Excess Nitrogen Loading

Excess nitrogen loading is the delivery of nitrogen to a water body beyond the rate the ecosystem can assimilate, the primary driver of coastal eutrophication. Reactive nitrogen arrives as nitrate and ammonium from fertilizer runoff, animal waste, sewage effluent, and atmospheric deposition from combustion. In most temperate coastal systems nitrogen, not phosphorus, is the limiting nutrient, so added nitrogen directly fuels phytoplankton blooms. The cascade runs from loading to algal bloom, to sinking and bacterial decay, to bottom-water hypoxia and anoxia. The EU Marine Strategy Framework Directive treats eutrophication as Descriptor 5, and OSPAR and HELCOM set nutrient-reduction targets to cut riverine and airborne nitrogen inputs.