Nutrient content and stoichiometry of pelagic Sargassum reflects increasing nitrogen availability in the Atlantic Basin

The pelagic brown macroalgae Sargassum spp. have grown for centuries in oligotrophic waters of the North Atlantic Ocean supported by natural nutrient sources, such as excretions from associated fishes and invertebrates, upwelling, and N-2 fixation. Using a unique historical baseline, we show that since the 1980s the tissue %N of Sargassum spp. has increased by 35%, while %P has decreased by 44%, resulting in a 111% increase in the N:P ratio (13:1 to 28:1) and increased P limitation. The highest %N and delta N-15 values occurred in coastal waters influenced by N-rich terrestrial runoff, while lower C:N and C:P ratios occurred in winter and spring during peak river discharges. These findings suggest that increased N availability is supporting blooms of Sargassum and turning a critical nursery habitat into harmful algal blooms with catastrophic impacts on coastal ecosystems, economies, and human health. The macroalgae Sargassum has grown for centuries in the oligotrophic North Atlantic supported by natural nutrient sources and cycling. Here the authors show that changes in tissue nutrient contents since the 1980s reflect global anthropogenic nitrogen enrichment, causing blooms in the wider Atlantic basin.

Automated summary

Research shows that since the 1980s, the nitrogen content of Sargassum spp. has increased, while the phosphorus content has decreased, leading to an increase in the N:P ratio, reflecting global anthropogenic nitrogen enrichment causing algal blooms in the wider Atlantic basin.