The central finding is that coastal arrivals follow a strikingly regular semiannual rhythm. Rather than a single annual peak, the northern Gulf of Guinea experiences two distinct influx seasons: one in boreal spring, between March and May, and a second, often larger, wave in autumn, between September and November. Both satellite-derived measurements of Sargassum coverage and the NEMO-Sarg computer model, which simulates the transport, growth and beaching of the seaweed, reveal this double-peaked cycle. The pattern contrasts sharply with the well-documented spring-summer maximum seen on the far side of the Atlantic, where Caribbean islands have endured recurrent golden tides since the emergence of the Great Atlantic Sargassum Belt, a vast expanse of floating algae stretching from West Africa to the Gulf of Mexico.

Crucially, the study shows that the seaweed arriving on West African beaches is not grown locally. Backward particle-tracking experiments, in which virtual drifters were released from Sargassum-rich coastal waters and traced upstream for 120 days, reveal a dominant west-to-east transport corridor. Biomass is supplied mainly from the eastern tropical Atlantic, funnelled toward the coast by the North Equatorial Countercurrent and the Guinea Current, a coastal flow that runs along the West African margin. The journey from the open eastern tropical Atlantic to the northern Gulf of Guinea takes roughly two to three months, while the final leg past the Cape Palmas gateway, at the Liberia-Côte d’Ivoire border, takes about a month.

The two seasonal peaks arise from different upstream configurations. Spring arrivals draw on biomass retained offshore of Guinea and Sierra Leone during the winter months, which is then carried southeastward by the Guinea Current, supplemented in some years by a second pool of seaweed sitting closer to the Equator. Autumn arrivals, by contrast, are fed by a much larger upstream accumulation that forms near the Intertropical Convergence Zone, the cloudy equatorial rain belt, during late summer, when strengthened trade winds and the monsoon push the North Equatorial Countercurrent to its annual maximum. Because of the two-to-three-month transit time, the autumn glut on the coast reflects transport conditions established weeks earlier far out at sea.

Winds emerge as the decisive force shaping what happens once the seaweed rounds Cape Palmas. Southerly winds, acting through direct wind drag on the floating mats and through Stokes drift, the wave-induced surface motion, push the biomass northward against the coastline and keep it trapped north of the Equator. Sensitivity experiments in which wind forcing was switched off paint a dramatically different picture: without winds, Sargassum shifts about two degrees of latitude southward, remains embedded in the core of the Guinea Current, spreads far more widely across the Gulf, and even finds a pathway southward along the Gulf margin toward the Southern Hemisphere, where it could potentially proliferate. Winds therefore perform three roles at once: they position the seaweed latitudinally, they drive it into the coastal accumulation zone off Côte d’Ivoire and Ghana, and they prevent leakage into the southern Gulf.

Beaching itself turns out to be a powerful regulator. In a companion experiment with stranding disabled, coastal Sargassum cover stayed high for much longer after the seasonal peaks, with the largest anomalies concentrated in nearshore waters between Côte d’Ivoire and Nigeria. Stranding, implemented in the model as a removal rate of 0.08 per day in coastal cells, acts as a sink that strips biomass from the floating pool and limits how long each influx lingers along the shore. The researchers tested several stranding rates before settling on the value that best matched the observed seasonal and spatial distribution of coastal coverage.

Year-to-year variability in the inundations is governed by the Atlantic Meridional Mode, a coupled ocean-atmosphere climate pattern defined by a north-south sea surface temperature gradient across the tropical Atlantic. In autumn, the link is remarkably strong, with the AMM index explaining 83 percent of the variance in coastal Sargassum cover. Negative AMM phases, marked by cooler waters in the tropical North Atlantic and warmer waters to the south, shift the Intertropical Convergence Zone and the seaweed band southward, closer to the latitude of Cape Palmas, while simultaneously strengthening the North Equatorial Countercurrent. Both effects favour eastward delivery of biomass into the Gulf. In spring, the AMM explains about 54 percent of the variance, with high-Sargassum years again showing a more southerly upstream distribution and a stronger countercurrent.

The study confronts real observational hurdles. Persistent cloud cover beneath the Intertropical Convergence Zone means that along much of the northern Gulf coast, fewer than fifteen valid satellite observations per month are available on average, and the satellite product measures areal coverage rather than biomass volume. The model was therefore nudged toward weekly satellite averages west of 15 degrees west, leaving the eastern tropical Atlantic and the Gulf free to evolve dynamically. Published stranding records, though sparse and uneven across countries, broadly corroborate the simulated timing, with documented events in Ghana, Côte d’Ivoire and Nigeria matching the spring and autumn peaks. Only 3.4 percent of research records on Sargassum since 2010 concern West Africa, underscoring how little studied this coastline has been compared with the Caribbean.

For coastal communities and governments, the findings offer a practical framework for seasonal risk assessment. Upstream Sargassum cover in the eastern tropical Atlantic, biomass approaching the Cape Palmas gateway, and the preceding state of the Atlantic Meridional Mode could together serve as early indicators of impending inundations, giving fisheries and cleanup operations weeks to months of lead time. The study also flags longer-term questions: the tropical Atlantic Sargassum baseline has remained elevated since 2011, and future shifts in the Intertropical Convergence Zone, regional winds or the Atlantic Meridional Overturning Circulation could either amplify or suppress the seaweed delivery to West Africa. A conditional southern export pathway, active only under unusual biomass surpluses or anomalous winds, could further reorganize the basin-wide distribution. What is certain is that the golden tides of the northern Gulf of Guinea are not random; they are the predictable product of currents, winds and Atlantic climate, now finally mapped.

Subject of Research: Seasonal and interannual drivers of pelagic Sargassum inundations along the northern Gulf of Guinea coast of West Africa

Article Title: Seasonal and interannual drivers of Sargassum inundations in the Northern Gulf of Guinea

Article References: Thouvenin-Masson, C., & Jouanno, J. (2026). Seasonal and interannual drivers of Sargassum inundations in the Northern Gulf of Guinea. Ocean Science, 22(5), 2835-2862. https://doi.org/10.5194/os-22-2835-2026

Image Credits: AI Generated

DOI: 10.5194/os-22-2835-2026

Keywords: Sargassum, Gulf of Guinea, West Africa, ocean currents, North Equatorial Countercurrent, Guinea Current, Atlantic Meridional Mode, satellite observations, Lagrangian transport, coastal inundations, Great Atlantic Sargassum Belt, seasonal forecasting

The research, published in the open-access journal Parasites & Vectors, was conducted by Daniel R. Matute of the Department of Biology at the University of North Carolina at Chapel Hill. Its central question is deceptively simple: do these mosquitoes behave the same way everywhere they live, or do populations that have colonized different thermal environments also differ in the temperatures they seek out? The distinction matters because temperature preference is not a trivial trait. It shapes microhabitat selection, which in turn influences how often a vector encounters the hosts it feeds on, including humans. A mosquito that chooses where to rest, seek blood, and deposit eggs on the basis of temperature is a mosquito whose biting risk is spatially structured by thermal behavior.

Aedes aegypti and Aedes albopictus are among the most intensively studied vectors on the planet, yet the study argues that one dimension of their ecology remains strikingly understudied: their altitudinal range, and the extent to which thermal behavior differs between lowland and highland populations. Both species are continuing to expand their geographic ranges year after year, driven by trade, urbanization, and a changing climate. Aedes aegypti is the primary vector of yellow fever, dengue, Zika, and chikungunya viruses, while Aedes albopictus, the Asian tiger mosquito, is a capable secondary vector of several of the same pathogens. Where these insects can live, and where they choose to live within the landscapes they occupy, directly determines which human communities sit in the path of transmission.

The fieldwork took place on Bioko and São Tomé, two islands that form a natural laboratory for questions of range and adaptation. Bioko, belonging to Equatorial Guinea, sits off the coast of Cameroon near the mouth of the Gulf of Guinea, while São Tomé lies farther southwest as part of the nation of São Tomé and Príncipe. Both are volcanic in origin, rising steeply from the ocean floor and offering compressed environmental gradients in which lowland rainforest transitions to montane habitat over a horizontal distance of only tens of kilometers. On such islands, a researcher can sample a 2000-meter elevation span that on a continental landmass might require journeys of hundreds of kilometers, all while holding insular context, isolation, and broad biogeographic history roughly constant.

The first stage of the study was distributional. Matute collected both Aedes species across the altitudinal cline on Bioko and mapped how the abundance of immature stages, the larvae and pupae found in aquatic containers, changed with elevation. The statistical picture that emerged was consistent: abundance follows a negative exponential decay with altitude. In practical terms, this means that most immature mosquitoes are concentrated near sea level, and populations thin rapidly as elevation increases, with a long tail of sparse occurrences reaching up the mountainsides. This functional form is more punishing than a linear decline; it implies that each additional increment of elevation removes a proportionally large share of the remaining mosquito population, rather than a fixed amount.

Comparison with neighboring São Tomé added an instructive wrinkle. The overall distribution patterns of the two species were similar across the two islands, suggesting that the same altitudinal filtering operates in both places. But the models indicated a higher abundance at sea level on São Tomé than on Bioko. Islands that look superficially similar, sharing volcanic origin, tropical latitude, and the same two invasive mosquito species, apparently differ in how many mosquitoes their coastal zones support. Whether that difference stems from ecology, history of colonization, availability of breeding containers, or other local factors is a question the distribution data raise but do not settle. What the comparison establishes is that the pattern is repeatable in shape while remaining variable in magnitude, a combination that invites hypothesis-driven follow-up work.

The second stage of the study moved from the field to the laboratory, and this is where the results become most provocative. Using live specimens collected during the survey, Matute tested temperature preference in a laboratory thermocline, an apparatus that presents an insect with a continuous gradient of temperatures and records where it settles. With such a device, preference can be measured under controlled conditions, free from the confounds of field sampling, where differences in abundance might reflect survival or reproduction rather than choice. The critical comparison was between mosquitoes whose ancestors came from different elevations, tested side by side under identical conditions.

Two results stand out. First, there were no significant differences in temperature preference between the two species. Aedes aegypti and Aedes albopictus, despite their distinct evolutionary origins and invasion histories, did not differ detectably in the temperatures they chose. Second, and more strikingly, the study detected an altitudinal cline in temperature preference: high-elevation populations preferred cooler temperatures on both islands. In other words, the trait varies with elevation rather than with species identity. This is the signature of phenotypic variation organized along an environmental gradient, the kind of pattern that classic ecological genetics has long used to identify locally adapted traits. Because the preference difference appears on two separate islands, it is unlikely to be a one-off idiosyncrasy of a single population; it looks like a systematic relationship between where mosquitoes live and what temperatures they favor.

The conclusion the author draws from this is carefully framed: the results indicate the presence of phenotypic variation in a key trait, temperature choice, that may alter the likelihood of contact between these vectors and humans. The word “may” carries real weight here. The study measures preference under controlled conditions and abundance along elevation; it does not demonstrate that highland mosquitoes bite people less often, nor that the preference difference is genetically based rather than plastic. Mosquito behavior can be shaped by developmental temperature, and laboratory-reared or field-collected adults may carry the imprint of the conditions in which they matured. Distinguishing heritable adaptation from environmental carryover would require common-garden or breeding experiments that the present study does not include. These are not weaknesses so much as the natural boundaries of a first, foundational survey, and the author’s framing respects them.

Even within those boundaries, the public health implications are worth spelling out. If temperature preference varies clinally with altitude, then models that assume a single, uniform thermal behavior for Aedes aegypti or Aedes albopictus may misestimate where human exposure actually occurs. Elevational gradients are also where climate change is expected to reshuffle vector distributions most visibly: as temperatures rise, thermal conditions once found only at low elevations move upslope, and the upper limits of vector ranges shift with them. A population of mosquitoes already preferring cooler temperatures at altitude could be positioned differently in that reshuffling than models predict. Conversely, the steep exponential decay of abundance suggests that highland communities currently enjoy a substantial buffer, with far fewer immature mosquitoes than coastal settlements on the same island. Whether that buffer will hold, and whether temperature preference itself will evolve or shift as the climate warms, are now empirically tractable questions. What this study provides is the baseline: two disease vectors, two islands, one clear altitudinal pattern of abundance, and a measured behavioral cline in the temperatures these insects choose. For a trait as consequential as thermal preference, establishing that the variation exists is the essential first step toward understanding what it will mean for the human communities living along these mountainsides.

Subject of Research: Altitudinal variation in temperature preference of Aedes aegypti and Aedes albopictus mosquitoes on the Gulf of Guinea islands of Bioko and São Tomé

Article Title: Two species of Aedes show altitudinal variation in temperature preference in the islands of the Gulf of Guinea

Article References: Matute, D. R. (2026). Two species of Aedes show altitudinal variation in temperature preference in the islands of the Gulf of Guinea. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07662-0

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07662-0

Keywords: Aedes aegypti, Aedes albopictus, temperature preference, altitudinal cline, Bioko, São Tomé, Gulf of Guinea, disease vectors, mosquito ecology, thermocline, vector-borne disease, phenotypic variation