Human activity is now contributing more nitrogen to about one in four coastal waterways worldwide than all natural sources combined, according to a new study led by Washington State University. Published in Global Biogeochemical Cycles the research examines how agriculture, wastewater, urban runoff and fossil fuel emissions are changing nutrient balances in coastal ecosystems, with the Gulf of Mexico identified as one of the major hotspots.
Nitrogen, phosphorus and silicon are essential for marine ecosystems. They support plankton, seagrasses and other organisms that form the foundation of aquatic food webs. However, excessive nutrient inputs can cause rapid algae growth followed by oxygen depletion as the algae decompose. This can create oxygen starved areas known as dead zones where fish and shellfish struggle to survive.
The study titled “Coastal Nitrogen, Phosphorus, and Silicon Sources: Integrating Land, Sea, and Human Inputs,” combined global data on natural and human sources of these nutrients and compared conditions from pre-industrial times with those of the modern era.
Although natural sources continue to dominate nutrient inputs globally, the researchers found that land based sources now account for the majority of nutrients along more than half of the world’s coastlines. Human activities contribute more nitrogen than all natural sources combined in about 25 per cent of coastal waterways and around 20 per cent of large marine ecosystems. For phosphorus, human sources exceed natural inputs in about 11 per cent of coastal waterways.
Agriculture is a major contributor. Fertiliser and manure can wash from farmland into rivers and eventually reach coastal waters. Wastewater from cities, runoff from urban areas and nitrogen containing emissions from vehicles and power plants also contribute to the growing nutrient load.
The Gulf of Mexico is among the most prominent hotspots. The Mississippi River basin carries large quantities of agricultural runoff and wastewater toward the Gulf, contributing to a seasonal dead zone where oxygen concentrations become too low to support many marine organisms. Such conditions can affect fish and shrimp populations and create challenges for fishing communities.
Other heavily affected areas include parts of the North Sea, Baltic Sea and Mediterranean, as well as coastal regions of China, India and Southeast Asia, where population growth, intensive agriculture, urbanisation and industrial activity have increased nutrient pressures.
The researchers also highlighted the importance of nutrient ratios. Human activity can increase nitrogen and phosphorus while dams can trap silicon rich sediments, preventing silicon from reaching coastal waters. Changes in the balance among these nutrients can favour certain algae over diatoms, potentially altering marine food webs and biodiversity.
Reducing nutrient pollution could involve improving wastewater treatment, using fertilisers more efficiently and adopting agricultural practices such as cover crops, buffer strips and reduced soil disturbance. Wetland and floodplain restoration can also help capture nutrients before they reach rivers and coastal waters.
The study’s global mapping approach could help identify areas where reductions in nutrient pollution may have the greatest effect. By tracing nutrients from their sources on land to their eventual destination in the sea, researchers can provide a clearer picture of how human activities are affecting coastal ecosystems.
The findings highlight the close connection between activities on farms and in cities and the health of marine environments. Excess nitrogen can contribute not only to ecological damage but also to disruptions in fisheries, tourism, food security and other activities that depend on healthy coastal waters.