A new study led by Washington State University has found that human activities are responsible for more harmful nitrogen than all natural sources combined in a quarter of the planet's coastal waterways. Published in the journal Global Biogeochemical Cycles, the research examines how human-driven nutrient inputs—such as agricultural runoff, wastewater discharges, and other activities—impact levels of nitrogen, phosphorus, and silicon in coastal waters. These excess nutrients can disrupt marine ecosystems and pose health risks to people who rely on these waters for food and recreation. The study reveals that land-based sources of nutrients, both natural and human-caused, contribute more than marine sources in roughly half of the world's coastal waters. In a quarter of coastal waterways and a fifth of large marine ecosystems—vast ocean areas extending from the coasts—human activities add more nitrogen than natural sources. For phosphorus, human activities are the dominant source in 11% of coastal waterways. The impact of human activity on silicon is different; it tends to lower silicon levels, often because river dams block the natural flow of silicon from inland to the coasts. This research is notable for its comprehensive approach, as it includes both land-based and marine sources of multiple nutrients and builds a new global database of nitrogen, phosphorus, and silicon sources over both preindustrial and modern times. The researchers assessed how land-based human activities influence nutrient ratios and the risk of eutrophication, a process where excess nutrients lead to rapid algal growth, which can deplete oxygen in the water and harm marine life. The findings suggest that while natural marine sources still provide the largest share of nutrients globally, land-based sources are the main contributors to nutrients in more than half of the world's coastlines. These effects are most noticeable in areas with high human activity, such as the Gulf of Mexico and coastal regions of Europe and Asia. To address this issue, the study proposes several strategies, including advanced technology in wastewater treatment, better fertilizer management, and no-till farming practices that reduce nutrient runoff. According to John Harrison, the study's corresponding author, reducing nutrient discharge can significantly improve coastal water health, and the research highlights where such efforts would be most effective.