Marine Pollution Science

Plastic Pollution

An estimated 8 to 12 million metric tons of plastic enter the ocean annually, accumulating in all ocean environments from surface waters to the deepest trenches. Plastic does not biodegrade but fragments into progressively smaller pieces through UV radiation, wave action, and mechanical abrasion. Microplastics (fragments smaller than 5 millimeters) now contaminate every marine environment sampled, from Arctic sea ice to deep-sea sediments, at concentrations reaching thousands of particles per cubic meter in polluted coastal waters.

Physical impacts of plastic pollution include entanglement (affecting over 700 marine species including seabirds, turtles, and marine mammals), ingestion (documented in over 900 species), and habitat alteration (smothering of seafloor communities by accumulated debris). Sea turtles mistake plastic bags for jellyfish prey, and seabirds like albatrosses feed plastic fragments to chicks, which die of starvation with stomachs full of indigestible material. Ghost fishing by abandoned nets and traps kills an estimated 640,000 tons of marine animals annually.

Chemical impacts arise because plastics absorb hydrophobic pollutants (PCBs, DDT, PAHs) from seawater at concentrations up to one million times higher than surrounding water. When organisms ingest contaminated microplastics, these concentrated pollutants can transfer to tissues. Additionally, plastic additives (phthalates, bisphenol A, flame retardants) leach from fragments into surrounding water and organisms. The full toxicological consequences of chronic microplastic exposure to marine food webs remain an active area of research.

Chemical Contaminants

Persistent organic pollutants (POPs) including PCBs, DDT metabolites, and PFAS compounds accumulate in marine food webs through biomagnification, reaching concentrations in top predators millions of times higher than ambient seawater levels. Orca populations in the northeast Pacific and European waters carry PCB body burdens high enough to cause reproductive failure and immune suppression, threatening population viability decades after these chemicals were banned from production.

Heavy metals (mercury, lead, cadmium, arsenic) enter the ocean through industrial discharge, atmospheric deposition from coal combustion, and mining runoff. Mercury is particularly concerning because microbial processes in ocean sediments convert inorganic mercury to methylmercury, a highly toxic organic form that biomagnifies through food webs. Tuna, swordfish, and sharks accumulate methylmercury at concentrations that prompt human consumption advisories in many countries.

Oil pollution from tanker accidents receives dramatic media coverage, but chronic inputs from land-based runoff, routine shipping operations, natural seeps, and offshore production collectively exceed catastrophic spill volumes. Oil toxicity varies by composition: lighter aromatic compounds (benzene, toluene, PAHs) are acutely toxic to marine larvae and early life stages, while heavier fractions cause physical smothering and chronic sublethal effects including immune suppression and reproductive impairment.

Nutrient Pollution and Dead Zones

Excess nitrogen and phosphorus from agricultural fertilizers, sewage, and atmospheric deposition cause eutrophication in coastal waters. These nutrients stimulate algal blooms that, upon dying and sinking, consume dissolved oxygen through bacterial decomposition. When oxygen drops below 2 milligrams per liter (hypoxia), most fish and invertebrates flee or die. Over 700 coastal dead zones have been identified globally, with the Gulf of Mexico dead zone reaching 22,000 square kilometers in severe years.

Harmful algal blooms (HABs) represent a particularly dangerous consequence of nutrient pollution. Certain phytoplankton species produce potent toxins that accumulate in shellfish (causing paralytic, diarrhetic, or neurotoxic shellfish poisoning in humans), kill fish directly, or produce aerosol toxins that cause respiratory distress in coastal residents. Red tides caused by Karenia brevis in the Gulf of Mexico kill millions of fish and cause significant economic losses to tourism and fisheries during bloom events.

Noise Pollution

Anthropogenic ocean noise from shipping, sonar, seismic surveys, and construction has doubled ambient noise levels in many ocean regions every decade since the 1950s. Marine mammals that depend on sound for communication, navigation, and foraging face increasing masking of their signals, behavioral disturbance, and in extreme cases (military sonar), physical injury or death. North Atlantic right whales show chronic stress responses correlated with shipping noise levels, and reduced vessel traffic during the COVID-19 pandemic measurably decreased whale stress hormone levels.