Coastal Nitrogen, Phosphorus, and Silicon Sources: Integrating Land, Sea, and Human Inputs

Land‐based human activities have greatly increased inputs of nutrients (nitrogen [N] and phosphorus [P]) to the coastal zone, while also decreasing silicon (Si) export. Here we present a novel synthesis of N, P, and Si sources to coastal waters and use that data set to evaluate: (a) the amount, global spatial distribution, and proportion of N, P, and Si delivered to coastal waters attributable to land‐based human activities, (b) the impact of land‐based and anthropogenic nutrient inputs on N:P ratios and an index of coastal eutrophication potential (ICEP) relative to marine‐derived sources, including implications for coastal nutrient mitigation efforts, and (c) critical uncertainties and unknowns that should be addressed in order to better inform future efforts to quantify and mitigate coastal nutrient inputs and their negative impacts. We show that although total global inputs of N, P, and Si from marine sources (net upwelling and net onwelling) to coastal waters are larger than land‐based inputs, land‐based nutrient sources exceed marine‐derived sources along roughly half of the world's coastline. In addition, anthropogenic sources of N and P are greater than all natural (marine‐derived plus land‐derived) sources in a substantial number of coastal segments (COSCATs; 25% of COSCATs for N and 11% of COSCATs for P). Also, human impact on N has been greater than on P, causing a shift toward P limitation globally, and both N and P loading have increased more than Si loading, resulting in increased ICEP, an indicator of the likelihood of harmful algal bloom occurrence.