# The Guiana current: a fast-lane to the Caribbean for holopelagic Sargassum contaminated in arsenic and cadmium

Tristan Gobert, Solène Connan, Donald R Johnson, Matthieu Waeles

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Tristan Gobert, Solène Connan, Donald R Johnson, Matthieu Waeles. The Guiana current: a fast-lane to the Caribbean for holopelagic Sargassum contaminated in arsenic and cadmium. Harmful Algae, 2025, 150, pp.102981. 10.1016/j.hal.2025.102981. hal-05302947

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https://hal.univ-brest.fr/hal-05302947v1

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# The Guiana current: a fast-lane to the Caribbean for holopelagic *Sargassum* contaminated in arsenic and cadmium

Tristan Gobert<sup>a</sup>, Solène Connan<sup>a, ID</sup>, Donald R. Johnson<sup>b</sup>, Matthieu Waeles<sup>a, * ID</sup>

## ARTICLE INFO

**Keywords:**

Arsenic

Backtracking

Cadmium

Metal contamination

Phaeophyceae

*Sargassum*

Trajectories

## ABSTRACT

To date, little key information about inorganic arsenic (iAs) and cadmium (Cd) has been promulgated to help manage the massive influxes of pelagic *Sargassum* that have inundated the Caribbean coasts since 2011 and are causing significant environmental, health and economic impacts. In particular, the levels of iAs and Cd, which often exceed regulatory levels and limit the seaweed valorization, have not been linked to an origin or route. By analyzing a large number of samples collected at different times in Guadeloupe and Martinique, we have highlighted for the first time a route that rapidly traces back through the Guiana, North-Brazil and equatorial currents, bringing *Sargassum* to the Lesser Antilles in <3 months from the equator. All the *Sargassum* morphotypes from this problematic fast track displayed high levels of iAs and Cd, very often exceeding 40 ppm and 1 ppm, respectively. The other routes, which bring *Sargassum* more slowly, either from the Cape Verde basin or from the North Brazil Retroflection area, have low or intermediate levels of iAs and Cd. Following an in-depth examination of the cadmium-phosphorus relationship in the tropical Atlantic Ocean, we provide here for the first time an overview of the spatial disparities in the Cd and P composition of *Sargassum*.

## 1. Introduction

Initially confined to the Sargasso Sea and the Gulf of Mexico, holopelagic *Sargassum* is now blooming in the tropical Atlantic, in what has been defined as the North Equatorial Recirculation Region (NERR) (Franks et al., 2016). This region (Fig. 1) is created during boreal Spring through Autumn, when the ITCZ (Inter-Tropical Convergence Zone) shifts northwards and the North Equatorial Counter Current (NECC) becomes established. *Sargassum* is seasonally retained in a broad area between the NECC and the South Equatorial Current (SEC), where it experiences elevated growth conditions in warm waters with substantial nutrient levels, before being dispersed by the northern hemisphere winter currents (December through March) into the tropical North Atlantic. Yearly composite satellite images describe a large swath of *Sargassum* extending from the NERR through the Caribbean Sea, known as the Great Atlantic *Sargassum* Belt (GASB) (Wang et al., 2019). Since 2011, unprecedented stranding of *Sargassum* have been reported on all coasts bordering the GASB, with very serious environmental, health and economic impacts (Milledge and Harvey al., 2016; van Tussenbroek et al., 2017). A better understanding of the composition of *Sargassum* according to its origin/season/route and its dependence on

environmental conditions is essential for managing these massive in-fluxes that are expected to persist over the coming decades (Abdool-Ghany et al., 2023; Milledge and Harvey, 2016).

Due to its high concentration in *Sargassum*, between 20 and 200 ppm, arsenic has been identified as the main obstacle for its valorisation as fertilizer, food, fuel or building material (Devault et al., 2021; Gobert et al., 2022, 2023; Oxenford et al., 2021; Ortega-Flores et al., 2022). However, international regulations now focus on inorganic arsenic (iAs), and no longer on total As, whose toxic potential is higher than that of other arsenicals. For example, the maximum authorized level for iAs in fertilizers in the European Union (EU) is 40 ppm (European Union, 2019) and the EU has recently recommended that business operators systematically monitor iAs in food and feed derived from seaweed (European Union, 2022). To date, few studies have exhaustively looked at iAs in *Sargassum*. While the proportion of iAs may vary considerably, from 15 % to 80 % of total arsenic (Gobert et al., 2022; Ortega-Flores et al., 2023), no link has yet been demonstrated between iAs levels and the origin of *Sargassum* (Alleyne et al., 2023b). Although less abundant, cadmium can also be a problem due to its toxicity and levels close to or exceeding regulatory levels (Devault et al., 2021). If some studies have reported Cd values in *Sargassum* (Gobert et al., 2022; Milledge et al.,

<footnote><sup>a</sup> University of Brest, CNRS, IRD, LEMAR laboratory F-29280, Plouzane, France</footnote>

<footnote><sup>b</sup> The University of Southern Mississippi, Center for Fisheries Research and Development, Gulf Coast Research Laboratory, Ocean Springs, MS, United States</footnote>

<footnote>* Corresponding author.
E-mail address: waeles@univ-brest.fr (M. Waeles).</footnote>
2020; Nielsen et al., 2021), no attention has been paid, to our knowledge, to the drivers behind them.

It has been previously shown that *Sargassum* collected in the open ocean in different areas of the northwest Atlantic have different compositions (Dassié et al., 2022; Gobert et al., 2022; McGillicuddy Jr. et al., 2023). The most obvious differences were lower P levels and higher As:P ratios for *Sargassum* from the Sargasso Sea compared with that from the GASB. Because *Sargassum* essentially proliferates in the GASB (Jouanno et al., 2023), it is now urgent to better constrain the composition variability of *Sargassum* along the different routes within the GASB.

In this study we focus on *Sargassum* washing ashore in the Caribbean with contaminants, particularly inorganic arsenic and cadmium, and establish its origins and pathways during its recent bloom in the Tropical Atlantic. We determined the chemical composition of *Sargassum* collected at different locations and dates around the islands of Guadeloupe and Martinique. Chemical analyses included total arsenic (As), inorganic arsenic (iAs) and cadmium (Cd) as well as phosphorus (P), iron (Fe) and aluminium (Al) concentrations. Examining the incorporation of P is important because marine plants, in their quest for this essential element, inadvertently assimilate arsenic and cadmium (Boyle et al., 1976; Cooney et al., 1978). In the case of *Sargassum*, a higher As:P ratio has been found in rafts of the Sargasso Sea (Gobert et al., 2022; McGillicuddy Jr. et al., 2023). This is due to the phosphate strong depletion of this area (Wu et al., 2000; Wurl et al., 2013) where surface waters display arsenic concentrations in the range 15–20 nM, similar than elsewhere (Cutter et al., 2001; Wurl et al., 2015). Iron is also an essential nutrient that can boost *Sargassum* production (Leemans et al., 2025). The reasons for the enhanced presence of iron and aluminium, sometimes observed in *Sargassum*, have been recently debated (Gobert et al., 2024; Machado et al., 2024a), but they could shed light on the life histories of the collected samples. In addition to the fact that the chemical composition can be controlled by an offshore route, significant changes are also likely to occur in the coastal zone depending on the time the *Sargassum* spends in the nearshore environment before being collected (Gobert et al., 2024).

# 2. Material and methods

## 2.1. Sample collection

*Sargassum* samples were collected in Guadeloupe and Martinique (Fig. 1) during four sampling campaigns corresponding to different periods of the year and on three different types of site: (1) stranded on the shore, (2) floating in protected bays and (3) offshore. Samples taken offshore and in the bay were collected directly in a water using small nets, while samples taken on the shore were collected on the beach in piles of dried *Sargassum*. The *Sargassum* thalli were then brought back to the laboratory where, for each station, they were sorted by morphotype, rinsed with seawater, surface dried, weighed and frozen at -25 °C. At each site, the three different morphotypes of *Sargassum* were collected in replicates (n = 2, 3 or 4) for a total of 105 samples (Table 1). Twelve samples (mixed morphotypes dominated by *S. fluitans*), were also collected in April 2023 at two stations off French Guiana (5.982°N, 52.803°W and 7.448°N, 53.610°W respectively). Immediately after collection, the *Sargassum* samples were frozen. Back to the laboratory, the samples were dried using a freeze-dryer (Christ, Germany) and then ground to a fine powder (MM400, Retsch, Germany) before being analyzed.

## 2.2. Composition analysis

Total concentrations of As, Cd, P, Al and Fe were measured as follows. After weighing 50 mg of dried and powdered material, digestions were carried out at 105 °C for 3 h in closed 30-mL Teflon screw-capped vials (Savillex, Minnetonka, MN, USA) with 1 mL suprapur 65 % nitric acid (Merck, Darmstadt, Germany) and 0.25 mL suprapur 30 % hydrogen peroxide (Merck, Darmstadt, Germany). Measurements were then carried out on diluted mixtures (2.3 % HNO₃) using an ICP-quadrupole mass spectrometer (X-series II, Thermo Scientific) operated at the Pole Spectrometry Ocean Brest (PSO, Brest, France) in collision cell mode (7 % H₂ and 93 % He). Detection limits (DL) and procedural blank ranges (PBR) were well below the concentration

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Maps of (a) Guadeloupe and (b) Martinique where *Sargassum* biomass was collected in 2020 and 2021 at stations located 3–4 km offshore, in bays or stranded. (c) Map of the Tropical Atlantic Ocean with a diagram of the main surface currents (adapted from Coles et al., 2023; Martín-Rey et al., 2023). NERR: North Equatorial Recirculation Region, NEC: North Equatorial Current, NECC: North Equatorial Counter Current, nSEC northern branch of the South Equatorial Current, NBC: North Brazil Current, NBR: North Brazil Retroflexion, GC: Guiana Current. The surface circulation is overlaid on the mean surface phosphate concentration from the World Ocean Atlas 2018 (Garcia et al., 2019). This map also shows the approximative boundaries of the Great Atlantic *Sargassum* Belt (GASB) as described previously (Franks et al., 2016) and the position of two sampling stations in the NBC-GC were additional *Sargassum* samples were collected in 2023.</figcaption></figure>
<caption>**Table 1**</caption>

Dates, types of site and locations where the 105 *Sargassum* samples of the three different morphotypes were collected during four sampling campaigns in Guadeloupe and Martinique.

<table><thead><tr><th rowspan="2">Date</th><th rowspan="2">Island</th><th rowspan="2">Type of site</th><th rowspan="2">Coordinates</th><th colspan="3">Number of samples per morphotype*</th></tr><tr><th>S. natans VIII</th><th>S. natans I</th><th>S. fluitans</th></tr></thead><tbody><tr><td>07-Dec-20</td><td rowspan="3">Guad</td><td>Offshore</td><td>16.175°N, 61.522°W</td><td>2</td><td>2</td><td>2</td></tr><tr><td>07-Dec-20</td><td>Bay</td><td>16.196°N, 61.573°W</td><td>2</td><td>-</td><td>2</td></tr><tr><td>14-Dec-20</td><td>Stranded</td><td>16.206°N, 61.443°W</td><td>2</td><td>2</td><td>2</td></tr><tr><td>09-Mar-21</td><td rowspan="3">Mart</td><td>Offshore</td><td>14.679°N, 60.872°W</td><td>3</td><td>4</td><td>4</td></tr><tr><td>09-Mar-21</td><td>Bay</td><td>14.683°N, 60.901°W</td><td>4</td><td>4</td><td>4</td></tr><tr><td>09-Mar-21</td><td>Stranded</td><td>14.668°N, 60.942°W</td><td>4</td><td>4</td><td>4</td></tr><tr><td>10-Jun-21</td><td rowspan="3">Guad</td><td>Offshore</td><td>16.184°N, 61.425°W</td><td>3</td><td>3</td><td>3</td></tr><tr><td>08-Jun-21</td><td>Bay</td><td>16.146°N, 61.571°W</td><td>3</td><td>3</td><td>3</td></tr><tr><td>07-Jun-2021</td><td>Stranded</td><td>16.164°N, 61.584°W</td><td>3</td><td>3</td><td>3</td></tr><tr><td>15-Jun-21</td><td rowspan="3">Mart</td><td>Offshore</td><td>14.655°N, 60.861°W</td><td>3</td><td>3</td><td>3</td></tr><tr><td>16-Jun-21</td><td>Bay</td><td>14.684°N, 60.920°W</td><td>3</td><td>3</td><td>3</td></tr><tr><td>14-Jun-21</td><td>Stranded</td><td>14.542°N, 60.829°W</td><td>3</td><td>3</td><td>3</td></tr></tbody></table>

*It has recently been recommended to change the names of *S. natans* VIII / *S. natans* I / *S. fluitans* morphotype to a more appropriate one (scientifically valid): i.e. *S. natans* var. *wingei* / *S. natans* var. *natans* / *S. fluitans* var. *fluitans* (Siuda et al., 2024). For the sake of simplicity, we kept here the former names of the species. -: no *S. natans* I observed.

ranges (CR) obtained for the diluted mixtures of the samples. Indeed, DL/PBR/CR values (in µg L⁻¹) were 0.04/0.2–0.4/6.6–310 for As, 0.0003/0.0003–0.0007/0.19–2.8 for Cd, 10/20–40/630–3600 for P, 0.1/2–10/40–8200 for Al and 1/7–20/70–6900 for Fe.

Inorganic arsenic (iAs) was analysed electrochemically using the method described in Gobert et al. (2023). Briefly, iAs was first extracted from algal powder with ultrapure water, under sonication for 30 min, using a water-to-biomass ratio of 100 (typically 50 mg algal powder in 5 mL ultrapure water). Electrochemical measurements were then carried out using 200 µL of extract, which was added to an electrolyte solution consisting of 11.8 mL ultrapure water, 1.2 mL CLAC electrolyte and 13 µL 10 mM KMnO₄ solution (final concentration 10 µM). The CLAC electrolyte (pH 4.7) consisted of 0.21 M acetic acid (6 mL), 0.6 M sodium acetate (23.4 g) and 2.5 M sodium chloride (73 g) dissolved in 500 mL ultrapure water. The electrochemical conditions applied were a deposition potential of –1.2 V, a deposition time of 30 s and linear scan voltammetry. Concentrations of iAs were then obtained after appropriate standard additions of a 1000 µg L⁻¹ As(V) stock solution. *Sargassum fusiforme* CRM (Hijiki 7405-b, National Metrology Institute of Japan) was routinely used to check the accuracy of the method for total element and iAs concentrations. Concentrations are given in µg per gram of algal dry weight and are denoted 'ppm'. For comparison with previous studies, elemental ratios have been expressed as mass ratios or molar ratios.

## 2.3. *Sargassum* backtracking

*Sargassum* backtracking was determined using satellite-tracked drifter locations with calculated current velocities which were obtained from the Global Drifter Program (now the Surface Velocity Program, SVP) (Lumpkin et al., 2017). The drifters are shown and described at https://gdp.ucsd.edu/ldl/svp/. Nominally the drifters, with small satellite transmitters at the ocean surface, have a holey sock drogue at 15 m depth to measure Lagrangian currents of the mixed layer. The program was launched in 1979 as a means of informing climate models and has been used in the development and validation of numerical ocean models. The program's success has resulted in approximately 1.5 million data points in our area of interest in the North Tropical Atlantic, spread over about 40 years. These current component data are interpolated to 1/12th degree longitude/latitude grid points (~9 km resolution) and day-of-year averaged. To bring the mixed-layer drifter currents to the surface, a correction using 0.5 percent wind at 10 m is applied. The result is a 365-day climatology of currents suitable for Lagrangian tracking. This drifter-based field of currents was validated (Johnson et al., 2020) for the Tropical Atlantic and compared to a numerical model and a diagnostic model. Windages were included in the

comparisons. The drifter model showed superior skill and has become the basic tool for *Sargassum* arrival prediction in the Lesser Antilles. Since it is a climatology, it shows the expected pathways, which is our objective. By 'expected' we mean the long-term averaged outcome similar to that gained by averaging the results of yearly model runs but without the uncertainties in sub-grid scale motions that are problematic in finite-difference models. Because drogued drifters run to ground offshore, our system does not include very shallow water currents. Hence, backtrack starting locations were moved to an offshore position in line with the approaching currents. This creates an along track uncertainty of about several days at most. In the year-long backtracks and the enormous distance covered, this uncertainty is minor.

## 2.4. Statistical analysis

The R program (R Core Team, 2021) was used for statistical analysis. Significant correlations between the different composition parameters were highlighted with a Spearman rank test. Significant differences in composition between the four different situations (December-2020, March-2021, June-2021-G, June-2021-M) were tested using a two-stage process. In the first stage, the normality and homogeneity of variances were verified by a Shapiro test and a Bartlett test, respectively. When both conditions were respected, the data were analyzed using an Analysis of Variance (ANOVA) followed by a Tukey post-hoc test. When the normality or homogeneity of the data were not met, non-parametric tests were applied (Kruskal-Wallis followed by a Dunn post-hoc test). A significance level of 95% ($p < 0.05$) was accepted for all the statistical analyses. Detailed results of these statistical analyses are presented in supplementary information (Table S1).

# 3. Results and discussion

## 3.1. *Sargassum* composition

For the composition parameters examined in this study, three highly significant correlations (Spearman rank test, $p < 0.0001$; Figure S1) were found between iAs and As ($R = 0.82$), Cd and P ($R = 0.83$) and Fe and Al ($R = 0.95$). These highly correlated parameters are plotted against each other in Fig. 2 to facilitate analysis of the different situations.

While As levels were not very different between the four situations, with values most often between 60 and 130 ppm, iAs levels were more differentiated (Table 2). Indeed, iAs concentrations in March-2021, with a range of 35–98 ppm and a mean (±sd) of 64±17 ppm, were significantly higher (ANOVA, $p < 0.0001$; Table S1, Figure S2) than those observed for the other three situations (29±9; 45±13 and 31±17 ppm
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (a) Inorganic As (iAs) content as a function of total As content, (b) Cd content as a function of P content and (c) Al content as a function of Fe content (log scale) in *Sargassum* collected in Guadeloupe and Martinique at different periods of the year and on three different types of site: offshore, in bay or stranded. The line added to panel (a) represents an iAs:As mass ratio of 50%; the line added on panel (c) represents an Al:Fe mass ratio of 1.</figcaption></figure>

<caption>**Table 2**</caption>

Composition parameters (mean±sd) for the four different situations. Significant differences between situations for each element or ratio are indicated by different letters (see also Figures S2, S3 and S4). The different corresponding routes and tracks (see Fig. 3) are also displayed. Concentrations are expressed in µg per gram of algal dry weight and are denoted ‘ppm’. For comparison with previous studies, elemental ratios have been expressed as mass ratio or molar ratio.

<table><thead><tr><th></th><th>Dec-2020<br>Guadeloupe</th><th>Mar-2021<br>Martinique</th><th>Jun-2021<br>Guadeloupe</th><th>Jun-2021<br>Martinique</th></tr></thead><tbody><tr><td>iAs (ppm)</td><td>Route I<br>Blue Tack<br>29±9 <sup>c</sup></td><td>Route II<br>Green Track<br>64±17 <sup>a</sup></td><td>Route III<br>Red Track<br>45±13 <sup>b</sup></td><td>Route III<br>Orange Track<br>31±17 <sup>c</sup></td></tr><tr><td>As (ppm)</td><td>77±20 <sup>bc</sup></td><td>107±27 <sup>a</sup></td><td>97±15 <sup>ab</sup></td><td>68±31 <sup>c</sup></td></tr><tr><td>Cd (ppm)</td><td>0.28±0.17 <sup>c</sup></td><td>1.09±0.30 <sup>a</sup></td><td>0.74±0.12 <sup>b</sup></td><td>0.70±0.14 <sup>b</sup></td></tr><tr><td>P (ppm)</td><td>500±130 <sup>c</sup></td><td>1340±210 <sup>a</sup></td><td>960±250 <sup>b</sup></td><td>950±180 <sup>b</sup></td></tr><tr><td>Al</td><td>380±640 <sup>b</sup></td><td>280±200 <sup>a</sup></td><td>480±850 <sup>ab</sup></td><td>100±70 <sup>b</sup></td></tr><tr><td>Fe</td><td>380±530 <sup>ab</sup></td><td>300±220 <sup>a</sup></td><td>520±750 <sup>ab</sup></td><td>140±100 <sup>b</sup></td></tr><tr><td>iAs:As (%)</td><td>37±5 <sup>c</sup></td><td>58±13 <sup>a</sup></td><td>46±10 <sup>b</sup></td><td>45±11 <sup>bc</sup></td></tr><tr><td>As:P (g:g)</td><td>0.15±0.03 <sup>a</sup></td><td>0.08±0.03 <sup>bc</sup></td><td>0.11±0.03 <sup>b</sup></td><td>0.07±0.03 <sup>c</sup></td></tr><tr><td>Cd:P (mmol:mol)</td><td>0.14±0.06 <sup>b</sup></td><td>0.22±0.04 <sup>a</sup></td><td>0.23±0.07 <sup>a</sup></td><td>0.21±0.04 <sup>a</sup></td></tr></tbody></table>

for December-2020, June-2021-G, June-2021-M, respectively) (Table 2). In line with these observations, significantly higher iAs:As ratios (KW, *p* < 0.0001; Table S1, Figure S3) were found for March-2021 (range 45–74 %) compared with the other situations, while December-2020 situation presented the lowest range (31–48 %). Fe and Al are also scattered over a wide range covering two orders of magnitude, from

a few tens to a few thousands of ppm, with a correlation slope (1.1) close to 1. Unlike the other compositional parameters, the different periods cannot be distinguished. On the other hand, the data clearly discriminate between *Sargassum* collected offshore, with levels below 100 ppm, and *Sargassum* collected in bays or stranded, with values generally above 100 ppm.

Cd and P values were dispersed over a wide range and differed significantly between the different situations (Fig. 3, Table 2, KW, *p* < 0.0001 for both elements; Table S1). For both parameters, the lowest values were observed in December-2020 (range 0.08–0.53 ppm and 270–720 ppm, respectively) while the highest values were found in March-2021 (range 0.67–1.84 and 900–1780 ppm, respectively). The data from June-2021 were intermediate and highly comparable between the two islands.

## 3.2. *Sargassum* routes

The trajectories of our *Sargassum* samples, determined using backtracking for 365 days from the four different sampling dates, showed remarkably wide variations in paths taken (Fig. 3). These estimates are consistent with those determined for backtracks from Barbados for the same periods (Alleyne et al., 2023a), but here we distinguish three routes instead of two.

The first route (blue track, Fig. 3) is backtracked to the Cape-Verde basin that has been earlier evidenced as one of NERR’s main source areas (Alleyne et al., 2023a). *Sargassum* travels directly E–W across the Atlantic, within the NEC. It should be noted that North Atlantic Trade
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Backtracks of *Sargassum* advected towards Guadeloupe and Martinique on different dates (see legend). The backtracks cover a 12-month period. The diamonds indicate the positions of *Sargassum* 9 months, 6 months and 3 months before it reached Guadeloupe or Martinique. The triangles (red and orange) show the predicted locations of *Sargassum* in mid-April-2021, prior to arriving off the coast of Guadeloupe and Martinique in mid-June. These locations are relevant to when and where *Sargassum* could have begun to share the sea surface with volcanic ash from the eruption of La Soufrière (St Vincent) (Machado et al., 2024b).</figcaption></figure>

Wind-driven inflow to the Caribbean Sea occurs principally north of 15°N (Johns et al., 2020), corresponding to the latitude of our collection point in Guadeloupe (16.2°N).

The second route (green track, Fig. 3), which is the longest, is backtracked to the equatorial currents and the Gulf of Guinea. On its way to the Lesser Antilles, *Sargassum* rafts rapidly followed the NBC and the GC along the South American coast. Sea-Surface Salinity (SSS) reconstruction along this route (Figure S5) indicates a short but important interaction of around one week with the Amazon plume (SSS in the range 32–36), ~70 days before collection in Martinique, and a more prolonged (over the February-2021 month) and recent (~1 month to ~10 days before collection in Martinique in March) interaction with the external plumes of the Amazon and the Orinoco (SSS~36). It should be noted that the North Brazil Retroflexion (NBR), in front of the Amazon mouth, does not form until June-July, and that the southern position of the ITCZ during the December-April season leads to onshore winds that blow the Amazon plume against the continental margin with an Amazon outflow pathway along the South American coast toward the Lesser Antilles (Coles et al., 2013).

The third and fourth tracks (red and orange, Fig. 3) are similar (Route III). They both ended after one year to the NBR area, which establishes the western extent of the NERR where the NBC is deflected into the NECC. The NBR and a captured anticyclonic eddy of ~300–400 km diameter, called a North Brazil Current ring (Johns et al., 1990), form in early boreal summer when the ITCZ shifts northwards, and entrains a high portion of the Amazon plume to the north during this period of increasing Amazon discharge (Coles et al., 2013). From this dynamic area, the tracks included a north-easterly course with the NECC, then, at 10°N, a westerly course with the NEC until reaching the lesser Antilles. It should be noted that the interaction of *Sargassum* rafts with the Amazon plume was much older for this route III (>6 months), than for route II. It should also be noted that *Sargassum* on this route III was present in mid-April 2021, ~2 months before collection, inside the area of volcanic ash deposition from the eruption of La Soufrière of St Vincent (Machado et al., 2024b) (see triangles in Fig. 3).

### 3.3. Inorganic As and Cd compared with regulation levels

Data on iAs indicate that *Sargassum* coming with route II ("Fast lane") is more problematic for valorization. Indeed, 33 of the 35 samples collected in March-2021 (i.e. 94%) exceeded the regulation set by the EU (40 ppm) for the admissible iAs levels in organic fertilizers (European Union, 2019). In contrast, only 12% of samples exceeded this limit in December-2020 (route I). For *Sargassum* collected in June, which followed route III, 30% and 44% (for June-2021-M and June-2021-G, respectively) were above this limit. It should be noted that the stranded algae collected in June 2021 were very depleted in As and iAs compared with those collected offshore or in the bay. This particular situation could be explained by a biomass that has undergone transudation, which can occur when *Sargassum* accumulates close to the shore and no longer benefits from sufficient mixing (Devault et al., 2022). Such a transudation process, which results in a significant reduction in arsenic, may seem interesting for obtaining a biomass below regulatory limits, but it is very unsatisfactory for a number of reasons. Firstly, the oxygen consumption linked to the transudation/degradation of these algae has a major impact on coastal organisms and ecosystems (van Tussenbroek et al., 2017). Secondly, the hydrogen sulphide emanations in the advanced degradation phase represent a nuisance but also a major risk for neighbouring populations (Resiere et al., 2021). Thirdly, *Sargassum* will also lose with transudation essential metabolites which would be of interest for agronomic applications (biostimulants or feed products). Finally, the potential impact and reactivity of the released arsenic where *Sargassum* decomposes, have not yet been studied, but could be damaging. While it seems vital to remove this *Sargassum* from the shoreline as quickly as possible, there are other important questions to be asked about arsenic. In the case of storage on land or application to agricultural soils of fresh *Sargassum*, transudation or exposure to heavy rainfall will lead to significant releases of arsenic (Ortega-Flores et al., 2023). Our data allow to estimate losses that could be of the order of 100 ppm of total arsenic (including 50 ppm of inorganic arsenic). This means that a hundred tons of Sargasso, corresponding to a typical collection effort for the Baie du Robert, could contaminate 0.2 million cubic meters
of water, based on the threshold of 10 µg/L provided by the World Health Organization. Preventing these algae from stranding and eliminating them at sea, relatively far off-shore, currently appears to be the least problematic option.

Regarding Cd, route II ("Fast lane") is also the most problematic, while the NEC route (route I) is associated with much lower Cd concentrations. Compared to the EU regulatory threshold of 1.5 ppm for organic fertilizers, a number of March-2021 values are close to or exceed this threshold (30 % of values above 1.2 ppm). On the other hand, Cd concentrations obtained in December-2020 were all below 0.5 ppm. Concentrations in June-2021, corresponding to the intermediate route III, remained also at values significantly below this threshold (0.74 ±0.12 and 0.70±0.14 ppm, respectively). It is important to note that the most problematic Cd levels in March-2021 correspond more to *Sargassum* collected in the bay or stranded. Although the biomass approaching the coast had relatively high Cd levels, on the order of 1 ppm, *Sargassum* can accumulate additional Cd during their transit through the coastal zone, where they interact with Cd-enriched waters/sediments. The coastal system in which we sampled in March-2021 is the Baie du Robert. This bay is influenced by the Mansarde River, whose mouth is characterized by relatively Cd-contaminated sediments, with values of 0.32 mg kg⁻¹ compared with a baseline of ~0.15 mg kg⁻¹ for other rivers in Martinique (Bertrand et al., 2009). This important Cd-related feature supports the need to harvest algae offshore, not only for health reasons, but also to avoid further accumulation of contaminants such as Cd or chlorodecone (Devault et al., 2022). Interestingly, Cd enrichment was also notable for *Sargassum* collected in June-2021 in the Baie du Robert, with values (0.84±0.14 ppm) slightly higher than those of the offshore biomass (0.62±0.07 ppm), whereas those collected stranded further south in Le Vauclin, an uncontaminated site (Bertrand et al., 2022), did not appear to be impacted (0.64±0.07 ppm).

### 3.4. Increase of Al and Fe content in the coastal area

The possible acquisition of additional Al, Fe or Mn within the coastal area has been recently highlighted by Gobert et al. (2024). Our Fe and Al data from the present study, with clear differences between offshore and bay/stranded biomass composition, further demonstrate the effectiveness of such a process, particularly in shallow waters affected by fluvial inputs. Such acquisition of Fe and Al within the coastal area, with Fe:Al ratio tending towards ~1.1 distinct from the Fe:Al ratio of 0.48 for continental crust (Rudnick and Gao, 2013), cannot be explained by contamination with inorganic material (sand). Such Fe:Al ratio close to 1 should rather correspond to an adsorption of colloidal material in the coastal area, as recently demonstrated by Barrat et al. (2024) for other brown algae (*Ascophyllum nodosum*) using rare earth elements. Such colloidal material is an important component of the Fe and Al speciation in freshwaters and coastal marine systems and consists of inorganic colloids of aluminosilicates and iron oxyhydroxides covered by organic debris, mostly fulvic/humic-like substances (Buffle et al., 1998). It should be noted that the *Sargassum* collected offshore in June-2021 (Fig. 3), which interacted with the ashes from La Soufriere volcano (St Vincent) ~60 days earlier, did not display particularly high levels in Al or Fe (<100 ppm). Thus, the important Al and Fe concentrations reported few months later for stranded *Sargassum* in Jamaica (Machado et al., 2024b) must rather be associated with the torrential rainfall in Jamaica (382 mm in August 2021) caused by Grace and Ida tropical storms (Meteorological Service of Jamaica, 2021).

### 3.5. Morphotype differences

Significant differences in iAs composition between the three *Sargassum* morphotypes have been clearly demonstrated previously (Ortega-Flores et al., 2023), with *S. natans* VIII and *S. fluitans* showing higher iAs concentrations than *S. natans* I. Our data confirm this result, with the most problematic levels from an operational point of view (>40

ppm) being essentially associated with these two morphotypes, and with *S. fluitans* in particular (Figure S6). This result can be related to the observations of *S. fluitans* dominance in massive GASB influx, particularly those from the NBR area (Alleyne et al., 2023a; Machado et al., 2024b). In the case of Cd, the data show that *S. natans* I is the only morphotype concerned by exceedances of the regulatory threshold of 1.5 ppm for organic fertilizers. This result needs to be tempered, however, as *Sargassum* sampled off French Guiana in the Guiana Current, largely dominated by *S. fluitans*, were characterized by higher values ranging from 1.7 to 2.0 ppm.

### 3.6. Relationship between As, iAs or Cd and P

Understanding the incorporation of As and Cd into *Sargassum* is crucial. This incorporation can be related to that of phosphorus. Firstly, because the arsenate ion, the main form of arsenic in seawater, has geometric similarities with the phosphate ion. Thus, seaweeds absorb arsenate in their quest for phosphate and have developed efficient procedures for arsenate detoxification (Cooney et al., 1978). On the other hand, Cd is a bioactive trace element in the ocean whose nutrient-like behavior mimics that of phosphate (Boyle et al., 1976).

It has been recently shown that the As:P ratio in *Sargassum* is a hyperbolic function of phosphorus content, demonstrating that As and P levels in the *Sargassum* tissues reflect their proportional uptake (McGillicuddy Jr. et al., 2023). This relationship between As:P ratio and P content was tested here, and we found a good adjustment between the theoretical model (As:P~P-1) and our data (Fig. 4a). Although this model is important in explaining why the As:P ratio in *Sargassum* can be high in the phosphate-depleted surface waters of the Sargasso Sea (Gobert et al., 2022; McGillicuddy Jr. et al., 2023), its use is limited because it does not address iAs content. Moreover, the massive influx of *Sargassum* is not linked to the Sargasso Sea but to the GASB. Our iAs data show a significant correlation with P content (*p* < 0.001; Fig. 4b) and the iAs:As ratio is also strongly correlated with P content (*p* < 0.001; Fig. 4c). This important result suggests that where *Sargassum* can readily incorporate phosphorus, such as in the NBC or GC influenced by river plumes, where the phosphate ion with levels above 100 nM (Fig. 1) is more abundant than the arsenate ion at ~20 nM, the arsenate is not too harmful to *Sargassum* and there is no need to biotransform it, resulting in biomass with high iAs:As ratios (cf. range 45–74 % for March-21 biomass, Fig. 4c). The corollary is that *Sargassum* exposed to higher proportions of arsenate:phosphate, as in the NEC, must enhance arsenate biotransformation and therefore exhibit lower iAs:As ratios (cf. range 31–48 % for December-2020 biomass, Fig. 4c). Such enhanced biotransformation under a high proportion of arsenate:phosphate in water is consistent with laboratory culture experiments with macroalgae, e.g. *Undaria pinnatifida* and *Porphyra haitanensis* (Al Mamun et al., 2019a; Lin et al., 2021). In macroalgae, arsenic detoxification strategies differ widely from species to species (Zhao et al., 2009). For example, arsenate biotransformation in *Sargassum horneri* involves reduction and methylation inside the cell, with subsequent release of As(III) and dimethylarsenic acid (DMA), whereas *Sargassum patens* does not use methylation and releases only As(III) (Al Mamun et al., 2019b). Arsenate biotransformation in holopelagic *Sargassum* has yet to be resolved, but these species share strong similarities in As speciation with *Laminaria digitata*, with high As(V) content and very low or undetectable levels of As(III), monomethylarsonic or DMA (Gobert et al., 2023; Lapointe et al., 2021; Taylor and Jackson, 2016), suggesting high tolerance to iAs, biotransformation to arsenosugars and limited release of arsenicals. Contrary to previous assumption (McGillicuddy Jr. et al., 2023), a decrease in phosphorus input to the GASB is not necessarily a cause for concern. While the arsenic content in the GASB would possibly rise to levels observed in the Sargasso Sea as explained by McGillicuddy Jr. et al. (2023), our results also indicate that this may be accompanied by decreasing iAs content and iAs:As ratio (Figs. 4b and 4c).

The flourishing of *Sargassum* in the NERR has been linked to an
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** (a) Arsenic to phosphorus (As:P) ratio, (b) inorganic As content and (c) proportion of iAs to total as a function of P content in the three different morphotypes of *Sargassum* collected in Guadeloupe and Martinique at different periods of the year and on three different types of site: offshore, bay or stranded. It should be noted that the June-2021 stranding data from Martinique have not been taken into account here due to the likely significant loss of As through transudation. For panel (a), the black line is the model As:P = 0.1*(P/1000)⁻¹.</figcaption></figure>

increased availability of nutrients in this area compared to their native habitat (Franks et al., 2016; Lapointe et al., 2021) and it has recently been demonstrated that *Sargassum* in the NERR and GASB are indeed enriched in nitrogen and phosphorus compared to *Sargassum* in the Sargasso Sea (McGillicuddy Jr. et al., 2023). Our results go further, showing differences according to the three routes followed from the NERR, with a higher P content for *Sargassum* from the “Fast lane” (route II) compared with those from the NEC (route I). Importantly, our results also show that Cd follows this pattern in all respects.

The remarkable analogy between Cd and P in *Sargassum* is clearly demonstrated here for the first time. It reflects the similar distributions of these two elements in oceanic waters, with a dissolved Cd-to-phosphate ratio typically between 0.05 and 0.2 mmol:mol in Atlantic surface waters (Pohl et al., 2011; Wu and Roshan, 2015; Xie et al., 2019), a ratio also found in the composition of phytoplankton (Twining et al., 2015). The Cd:P ratios of our *Sargassum* samples are similar to this range, with values close to 0.2 mmol:mol, particularly for samples collected offshore (Fig. 5). The greatest deviations from this ratio are observed for low-concentrated samples (December-2020), with a ratio close to 0.1. For these samples from the NEC (route I), this is consistent with the lower dissolved Cd-to-phosphate ratio of NEC surface waters (Middag et al., 2018; Wu and Roshan, 2015). Higher ratios, close to 0.3, were found for some samples from Guadeloupe and Martinique. These ratios, which mainly concern bay/stranded samples, are probably acquired during the transit of *Sargassum* in the coastal zone. It the same trend, we also observed high Cd and P values and Cd:P ratios close to 0.3 for *Sargassum* collected off French Guiana (see Fig. 1 for location of the

sampling points), in an area strongly influenced by the Amazon plume (Coles et al., 2013). Such ratios are consistent with recently reported concentrations of dissolved Cd and phosphate in the waters of the Amazon plume (Hollister et al., 2021, 2022).

Our study provides a number of key insights for the management of massive *Sargassum* influxes. Firstly, it is preferable to exploit *Sargassum* from the NEC (route I), which has lower iAs and Cd contents than *Sargassum* from more southerly routes. The “fast lane” (route II), which freshly transfers *Sargassum* from the Amazon and Orinoco plumes via the Guiana current and is dominated by *S. fluitans*, more prone than other morphotypes to high iAs levels, is by far the most problematic. The biomasses richest in iAs and Cd are also the richest in phosphorus. The highest enrichments of this nutrient are clearly stimulated by interactions with P-enriched waters along the South American coast (Fig. 1). They are also likely to be stimulated by equatorial upwelling in the Gulf of Guinea and in the more easterly equatorial zone connected to the NBC, where P concentrations in surface waters are also substantial (Fig. 1). In these systems, which not only provide excellent growing conditions for *Sargassum*, persistent ocean currents also enable them to consolidate on significant scales (Johnson et al., 2020). The southern islands of the Lesser Antilles, which are under the direct influence of the Amazon and Orinoco plumes at certain times of the year, and the coasts of the Gulf of Guinea are probably the most exposed to the most problematic influxes.

Finally, it should be pointed out that *Sargassum* rafts are likely to change composition over a period of a few weeks, depending on changing chemical conditions. Indeed, these algae develop with
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Cd content as a function of P content in *Sargassum* collected in Guadeloupe and Martinique at different periods in 2020 and 2021 and at three different types of site: offshore, in bay or stranded (colored dots, same legend as in **Figure 2**), in the North Brazil Current (NBC) / Amazon plume (black diamonds), in the North Equatorial Counter Current (NECC) / Eastern tropical Atlantic Ocean (+ symbols, data from Gobert et al., 2022), in the North Equatorial Current (NEC) / Western tropical Atlantic Ocean (× symbols, data from Gobert et al., 2022), in the coastal waters off Guadeloupe (open dots, data from Gobert et al., 2022), on the beaches of the Dominican Republic, the Caicos islands and South Florida (black dots, data from Fernández et al., 2017; Hatt et al., 2024; Milledge et al. 2020) and in the Sargasso Sea (open triangles, data from Gobert et al., 2022).</figcaption></figure>

doubling times ranging from 15 to 30 days (Magaña-Gallegos et al., 2023). On the other hand, significant wind-induced sinking has been observed beneath *Sargassum* rafts (Johnson and Richardson, 1977). Thus, a composition observed at a given time cannot be representative of a composition that is too old, e.g. over 100 days. For *Sargassum* taking route I or route III, there are probably significant changes in composition as they slowly advect towards surface waters relatively low in P and Cd. In contrast, it takes only 2 months to travel from the Amazon plume to the Lesser Antilles via the "fast lane" (route II; see also Putman et al., 2018). In addition, *Sargassum* must probably still be fed into the Guiana Current by other river plumes, including that of the Orinoco. An earlier study (Edmond et al., 1985) indicated particularly high dissolved Cd levels in estuarine waters of the Orinoco, but update and deeper examination of this is needed. Our study also highlights the fact that substantial alterations can also occur in coastal environments during the trapping of rafts in bays. Future studies should seek to better define the kinetics of contaminant accumulation or release in these areas.

**Supporting Information**

PDF file showing correlation matrix between the elements measured, composition levels and elemental ratios for the different situations with statistical differences and Sea surface salinity associated with the "fast lane".

**Funding**

We acknowledge financial support from the French ANR SAVE-C project (ANR-19-SARG-0008). This work was supported by ISblue

project, Interdisciplinary graduate school for the blue planet (ANR-17-EURE-0015) and co-funded by a grant from the French government under the program "Investissements d'Avenir" embedded in France 2030.

**CRediT authorship contribution statement**

**Tristan Gobert:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. **Solène Connan:** Writing – review & editing, Methodology, Investigation, Formal analysis, Conceptualization. **Donald R. Johnson:** Writing – review & editing, Investigation, Formal analysis. **Matthieu Waeles:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization.

**Declaration of competing interest**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Acknowledgement**

We are particularly grateful to the following persons: V. Stiger-Pouvreau (Lemar, Univ Brest), M.L. Rouget (Pôle Spectrométrie Océan, Univ Brest), C. Renoul (Lemar, Univ Brest), T. Gorgues (LOPS, Univ Brest), Fabrice Collard (OceanDataLab), Sébastien Guimbard
(Oceanscope), A.-S. Podeur (Lemar, Univ Brest), M.-A. Poullaoeuc (Lemar, Univ Brest), P. Salaün (Univ Liverpool), M. Eikelboom (Univ Liverpool); T. Lockhart (Laudato Sea), M. Nicolas (Port de Guyane) and Guyavoil crew.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.hal.2025.102981.

## Data availability

Data will be made available on request.

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