Marine chemicals reach land animals when pollutants enter seawater, are taken up by marine organisms, move through some food webs, and are carried ashore by mobile animals—most clearly seabirds. A bird may feed at sea, accumulate contaminants in its body or prey, and then deposit marine-derived material as guano, eggs, feathers or carcass remains at a breeding, roosting or nesting site on land.
This is a cross-ecosystem pathway, not a claim that every ocean pollutant reaches every terrestrial species. The chemical involved, its persistence, the food-web structure, the bird’s movements and the receiving habitat all determine whether transfer occurs and how important it is.
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The five-step pathway from seawater to a land animal
1. Pollutants enter the ocean
Chemicals associated with land and human activity can reach seawater through atmospheric transport, rivers, runoff and direct discharges. Ocean currents and climate-related circulation patterns then redistribute them. The broad review by Landrigan and colleagues (2020), hosted by NOAA, describes those routes for most of the contaminants it considers; crude oil is an important exception because spills and petroleum releases follow different source patterns.
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2. Marine organisms take up the chemicals
Plankton, shellfish, fish and other marine organisms can absorb contaminants from water, sediment or food. Uptake is not identical for all chemical classes. Some compounds bind strongly to tissues or fats and persist for long periods; others are rapidly transformed or excreted. Exposure through diet is especially important for predators that repeatedly consume contaminated prey.
A chemical’s concentration in an organism therefore reflects more than the concentration measured in surrounding seawater. Age, body size, species, diet, metabolic ability and geographic origin can all change the amount present. Landrigan et al. emphasize these sources of variation when comparing marine species.
3. Contaminants move through marine food webs
Predators can receive contaminants when they eat contaminated prey. Two related processes must be separated:
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- Biomagnification is an increase in concentration from one trophic level to the next, so predators may contain more than their prey.
Methylmercury is a well-known example of a contaminant that can increase through aquatic food webs. It occurs in tissues of top-predator fish and marine mammals, and fish that feed higher in the web often contain more than lower-trophic fish. That pattern is not universal: the NOAA-hosted review notes geographic differences and exceptions, including situations in which sardines had higher PCB concentrations than cod or salmon.
The older aquatic-toxicology review by Borgå and colleagues (1994) reached an essential caution: most of the chemicals it evaluated did not biomagnify, while a subset had the potential to do so. Biomagnification must therefore be assessed chemical by chemical and food web by food web, rather than treated as a property of all marine pollution.
4. A mobile animal crosses the marine–terrestrial boundary
Seabirds are the clearest documented biological link. Many species forage at sea but breed, roost or nest on land. During those movements they carry material acquired from marine food webs across the ecosystem boundary. The material can be part of the bird itself, such as feathers or an egg, or material it produces after feeding, especially guano.
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Because a colony may receive deposits from thousands or millions of birds over repeated seasons, a small amount transported by one bird can become a measurable input at the site level. This is different from saying that every individual terrestrial animal receives a known dose.
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5. Deposits expose terrestrial soils and food webs
Guano, eggs, feathers and other allochthonous inputs land on soil, vegetation, freshwater margins or shoreline sediments. Chemicals in those materials can remain in the receiving habitat, bind to soil or organic matter, be taken up by plants and invertebrates, or enter local predator–prey interactions. The strongest evidence concerns changes in colony soils and associated terrestrial communities.
The pathway can also end without substantial onward transfer. A compound may degrade, become tightly bound to sediment, be diluted, or be metabolized by organisms before a terrestrial predator encounters it. Detecting a marine-derived contaminant in a deposit does not by itself establish the dose received by a particular land animal.
Why seabirds are the best-documented bridge
Grant, Bond and Lavers’ 2022 systematic review and meta-analysis evaluated 181 publications 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 material to breeding, roosting and nesting grounds through guano and other inputs such as eggs and feathers.
Guano is the most direct route. A bird eats at sea, digests part of its meal and excretes the residue on land. Repeated deposition can concentrate marine-derived substances in colony soils relative to nearby areas that receive little seabird input. Eggs and feathers provide additional pathways: they are formed from materials in the bird’s body and remain at the terrestrial site when they are laid, shed or left behind.
Seabirds are not the only possible connectors. The appraisal by Subalusky and Post (2017) also discusses seabirds, marine mammals, reptiles and anadromous fish as pathways among sea, coast, fresh water and land. However, the seabird evidence is particularly visible because colonies create discrete, repeatedly used receiving areas that can be sampled.
How chemical behavior changes the pathway
The same movement—from ocean to bird to land—can have very different consequences depending on the contaminant. A useful assessment keeps four questions separate: how the chemical enters the ocean, whether it persists in organisms, whether it increases with trophic level, and how it leaves the bird’s body.
| Question | What to examine | Why it changes the outcome |
|---|---|---|
| Does it persist? | Breakdown rate, metabolic transformation and binding to tissues or sediment | Persistent compounds can remain available during long migrations or repeated feeding seasons; rapidly transformed compounds may not. |
| Does it bioaccumulate? | Uptake compared with excretion and metabolism in the exposed organism | An individual can retain a contaminant even when concentrations do not rise at every trophic step. |
| Does it biomagnify? | Concentration changes between prey and predator | Predators may receive elevated concentrations, but many chemicals do not show this pattern. |
| Where is it stored? | Muscle, fat, liver, blood, feathers, eggs or excreta | Storage determines which material is transported to land and how readily it can be released. |
| What is the receiving habitat? | Soil, vegetation, freshwater, sediment and resident food-web species | Local chemistry and biology determine whether a deposit is retained, transformed or taken up. |
Methylmercury illustrates why trophic position can matter: it may increase in aquatic food webs and be present in top predators. Persistent organic pollutants and chemicals associated with plastics require more cautious wording. Reviews identify trophic transfer and seabird transport as relevant concerns, but the evidence does not establish one universal terrestrial exposure dose or a uniform response across compounds.
Marine food-web accumulation is not the same as transport onto land
These are two distinct stages that are often collapsed into one claim:
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match- Within the ocean, a chemical may be concentrated as organisms consume one another.
- Across the boundary, a mobile organism may carry a portion of that chemical or contaminated material to land.
- Within the terrestrial system, the deposit may be retained, transformed or passed to another organism.
A contaminant can biomagnify in a marine food web without being exported in a meaningful amount if the organisms involved remain at sea. Conversely, a seabird can transport marine-derived material even when the chemical does not biomagnify, because the bird may still ingest, retain or excrete it.
What controls the amount reaching land animals?
Bird movement and colony size
Transport is greater where birds repeatedly commute between productive marine feeding grounds and dense land colonies. Colony size, residence time, feeding range and the number of breeding seasons all affect cumulative deposition. A remote colony can receive a strong local signal even when the transported quantity is small compared with the total mass of chemicals in the ocean.
Diet and trophic position
Birds eating long-lived or higher-trophic prey may encounter different contaminant concentrations from birds eating smaller, lower-trophic prey. Species that switch diets by season or location can therefore show changing burdens. The same species may also differ between colonies because prey fields and pollution sources differ geographically.
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Age, size and physiology
Concentrations vary with age, body size and species. Older animals may have had more time to accumulate persistent compounds, while growing birds can dilute some concentrations as tissue mass increases. Metabolism, detoxification and the allocation of chemicals to eggs or feathers also influence what is ultimately deposited on land.
Properties of the receiving site
Soil texture, organic carbon, moisture, pH and microbial activity affect whether a pollutant remains available. Vegetation and invertebrates can incorporate some compounds, creating a local terrestrial route to predators. Other compounds may bind tightly or break down before they move beyond the colony soil.
Local effects can be clear even when global effects are limited
The reviewed evidence supports a strong distinction between local concentration and global redistribution. Seabird colonies can receive repeated, concentrated inputs and show measurable changes in soils and nearby organisms. At the scale of the entire planet, however, the amount transported by birds is small relative to the volume and mixing of the oceans.
Subalusky and Post (2017) judged that local effects may be substantial while the global influence of these animal-mediated transfers is likely minor. That conclusion does not make local contamination unimportant. A pollutant concentrated in a colony can matter to plants, invertebrates, scavengers and predators living there, even if it barely changes the global chemical inventory.
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The evidence base is larger for nutrients than for pollutants
Grant, Bond and Lavers’ 181-publication review found that seabird-mediated nutrient transfer has received more attention than pollutant transfer. Studies of pollutants have focused especially on toxic, bioaccumulative metals. Persistent organic pollutants and plastics-related chemicals are less thoroughly characterized as seabird-borne transfers.
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Bird species and chemical coverage are uneven
Kuo and colleagues’ 2022 U.S. Geological Survey critical review compiled more than 500 peer-reviewed studies, more than 25,000 data entries, information on more than 700 organic compounds, about 320 wild bird species and 60 domestic breeds. Despite that scale, the review identified substantial gaps in organic-chemical bioaccumulation and biotransformation data, particularly for shorebirds, seabirds and raptors.
Those gaps mean that a measured concentration in one species or location should not be generalized to all land-feeding birds. Researchers may know that a compound is present without knowing how efficiently a species metabolizes it, how much reaches eggs or feathers, or what fraction is released in guano.
Exposure is not the same as harm
Finding a marine-derived chemical in colony soil or a terrestrial organism demonstrates transport or exposure, not necessarily toxicity. Effects depend on concentration, duration, sensitivity, mixtures and the organism’s ability to detoxify or eliminate the compound. The available reviews do not support a blanket claim that every terrestrial animal near a seabird colony is harmed.
How to evaluate a reported ocean-to-land pathway
When reading a study or environmental report, check the following sequence:
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errors- Source: Is the chemical linked to atmospheric deposition, a river, runoff, discharge, a spill or another route?
- Marine entry: Was it measured in seawater, sediment or prey, or inferred from a source?
- Trophic evidence: Were concentrations measured across several food-web levels, or only in one species?
- Transporter: Is there direct evidence that seabirds or another animal moved the material from sea to land?
- Deposit: Was the chemical measured in guano, eggs, feathers, carcasses or colony soil?
- Recipient: Was uptake demonstrated in a terrestrial organism, and was a dose or effect measured?
- Scale: Does the result describe one colony, a coastline, a region or a global estimate?
This checklist prevents a common logical error: treating evidence of contamination in marine prey as proof that a particular land animal received a harmful dose.
The practical conclusion
Marine chemicals can reach land animals through a chain of linked events: pollutants enter seawater, some are taken up by marine organisms, selected chemicals accumulate or biomagnify in parts of the food web, and mobile animals—especially seabirds—carry marine-derived material ashore. Guano is the clearest route, with eggs and feathers providing additional transfers.
The pathway is real but conditional. Chemical properties, species, geography, diet, movement and receiving-habitat chemistry determine its importance. Local seabird colonies are the best-established examples of concentrated transfer, while evidence for many organic pollutants, plastics-associated chemicals and several bird groups remains limited.
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