Macroalgal Blooms in Coastal Ecosystems

Macroalgal blooms are episodic proliferations of large seaweeds, notably brown and green algae, that can transform coastal ecosystems worldwide. They arise when nutrient enrichment, often from agricultural and urban runoff, combines with favourable temperature regimes, ocean currents and upwelling to fuel rapid algal growth. Floating genera such as Sargassum and Ulva produce extensive mats that alter biogeochemical cycles, disrupting food webs and generating hypoxic or acidified conditions upon decay. These blooms impose substantial economic burdens through beach fouling, harm benthic habitats and pose health risks via gas emissions. Recent advances underscore the interplay between nutrient stoichiometry, microbial symbionts and physical transport, facilitating bloom emergence across the tropical Atlantic, the Yellow Sea and other vulnerable coastlines. Improved forecasting models now integrate satellite observations with mechanistic understanding of nutrient sources and algal physiology, offering pathways to mitigation. Simultaneously, valorisation strategies explore converting stranded biomass into biofertilisers or biogas feedstock, highlighting opportunities within a circular economy while addressing environmental and social challenges.

Research from Nature Portfolio

Recent studies have revealed shifts in the nutrient composition of pelagic macroalgae, showing a marked increase in nitrogen content and a decline in phosphorus over recent decades. Such changes have driven a doubling of the nitrogen-to-phosphorus ratio, exacerbating phosphorus limitation and promoting bloom occurrence even in traditionally oligotrophic regions. Investigations into arsenic biogeochemistry demonstrate that arsenic accumulation within floating Sargassum correlates inversely with phosphorus availability, consistent with nutrient uptake kinetics. These findings allow the use of elemental and isotopic signatures to trace nutrient sources—be they atmospheric, riverine or upwelled waters—shedding light on the drivers sustaining large-scale blooms like the Great Atlantic Sargassum Belt and informing targeted nutrient management strategies along affected coasts.

Macroalgal Blooms in Coastal Ecosystems publication trend

The graph below shows the total number of articles in macroalgal blooms in coastal ecosystems across all publications each year (not limited to Nature Index journals).

Technical terms

Macroalgal bloom: A rapid increase in the biomass of large multicellular algae in a coastal or open-ocean environment.

Holopelagic: Referring to algae that remain free-floating throughout their entire life cycle rather than attaching to substrates.

Diazotroph: A microorganism capable of fixing atmospheric nitrogen into biologically available forms.

Stoichiometry: The balance of chemical elements, such as nitrogen and phosphorus, within algal tissues or ecosystems.

References

1. Nutrient and arsenic biogeochemistry of Sargassum in the western Atlantic. Nature Communications (2023).
2. Predominant heterotrophic diazotrophic bacteria are involved in Sargassum proliferation in the Great Atlantic Sargassum Belt. The ISME Journal: Multidisciplinary Journal of Microbial Ecology (2024).
3. Misperception of drivers of risk alters willingness to adapt in the case of sargassum influxes in West Africa. Global Environmental Change (2024).
4. Changes in holopelagic Sargassum spp. biomass composition across an unusual year. Proceedings of the National Academy of Sciences of the United States of America (2024).
5. Golden Tides: Problem or Golden Opportunity? The Valorisation of Sargassum from Beach Inundations. Journal of Marine Science and Engineering (2016).
6. Ulva prolifera green-tide outbreaks and their environmental impact in the Yellow Sea, China. National Science Review (2019).
7. Simulating transport pathways of pelagic Sargassum from the Equatorial Atlantic into the Caribbean Sea. Progress In Oceanography (2018).
8. From In Situ to satellite observations of pelagic Sargassum distribution and aggregation in the Tropical North Atlantic Ocean. PLOS ONE (2019).
9. Nutrient content and stoichiometry of pelagic Sargassum reflects increasing nitrogen availability in the Atlantic Basin. Nature Communications (2021).