heavy metals

Sargassum Fertilizer Transfers Heavy Metals to Vegetables

Nederlands below.


A joint experiment between WWF-Mexico and STINAPA Bonaire found that vegetables grown in soil enriched with sargassum had higher levels of arsenic and cadmium, heavy metals that can be toxic to humans and animals.  Researchers warn that sargassum should not be used to compliment animal fodder, nor used as a fertilizer for consumables until further investigated.

Sargassum influx in Lac Cai

Sargassum is a floating brown seaweed that plays several important ecological roles. Although sargassum occurs naturally, due to shifting ocean currents and increased pollution, the Atlantic is experiencing episodic sargassum blooms.  Since 2011, the Caribbean has experienced several significant sargassum events, leading to a number of social, environmental and economic issues, particularly in the hospitality and fisheries sectors.  Sargassum influxes threaten the already fragile coral reefs, mangroves and seagrass beds.

The Study

To better understand the impact of disposed sargassum, a joint project between WWF-Mexico and STINAPA Bonaire explored whether sargassum-enriched fertilizer promoted faster seed development and if any heavy metals were detectable in the vegetables after harvest. Two planter boxes were used, one filled with 50/50 dried sargassum and potting soil and one with only potting soil.


The Results

Sargassum enriched soil testing set up

Although, in general, there appeared to be no significant physical differences (shape or quantity of vegetable production) between plants grown with or without the presence of sargassum, samples analyzed at the Radboud University laboratory found that arsenic levels were higher in vegetables grown in soil with sargassum. More specifically, bok choy had 37 times, zucchini 21 times, spinach 4 times and soil 13.5 times more arsenic than their counterparts grown in plain potting soil.  Cadmium levels were also higher in plants grown in sargassum enriched soil, with chemical analysis showing bok choy having 2.5 times, zucchini with 3 times, spinach with 1.3 times and soil with 2.7 times the amount of cadmium than samples without sargassum enrichment.

Furthermore, a Wageningen University and Research report titled “Opportunities for valorization of pelagic Sargassum in the Dutch Caribbean”, analyzed sargassum from the same source and found it to have high levels of heavy metals.  This full report is available from the Wageningen University and Research website (https://edepot.wur.nl/543797).


Decomposing sargassum in water

The health implications of these findings are still unclear. Arsenic can take several forms, namely organic and inorganic, where organic levels can be much higher before negative impacts are observed in people.  It should be noted that the European Food Safety Authority (EFSA) has not yet set official thresholds for arsenic. In fact, the EFSA Panel on Contaminants in the Food Chain (CONTAM) published data in 2010 which stated that there are no ‘safe’ levels of arsenic.  Long term ingestion of inorganic arsenic has been connected to skin lesions, cancer, developmental toxicity, neurotoxicity, cardiovascular disease, abnormal glucose metabolism and diabetes (CONTAM, 2010). More research is needed to understand impacts of these higher levels of heavy metals and the long -term effects when ingested.

As influxes of sargassum are becoming increasingly common, countries and individuals will search for innovative ways to use and dispose of this nuisance. Already, some reports have highlighted its use as a building material, animal fodder or fertilizer for home gardens. Until the health implications are more widely understood, it would be wise to limit sargassum use to non-consumable options.  This leaves the door open for sargassum to be used as building material (dried and pressed into bricks), biofuel or perhaps fertilizer for decorative plants or construction material, such as bamboo.

Submitted by: Jessica Johnson and Sabine Engel, researchers for STINAPA. This project was funded by WWF-Netherlands and received support from Radboud University.




Zware metalen in sargassum-mest worden door planten opgenomen

De bevindingen van een experiment uitgevoerd door STINAPA in het kader van een gezamenlijk project met WWF Mexico tonen aan dat grond verbeterd met sargassum hogere arseeen (As) en cadmium (Cd) waardes heeft. Arseen en cadmium zijn zware metalen die schadelijk zijn voor mens en dier. De onderzoekers waarschuwen dat sargassum niet gebruikt moet worden als aanvulling voor dier voedsel of als bemesting van groenten, voordat verder onderzoek heeft plaatsgevonden.

Sargassum bij Lac, Bonaire

Sargassum is een drijvend bruinwier dat een aantal belangrijke ecologische functies vervult. Sargassum komt natuurlijk voor, maar door veranderde zeestromingen en toegenomen vervuiling komen er nu periodieke sargassum ‘blooms’ (woekeringen) voor. Sinds 2011 zijn er in de Caraïben verschillende heftige sargassum ‘blooms’ geweest die gevolgen hadden op sociaal-, milieu- en economische gebied, voornamelijk in de horeca, toeristen en visserij sector. De periodieke sargassum aanvoer bedreigt daarnaast ook de toch al kwetsbare koraalriffen, mangroves en zeegras bedden.

De studie

Om meer te weten over het lot of de mogelijke toepassing van het afgevoerde sargassum werd in een gezamenlijk WWF Mexico – Bonaire project door STINAPA onderzocht of planten beter ontkiemden op grond waar sargassum-mest aan toe was gevoegd, en of de zware metalen voor kwamen in de geoogste planten. Twee plantenbedden werden klaargemaakt: één met potgrond, en één met 50/50 potgrond en gedroogd sargassum.

De resultaten

Het kweek experiment

Over het algemeen was er geen zichtbaar verschil tussen de planten gekweekt in de twee bedden maar monsters geanalyseerd aan de Radboud Universiteit toonden hogere arseen waardes aan in groentes uit de bedden met sargassum. Om precies te zijn, bok choy had 37 keer, zucchini 21 keer, malabar spinazie 4 keer, en het groeimedium potgrond sargassum 13.5 keer meer arseen dan de planten en grond uit de bedden met alleen potgrond. Het cadmium gehalte was ook hoger in planten die gekweekt waren in de ‘sargassum’ grond. Bij bok choy was dat 2,5 keer zoveel, zucchini 3 maal spinazie 1,3 maal en het medium 2,7 keer de hoeveelheid cadmium dan de controle planten.

Aan de Wageningen Universiteit werden monsters sargassum van dezelfde bron onderzocht in het kader van de studie “Opportunities for valorization of pelagic Sargassum in the Dutch Caribbean”, en werden ook hoge waardes voor zware metalen aangetroffen. Het volledige rapport is te vinden op de site van Wageningen University and Research (https://edepot.wur.nl/543797)


De gezondheids implicaties zijn nog niet helemaal duidelijk. Er zijn verschillende organische en anorganische vormen van arseen. Pas bij hogere waardes van organisch arseen worden negatieve gevolgen waargenomen voor mensen. De Europese Voedsel Veiligheid authoriteit (EFSA) heeft nog geen drempelwaardes vastgesteld. In feite publiceerde het EFSA Panel Verontreinigingen in de Voedselketen (EFSA Contaminants in the Food Chain (CONTAM)) in 2010 gegevens en stelde dat er geen ‘veilige’ arseen waarden zijn. Verband is gelegd tussen langdurige blootstelling aan anorganisch arseen en huidafwijkingen, kanker, ontwikkelingstoxiciteit, neurologische toxiciteit, cardiovasculaire ziektes, abnormale glucose vertering en diabetes (CONTAM, 2010). Er is meer onderzoek nodig om te bepalen wat de effecten zijn van deze zware metalen bij langdurige blootstelling.

Nu het op de kust ophopen van sargassum steeds vaker voorkomt zoeken landen en organisaties naar innovatieve manieren om van deze sargassum-overlast af te komen. Er zijn al publicaties die hebben aangegeven dat sargassum gebruikt kan worden als bouwmateriaal, veevoer of als huis & tuin grondverbeteraar. Maar zolang de gezondheidseffecten niet duidelijk zijn is het verstandig om sargassum niet te gebruiken voor voedselproductie. Het biedt dus alleen mogelijkheden voor bouwmateriaal (gedroogd en in blokken samengeperst), biofuel en misschien bemesting voor decoratieve planten of bouwmateriaal zoals bamboe.

Samengesteld door Jessica Johnson en Sabine Engel, onderzoekers voor STINAPA. Dit project is gefinancierd door WWF – Nederland, met bijdrages van de Radboud Universiteit.


Published in BioNews 54


Data type
Research and monitoring
Geographic location

Sharks found with dangerously high levels of heavy metals in The Caribbean

In a new study, researchers from the non-profit research institute Beneath the Waves (BTW) documented and revealed alarmingly high levels of 12 heavy metals, including mercury, in the muscle tissues of large reef and tiger sharks sampled throughout The Bahamas. Published in Scientific Reports, the new findings carry important implications for human health in the Greater Caribbean region, where sharks are occasionally consumed by humans, even though strictly prohibited around several island such as in the Yarari Sanctuary (Bonaire, Saba, St. Eustatius) and around St. Maarten.

Over the last century, human activities have rapidly accelerated the influx of metals and metalloids entering the marine environment, posing potential risks to biodiversity and food security. Evaluating muscle tissues of 36 individual sharks from six species, the results from this study provide the first account of metal concentrations in sharks sampled in The Bahamas, a relatively pristine marine ecosystem where sharks live risk-free in a large marine protected area. Sharks are not commonly consumed by humans in The Bahamas.

As apex predators, sharks naturally bioaccumulate toxins in their bodies from eating other species of fish. While the impacts on shark health remain unknown, the concentrations of metals quantified as present in the study were determined to exceed the levels that are considered toxic for human consumption. The study also found that reef sharks, the more resident species, had higher mercury levels than tiger sharks, and that reef sharks’ mercury levels increased as they matured and grew larger.

Understanding how sharks are affected by humans is critical for ongoing conservation efforts of these ecologically and economically important species,” says Dr. Oliver Shipley, the study’s lead author, Research Associate at Beneath the Waves and postdoctoral researcher at The University of New Mexico. “Working in areas such as The Bahamas where shark abundance is relatively stable and healthy due to effective long-term protection, is important for us to be able to establish these baseline studies. If the levels are high in The Bahamas, imagine what they could be in other parts of the world where sustainability and environmental conservation are not a priority.

“This work underscores the benefits of the Bahamas shark sanctuary for conducting important baseline studies on the health of our marine resources,” says Eric Carey, Executive Director of Bahamas National Trust.  “It also highlights the need for sustained conservation efforts of sharks regionally, which are important to the Bahamian economy and reef health,” he added.

While the researchers in the study identified the need for future studies to understand the pathways for how these metals ultimately enter into the marine food web, the human health risks of ingesting heavy metals by consuming Caribbean sharks species are clear.

“Shark fisheries are not very prevalent in most of the Greater Caribbean region, but eating sharks can be culturally important to some nations,” says study co-author Dr. Austin Gallagher, Chief Scientist at Beneath the Waves and co-founder of The Caribbean Shark Coalition. “Yet with a strong demand for shark products worldwide, this is another piece of evidence to steer people away from consuming sharks,” he adds.

“Humans and oceans are intricately connected, and this work highlights the notion that science can and should guide decisions that improve ocean and human health.”


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To download an open-access PDF of the full research paper, please visit: www.nature.com/articles/s41598-020-79973-w

Shipley ON, Lee CS, Fisher NS, Sternlicht JK, Kattan S, Staaterman ER, Hammerschlag N, Gallagher AJ (2021) Metal concentrations in coastal sharks from The Bahamas with a focus on the Caribbean Reef shark. Scientific Reports

About Beneath the Waves

Founded in 2013, Beneath the Waves is an ocean NGO using cutting-edge science to advance scientific discovery and catalyze ocean policy, with a focus on threatened species and marine protected areas.


Article included in BioNews 41


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Education and outreach
Research and monitoring