At a time when the world’s population is growing and demand for food increasing, agricultural practices are taking a heavy toll on marine environments. Agricultural runoff, laden with fertilisers, pesticides and animal waste, flows unchecked from farms into rivers, streams and, eventually, the ocean. This potent cocktail of contaminants disrupts fragile ecosystems in coastal waters and open seas alike. Nutrient pollution from nitrogen and phosphorus fuels the proliferation of “dead zones”, which kills marine life. Additionally, the impacts of pesticide residues, bacteria and viruses are affecting marine life in ways that have not yet been entirely evaluated.

The consequences of agricultural pollution ripple outwards, jeopardising sensitive marine habitats like coral reefs and kelp forests while tainting seafood with toxins. Coastal economies and food security for populations that depend on the ocean are under threat. Solving this growing crisis requires radically rethinking conventional agricultural systems and shifting towards more sustainable practices that reduce nutrient runoff and restrict pesticide use, as well as better managing water use and waste.

As the earth’s water cycle is intricately connected, every contaminant that encounters water on land can eventually end up in the ocean. Indeed, 80% of marine pollution originates from land-based sources, with agriculture a major contributor. Agricultural pollution, including runoff from farms, ranches and forest areas, is classified as nonpoint source pollution, meaning that it comes from many diffuse sources and is caused by rainfall or snowmelt moving over and through the ground. Unlike pollution from factory waste or sewage treatment plants, it is diffuse and difficult to track and prevent. As runoff moves, it picks up pollutants across miles of land, depositing them into lakes, rivers, wetlands, coastal waters and ground waters, moving contaminants far from their original source.

Agricultural runoff pollution includes nutrients like phosphorus, nitrogen and potassium from chemical fertilisers, manure and sludge. Additionally, pollutants like fertilisers, pesticides, heavy metals, pathogens from animal feeding operations, and organic matter from livestock-related wastes are carried into water bodies through agricultural runoff. Others include oil and toxic chemicals from energy production, sediment from croplands, and bacteria or viruses from livestock.

Fertilisers provide essential nutrients like nitrogen and phosphorus to support plant growth. However, excessive levels of these nutrients are proving deeply disruptive to aquatic ecosystems. David Kanter, associate professor of environmental studies at New York University, describes our relationship with these nutrients, particularly nitrogen, as complicated. “The same thing that is so essential is at the heart of such an important problem. This relationship makes nitrogen a more challenging societal and environmental problem; it is essential to life, it is in the building blocks of DNA, there is no world and especially no food system without nitrogen,” he said.

Fertiliser use has increased massively, leading to important gains in food security and production. However, half of this becomes nitrate runoff and makes its way into the aquatic ecosystem, including both coastal and oceanic ones.

High nitrate concentrations can trigger a process known as eutrophication in marine environments, which is the prolific growth of algae. When this excessive bloom decomposes, it depletes oxygen levels in the water, adversely impacting marine health on a large scale. Harmful algal blooms, known as red tides, brown tides and green tides, produce potent toxins like ciguatera and domoic acid, which accumulate in fish and shellfish. Ingestion by humans can cause dementia, amnesia, paralysis and death. A combination of coastal pollution and warming ocean temperatures promotes the spread of dangerous microorganisms. There is a high risk that harmful bacteria like the vibrio species will start spreading to new regions, leading to cholera in previously unaffected areas.

In many parts of the Caribbean, algal blooms are creating an unsightly coastal feature, impacting tourism. Human health is also affected, as harmful gases are released when seaweed breaks down, with deaths reported as a direct result of this in Western Europe.

Agriculture causes 78% of global ocean and freshwater eutrophication, and nutrient pollution worldwide is linked to agricultural activities. The potential for such pollution is exacerbated when fertilisers are used inefficiently, leading to excess unused nutrients being washed away into the environment. Other poor agricultural practices, like directly discharging untreated livestock effluents into water bodies, also contribute substantially to the problem, due to nitrogen’s unique chemistry. “One nitrogen atom can go on a journey of environmental destruction, starting off as fertiliser and contributing to air pollution, eutrophication and nitrous oxide, the third most important greenhouse gas,” said Professor Kanter.

There is good news. Several management practices exist to better manage nitrogen and ensure that it better reaches crops. These practices relate both to how the nitrogen is applied to the soil and the type used.

Strategies include applying fertiliser closer to the time of planting, using smaller doses over the growing season and adopting innovative machinery with greater precision. These all work towards better availability and use of nitrogen instead of applying at a time when the crops are unable to use it effectively. “It’s really about using the same product smartly,” explained Professor Kanter.

Technologies such as controlled release fertilisers also help manage the problem of nitrate runoff. These might be fertiliser granules coated with a polymer that helps delay nitrogen release, or inhibitors that keep nitrogen in a form that is less likely to runoff. Other innovative technologies are being developed, which Professor Kanter described as the holy grail of agronomic research. “There’s a number of start-ups working on genetic engineering solutions, where they are trying to engineer crops to act more like legumes, soyabeans or pulses, which fix nitrogen from the air and do not need much fertiliser,” he explained. Such start-ups are attracting venture capital interest. Changing our eating habits would also go some ways towards reducing nitrogen; reducing meat and dairy consumption alongside improving farm and food chain management could lead to a 49% reduction in nitrogen losses.

Professor Kanter is chair of the International Nitrogen Initiative, a network that convenes the nitrogen community and advances new science. In the last five years he has seen the issue attract more attention from policymakers. One area of focus is pushing farmers to adopt more efficient practices, but there are other areas that could be targeted. “There has been significant pushback from farmers who feel really put upon to achieve increasingly ambitious environmental targets,” he said. Their protests bolster the argument that the regulatory burden needs to be shared across the food system and not just placed on the farmers. This could be achieved through regulating the fertiliser industry to impose design standards on fertiliser companies.

With protests taking place in the likes of the Netherlands, France and Germany, more countries are taking the issue of nitrogen more seriously. “There is a realisation that fertiliser is a significant part of our major environmental goals,” explained Professor Kanter. “Dealing with nitrogen is a microcosm of sustainable development. If we manage our relationship with land and with nitrogen, we get multiple societal and environmental wins, not just the climate problem, but also social and economic outcomes,” he continued.

There are further linkages between agriculture and ocean pollution. In arid and drought-prone areas, wastewater is often used in agriculture to conserve and reuse scarce water resources. This practice can be an important component of a circular economy approach, where waste products are treated and repurposed. When implemented with proper infrastructure and regulatory frameworks, wastewater can provide a reliable source of water for agriculture.

Dr Anne Thebo, research scientist in the Climate Impacts Group at the University of Washington, describes wastewater as a reliable source. “Historically, one of the benefits of reuse is related to water supply. Wastewater is less sensitive to impacts from stressors such as drought and climate change, and provides benefits to reuse and to sustainable agriculture,” she noted.

Municipal wastewater is a product of household activities such as washing dishes and flushing toilets, as well as water used by businesses and institutions like schools, hospitals and manufacturing companies. This makes the flow of wastewater predictable, preventing the economic impacts of inadequate water at important times of the crops’ lifecycle.

But there are benefits beyond its reliability. “Plants can use the nutrients in the wastewater, which has the double benefit of fertiliser for the crops and avoiding discharge of nutrients to marine and surface water,” Dr Thebo further explained. The use of recycled water therefore reduces or even eliminates the need for chemical fertilisers, reducing the pollution that follows their use.

Despite this untapped potential, its use in agriculture is not widespread due in part to concerns of its suitability and safety. The quality of the water needs to align with the level of risk posed by the consumption of some crops. “Crops like lettuce or strawberries are typically consumed raw and require a higher quality of water than those not consumed raw,” Dr Thebo explained. The ongoing research into the fate of emerging contaminants such as forever chemicals is trying to answer questions about wastewater’s safety, and regulatory agencies are watching this closely.

The biggest obstacles to wastewater adoption are cost, capacity and co-operation. In many regions, and particularly in low-income countries, the lack of adequate infrastructure and institutional arrangements for wastewater treatment exacerbates the issue of ocean pollution from agriculture. Without proper decontamination processes, untreated or partially treated wastewater used in agriculture can lead to additional contaminants reaching the oceans. This pollution can take various forms, including harmful pathogens and industrial chemicals, compromising food safety and introducing more waste products directly into water systems. Urban and industrial contaminants ending up in agricultural contexts lead to new and cross-contaminating forms of nutrient pollution in the ocean.

Another harmful practice comes in the form of sewage sludge. When applied as fertiliser on agricultural land, it can be a major pathway for microplastics to enter soil. In Europe, an estimated 8-10 million tonnes of sewage sludge is generated each year, with around 40% sent to farmland. Its use on farmlands is widely adopted due to the nitrogen and phosphorus it provides for crop growth.

However, during wastewater treatment, microplastics present in sewage become concentrated in the sludge. As a result, between 31,000 and 42,000 tonnes of microplastics are introduced annually into European agricultural soils; this is similar to the quantity of microplastics found in ocean surface waters. The microplastics spread on farmlands eventually make their way back into natural water bodies as surface runoff from rainfall carries them into rivers or as they infiltrate into groundwater over time. At one wastewater treatment facility in Wales, each gram of sewage sludge contained up to 24 microplastic particles, which is roughly 1% of its weight.

There are additional links between agriculture and ocean plastic pollution. Plastics in agriculture are a source of several chemical pollutants and soil contamination. Plastic packaging for animal feedstock has been found to contain several bisphenol compounds. BPA is the predominant form, and there is evidence of it leaching. Microplastics have also been detected in animal feedstock such as fish and soybean meal, as well as in animals’ blood, meat and milk. Agricultural plastic is a major component of land-based plastic waste—some of which will inevitably wind up in waterways.