

Global disparity in impact

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Wastewater Treatment

Land-Based

Treatment plants remove most large particles but microfibers and nanoplastics pass through. Effluent carries millions of particles per liter directly into surface water.

700,000+ fibers per laundry load enter system

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Tire & Road Wear

Land-Based

Tire abrasion against road surfaces generates rubber and plastic particles that accumulate on roadsides. Stormwater carries these into drainage systems and ultimately waterways.

Leading source by mass in some regions

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Fishing Gear (Ghost Gear)

Ocean-Based

Lost, abandoned, or discarded fishing nets, lines, and traps degrade at sea into microplastics. An estimated 640,000 tons of fishing gear are lost annually.

Largest plastic type by mass in ocean

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Shipping & Cargo

Ocean-Based

Containers lost overboard release large quantities of goods (including plastic pellets, called nurdles) directly to the ocean. Paint flaking from ship hulls is also a significant microplastic source.

~10,000 containers lost per year globally

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Offshore Platforms

Ocean-Based

Oil and gas platforms use plastic components, piping, and coatings that degrade and fragment into marine environments over decades of operation.

Persistent point sources

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Atmospheric Deposition

Atmospheric

Microplastics have been found in rainfall, snow, and air samples at remote mountain summits and in the Arctic. Fibers and fragments are transported globally by wind before depositing on ocean surfaces.

Found in Pyrenees, Himalayas, Arctic

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Industrial Emissions

Atmospheric

Plastic manufacturing and incineration release particles and fibers into the atmosphere. These can travel hundreds to thousands of miles before settling onto ocean surfaces.

Emerging area of research

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Personal Care Products

Consumer Products

Microbeads in facial scrubs, toothpastes, and exfoliants wash directly down the drain. Though banned in many countries, they remain in use globally and in legacy contamination.

Banned in US, UK since 2018

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Synthetic Textiles

Consumer Products

Polyester, nylon, and acrylic clothing shed hundreds of thousands of microfibers per wash cycle. These fibers are extremely abundant in ocean sediment worldwide.

35% of all ocean microplastics

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Single-Use Packaging

Consumer Products

Bottles, bags, straws, and food wrappers are the most visually obvious plastic pollutants. They fragment into microplastics over months to years when exposed to UV and wave action.

8 million tons enter ocean annually

Transport & Accumulation

How Microplastics Move Through Marine Systems

Once in the water, microplastics don't stay put. They travel through complex physical and biological processes, eventually accumulating in unexpected places — from deep sea trenches to Arctic ice. Click each stage below to learn more.

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Land Sources

Entry point

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Runoff & Rivers

Transport

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Coastal Zone

First accumulation

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Ocean Gyres

Concentration

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Sinking & Settling

Deep sea sink

🏙️ Land Sources — Where It All Begins

The majority of ocean plastic comes from land — estimates range from 70–80%. Rivers act as the primary delivery mechanism, with the most plastic-polluted rivers in Asia and Africa contributing disproportionately to global ocean input. In coastal cities, stormwater runoff can deliver millions of particles directly to the sea within hours of a rain event.

Vertical Distribution

Where in the Water Column Do They Go?

Density differences between plastic types determine whether particles float, sink, or stay suspended — with major consequences for which organisms are exposed.

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Surface Microlayer

Low-density plastics (PE, PP) concentrate at the very surface. This thin layer is a critical nursery habitat for many marine larvae. Surface concentration can be 2–6× higher than even 50 cm below.

Density < 1.02 g/cm³ → floats

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Photic Zone (0–200m)

Where photosynthesis occurs — and where most commercially fished species live. Biofouling (organisms colonizing plastic surfaces) adds weight, causing particles to sink deeper over time.

Most ecological exposure occurs here

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Deep Sea Sediment

Deep sea trenches contain some of the highest microplastic concentrations ever recorded. Particles sink through biological pump processes (fecal pellets, marine snow) or when biofouled. The deep sea may be the ultimate plastic sink.

Mariana Trench: ~2,000 pieces/L sediment

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Sea Ice (Cryosphere)

Arctic sea ice contains exceptionally high concentrations of microplastics — up to 12,000 particles per liter of ice meltwater. As climate change accelerates ice melt, this frozen reservoir is being released.

Arctic as release valve for stored plastic

The Garbage Patches: The Great Pacific Garbage Patch is often described as a "floating island of trash" — but the reality is more insidious. It is a diffuse soup of mostly microplastics spread across an area larger than Texas. Much of it is invisible to the naked eye. You can't see it from a boat, but the fish certainly can.

Ecological & Human Health

Why Microplastics Are an Ecological Emergency

Size is the trick. Organisms can't distinguish microplastics from food. The smaller the particle, the deeper it penetrates into tissues, cells, and biological barriers — including barriers we thought were impenetrable.

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Microplastics Have Been Found In:

Human blood, lung tissue, placental tissue, breast milk, fetal meconium, stool, and — in a 2024 study — arterial plaque. People with higher microplastic concentrations in their arterial plaques had a significantly elevated risk of heart attack and stroke. We are, in the most literal sense, part of the pollution cycle now.

Human blood (2022) Placenta (2021) Breast milk (2022) Lung tissue Fetal tissue Arterial plaques (2024)

Mechanisms of Harm

How Microplastics Cause Damage

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Physical Ingestion

Particles are mistaken for food at every trophic level. Phytoplankton consume nanoplastics. Zooplankton eat small microplastics. Fish eat larger fragments. Seabirds feed plastic to chicks. Physical blockage, false satiety, and gut damage are the direct results.

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Chemical Toxicity

Plastics contain additives — plasticizers, flame retardants, colorants, stabilizers — many of which are endocrine disruptors. Particles also act as vectors, carrying adsorbed pollutants (PCBs, DDT, heavy metals) into organisms that ingest them.

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The Plastisphere

Microplastic surfaces are colonized by microbial biofilms — the "plastisphere." These communities include pathogenic bacteria and harmful algae, which hitchhike on plastic to new environments, potentially spreading disease and disrupting native microbiomes.

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Biomagnification

Chemical contaminants associated with microplastics accumulate and concentrate as they move up the food chain. An apex predator may carry concentrations millions of times higher than surrounding water. You'll model this process in the next lab.

→ Explore in the Simulator Lab

Preview — Biomagnification: When an organism eats contaminated food, it retains most of the contaminant but excretes the food mass. At each trophic level, the concentration increases — this is called biomagnification. A zooplankton eating contaminated phytoplankton accumulates concentration. A small fish eating 10,000 zooplankton accumulates far more. A tuna eating thousands of small fish carries a staggering load. A human eating that tuna sits at the top of a very toxic pyramid.