Surface-floating, holopelagic Sargassum in the Atlantic Ocean presents scientists and society with a conundrum. On one hand, within its historic Sargasso Sea range, holopelagic Sargassum is recognized as a unique high seas ecosystem valued for the critical habitat it provides for iconic species from endemic invertebrates to sea turtles to economically-important fishes. Conversely since 2011, recurring blooms in the equatorial Atlantic have driven holopelagic Sargassum inundation events with significant, persistent ecological and economic impacts from west Africa to South America, the wider Caribbean, and the southern United States. Given this regime shift, the path forward must balance the competing goals of international conservation, management, and mitigation of this enigmatic drifting ecosystem. A better understanding of annual bloom dynamics, driving factors, and temporal variability will improve beaching forecasts and mitigation of coastal impacts. Furthermore, foundational research on holopelagic Sargassum physiology, ecology, shifting distribution patterns, and transport mechanisms in both offshore and coastal environments are necessary to inform remote sensing and modeling efforts. This session seeks to coalesce work occurring across the Atlantic basin to advance our knowledge of holopelagic Sargassum ecology and distribution and to build a holistic view of the Great Atlantic Sargassum Belt. Submissions reporting on field studies, experimental and taxonomic work, in situ observations, remote sensing, and modeling are all welcome.
Lead Organizer: Amy Siuda, Eckerd College (siudaan@eckerd.edu)
Co-organizers:
Amy Siuda, Eckerd College (siudaan@eckerd.edu)
Jeffrey M. Schell, PhD, Sea Education Association (jschell@sea.edu)
Thierry Thibaut, Mediterranean Institute of Oceanography (thierry.thibaut@univ-amu.fr)
Charlotte Dromard, Université des Antilles, Campus de Fouillole, UFR Sciences Exactes et Naturelles (Charlotte.Dromard@univ-antilles.fr)
Presentations
08:30 AM
Carbon and nitrate pathways in pelagic Sargassum (6648)
Primary Presenter: Thierry THIBAUT, Aix-Marseille University (thierry.thibaut@univ-amu.fr)
The impact of latitude and longitude on isotopic ratios (δ15N and δ13C) on the three morphotypes of Sargassum were analyzed from data collected during two oceanographic Atlantic cruises in 2017 and supplemented by coastal sampling at West Indies. Longitude and morphotypes have an influence on the δ13C content. The δ13C is lower in S. fluitans III (-17.5‰) than in S. natans I and VIII (-15.2‰). The variation of δ13C varies according to the state of the sea and the presence of large rafts which influence the absorption of CO2 and HCO3-. The δ15N levels are very negative and do not differ from east to west between the morphotypes. This show the importance of diazotrophs in nitrogen fixation. The δ15N levels are higher towards the coasts or in the up-welling zones. Ammonium from excretion by metazoans is al so a source of nitrogen for Sargassum off Africa. Whatever the position of the Sargassum in the ocean, the nitrogen is not limiting and comes from different sources, diazotrophy, terrigenous inputs, upwellings and contribution by metazoans associated with rafts.
08:45 AM
Nitrogen fixers in the GASB are involved in pelagic Sargassum brown tides in the Caribbean (5130)
Primary Presenter: Matéo Léger-Pigout, Aix-Marseille University (mateo.leger-pigout@mio.osupytheas.fr)
Since 2011, blooms of holopelagic Sargassum have been observed in the Tropical North Atlantic Ocean which caused important beach stranding along the Caribbean and African shores. Main microbial contributors of the nitrogen cycle were studied in holopelagic or benthic Sargassum biofilm and in adjacent waters to understand their relative contribution to seaweed growth in the Great Atlantic Sargassum Belt (GASB) and in Caribbean coastal zones. In GASB Sargassum biofilm, the proportion of diazotrophs (composed of 76% of non-cyanobacterial-nifH sequences) predominated and declined in coastal samples. Denitrifying and nitrifying counterparts showed an opposite trend. Biofilm diazotrophs also exceeded those found in the nearby waters, whereas nitrifiers and denitrifiers presented the same trends in both sample types. Benthic Sargassum biofilm showed lower values for all investigated communities. Significant differences were observed between diazotroph diversity of pelagic and benthic biofilms or adjacent water microbiomes. The GASB seaweed mean δ15N value was very low (-2.04‰), and increased in coastal samples (-1.17‰), whereas lower values were recorded for benthic Sargassum (0.03‰) or POM (1.73-4.95‰). These results highlighted the atmospheric origin of nitrogen used during the pelagic Sargassum growth in the GASB and extended the role of biofilm diazotrophs to non-Cyanobacteria.
09:00 AM
Carbon fixation in holopelagic Sargassum is partially decoupled from nitrate and phosphate availability (5669)
Primary Presenter: Miriam Philippi, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (miriam.philippi@awi.de)
Since the first occurrence of the Great Atlantic Sargassum Belt (GASB) in 2011, scientists have been searching for explanations for this yearly recurring phenomenon. Climate warming-induced changes in winds and currents, in combination with increased anthropogenic nutrient inputs to the tropical Atlantic, have been identified as potential drivers of bloom formation. Yet, a detailed understanding of the physiological response of holopelagic sargassum to varying nutrient conditions is lacking. In summer 2022, we studied the effect of different nitrate (0 – 35 µM) and phosphate (0 – 2.2 µM) conditions on carbon fixation and growth of Sargassum fluitans III in ex situ culture systems. Stable isotope incubation experiments (13C-labelled dissolved inorganic carbon and 15N-labelled nitrate) were performed to quantify carbon and nutrient uptake rates under different conditions. While nutrient uptake rates were directly linked to nutrient availability, increasing nutrient conditions did not result in increasing carbon fixation and growth rates. Instead, it let to a decreasing biomass carbon to nitrogen ratio, potentially hinting at internal nutrient storage. Our results revealed that the effect of nitrate and phosphate on the growth of sargassum is more complex than previously assumed, posing new questions regarding the role of anthropogenic nutrient inputs in the GASB formation. Further, our results provide new insights into the regulation of carbon fixation and therewith the overall carbon dioxide removal and climate mitigation potential of holopelagic sargassum.
09:15 AM
BIOAVAILABILITY AND PHOTOLABILITY OF DISSOLVED ORGANIC CARBON FROM SARGASSUM NATANS (6140)
Primary Presenter: Chance English, University of California, Santa Barbara (cje@ucsb.edu)
Recent increases in the intensity of macroalgal blooms and calls for its cultivation for ocean carbon dioxide removal require greater understanding of the production and fate of this carbon. In addition to large amounts of biomass produced by macroalgae, a substantial amount of net primary production can be released in the form of dissolved organic carbon (DOC), some of which may resist remineralization and contribute to carbon export and sequestration. Here we characterize the DOC released by mature and senescent S. natans and show that its composition determines its bioavailability to natural bacterioplankton communities. S. natans derived DOC is enriched in carbohydrates and polyphenolic compounds and is low in hydrolyzable amino acids; reflected in its high carbon-to-nitrogen ratio. Remineralization bioassays demonstrate that the fraction of this DOC that resists biotic degradation over several months is driven, in part, by the total polyphenolic carbon portion of Sargassum DOM amended. Polyphenols released by S. natans are sensitive to light and have an absorption peak in the UV-C range around 270nm. We demonstrate that light can enhance the remineralization of these refractory compounds and quantify the formation of photooxidation products such as CO2. Our results demonstrate that DOC is an important component of S. natans ecology and contribution to carbon cycling. As polyphenols are ubiquitous among brown macroalgae, our results can be extended to environments or aquaculture dominated by other species.
09:30 AM
The ecophysiology of Sargassum holobiomes in the Great Atlantic Sargassum Belt and coastal Caribbean (5135)
Primary Presenter: Tom Theirlynck, University of Amsterdam, NIOZ Institute (tom.theirlynck@nioz.nl)
Holopelagic species of the brown macroalga Sargassum create a vital and biodiverse habitat in the open Atlantic Ocean. However, over the past decade, floating Sargassum has started to produce nuisance blooms resulting in mass strandings on continental coastlines across the tropical and subtropical Atlantic. Hypotheses regarding the causes of these blooms, include the influence of nutrient runoff from the Amazon and Congo rivers, and a changing climate and oceanographic regimes, with the latter recently being experimentally supported. To understand the influence of nutrient increases on the growth of holopelagic Sargassum, we conducted ecophysiology experiments on board of a research vessel in the Great Atlantic Sargassum Belt (GASB), and ex-situ experiments on the Caribbean island of Curaçao. Our initial results reveal increases in the growth rates of Sargassum natans VIII and Sargassum natans I after the addition of nitrate, phosphate, and Saharan dust. Furthermore, the holopelagic Sargassum microbiomes undergo significant changes after nutrient pulses of phosphate and nitrate, possibly due to physiological changes in the Sargassum host. We combined our growth measurements with measurements of physiological and stoichiometric changes in Sargassum to better quantify the influence of varying nutrients on Sargassum ecophysiology and how this may differ per Sargassum species.
09:45 AM
ENVIRONMENTAL TOLERANCES AND GROWTH RATES OF HOLOPELAGIC <em>SARGASSUM</em> MORPHOTYPES (4815)
Primary Presenter: Jeffrey Schell, Sea Education Association (jschell@sea.edu)
To predict temporal dynamics of holopelagic <em>Sargassum</em> blooms and their development across the Great Atlantic <em>Sargassum</em> Belt, one must understand the environmental tolerances and growth rates of different morphotypes under varying environmental conditions. We conducted a series of experiments exposing three common holopelagic <em>Sargassum</em> morphotypes to different temperature (21 to 30 degrees C) and salinity (26 to 40 ppt) conditions. Tolerance of <em>Sargassum</em> was assessed by two methods: a daily health metric was calculated from changes in a specimen’s blade and float count and coloration patterns, and growth rates were estimated from pre- and post-treatment wet weights. Morphotypes exhibited different responses to treatment conditions. <em>S. natans I<em> has a narrow salinity tolerance (32 to 40 psu) and a wide tolerance for colder temperatures (20 to 28 degC). <em>S. natans I<em> does not tolerate salinities below 30 psu and temperatures above 32 degC. <em>S. natans VIII<em> has a wide salinity tolerance (28 to 40 psu) and narrow temperature (24 to 32 degC) tolerance favoring warm conditions. <em>S. fluitans III<em> has a wide salinity (28 to 40 psu) and temperature (20 to 28 degC) tolerance. <em>S. fluitans IIII<em> does not tolerate salinities below 26 psu and temperatures above 32 degC. Additional experiments of longer duration and with varying nutrients, using specimens from across holopelagic <em>Sargassum</em>’s geographic range, are necessary to understand growth ecology and parameterize models.
SS118 Ecology, Distribution, and Dynamics of Holopelagic Sargassum spp.
Description
Time: 8:30 AM
Date: 8/6/2023
Room: Sala Portixol 1