In recent years, the Atlantic Ocean has witnessed an unprecedented ecological phenomenon: the explosive growth and widespread distribution of pelagic sargassum, a free-floating brown seaweed once thought to be confined primarily to the nutrient-poor Sargasso Sea. A groundbreaking comprehensive review published in the journal Harmful Algae by researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute (HBOI) reveals critical insights into the complex biological, chemical, and physical processes driving these transformations over the past four decades. This extensive analysis reshapes our understanding of the interface between marine ecosystems and human impacts, framing sargassum as not only a keystone species but also a bioindicator of shifting oceanographic and anthropogenic forces.
Traditionally, the Sargasso Sea has been characterized by vast expanses of warm, clear waters with low concentrations of nutrients, creating an environment often described paradoxically as both a “biological desert” and a refuge for sargassum. Early oceanographers identified sargassum mats through surface observations, suggesting these algae thrived in nutrient-poor conditions. However, mid-20th century studies struggled to reconcile such productivity with the apparent scarcity of nutrients, revealing a paradox now addressed through modern satellite data and oceanographic modeling. This emerging understanding reveals that sargassum populations are significantly influenced by nutrient influxes from coastal sources, challenging long-standing assumptions of the species’ ecological niche.
Since 2011, a massive and recurring bloom known as the Great Atlantic Sargassum Belt (GASB) has extended from the western coast of Africa across the tropical Atlantic to the Gulf of Mexico, reaching staggering biomass levels never before documented. This “belting” phenomenon was absent only in 2013 and has since intensified, with the May 2025 bloom reaching a record 37.5 million tons. This amount vastly exceeds the natural baseline biomass of 7.3 million tons historically estimated within the Sargasso Sea, underscoring the scale of this emergent ecological event. The GASB exemplifies the intersection of natural oceanic dynamics and escalating human-driven nutrient enrichment, leading to dramatic ecological consequences.
Central to this review is the examination of sargassum’s biogeochemical composition, particularly shifts over time in key elements such as nitrogen, phosphorus, and carbon. Researchers have documented a more than 50% increase in nitrogen content from the 1980s to the 2020s, contrasted by a slight decline in phosphorus concentrations, a change that has substantially raised the nitrogen-to-phosphorus (N:P) ratio within sargassum tissue. These stoichiometric shifts indicate a departure from traditional nutrient sources like oceanic upwelling and vertical mixing, toward land-based contributions including agricultural runoff, wastewater effluent, and atmospheric nitrogen deposition. This chemically enriched profile enhances sargassum growth rates, biomass accumulation, and reproductive potential, fundamentally altering its ecological role.
Laboratory and field studies conducted over the past four decades reveal that sargassum growth is highly responsive to nutrient availability, particularly phosphorus and nitrogen, exhibiting the capacity to double biomass in as little as 11 days under optimal conditions. This rapid productivity is more pronounced in nutrient-enriched coastal waters compared to the oligotrophic open ocean. Such findings spotlight the vulnerability of coastal and nearshore environments to nutrient pollution and emphasize the potential for these inputs to catalyze widespread blooms with ramifications spanning marine biodiversity, fisheries, and human communities. Sargassum’s response to nutrient dynamics serves as a bellwether for broader ocean health amid anthropogenic pressures.
The mechanisms supporting sustained sargassum growth in diverse and sometimes nutrient-limited environments extend beyond mere nutrient input. The review highlights the critical role of nutrient recycling within sargassum windrows—linear aggregations of floating sargassum mats. These windrows facilitate localized microenvironments where associated marine organisms excrete nutrients, and microbial communities break down organic matter, effectively sustaining sargassum populations even when external inputs are scarce. This intricate nutrient cycling underscores the resilience and adaptability of sargassum ecosystems and informs future predictions for bloom persistence and dispersal patterns under varying oceanographic conditions.
The geographical origins of the Great Atlantic Sargassum Belt are intricately tied to nutrient-rich river systems, notably the Amazon River. Sargassum samples collected near the Amazon River mouth exhibit chemical signatures consistent with terrestrial nutrient influx, implicating episodic flood and drought cycles in driving variations in bloom intensity and spatial distribution. These terrestrial-marine linkages illustrate a complex interplay where watershed land use, hydrological variability, and climate phenomena converge to influence large-scale ocean productivity. The review further suggests that atmospheric and oceanic circulation patterns, including shifts related to the North Atlantic Oscillation, may create conditions conducive to initiating and sustaining the GASB, although genetic evidence points to the tropical Atlantic as an important early and ongoing habitat.
The societal and ecological impacts of massive sargassum blooms are profound and multifaceted. Coastal communities spanning from West Africa to the Gulf of Mexico contend with beach closures, degradation of tourism economies, disruptions to fisheries, and public health concerns stemming from the decay and toxicity of stranded seaweed. Notably, such blooms have necessitated extraordinary responses, such as the emergency shutdown of a Florida nuclear power plant in 1991 due to sargassum clogging cooling water intakes. These events highlight the critical need for integrated monitoring, early warning systems, and coordinated management strategies that bridge scientific understanding and policy actions.
Technological advances in remote sensing have been instrumental in revealing the dynamics of sargassum distribution. Satellite imagery collected since the early 2000s has detected extensive sargassum accumulations, or windrows, particularly in the western Gulf of Mexico. This technology facilitates near real-time surveillance of bloom development and dispersal across ocean basins, enabling researchers to correlate satellite data with in situ observations and oceanographic models. The integration of these data streams provides a powerful framework for elucidating the spatial-temporal dynamics of pelagic sargassum and its responses to environmental variability and anthropogenic influences.
This comprehensive review underscores a paradigm shift in understanding pelagic sargassum from a static, isolated organism confined to oligotrophic waters into a dynamic, ecosystem-engineering species influenced by regional nutrient dynamics and global change. The implications extend beyond academic interest: they challenge existing ocean management frameworks and highlight the imperative for interdisciplinary approaches that consider terrestrial inputs, ocean circulation, chemical ecology, and socio-economic impacts. These insights provide a foundation for developing predictive models and mitigation strategies essential for coastal resilience in the face of ongoing environmental change.
Furthermore, the rise in sargassum biomass signals broader environmental shifts, including increased nutrient pollution linked to expanding agricultural practices and urbanization. The altered stoichiometry of sargassum tissue reflects these anthropogenic changes in nutrient availability, potentially affecting the quality and quantity of organic matter supplied to marine food webs. These biogeochemical transformations may have cascading effects throughout trophic levels, from microbial assemblages to commercially important fish species, thereby influencing ecosystem structure and function on a basin-wide scale.
The interdisciplinary team at FAU Harbor Branch Oceanographic Institute, led by Dr. Brian Lapointe, combines decades of historical data, satellite imagery, and advanced biogeochemical analyses to articulate this pressing environmental narrative. Their work synthesizes oceanography, marine ecology, chemistry, and climatology, providing an integrative understanding necessary for addressing the challenges posed by pelagic sargassum expansion. This review serves as a critical resource for scientists, environmental managers, and policymakers striving to balance ocean health with human development amid accelerating global change.
In conclusion, the burgeoning presence of pelagic sargassum across the Atlantic Ocean exemplifies the intricate connections among human activities, nutrient cycles, and marine ecosystems. This transformative phenomenon necessitates vigilant scientific exploration and collaborative management to mitigate negative impacts while appreciating the fundamental role sargassum plays within the ocean’s ecological fabric. As the Great Atlantic Sargassum Belt continues to evolve, the insights from this landmark review chart the course for future research priorities and adaptive strategies vital for sustaining the health and productivity of marine environments in an era of rapid planetary change.
Subject of Research: Not applicable
Article Title: Productivity, growth, and biogeochemistry of pelagic Sargassum in a changing world
News Publication Date: 8-Aug-2025
Web References:
- Florida Atlantic University Harbor Branch Oceanographic Institute: www.fau.edu/hboi
- Florida Atlantic University: www.fau.edu
- Article DOI: 10.1016/j.hal.2025.102940
References:
- Lapointe, B., Webber, D. F., Brewton, R., et al. (2025). Productivity, growth, and biogeochemistry of pelagic Sargassum in a changing world. Harmful Algae, [Article].
Image Credits: Credit: FAU Harbor Branch
Keywords: Seaweeds, Ecology, Environmental sciences, Pollution, Nitrogen deposition, Water pollution, Microbial ecology, Ecosystems, Aquatic ecosystems, Coastal ecosystems, Tropical ecosystems, Environmental chemistry, Hydrogeochemistry, Nitrogen, Phosphorus, Carbon
