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Marine chemicals reach land animals through a chain of transport rather than by moving directly from seawater onto every nearby species. Pollutants enter the ocean, some accumulate in marine organisms, and mobile animals—most clearly seabirds—carry contaminated material back to land. Guano is the main documented route, with eggs, feathers and other biological deposits providing additional pathways.
The strongest evidence concerns seabird colonies, where marine-derived material can alter terrestrial soils and local food webs. The transfer can be locally important, but evidence is not strong enough to claim that every marine chemical biomagnifies, that every terrestrial animal is exposed, or that seabirds cause a major worldwide redistribution of pollutants.
The five-step pathway from seawater to land
Keeping the steps separate matters. Accumulation inside a marine food web and transport across the ocean–land boundary are related, but they are not the same process.
1. Pollutants enter seawater
Chemicals reach marine systems through atmospheric transport, river deposition, runoff and direct discharges. The NOAA-hosted review by Landrigan and colleagues (2020) describes these as routes for most of the contaminants it examined; crude oil is treated as an exception because its release and behavior differ from those of many other chemical pollutants. Land-based sources, ocean currents and climate-related circulation patterns help determine where contaminants occur.
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Because sources and circulation vary geographically, two coastlines can receive very different mixtures even when they are exposed to the same broad class of pollutant. A chemical’s concentration in a fish or bird therefore cannot be interpreted without considering its location and the local source history.
2. Marine organisms take up some of those chemicals
Once in seawater or marine sediments, contaminants can enter organisms through food and other exposure routes. The reviewed evidence spans several contaminant classes, so no single uptake rule applies to all of them. Some compounds are readily stored in tissue, some are transformed or eliminated, and others move through organisms with little long-term retention.
Bioaccumulation means that an organism retains a chemical from its surroundings and diet over time. It does not automatically mean that the concentration will rise at every succeeding trophic level.
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3. Predators consume contaminated prey
Food-web transfer occurs when a predator eats organisms that contain a contaminant. Methylmercury is a clear example discussed in the literature: concentrations can increase through parts of a marine food web, and the compound occurs in tissues of top-predator fish and marine mammals. More generally, predator fish often contain higher concentrations than lower-trophic fish, but that pattern has geographic differences and exceptions.
The distinction between bioaccumulation and biomagnification is essential. Bioaccumulation describes retention within an organism over time. Biomagnification describes an increase in concentration between trophic levels. A chemical can bioaccumulate without biomagnifying, and the extent of either process depends on the compound, the organisms and the food web.
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4. A mobile animal crosses the ecosystem boundary
Seabirds are the best-documented biological link. They forage in marine waters, then return to terrestrial breeding, roosting or nesting grounds. The 2022 systematic review by Grant, Bond and Lavers identifies guano and other allochthonous inputs—including eggs and feathers—as routes for bringing marine-derived pollutants onto land.
This is a physical transfer performed by the animal’s movement. The bird does not need to release a chemical directly into soil for the transfer to begin: contaminated prey can become contaminated feces, shed feathers or an egg deposited at a colony.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minute5. Deposited material enters terrestrial systems
Guano, eggs, feathers and carcass material can contact soil, plants, invertebrates and freshwater at or near a colony. The systematic review most clearly establishes effects on colony soils. From there, a contaminant may remain near the deposition site, bind to soil or sediment, be taken up by terrestrial organisms, or move through a local food web.
The evidence does not establish a uniform dose for every land animal. Exposure depends on the amount deposited, chemical persistence, soil and water conditions, distance from the colony, and whether a terrestrial species actually contacts or consumes the material.
| Stage | Process | What the evidence establishes |
|---|---|---|
| Source | Atmospheric transport, rivers, runoff or direct discharge deliver contaminants to the sea. | These are documented routes for most contaminant types reviewed by Landrigan et al. (2020); crude oil is an exception in that review. |
| Marine uptake | Organisms absorb or ingest chemicals. | Uptake varies by chemical and exposure route. |
| Trophic transfer | Predators eat contaminated prey. | Methylmercury can increase through food webs; increases are not universal for all chemicals. |
| Cross-boundary transport | Seabirds carry marine-derived material to land. | Guano, eggs and feathers are documented pathways. |
| Terrestrial deposition | Deposits contact colony soil and nearby organisms. | Colony-soil effects are better established than a dose for every terrestrial species. |
Why seabirds are the clearest example
Seabirds combine three properties that make cross-ecosystem transport visible: they feed at sea, travel repeatedly between marine and terrestrial environments, and gather in dense colonies. A colony can therefore receive repeated inputs from many foraging trips over a breeding season.
Grant, Bond and Lavers evaluated 181 publications in the first systematic literature review and meta-analysis focused on seabirds as vectors of nutrients and pollutants to terrestrial ecosystems. Their abstract describes seabirds as connecting marine and terrestrial systems by transporting marine-derived nutrients and pollutants to breeding, roosting and nesting grounds through guano and other allochthonous inputs such as eggs and feathers.
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Guano is the most direct route. A bird eats at sea, metabolizes or retains some portion of a contaminant, and excretes waste at its land-based colony. The deposit can change the chemical composition of soil and become available to organisms that live in or on that soil. Because many birds use the same colony repeatedly, the process can create a localized concentration gradient rather than a uniform signal across an entire region.
Eggs and feathers
Eggs can transfer chemicals from a female’s body to a nest site. Feathers can carry contaminants incorporated during growth and later become part of the colony’s organic debris. These routes are slower or less continuous than daily guano deposition, but they demonstrate that transport is not limited to feces.
Other animal-derived material
Dead chicks, prey remains and other biological debris can add marine-derived chemicals to terrestrial habitat. The reviewed literature specifically emphasizes guano, eggs and feathers; the importance of any additional material depends on the species and colony conditions.
Not every chemical biomagnifies
The phrase “pollutants biomagnify” is too broad. The 1994 review by Borgå and colleagues found that most chemicals it evaluated did not biomagnify, while identifying a subset with potential to do so and emphasizing substantial variation and uncertainty. That review is older, so it is best used for this general caution rather than as a current ranking of contaminants.
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Several factors can reverse an expected trophic pattern:
- Chemical properties: persistence, fat solubility, binding to proteins and metabolic breakdown affect whether a compound is retained.
- Food-web structure: a predator’s diet, prey size and feeding location determine which contaminants it receives.
- Age and size: concentrations can change as animals grow or accumulate exposure.
- Geography: source strength, currents and local chemistry alter environmental concentrations.
- Species biology: organisms differ in absorption, storage and detoxification.
The NOAA-hosted review notes that contaminant concentrations vary with geographic origin, age, size and species. It also gives a useful counterexample to a simple “larger predator equals more contamination” rule: sardines had higher PCB concentrations than cod or salmon in the comparison discussed by the authors.
What happens after deposition on land?
The clearest recipient is the soil beneath or near a seabird colony. Deposited material can change soil chemistry and expose terrestrial invertebrates, plants and microbes. Animals that eat those organisms, drink affected water or ingest soil may then encounter the contaminant.
This does not mean that a chemical necessarily moves through a long terrestrial food chain. A compound may bind tightly to soil, break down, be diluted by rainfall or remain concentrated near the colony. The pathway must therefore be described locally: marine prey to seabird, seabird deposit to colony habitat, and then contact with particular terrestrial organisms.
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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Anadromous fish provide another possible bridge. These fish grow partly in the ocean and migrate into rivers or lakes, where predators and scavengers can consume them. Subalusky and Post’s 2017 review appraisal discusses seabirds and anadromous fish as pathways linking sea, coast, freshwater and land. The evidence supports these as mechanisms, but not a single universal effect size for all landscapes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Local hotspots are better supported than global redistribution
A breeding colony can receive concentrated inputs because many birds repeatedly use the same ground. That local concentration is compatible with a much smaller effect at the scale of the whole ocean or planet. The 2017 appraisal cautions that local effects may be substantial while global influence is likely minor, given the limited evidence and the mixing and volume of ocean water.
In practical terms, distance from a colony, colony size, residence time, species diet and prevailing weather can matter more for a nearby terrestrial animal than the average contaminant concentration measured across a large marine region. A claim about a specific island or colony should not automatically be generalized to all land animals.
What scientists know—and what remains uncertain
Pollutant transfer has received less attention than nutrient transfer
The seabird systematic review found that research on pollutant and plastic transport is less developed than research on nutrient transfer. The pollutant literature has focused especially on toxic, bioaccumulative metals. Evidence involving persistent organic pollutants and chemicals associated with plastics is thinner, so the existence of a pathway should not be converted into a precise exposure estimate without measurements from the relevant species and site.
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A 2022 U.S. Geological Survey critical review by Kuo and colleagues compiled more than 500 peer-reviewed studies, more than 25,000 data entries, more than 2 million information bytes, more than 700 organic compounds, about 320 wild bird species and 60 domestic breeds. Even at that scale, the authors identified substantial data gaps and limited characterization for shorebirds, seabirds and raptors.
Those gaps affect both exposure and metabolism. A measured chemical in one bird species cannot automatically be used to predict how another species stores, transforms or eliminates it. Missing data are particularly important when a contaminant is less studied than metals or when a population feeds in a different marine region.
Plastic-associated chemicals need careful wording
Law’s 2017 review of plastic as a persistent marine pollutant discusses trophic transfer and associated chemicals. That background supports concern about movement through marine food webs, but it does not by itself establish a particular dose delivered to a terrestrial animal through seabird deposition.
How to evaluate a claim about marine chemicals reaching land animals
- Identify the source. Ask whether the chemical entered through air, rivers, runoff or a direct discharge, and whether the source is local or transported from elsewhere.
- Name the chemical. Do not treat methylmercury, PCBs, persistent organic pollutants, plastics-associated chemicals and crude oil as interchangeable.
- Separate accumulation from biomagnification. Determine whether the evidence shows retention in one organism, an increase between trophic levels, or both.
- Identify the carrier. Seabirds and anadromous fish move between ecosystems in different ways; a stationary marine organism does not create the same pathway.
- Specify the deposit and habitat. Guano, eggs and feathers reach different places and may expose different organisms.
- Check the geographic scale. Colony-level evidence supports a local conclusion unless broader measurements show otherwise.
- Check species coverage. Results from one seabird, fish or terrestrial consumer should not be generalized without comparable data.
The practical takeaway
Marine chemicals can reach land animals when contamination enters marine food webs and a mobile animal carries contaminated tissue or waste onto land. Seabirds provide the strongest documented example: they forage at sea, return to colonies and deposit guano, eggs, feathers and related material that can affect terrestrial soils and organisms.
The scientifically accurate qualification is just as important as the pathway itself. Chemical behavior differs, biomagnification is not universal, concentrations vary by species and geography, and evidence for pollutant transfer is smaller than the evidence for nutrient transfer. The most defensible conclusion is therefore one of a real, measurable cross-ecosystem pathway that can create local exposure—not a claim that every ocean pollutant reaches every land animal or that the global effect is necessarily large.
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