Why forecastable Sargassum matters

The Great Atlantic Sargassum Belt is the broad accumulation zone of floating Sargassum macroalgae that develops across much of the tropical Atlantic and adjoining Caribbean region (Wang et al. Reference Wang, Hu, Barnes, Mitchum, Lapointe and Montoya2019). Its recurrent large accumulations have created conservation, public health and coastal management problems across the Caribbean, Gulf of Mexico, tropical Atlantic and parts of West Africa. Offshore, floating Sargassum mats can provide habitat, shelter and trophic support. Nearshore, the same biomass can accumulate in large volumes, decay, generate odour and hydrogen sulphide exposure, reduce beach amenity, stress seagrass and coral habitats, interfere with fisheries and tourism and create costly disposal problems. The policy issue is therefore not whether Sargassum is beneficial or harmful in general; the issue is where the biomass is, where it is likely to go, what ecological function it is performing and what will happen to it if managers intervene.

This problem already involves strategic and local decisions. Coastal authorities and communities must decide when to issue warnings, whether to install barriers or booms, whether to collect biomass offshore or after beaching, where to land and store collected material, how to protect workers from decay gases, who pays for beach cleaning and whether biomass can be used safely. These decisions are often made under emergency conditions, after landfall has already begun. A forecast-based approach changes the timing: it asks whether offshore information can support earlier decisions regarding monitoring, interception, port allocation, biomass handling, public health preparedness and conservation safeguards.

The aim of this Comment is to define a simple policy triage framework for this new situation. The framework is intended for readers outside the specialist Sargassum community. It separates three claims that are often conflated: that a bloom can be forecast; that biomass can be harvested or used; and that the resulting pathway creates verified climate benefit. It also makes clear that a good coastal management action and a valid carbon removal claim are not the same thing.

Why does carbon removal enter a discussion of forecasting? Macroalgae have been proposed as a carbon dioxide removal pathway through large-scale cultivation and deep-ocean sinking, and the Great Atlantic Sargassum Belt has itself been analysed as a natural analogue for ocean afforestation (Bach et al. Reference Bach, Tamsitt, Gower, Hurd, Raven and Boyd2021, Wu et al. Reference Wu, Keller and Oschlies2023). Forecasting therefore raises a practical possibility: recurring natural biomass could be intercepted and routed into pathways described as ‘climate mitigation’ or ‘carbon removal’. The central caution of this Comment is that forecastability does not by itself validate such claims.

A simple example shows the problem. Suppose a Caribbean island receives a 2-week warning that a large offshore Sargassum patch may reach a tourism coast. The choices are not simply to harvest or ignore it. Managers may warn communities, prepare beach cleanup, leave part of the mat offshore because it supports fish and other organisms, intercept only the portion most likely to beach, send wet or contaminated biomass to secure disposal or use clean measured biomass in a pathway with audited benefits. The useful warning time is action-specific: even hours to a day may be enough to issue warnings or deploy floating barriers at a beach, whereas mobilizing vessels, permits, ports, storage and processing may require days or weeks. The policy question is therefore a sequence of decisions, not a single decision to collect as much biomass as possible.

Recent modelling of the Great Atlantic Sargassum Belt sharpens the policy question. Zhou et al. (Reference Zhou, Novi, Hay, Montoya, Aliu, Realff and Bracco2026) argue that the system has shifted from dynamics driven mainly by physical forcing, such as wind and mixed-layer deepening, towards stronger ecological control. In plain terms, ecological control means that the floating mat is not just passively moved by winds and currents. The living mat and its associated organisms can recycle nutrients locally through excretion, decay and biological processing, helping maintain nutrient availability around the algae and supporting further growth. This interpretation is consistent with evidence of increasing nitrogen availability in the Atlantic Basin (Lapointe et al. Reference Lapointe, Brewton, Herren, Wang, Hu and McGillicuddy2021).

Predictability also needs clarification. This does not mean that managers can know the exact position of every patch or prevent blooms from forming; it means that past Sargassum concentration, mixed-layer conditions and other oceanographic information may provide useful lead time regarding future biomass abundance, broad distributions and likely impact windows (Zhou et al. Reference Zhou, Novi, Hay, Montoya, Aliu, Realff and Bracco2026). For management decision-making, the relevant question is not whether prediction is perfect but whether it is early and reliable enough for the action being considered.

From forecast to triage, not automatic carbon removal

Predictability should not be confused with justification. A forecast of biomass does not justify harvesting every forecast patch. Harvesting does not imply successful biomass valorization. Biomass valorization does not imply marine carbon dioxide removal. A tonne of wet Sargassum can become beach waste, a poorly handled pile that releases gases, a low-grade feedstock, a fertilizer risk, a substituted fuel, a durable product or a speculative carbon removal claim (Bach et al. Reference Bach, Tamsitt, Gower, Hurd, Raven and Boyd2021, Wu et al. Reference Wu, Keller and Oschlies2023). Its climate and conservation meaning depends on the complete pathway from detection to final fate.

The first triage decision is whether an offshore patch should be left in place or harvested. Biomass with low landfall risk, high habitat value or high bycatch sensitivity should remain offshore. Biomass forecast to generate severe coastal damage may become eligible for selective interception, but only if intervention can occur before landfall and if minimum habitat retention and bycatch safeguards are respected. Selective intervention is therefore not maximum extraction. It is a decision to remove particular biomass because the expected coastal harm avoided is greater than the ecological value lost by removing it.

The second triage decision begins only if biomass has been harvested. Collected Sargassum should not automatically be treated as feedstock. ‘Conversion’ refers here to routing biomass into uses such as biogas, fuels, fertilizers, materials, soil amendments or other products. Suitability depends on wet mass, dry mass, salt, sand, plastics, heavy metals, moisture, storage time and transport distance. Sargassum biogeochemistry also raises contaminant-accounting concerns, including arsenic and nutrient issues, for some downstream uses (McGillicuddy et al. Reference McGillicuddy, Morton, Brewton, Hu, Kelly, Solow and Lapointe2023). When these quality conditions are poor, controlled disposal may be preferable to forced conversion.

In plainer terms, the relevant test is verified net benefit after measurement and accounting. Monitoring, reporting and verification (MRV) is the system that makes a claim auditable: what was collected, where it went, what emissions or ecological harms were created and what benefit remained. A conversion pathway passes this test only when its climate, product or avoided-damage benefit remains positive after operational emissions, processing impacts, ecological damage, leakage, uncertainty and verification limitations are counted. This matters because weakly measured pathways can create attractive narratives while delivering little conservation or climate benefit.

Figure 1 presents the proposed triage framework in two decisions. First, offshore biomass is left in place or harvested selectively according to coastal damage risk and offshore habitat value. In Fig. 1a, the illustrative decision boundary slopes upward: at low coastal damage risk, harvesting would require very low habitat value and an independent benefit; as expected coastal damage rises, selective harvesting may be considered at higher habitat values, subject always to ecological no-go and bycatch safeguards. The boundary is a policy heuristic, not a fitted ecological threshold. Second, harvested biomass is converted or disposed of according to usable biomass quality and verified net benefit after accounting (Fig. 1b). Figure 1 is a guardrail against a common policy error: treating collection as proof of climate benefit. Beach or offshore collection may reduce local harm; biofuel production may create substitution benefits if it displaces fossil energy; and durable storage or sinking may support removal claims only if permanence, leakage, ecological impacts and accounting boundaries are credible (Bach et al. Reference Bach, Tamsitt, Gower, Hurd, Raven and Boyd2021, Wu et al. Reference Wu, Keller and Oschlies2023).

Figure 1.

Policy triage diagram for forecastable Sargassum. Offshore biomass is first assigned to be left or to be harvested according to predicted coastal damage risk and offshore habitat value. The upward-sloping boundary in (a) is illustrative rather than empirical: greater expected coastal damage can justify considering selective harvest at higher habitat values, while ecological no-go constraints still apply. (b) Harvested biomass is then assigned to convert or dispose according to usable biomass and processing suitability and verified net benefit after measurement and accounting. The diagram separates coastal risk reduction from biomass use and from verified climate benefit.

What a credible pilot must prove

A credible pilot should report more than tonnes collected. It should report forecast lead time, decision thresholds, ecological no-go zones, habitat retention assumptions, bycatch monitoring, wet and dry mass, contamination, transport energy, processing energy, product fate, disposal fate and uncertainty. For example, a pilot that removes biomass from a predicted landfall corridor should say how much habitat was deliberately left offshore, what fraction of collected biomass was too wet or contaminated for conversion and whether the final pathway reduced emissions or merely shifted waste. It should distinguish avoided coastal damage from substituted emissions and from durable carbon storage. Language referring to ‘blue carbon’ and ‘marine carbon dioxide removal’ should be reserved for pathways whose benefits remain positive after operations, processing, leakage, ecological disturbance and uncertainty are accounted for.

Regional governance is also necessary. The Great Atlantic Sargassum Belt crosses exclusive economic zones, high seas areas, tourism economies, fishing grounds and vulnerable island states. Forecasts, harvest decisions and product claims may be made in different jurisdictions. Without shared standards, carbon or biofuel incentives could encourage extraction in one place while ecological costs, beaching risks or waste burdens are displaced elsewhere. A minimum governance system should include shared forecasts, transparent permits, liability rules, ecological safeguards, benefit-sharing mechanisms and public MRV accounts.

Forecasts may support navigation, recreation, beach access, warnings and other day-to-day choices even when no biomass intervention is undertaken. The practical policy test considered here is narrower: a forecast creates significant management value for an intervention when it can trigger a feasible action before the predicted impact window and when that action has positive verified net benefit under ecological constraints. This links science to capacity. If vessels, permits, ports, labour or processing capacity cannot be mobilized within the relevant forecast horizon, prediction remains informational rather than operational for that intervention. If harvesting creates ecological damage, carbon accounting uncertainty or excessive operational emissions, prediction may accelerate bad decisions rather than improve management.