The proliferation of holopelagic Sargassum in the Atlantic Ocean and Caribbean Sea has significant ecological, economic, and environmental impacts. This session aims to bring together researchers from the international community to share their latest research on the causes, origins, and predictability of Sargassum accumulation and stranding events. The session will focus on several key areas to increase our knowledge and understanding of Sargassum ecology: 1. Insights into the genetic diversity of Sargassum across the Atlantic, including microbiome and geographic distribution, 2. Examination of the role of nutrient inputs from diverse sources, such as Amazon river outflow, upwelling systems, and atmospheric deposition (e.g., Saharan dust), in stimulating Sargassum growth, 3. Exploration of changing ocean currents and temperature on the patterns of distribution and intensity of Sargassum accumulations. 4. Development of models and predictive tools to forecast bloom events, integrating data from satellite observations, oceanographic measurements, and climate models to improve the accuracy of bloom predictions. 5. Assessment of the ecological consequences of large-scale Sargassum events on marine ecosystems, including impacts on biodiversity, fisheries, and coastal environments.
Lead Organizer: Fabio Nauer, Royal Netherlands Institute for Sea Research (NIOZ) (fabio.nauer@nioz.nl)
Co-organizers:
Linda Amaral-Zettler, Royal Netherlands Institute for Sea Research (NIOZ) (linda.amaral-zettler@nioz.nl)
Pierre-Yves Pascal, Université des Antilles (pierreyves.pascal@gmail.com)
Presentations
02:30 PM
Quantifying Sargassum and Carbon Export from Satellites to the Seafloor in the Great Atlantic Sargassum Belt (GASB) (8801)
Tutorial/Invited: Tutorial
Primary Presenter: Linda Amaral Zettler, NIOZ Royal Netherlands Institute for Sea Research / Univ. of Amsterdam, Netherlands (linda.amaral-zettler@nioz.nl)
For over a decade, unprecedented amounts of the brown seaweed Sargassum have been accumulating in the Great Atlantic Sargassum Belt (GASB), acting as a source of Sargassum strandings on shores of the Caribbean, Brazil, western Africa and parts of the Gulf of Mexico. Quantifying holopelagic Sargassum biomass in this large region is challenging and has relied primarily on satellite imaging providing synoptic estimates of surface coverage, but with limited ground-truthing or understanding of carbon export to the seafloor. Here we provide an in-situ estimate of the amount of holopelagic Sargassum biomass from the surface to the seafloor using data collected during a summer 2024 research expedition from Guadeloupe to Cape Verde through the GASB. We used satellite imagery linked to shipboard camera surveys of surface coverage, measurement of biomass inside mats, sub-surface traps to quantify sinking Sargassum, and tethered camera surveys to quantify the amount of Sargassum reaching the sea floor. Elemental analyses of selected Sargassum samples allow us to provide an estimate of the amount of carbon that Sargassum mats sequester and how much of this carbon is exported to the sea floor.
03:00 PM
Growth experiments of two Sargassum genotypes (S. fluitans III and S. natans VIII) and study of the impacts of exposure to metals (iron, arsenic) in the Great Atlantic Sargassum Belt (GASB) (8968)
Primary Presenter: Marine Guilbaud, La Sorbonne Université (guilbaudmarine.pro@gmail.com)
Since 2011 the northern tropical Atlantic in the North Equatorial Recirculation Region (NERR) has seen repeating blooms of holopelagic Sargassum species, creating the ‘Great Atlantic Sargassum Belt’ extended for 8850-km length which stretches from tropical Africa to the Gulf of Mexico. Sargassum's ability to absorb metals, including arsenic (As) and iron (Fe), raises concerns about potential physiological impacts, especially on its growth. Arsenic is a toxic metalloid, and iron, though essential for cellular processes like photosynthesis, can be harmful in high concentrations. This study aimed to (i) develop a growth system for Sargassum onboard a research cruise, (ii) investigate the effects of metal bioaccumulation on growth rates, and (iii) determine the speciation of bioaccumulated arsenic. Morphotype S. fluitans III and S. natans VIII were grown for five days under controlled conditions with seawater from different locations along the cruise and various iron and arsenic concentrations. Growth parameters such as wet weight, number of bladders, number of secondary branches, thickness of primary strand and branch lengths were evaluated, while metal concentrations were measured in sargassum tissues as well as in water using simultaneously DGTs and water samples. Growth rates were determined as a percentage of growth. Initial findings indicate that iron and arsenic contamination result in complex growth adaptations in Sargassum, with different responses in each growth parameter depending on the presence or absence of contamination.
03:15 PM
The influence of environmental factors on the growth of holopelagic Sargassum in the Great Atlantic Sargassum Belt (GASB) (9051)
Primary Presenter: Fabio Nauer, Royal Netherlands Institute for Sea Research (NIOZ) (fabio.nauer@nioz.nl)
In 2011, the first massive holopelagic Sargassum spp. (herein Sargassum) bloom was reported in the central Atlantic. Over the following years, the bloom developed into the Great Atlantic Sargasum Belt (GASB), extending from West Africa to the Gulf of Mexico. The Sargassum establishment in the GASB has raised questions about the main drivers associated with the blooms. It is hypothesized that an increase in nutrient input from rivers (e.g. Amazon, Orinoco, Congo), atmospheric dust (e.g. Saharan dust), or upwelling waters could significantly contribute to the rise in open-ocean Sargassum accumulations. However, direct evidence of the impact of these nutrient sources on Sargassum performance remains lacking. Access to the GASB from our research cruise, during the summer of 2024 from Guadeloupe to Cape Verde, gave us the opportunity to study these regional drivers. We performed controlled physiological experiments onboard, comparing the performance of oligotrophic “open ocean” Sargassum against its exposure to the Amazon River plume water, ocean water plus Saharan dust, and nutrient-rich “upwelled water” collected from below the mixed layer. By analyzing data on nutrient concentrations in the water, growth rate determination and photosynthetic efficiency through different experiments enabled us to assess the influence of these sources on the Sargassum performance. The results of this study explore the impact of different nutrient sources on the Sargassum growth and physiology and provide essential insights into the mechanisms underlying their dramatic proliferation in the region.
03:30 PM
SARGASSUM (S. FILIPENDULA) GROWTH, PHOTOSYNTHESIS, AND BIOCHEMICAL RESPONSE TO TEMPERATURE INCREASE (9479)
Primary Presenter: Jessalin Nguyen, University of Chicago (jessalinnguyen@uchicago.edu)
Rising global temperatures from climate change have altered ecosystems. This effect may be triggering observations of brown macroalgae bloom formations of the genus Sargassum. The response of the Cape Cod species, S. filipendula, was observed over a 16-day tank experiment testing increasing temperature on growth and physiology. Biomass accumulation and loss, specific growth rates, photosynthetic performance via PAM chlorophyll fluorometry, and biochemical tissue, components of S. filipendula were measured. Lower temperature treatments of 14°C and 18°C had the most accumulated biomass compared to higher temperatures, as well as less biomass loss. There was no statistical significance between treatments and photosynthetic yields (Fv/Fm), while the treatment of 23°C and 28°C showed the highest maximum relative electron transport rates (rETRmax). Treatments of 18°C and 23°C had the highest rates of non-photo quenching (NPQ). Biochemical components showed variation with temperature: 18°C having the highest percent carbon content, 14°C having the highest percent of nitrogen, carbohydrate, and protein content. These results suggest that an extended growing season with temperatures ~14-18 °C could increase growth and carbon assimilation by S. filipendula. If there happens to be a heat wave late in the season, S. filipendula could remain resilient, although at the cost of a higher turnover rate. The lack of statistical significance for the different treatments alludes that temperature may not be the most significant factor contributing to the changes in Sargassum proliferation.
03:45 PM
Understanding carbon fixation rates and biodiversity in and around wild and aquafarmed holopelagic Sargassum as crucial step towards resource valorisation (9025)
Primary Presenter: Katharina Paetz, Alfred-Wegener-Institut (katharina-paetz@gmx.net)
While Sargassum accumulations cause economic and environmental issues in the Caribbean, the macroalgae is discussed as a carbon dioxide removal option. Early cultivation attempts were made but the impact of Sargassum aquafarms on the surrounding biodiversity is poorly understood. To better understand its potential, it is crucial to quantify carbon fixation under different conditions both in the wild and in aquaculture. We investigated the carbon fixation rates of the three Sargassum morphotypes both during a cruise across the GASB and within an aquafarm in St Vincent and the Grenadines using 13C measurements. While controlled conditions lead to similar carbon fixation rates in GASB S. fluitans III and S. natans I, S. natans VIII showed significantly lower carbon uptake rates (p < 0.05). No significant differences between carbon fixation rates of aquafarmed and wild Sargassum could be observed in St Vincent. Using eDNA, we investigated the impact of wild and aquafarmed Sargassum on biodiversity of adjacent waters. While the wild algae present a unique habitat for a Sargassum-specific community, they do not impact the biodiversity of adjacent waters. We found that a Sargassum aquafarm does not change the biodiversity of surrounding waters in St Vincent. Our results indicate that aquafarmed Sargassum efficiently fixes carbon, even for an extended time period, without impacting the biodiversity of its surrounding environment. Our findings support the algae’s potential as a candidate for carbon dioxide removal, shifting its perception from harmful algae to valuable resource.
SS31A - Sargassum Accumulations in the Atlantic: Ecological Drivers, Impacts, and Predictive Models
Description
Time: 2:30 PM
Date: 28/3/2025
Room: W206A