Changes in holopelagic Sargassum spp. biomass composition across an unusual year

Abstract

The year 2021 marked a decade of holopelagic sargassum (morphotypes Sargassum natans I and VIII, and Sargassum fluitans III) stranding on the Caribbean and West African coasts. Beaching of millions of tons of sargassum negatively impacts coastal ecosystems, economies, and human health. Additionally, the La Soufrière volcano erupted in St. Vincent in April 2021, at the start of the sargassum season. We investigated potential monthly variations in morphotype abundance and biomass composition of sargassum harvested in Jamaica and assessed the influence of processing methods (shade-drying vs. frozen samples) and of volcanic ash exposure on biochemical and elemental components. S. fluitans III was the most abundant morphotype across the year. Limited monthly variations were observed for key brown algal components (phlorotannins, fucoxanthin, and alginate). Shade-drying did not significantly alter the contents of proteins but affected levels of phlorotannins, fucoxanthin, mannitol, and alginate. Simulation of sargassum and volcanic ash drift combined with age statistics suggested that sargassum potentially shared the surface layer with ash for ~50 d, approximately 100 d before stranding in Jamaica. Integrated elemental analysis of volcanic ash, ambient seawater, and sargassum biomass showed that algae harvested from August had accumulated P, Al, Fe, Mn, Zn, and Ni, probably from the ash, and contained less As. This ash fingerprint confirmed the geographical origin and drift timescale of sargassum. Since environmental conditions and processing methods influence biomass composition, efforts should continue to improve understanding, forecasting, monitoring, and valorizing sargassum, particularly as strandings of sargassum show no sign of abating.

Keywords: algal blooms; biochemical composition; elemental composition; processing; volcanic ash.

Fig. 1.

Regionally aggregated areas (km²) of sargassum: (A) Central Equatorial North Atlantic; (B) Central Tropical North Atlantic; (C) West Tropical North Atlantic; (D) East Caribbean–East; (E) East Caribbean–Central; (F) East Caribbean–West; (G) North of Jamaica; (H) South of Jamaica. Thick curves indicate 2021, thin curves indicate 2011 to 2020, and green shading indicates the timing and duration of the major La Soufrière eruption (9 to 18 April). In G and H, red vertical lines indicate the timing of beach sampling over May–October in the south of Jamaica.

Fig. 2.

Statistics of simulated 180-d forward trajectories from the triangular area representative of volcanic ash deposition in early April: (A) fractional presence and (B) mean age, drifting with surface currents only (simulating ash dispersal); (C) fractional presence and (D) mean age, drifting with surface currents and subject to 1% windage (simulating sargassum dispersal).

Fig. 3.

Statistics of simulated 120-d backward trajectories for August released over 1988 to 2010 off east Jamaica (white circle): (A) fractional presence; (B) mean age (days before arrival off Jamaica). St Vincent is indicated with a red circle.

Fig. 4.

Relative abundance of the three sargassum morphotypes across the 2021 season in Jamaica. Results of statistical analysis, including of the post hoc test, are provided in Dataset S1 A–C.

Fig. 5.

Elemental composition of sargassum harvested in Jamaica across the 2021 sargassum inundation season. Samples were all frozen prior to analysis. For ease of visualization, elements were grouped by panel according to concentrations in seaweed biomass: (A) very high, (B) high, (C) medium, (D) low. The mean ± SD is reported for each measurement. Results of statistical analysis, including of the post hoc test, are provided in Dataset S2 A–C.

Fig. 6.

Biochemical composition of sargassum harvested in Jamaica across the year 2021. Results show contents in ash and moisture (A), proteins (B), phenolics and phlorotannins (C), fucoxanthin (D), and monosaccharides (E). Results of statistical analysis, including of the post hoc test, are provided in Dataset S3 A–C for panel A, Dataset S4 A and B for panel B, Dataset S5 A–C for panel C, Dataset S6 A and B for panel D, and Dataset S7 A–E for panel E.

Fig. 7.

Heat map showing potential correlations between the contents of biochemical compounds and of elements in freeze-dried sargassum biomass harvested at different times of the year 2021. Only significant correlations (P-value ≤ 0.05) are plotted. Alginate_bio, percentage of biomass accounted for by alginate (mannuronic and guluronic acids); Alginate_mono, percentage of the total sugars accounted for by alginate (mannuronic and guluronic acids); MG_ratio, mannuronic acid:guluronic acid (M:G) ratio; Perc_DW, percentage of the dry weight accounted for by the total quantities of sugars; Total sugars, total quantities of mannitol, monosaccharides, and uronic acids. An extended version of this figure is presented in SI Appendix, Fig. S4.