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• Since 2014, the Mexican Caribbean has received massive influxes of pelagic Sargassum species (sargasso). • The biomass of beached sargasso varied interannually and seasonally. • Variability was related to presence of sargasso in the ocean and trade winds. • A shift in species and morphotypes occurred from 2016 to 2020. Since 2011, unusually large quantities of pelagic Sargassum fluitans and S. natans (sargasso) have been washing ashore along the coasts of some African countries and the Greater Caribbean, impacting ecosystems and economies. We estimated biomass and composition of sargasso arriving to a Mexican Caribbean coast from September 2016 until May 2020. In 2016, the beached masses comprised S. natans VIII and S. fluitans III. S. fluitans III was the predominant form throughout the study period, comprising on average >60 % of total wet biomass. The relative abundance of S. natans VIII decreased in time from 2016 to 2019 (41 to 3 %), although it became prevalent again in the first months of 2020. The third morphological form, S. natans I, was not registered until February 2018, and its relative abundance increased from 23 % in 2018 to 31 % in 2019. The initial composition of Sargassum species and morphotypes of the beached sargasso in Mexico differed from that commonly reported in the Sargasso Sea. The total biomass of beached sargasso varied considerably among years and seasons, with peaks during the summer months of 2018 and 2019. Seasonal variations in biomass were explained by 1) presence of sargasso in the Yucatan Current and 2) prevailing trade winds. This study is the first report on biomass and species composition of beached sargasso in the Western Caribbean and may help to understand patterns of the massive influxes; which will aid in the management of this new phenomenon. [ABSTRACT FROM AUTHOR]

The recent recurring blooms of pelagic Sargassum spp. (sargasso) in the tropical Atlantic forming the Great Atlantic Sargasso Belt, are composed of distinct algal species and morphological forms, for which little understanding of basic development exists. Growth rate measurements across a range of environmental conditions in semi-controlled settings may aid in understanding the blooms. However, measuring the growth of this pelagic seaweed has proved to be challenging, as it tends to lose vigor in standard culturing systems. This work describes in-situ and ex-situ culture systems, that mimicked the pelagic condition by keeping the algae in motion. Growth rates measured in both systems were comparable, and in the same order of magnitude as those measured for sargasso in the Sargasso Sea. The morphotype VIII of the species S. natans , which became abundant with the blooms, had a higher in-situ growth rate (doubling time 17.9 – 26.3 d) than S. fluitans III (doubling time 25.6 – 83.3 d) under the studied conditions and temperatures (27–29 °C). Fertilizer addition to the in-situ trials did not result in higher growth rates, even though tissue content analysis showed that both N and P were taken up by the algae. In the ex-situ trials, S. natans VIII responded unfavorably to higher temperature (31 °C), whereas S. fluitans III did not. This preliminary study showed that both in-situ and ex-situ culture are feasible to determine growth rates of sargasso, paving the way for future (semi-) controlled studies on bloom dynamics in relation to abiotic and biotic conditions. [ABSTRACT FROM AUTHOR]