Ocean Nutrient Pollution: Causes And Human Impacts

Nutrient pollution is one of the most challenging environmental issues in the world, particularly in coastal areas. It is caused by an excess of nutrients, mainly nitrogen and phosphorus, in the air and water, which can act as fertilisers, leading to excessive growth of algae and aquatic plants. This process, known as eutrophication, can be attributed to various human activities, including agricultural practices, urbanisation, and industrial processes. The excessive growth of algae and plants disrupts marine ecosystems, destroys habitats, and depletes oxygen levels in the water, causing detrimental effects on marine life and industries that depend on healthy coastal environments, such as fisheries and tourism.

Agricultural pollution

Agriculture is a primary source of nutrient pollution in the ocean. This is mainly due to the use of chemical fertilisers and animal manure, which contain high levels of nitrogen and phosphorus. When these chemicals are applied to agricultural land, they can be washed away by rainwater and carried by rivers and streams into the ocean. This process is known as agricultural runoff.

On average, around 20% of nitrogen fertiliser is lost through surface runoff or leaching into groundwater, and up to 60% can vaporise into the atmosphere, later falling into the ocean as rainwater. This has led to a tenfold increase in global fertiliser use since the mid-20th century and a 43% increase in nitrogen discharges from rivers into the sea between 1970 and 2000. Agriculture is the leading source of nutrient pollution in the United States and the European Union, with more than three times as much nitrogen pollution coming from agriculture as from sewage.

The excess nutrients in agricultural runoff cause an explosion of plant and algal life in the ocean, disrupting marine ecosystems and destroying habitats. This process is called eutrophication and can lead to oxygen depletion in the water, causing the death of marine life and the creation of "dead zones" where no life can exist. Eutrophication can also cause reduced water clarity and toxic algal blooms, resulting in critical habitat losses such as coral reefs, seagrass meadows, and mangrove forests.

To combat agricultural pollution and its contribution to nutrient pollution in the ocean, Professor Donald Boesch of the University of Maryland suggests implementing more effective regulations and incentives to reduce the amount of nutrient runoff from agricultural land. Additionally, Professor Thomas Malone of the University of Maryland recommends better monitoring of nutrient pollution on a global scale and taking practical steps such as restoring habitats like mangroves and marshes that can remove nutrients from water before it reaches the sea.

Sewage and wastewater

Sewage, a subset of wastewater, is primarily contaminated with faeces and urine, carrying a range of harmful substances. These include nutrients, organic matter, bacteria, viruses, parasites, pathogens, solids, plastics, microfibres, chemicals, pesticides, heavy metals, and more. These contaminants can have detrimental effects on ocean ecosystems, even when released in small quantities.

There are several reasons why sewage ends up in the oceans. Firstly, many areas worldwide lack proper sanitation and wastewater treatment facilities. This issue disproportionately affects low-income communities and countries, where the lack of access to adequate sanitation infrastructure has severe health implications, such as diarrheal diseases. However, even in wealthy nations, communities suffer from inadequate wastewater treatment due to ageing infrastructure, facility malfunctions, and heavy rainfall that overwhelms systems, leading to combined sewer overflows.

Additionally, some treatment plants exist but may not function properly due to a lack of maintenance or repairs. In these cases, when the capacity of the treatment plants is exceeded, there is no alternative but to release the excess wastewater into nearby streams, bays, and rivers, which eventually flow into seas and oceans. Furthermore, sewage can be absorbed into the ground and soil, which does not effectively filter out all pollutants, depending on the terrain.

Another factor contributing to sewage pollution is the lack of regulation and monitoring in many parts of the world. While some countries have strict rules to prevent sewage discharge, other coastal areas lack the necessary laws and enforcement mechanisms. For example, cruise ships are required to treat their wastewater according to the laws of the Environmental Protection Agency, but some ships have been known to flout these regulations.

The impact of sewage pollution on marine environments is a significant threat to ocean health. It is one of the top three threats to marine ecosystems and has severe consequences for coastal environments, including coral reefs, seagrasses, mangroves, and intertidal ecosystems. Sewage pollution can lead to harmful algal blooms, which, upon decomposition, deplete the dissolved oxygen in the water, creating "dead zones" where marine life struggles to survive.

Fossil fuel burning

Nutrient pollution in the oceans is primarily caused by an excess of nitrogen and phosphorus, which act as fertilisers, causing an explosion of plant and algal life that disrupts marine ecosystems and destroys habitats. While agriculture is the leading source of nutrient pollution in the US and the EU, non-agricultural sources include stormwater, wastewater, fossil fuel burning, aquaculture, and domestic waste.

The combustion of fossil fuels releases carbon dioxide, which reacts with water to form bicarbonate and hydrogen ions, increasing the number of hydrogen ions and thus lowering the pH of seawater—a process known as ocean acidification. This change in ocean chemistry has already had devastating effects on oyster hatcheries in the Pacific Northwest, threatening an industry worth over $110 million. As carbon dioxide levels continue to rise with increased fossil fuel burning, larger drops in ocean pH are projected in the coming decades, particularly with the expanding use of fossil fuels in rapidly developing countries like China and India.

Ocean acidification poses a significant threat to marine life, as even relatively small alterations in seawater chemistry can have substantial effects on marine plants, animals, and microbes. Coral reefs, for example, are at particular risk from rising atmospheric carbon dioxide, facing the dual challenges of acidification and global warming. In addition, low oxygen environments caused by nutrient pollution exacerbate ocean acidification by releasing additional carbon dioxide into the ocean, further endangering corals, shellfish, and plankton.

Atmospheric deposition

The deposition of sulfur and nutrients contributes to the acidification and eutrophication of various water bodies. Eutrophication is the process where too many nutrients, mainly nitrogen and phosphorus, are added to bodies of water and act as a fertilizer, causing excessive growth of algae. This can lead to low levels of oxygen dissolved in the water, as when the algae and seagrass die, they decay and use up oxygen in the water. This can be detrimental to marine life.

In the context of oceans, atmospheric deposition of nitrogen compounds can be transported over the ocean. Studies have shown that atmospheric nitrogen has minimal effects on euphotic-zone productivity or on nitrate at the Atlantic thermocline. However, evidence suggests that areas with strong indications of phosphorus limitation in the North Atlantic and North Pacific are downwind of the most populated and urbanized regions of eastern Asia and North America. This indicates a potential mechanism where airborne pollutants are transported through atmospheric deposition.

Industrial and urban runoff

Urban stormwater and snowmelt significantly impact the quality of surface waters. Vehicular transportation-related activities, atmospheric deposition, and metallic building envelopes are major sources of pollution in this context. As stormwater flows through urban environments, it picks up accumulated debris, residues, and wastes, flushing them into nearby bodies of water. This runoff also carries chemicals that have accumulated in the environment, such as roads, building roofs, residential lawns, and parks.

In larger metropolitan areas, industrial facilities and construction sites contribute significantly to nutrient runoff. The water in these regions is often required to undergo treatment, either at regional treatment plants or through storage in large containment structures before release. However, not all communities have the financial means to implement these treatments, leading to the exploration of alternative methods like injecting aluminum sulfate into stormwater conduits to remove pollutants.

Agricultural activities, including the use of synthetic nitrogen fertilizers, legume and rice cultivation, and animal farming, are major sources of nutrient runoff. The expansion of agricultural lands has led to the loss of natural "sinks" for nutrients, such as wetlands and riparian zones, exacerbating the problem of coastal eutrophication. Additionally, the runoff from agricultural lands carries excess nutrients from fertilizers, manure, and chemical fertilizers into nearby water bodies, contributing to the overall nutrient load that eventually reaches the oceans.

To address the issue of industrial and urban runoff, comprehensive plans and strategies are being developed. These include watershed implementation plans, best management practices, and modeling tools for nutrient and sediment reduction. Restoration projects, such as those focused on wetlands, aim to reduce nutrient loading in affected areas. Additionally, the restoration of key wetlands on agricultural lands has made progress in mitigating nutrient runoff.

Frequently asked questions