To predict spatial and temporal dynamics of macroalgal blooms, including the Great Atlantic Sargassum Belt, understanding the environmental tolerances and growth rates of different species under varying conditions is essential. A series of preliminary experiments were conducted exposing three common holopelagic Sargassum morphotypes to different temperature (21.6-30.6 degrees C) and salinity (26.2-40.0 psu) conditions. Sargassum tolerance was assessed by two methods: a Health Metric calculated daily from changes in a specimen's blade and vesicle count and coloration patterns, and growth rates determined from pre-and post-treatment wet weights. Morphotypes exhibited different responses to treatment conditions. Health Metric values and growth rates for S. fluitans III were not significantly impacted by tested temperatures or salinities. S. natans I tolerated a wide range of temperatures but only moderate salinities. While S. natans VIII grew well and maintained strong Health Metrics across examined salinities, it was less tolerant of cold temperatures and exhibited overall lower growth rates than other morphotypes. Given inconsistent findings among recent growth rate studies, including this one, additional experiments of longer duration that continue to explore temperature and salinity effects and use specimens from across holopelagic Sargassum's geographic range are necessary to understand growth ecology and parameterize models.

Since 2011, massive new strandings of holopelagic Sargassum have been reported on the coasts of the Caribbean, northern Brazil, Guiana, and West Africa, causing severe economic and ecological damage. Three common morphotypes (S. fluitans III, S. natans I, and S. natans VIII) were identified as responsible for these catastrophic events, with dominance shifts between them over time. However, the taxonomic status of these holopelagic Sargassum morphotypes remains unclear. Using an integrative taxonomy framework, combining a morphological study and molecular analyses, this study aimed to clarify their taxonomic status. Morphological analyses of 54 characters revealed no intermediate form between the three morphotypes, with the overall shape, nature of the axis, and size and shape of blades and vesicles being the most discriminating. An analysis of mitochondrial (IGS, cox2, cox3, mt16S rRNA, and nad6) and plastid (rbcL) markers confirmed the genetic divergence among the three morphotypes, with a lower level of divergence between the two S. natans morphotypes. Without additional molecular characterization, these morphotypes cannot be classified as three distinct species. However, due to their distinct morphological characteristics and sympatry within drifting aggregations, a revision of holopelagic species names is proposed, with Sargassum fluitans var. fluitans (for S. fluitans III), Sargassum natans var. natans (for S. natans I), and S. natans var. wingei (for S. natans VIII). This revision provides necessary clarity on the species involved in inundations of the tropical Atlantic.

Between 2011 and 2020, 6,790 visual observations of holopelagic Sargassum were recorded across the North Atlantic Ocean to describe regional distribution, presence, and aggregation state at hourly and 10 km scales. Influences of oceanographic region and wind/sea conditions as well as temporal trends were considered; marine megafauna associates documented the ecological value of aggregations. Holopelagic Sargassum was present in 64% of observations from the western North Atlantic. Dispersed holopelagic Sargassum fragments and clumps were found in 97% of positive observations whereas aggregated windrows (37%) and mats (1%) were less common. Most field observations noted holopelagic Sargassum in quantities below the AFAI algorithm detection limit for the MODIS sensor. Aggregation state patterns were similar across regions; windrow proportion increased with higher wind speeds. In 8 of 10 years in the Sargasso Sea holopelagic Sargassum was found in over 65% of observations. In contrast, the Tropical Atlantic and Caribbean Sea exhibited greater inter-annual variability (1–88% and 11–78% presence, respectively) that did not align with extremes in central Atlantic holopelagic Sargassum areal coverage determined from satellite observations. Megafauna association patterns varied by taxonomic group. While some study regions were impacted by holopelagic Sargassum dynamics in the equatorial Atlantic, the Sargasso Sea had consistently high presence and operated independently. Field observations capture important dynamics occurring at fine spatiotemporal scales, including transient aggregation processes and ecological value for megafauna associates, and therefore remain essential to future studies of holopelagic Sargassum.

Sargassum natans and Sargassum fluitans are uniquely holopelagic macroalgae, providing open ocean nursery and foraging habitat for commercially and ecologically important species. Recent basin-wide changes in pelagic Sargassum diversity and distribution have manifested in proliferation of a previously rare morphotype, Sargassum natans VIII, to rival biomass levels of historically dominant S. natans I and S. fluitans III. Precise genetic identification of these morphotypes can improve accuracy and interpretation of ecological studies as well as clarify evolutionary history and population connectivity. For 139 field samples collected from the subtropical and tropical North Atlantic, three mitochondrial genes (cox3, nad6, and mt16S rRNA) were used to examine genetic divergence among the three common pelagic Sargassum morphotypes. These gene sequences successfully differentiated among morphotypes regardless of geographic origin, confirming in situ morphology-based identifications. Sargassum natans I and S. natans VIII exhibited divergence consistent with that between the S. natans-complex and S. fluitans III. Phylogenetic analysis of these samples also indicated evolutionary divergence between Sargassum morphologies. The genetic divergence among morphotypes, compared with benthic Sargassum species, suggested that taxonomic reclassification of the three most common pelagic morphotypes may be warranted.

Pelagic Sargassum macroalgal rafts in the North Atlantic support sessile and motile epifauna that attract ecologically and economically important migratory organisms. Three prevalent pelagic Sargassum morphotypes vary in their degree of branching and foliation, and thus have different structural complexities that can influence their respective value as motile epifauna habitat. Sargassum fluitans III and S. natans I have denser foliation, creating a complex habitat; in contrast, S. natans VIII is more open and architecturally simple. In 2015/2016, 373 dip net samples of algae were collected from the Tropical Atlantic, Greater Caribbean, Gulf of Mexico, Gulf Stream, and Sargasso Sea. 20,975 individual motile epifauna from 32 taxa were recorded. Sargassum fluitans III supported higher densities of individuals and greater numbers of taxa than S. natans VIII or S. natans I, a pattern attributed to its more complex architecture and consistent with communities on benthic and floating macroalgae. Most assemblages comprised a few dominant and many rare motile epifauna; when compared to historical studies, dominant motile epifauna had shifted. These findings suggest important differences in ecological value between pelagic Sargassum morphotypes with implications for coastal and pelagic conservation strategies, which warrant consideration given recent shifts in morphotype distribution and recurring pelagic Sargassum inundation events.

Bopyrid isopod parasitic infestation of a variety of decapod definitive hosts is common worldwide. We report frequencies of a parasite infestation in the shrimp Latreutes fucorum associated with the pelagic macroalgae Sargassum in the Gulf of Mexico, Sargasso Sea and Eastern Caribbean. Average Probopyrinella latreuticola infestation frequency was 6.7% and did not significantly vary between regions. The presence of the ectoparasite appeared to impact fertility with only one infested individual found carrying eggs. In contrast, across all three regions, 13% (of n = 4001) of the non-infested shrimp were carrying eggs. With L. fucorum accounting for three quarters of lower trophic level biomass in the pelagic Sargassum-associated faunal community, parasite infestation may have negative consequences for ecologically and commercially important populations that rely directly or indirectly on the host as a food source.

Abstract Over the past 5 years, massive accumulations of holopelagic species of the brown macroalga Sargassum in coastal areas of the Caribbean have created “golden tides” that threaten local biodiversity and trigger economic losses associated with beach deterioration and impact on fisheries and tourism. In 2015, the first report identifying the cause of these extreme events implicated a rare form of the holopelagic species Sargassum natans (form VIII). However, since the first mention of S. natans VIII in the 1930s, based solely on morphological characters, no molecular data have confirmed this identification. We generated full‐length mitogenomes and partial chloroplast genomes of all representative holopelagic Sargassum species, S. fluitans III and S. natans I alongside the putatively rare S. natans VIII, to demonstrate small but consistent differences between S. natans I and VIII (7 bp differences out of the 34,727). Our comparative analyses also revealed that both S. natans I and S. natans VIII share a very close phylogenetic relationship with S. fluitans III (94‐ and 96‐bp differences of 34,727). We designed novel primers that amplified regions of the cox2 and cox3 marker genes with consistent polymorphic sites that enabled differentiation between the two S. natans forms (I and VIII) from each other and both from S. fluitans III in over 150 Sargassum samples including those from the 2014 golden tide event. Despite remarkable gene synteny and sequence conservation, the three Sargassum forms differ in morphology, ecology, and distribution patterns, warranting more extensive interrogation of holopelagic Sargassum genomes as a whole.

The Sargassum Watch System processes satellite data and feeds results to a Web portal, giving decision makers timely information on seaweed location and warnings for potential beaching events.

During June 2011, pelagic Sargassum began washing ashore along Caribbean, Gulf of Mexico, West African, and Brazilian coastlines in unprecedented quantities.Tourist beaches were covered by more than a meter of seaweed.Economic impacts of this Atlantic basin-scale inundation event drew international media attention (Higgins, 2011).By summer 2012, our shipboard observations suggested the Caribbean portion of the event had run its course.However, another similarly extensive Sargassum inundation was underway by April 2014, persisting through 2015 (MercoPress, 2015).Did the invading pelagic Sargassum drift out of the Sargasso Sea, a vast region bounded by the currents of the North Atlantic gyre (Smetacek and Zingone, 2013)?Alternatively, is its source the North Equatorial Recirculation Region (NERR), as suggested by satellite-derived observations of Sargassum mats (Gower et al., 2013) and hindcast models of Sargassum landfalls (Johnson et al., 2013)?Our recent net sampling indicates that the invading Sargassum did not come from the Sargasso Sea.In late November 2014, Sea Education Association's (SEA's) SSV Corwith Cramer departed the Canary Islands.We sailed across the eastern Sargasso Sea without a sighting, but on day 15, after heading south into the tropics, we were surrounded by Sargassum.For the next three weeks, twice-daily surface net tows contained more Sargassum than ever recorded by SEA voyages.We noticed the seaweed looked different from the Sargassum fluitans or S. natans with which we were familiar from 20 years of sailing in the Sargasso Sea, the Caribbean, and Florida Straits (Figure 1a).Most resources assert pelagic Sargassum is composed of two species, S. fluitans and S. natans.However, each species exhibits a diversity of