Nutrient runoff from the Mississippi watershed and ecosystem pollution risk in the Northern Gulf of Mexico

1 Introduction

The development of ammonia synthesis by Fritz Haber in 1909 made synthetic fertilizers widely available and played a major role in the global expansion of agriculture during the Green Revolution (Charles, 2005). Although synthetic fertilizers significantly increased crop production, their extensive use has also contributed to nutrient pollution in aquatic environments, a process known as eutrophication. The term eutrophication comes from the Greek words eu (“well”) and trophe (“nourishment”) (Yang et al., 2008). Excess fertilizers, particularly those containing nitrogen and phosphorus, are transported from agricultural fields into rivers, lakes, and coastal waters through rainfall, snowmelt, surface runoff, and drainage systems. Animal waste and untreated sewage further increase nutrient inputs into these aquatic ecosystems (Mateo-Sagasta et al., 2017; Ullah Bhat and Qayoom, 2022).

The accumulation of excessive nutrients stimulated the rapid growth of algae and phytoplankton, disrupting the natural balance of aquatic ecosystems (Devlin and Brodie, 2023). As algal populations increase, biodiversity declines, food webs are altered, and aquatic habitats are degraded. When these algae eventually die, they are decomposed by microorganisms that consume large amounts of dissolved oxygen. This process can create hypoxic (low-oxygen environment) conditions, where dissolved oxygen falls below 2 mg/L (Benway et al., 2019), or even anoxic conditions in which oxygen is completely depleted. Such low-oxygen environments often result in the death of aquatic plants and animals and severely reduce the ecological health of affected water bodies (National Oceanic Atmospheric Administration, 2024a).

Nitrogen is generally more abundant than phosphorus in agricultural runoff, making it a major driver of eutrophication in many coastal waters (Howarth et al., 2000). Elevated nitrogen concentrations promote the rapid growth of phytoplankton and algae (U.S. Environmental Protection Agency, 2026a). As these blooms decompose, microbial respiration further depletes dissolved oxygen, creating hypoxic “dead zones” in coastal ecosystems (National Oceanic Atmospheric Administration, 2024b). These areas cannot support most marine organisms and experience dramatic declines in biodiversity. Bottom-dwelling species, including shrimp, crabs, and bottom-feeding fish, are particularly vulnerable because they cannot easily escape low-oxygen conditions (Hale et al., 2016). In contrast, more mobile species may temporarily migrate to oxygen-rich waters. It has been estimated that approximately 30% of the nitrogen applied to agricultural fields is transported into surrounding water bodies through surface runoff (U.S. Department of Agriculture, 2017).

Beyond oxygen depletion, eutrophication can also promote harmful algal blooms (HABs) that produce toxic compounds capable of contaminating seafood (Anderson et al., 2008). Shellfish and filter-feeding organisms accumulate these algal toxins by filtering contaminated water, while fish may become contaminated through the food web (U.S. National Office for Harmful Algal Blooms, 2026). Consumption of contaminated seafood can expose humans to algal toxins that cause illnesses ranging from gastrointestinal symptoms, cardiovascular issues, and neurological disorders such as paresthesia, memory loss, disorientation, and paralysis (Centers for Disease Control Prevention, 2023; U.S. Environmental Protection Agency, 2025). Nutrient-rich waters also favor the growth of pathogenic bacteria, further increasing health risks (Quigg et al., 2025). In addition to threatening public health, harmful algal blooms reduce water quality, decrease biodiversity, and negatively affect coastal fisheries and tourism. Commercially valuable fish populations decline, recreational fishing opportunities diminish, and coastal economies suffer significant losses (U.S. National Office for Harmful Algal Blooms, 2026). Therefore, reducing nutrient runoff and controlling eutrophication are essential not only for protecting aquatic ecosystems and biodiversity but also for preventing seafood contamination, safeguarding public health, and sustaining coastal economies (National Oceanic Atmospheric Administration, 2026).

Gulf of Mexico (GOM) is an oceanic basin located on the southeastern side of North America, covering an area of 1,507,639 km² (WorldAtlas, 2026). GOM, which covers the southern coast of the US, is threatened by massive eutrophication caused by the polluted runoff of the Mississippi River watershed (The Nature Conservancy, 2025). The river flows about 3,782 km from the state of Minnesota to Louisiana before it flows into GOM (Figure 1A). On its path, the river connects to five large river basins, draining an estimated over 3 million km². This paper discusses the overwhelming effects of nutrient fertilizers on the Gulf Coast ecology and how this colossal problem could be mitigated.

GOM represents a diverse aquatic ecosystem that includes various species of marine mammals (including dolphins, whales, and manatees), sea turtles (including endangered green turtles), sharks (including hammerheads, bull sharks, and tiger sharks), fish, and a wide range of seaweed and sea plants (National Oceanic Atmospheric Administration, 2025). This ecosystem also supports extensive coral reefs (National Oceanic Atmospheric Administration, 2026). Commercially valuable crustaceans like shrimp, crabs, and lobsters flourish here, along with mollusks such as oysters and clams (United Nations Industrial Development Organization, 2025). Economically important fish such as red snapper, grouper, tarpon, and various types of tuna also thrive here. In the coastal US alone, GOM generates over $200 billion annually in economic activity from fishing, shipping, tourism, and oil industries.

However, for the past several decades, this large coastal ecosystem has been threatened by a consistent flow of nutrient pollution from the largest watershed in North America (U.S. Environmental Protection Agency, 2026c). The Mississippi River runs through highly fertilized agricultural lands of 10 states of the USA: Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana (Figure 1A). However, the vast watershed involves five river basins: The Missouri River basin, the upper Mississippi River basin, the Ohio and the Tennessee River basins, the Arkansas and the Red River basins, and the lower Mississippi River basin (Figure 1B). These five basins connect 31 states and together drain 41% of the U.S. (U.S. Environmental Protection Agency, 2023).

The Mississippi River discharges an average of about 16,800 m³/sec of water and carries an estimated 1.6 million metric tons of nutrient fertilizer into the GOM per year (Johnson and Harrison, 2015). The excessive nutrient triggers extreme algal blooms in GOM, primarily off the coasts of Texas and Louisiana. Algae die, decompose, and deplete dissolved oxygen in water. This causes hypoxia in ocean water, leading to the formation of a vast area of dead zones in the Gulf (Figure 1C). Dead zones are associated with the degradation of ocean habitats, disturbing the Gulf ecology (The Nature Conservancy, 2025).

Considerable interannual variability is observed in the extent and intensity of the GOM dead zone. This seasonal manifestation occurs during the summer months, and the largest oceanic dead zone in the GOM measures about 23,000 km² (National Oceanic Atmospheric Administration, 2026). The size of the annual extent exhibits a significant difference from year to year (Mean area: about 14,325 km² (Figure 1D). Although a slow downward long-term trend can be seen based on collected data, the annual estimates of the dead zone area far exceeded the desired government target (5,000 km²) every year except 2000 and 2020 (National Oceanic Atmospheric Administration, 2024b). Such gigantic dead zones have been linked to the destruction of the marine habitats, including massive fish kills and the loss of other marine animals (The Nature Conservancy, 2025).

2 Eutrophication effects on the Gulf of Mexico

The release of excessive nitrogen and phosphorus in the GOM water by the Mississippi River causes a massive growth of algae faster than the ocean ecosystem can handle. This depletes dissolved oxygen (DO) and degrades the water quality, essential for the survival of fish and other aquatic animals (U.S. Environmental Protection Agency, 2026b). The algae overgrowth also deprives phytoplankton and subaquatic plants of sunlight. The death of algae further depletes oxygen in water, creating massive hypoxic and anoxic dead zones, spreading over thousands of km² (Figure 1D). Lack of oxygen makes the water unsuitable for aquatic life across a vast region of the Gulf. Dead zones force the migration of marine fauna to higher DO areas, and fauna that are sensitive or unable to escape perish. As depicted in Figure 2, this process causes massive loss of fauna, including fish and other commercial marine species, which affects the seafood industry (The Nature Conservancy, 2025).

GOM is a major source of seafood for the USA. It supplies 72% of the harvested shrimp, 66% of the harvested oysters, and 16% of the commercial fish (Agritecture, 2019). A recent report suggests that catches from GOM account for 40% of the annual fishing yield in the US, with an estimated annual value of $680 billion (Chen, 2017). A 2026 NOAA report based on 2023 data indicated that saltwater fishing provided 1.7 million jobs and generated $319 billion in sales in the U.S, (National Oceanic Atmospheric Administration, 2026).

Eutrophication alters GOM water chemistry, impacting aquatic life forms and severely impacting the fishing industry. Since eutrophication causes mass killing of fish and other aquatic organisms, it hurts the fishermen's yields (The Nature Conservancy, 2025). Furthermore, since some fish species move out of the dead zones, fishermen are also forced to travel further distances to make their catches, spending more time and resources. This adds additional stress to the fishing industry and compounds costs for the consumers at seafood markets. The Union of Concerned Scientists (2020) reports that eutrophication costs an annual $2.4 billion in damage to Gulf fish stocks (Union of Concerned Scientists, 2020).

3 Health effects of coastal eutrophication

Coastal eutrophication has significant human health effects. Poor water quality resulting from eutrophication may lead to skin and eye irritations and gastrointestinal illnesses in people encountering contaminated water. Some harmful algal blooms (HABs) release toxins that may accumulate in shellfish, and subsequent consumption may affect consumers' health causing nausea, vomiting, diarrhea, pneumonia, gastroenteritis, muscular cramps, and poisoning syndromes (Health Canada, 2021; HHS, 2017; Centers for Disease Control Prevention, 2023; U.S. Environmental Protection Agency, 2025; Quigg et al., 2025).

HABs have been documented in all U.S. coastal states, with indications of a potential rise in their occurrence. Due to their adverse impact on human health, marine ecosystems, and the economy, HABs bear national significance and every coastal state in the U.S. monitors the safety of fish and shellfish harvested for commercial use (National Oceanic Atmospheric Administration, 2025). HAB toxins can cause paralytic shellfish poisoning (PSP) or amnesic shellfish poisoning (ASP), which can be severe to fatal (Washington State Department of Health, 2023).

Nitrites and nitrates, the primary nitrogen-containing components of fertilizer nutrients, have been associated with a range of adverse health effects, such as gastrointestinal cramps and vomiting, methemoglobinemia (reduced oxygen-carrying capacity of hemoglobin), hypothyroidism, type 1 diabetes, hypertension, and neurological effects such as dizziness, loss of consciousness, and convulsions. Exposure has also been linked to an increased risk of several cancers, including those of the larynx, liver, lung, mouth, pancreas, pharynx, urinary tract, urinary bladder, and brain, as well as non-Hodgkin's lymphoma and certain birth defects (HHS, 2017; Ward et al., 2018). Because agricultural runoff can contaminate both surface water and groundwater, these nitrogenous compounds may enter drinking water supplies in coastal areas, posing potential risks to human health.

4 Recommendations for remediation

The remediation of ocean eutrophication in the coastal GOM region involves an intricate approach aimed at mitigating the nutrient runoff into marine ecosystems. The approach relates to coastal monitoring and ocean management to terrestrial land cover management, and agricultural practices that will improve the quality of the runoff flowing down from the vast Mississippi watershed. Here are some key strategies and solutions to address this significant issue.

4.1 Sustainable farming practices

One necessary step is to provide farmers with training and discourage them from using excessive fertilizer in the farmlands (U.S. Department of Agriculture, 2017). The promotion of sustainable farming practices could significantly reduce the influx of nutrients into nearby water bodies. This would not only benefit the environment, but it would also reduce agriculture costs and ensure long-term agricultural productivity (Benway et al., 2019).

4.2 Wetland preservation and reforestation

Wetlands act as natural filters for water, slowing down its flow and allowing wetland plants to efficiently absorb excess nutrients (Wetlands Work, 2025). Reforesting and preserving wetlands in the Mississippi River watersheds could prevent these nutrients from rapidly draining into rivers and subsequently, the ocean. This approach would help maintain the health of both wetland ecosystems and coastal waters in a sustainable and long-term manner (The Nature Conservancy, 2026).

4.3 Aquaculture of clams and oysters

Clams and oysters play a pivotal role in water filtration and the regulation of plankton populations (Koenig, 2018; Asha et al., 2023). Aquaculture of shellfish would not only control nutrient levels in coastal areas to improve water quality but would also enhance the productivity and economic value in the seafood market (National Center for Coastal Ocean Science, 2016).

4.4 Seaweed cultivation

Seaweed harnesses and utilizes nutrients present in seawater. Cultivating edible seaweed in the GOM states would not only conserve these nutrients but also could provide a sustainable source of commercially valuable seaweed products (Racine et al., 2021). This innovative solution would help combat eutrophication and support the development of a seaweed industry and subsequent economic benefits.

4.5 Use of advanced technologies

Mitigation strategies should also include the advanced monitoring of coastal water quality, such as earth observations using satellite technologies. Traditional water quality assessment techniques are labor-intensive, costly, and only capable of assessing small sections of a targeted waterbody at a time. This approach falls short when evaluating larger geographic areas (Murray et al., 2022). In contrast, remote sensing technology (1) provides comprehensive observations across extensive areas and (2) offers flexibility to conduct both short-term and long-term assessments (Binding et al., 2020). Newer satellite missions, such as the NASA Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE) project offers higher accuracy and temporal frequency suitable for continuous monitoring of the GOM dead zone (Werdell and deCharon, 2022).

4.6 Regulatory guidelines

Implementing regulatory guidelines is essential in controlling nutrient inputs into coastal waters. The European Union mandated reductions in both nitrogen and phosphorus inputs into coastal ecosystems. Similar measures were also recommended in the United States (Howarth and Paerl, 2008). Such regulations could serve as critical measures to safeguard the health of coastal environments and prevent eutrophication, which is evidenced by a reduction in nutrient inputs into Chesapeake Bay (Lefcheck et al., 2018).

4.7 Public awareness and education

Public awareness can play a vital role in reducing eutrophication by promoting informed actions and boosting collective responsibility for protecting water quality. When informed, people will adopt environmentally responsible practices, such as limiting the overuse of fertilizers, and careful about improper waste disposal, and urban runoff. This will minimize nutrient runoff into nearby water bodies (Sinclair et al., 2023).

Furthermore, campaigns aimed at raising public awareness can play a crucial role in promoting policies and community-driven initiatives to reduce eutrophication. Adopting educational programs in schools, media outreach, and community workshops can help individuals better understand the impact of their actions on overall ecosystem health (The Nature Conservancy, 2023). Well-informed individuals are more likely to advocate for green infrastructure, endorse funding for upgrading wastewater treatment facilities, and actively engage in local efforts to protect watersheds. Ultimately, cultivating an informed public not only helps mitigate nutrient pollution from its sources but also fosters a culture of environmental stewardship essential for preventing eutrophication in the long term.

5 Case studies

Like eutrophication in GOM, coastal water pollution affects many other large water bodies around the world in similar ways. Table 1 compares water pollution in GOM with two other major water bodies, the Chesapeake Bay and the Baltic Sea. The comparison examines key factors of water quality challenges, including major pollution sources, types of pollution, frequency of water column stratification, hypoxia and dead zone formation, impacts on biodiversity, public health effects, and economic losses (Table 1).

Table 1. Comparative overview of eutrophication: Gulf of Mexico, Chesapeake Bay, and Baltic Sea.

This table compares the Gulf of Mexico, Chesapeake Bay, and the Baltic Sea in terms of major pollution sources, types of pollutants, environmental consequences, and the restoration efforts (Boesch et al., 2001; U.S. Department of Agriculture, 2015; Batiuk et al., 2025).

5.1 Eutrophication of the Chesapeake Bay

The Chesapeake Bay is an estuary on the Atlantic coast of the U.S., spanning about 165,750 sq. km. extending into six states (Maryland, Virginia, West Virginia, Delaware, Pennsylvania, and New York) and Washington, D.C. It is the largest estuary in the US and receives water from the Potomac, Susquehanna, and the James rivers, and other smaller rivers and streams. The bay is suffering from eutrophication fueled by excessive input of nitrogen and phosphorus from urban stormwater, wastewater discharge, and agricultural runoff (Chesapeake Bay Program, 2026). As observed in GOM, these nutrients cause harmful HABs, hypoxia, and seasonal dead zones in the deeper waters of the Chesapeake Bay, triggering ecological disturbances, such as loss of aquatic vegetation, fish, oysters, blue crabs, other shellfish, and overall biodiversity in the coastal waters (Harding et al., 2016).

Eutrophication of the bay also affects human health and the coastal economy by depleting commercially important seafood species and tourism. While the federal and state partnership efforts (Chesapeake Bay Program) are helping to reduce the point-source pollution, managing nonpoint-source pollution remains a challenge (Chesapeake Bay Program, 2026).

5.2 Eutrophication of the Baltic Sea

The Baltic Sea in Northern Europe, stretching for about 385,900 sq. km., is the largest expanse of brackish water in the world. It receives water from numerous rivers of the surrounding nine countries (Denmark, Germany, Poland, Russia, Lithuania, Estonia, Finland, and Sweden) (Andersen et al., 2017). Since the mid-1900 s, the sea has been receiving large amounts of nitrogen and phosphorus pollutants from agricultural lands and untreated sewage, leading to eutrophication (Iho et al., 2023). This triggers excessive algal growth, including HABs, and dead zones. This has led to serious ecological damage, including loss of biodiversity, severe damage to benthic ecosystems, and a decline in important fish species (Andersen et al., 2017).

It also affects human health through contaminated seafood. To address this issue, in 2007, the Helsinki Commission (HELCOM) introduced the Baltic Sea Action Plan (HELCOM, 2021). Although some progress has been made in reducing phosphorus pollution, the overall process has been very slow due to continued runoff from farmlands. To achieve a long-term solution, stronger cooperation between countries, better farming practices to manage nutrients run off, and climate-smart strategies will be needed.

6 Summary and conclusions

Eutrophication, a prevalent water quality issue impacting both freshwater and coastal environments, can be predominantly attributed to the increased application of nitrogen and phosphorus-rich synthetic fertilizers in agriculture and the discharge of livestock waste (National Oceanic Atmospheric Administration, 2024a). Over the past several decades, agricultural practices have intensified to meet the demands of a growing global population, resulting in elevated fertilizer use in farmlands. The eutrophication of the GOM by the Mississippi River serves as a prominent example of this phenomenon.

The issue of GOM eutrophication is a colossal challenge with profound outcomes for the vitality of its marine ecosystem. The ecological balance of this vital marine ecosystem is at stake. The consequences of eutrophication, from oxygen-deprived dead zones to effects on marine life, and coastal economies, are far-reaching. Moreover, the consumption of seafood tainted with toxins poses a significant threat to human health.

The mitigation strategies of coastal eutrophication require a multifaceted effort. This involves implementing strict fertilizer management practices in agriculture, reducing urban runoff, wetland management, and promoting sustainable fishing practices. Wetland restoration could also potentially benefit soil and flood management, as well as enhance agricultural productivity. The Nature Conservancy suggested that implementing nutrient management practices could significantly reduce riverine nutrient runoff (The Nature Conservancy, 2023).

Implementation of such practices requires collaborative efforts among U.S. Department of Agriculture, farmers, industries, scientists, and local communities. Farmers must be taught how to use fertilizers wisely and effectively in the fields, and strict guidelines on the overuse of fertilizers must be implemented. Adaptation of such measures will help reduce the amount of extra nutrients in the waterbodies and restore a more resilient GOM, safeguarding its precious biodiversity and sustainability.

Addressing ocean eutrophication requires a combination of sustainable practices, ecosystem restoration, the use of innovative technologies, and regulatory measures. Implementation of these strategies would facilitate restoring the balance in marine ecosystems, preserving aquatic habitats, and ensuring the long-term health of the oceans.

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Footnotes

Edited by: Ram Kumar, Central University of South Bihar, India

Reviewed by: Lucia Gastoldi, New York University Abu Dhabi Water Research Center, United Arab Emirates