Abstract
The Gulf of Guinea, in the Atlantic coast of Central Africa, has three oceanic islands that arose as part of the Cameroon Volcanic Line. From northeast to southwest these are Príncipe (139 km2), São Tomé (857 km2), and Annobón (17 km2). Although relatively close to the adjacent mainland, the islands have distinct climactic and geomorphologic characteristics, and have remained isolated throughout their geological history. Consequently, they have developed a unique biodiversity, rich in endemic species. We provide an integrated overview of the physical setting of the islands, including their geographic location, geological origin, topography, geology and soils, climate zones, and prevailing wind and ocean currents—key features that underlie the evolution of their biodiversity.
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Introduction
The Gulf of Guinea is a major topographical feature of western equatorial Africa that marks the distinctive shape of the continent on its Atlantic coast (Fig. 2.1). The Gulf of Guinea has three oceanic islands (Príncipe, São Tomé, and Annobón), one land-bridge island (Bioko), and two seamounts, which together comprise the offshore part of the Cameroon Volcanic Line. The biodiversity of the oceanic islands is characterized by a small number of species but exceptional endemism (Jones 1994; Gascoigne 2004; Ceríaco et al. 2022). This chapter provides an introduction to the physical setting of the islands that created the conditions for the evolution of their unique biodiversity, including their geography and topography, geological history, geological substrates and soils, climate, and prevailing patterns of ocean sea currents.
Map of the Gulf of Guinea islands, western Central Africa. This system includes an ecological island (Mount Cameroon), a land-bridge island (Bioko), and the three oceanic islands, which are the focus of this book. Adapted from Jones and Tye (2006)
Some of the most complete sources of data for these topics are found in works published under the seal of the Portuguese scientific colonial institute—the Junta de Investigações do Ultramar—during the 1950s, 1960s, and 1970s. Of these sources, Lains e Silva (1958) provides key information on climate, soils, vegetation, and agricultural potential of São Tomé and Príncipe islands (see also Lains e Silva and Cardoso 1958). Building on earlier work, Tenreiro (1961) further addressed some of these topics for São Tomé Island. Cardoso and Garcia (1962) is a key reference for the soils of São Tomé and Príncipe—providing detailed maps of the soils of each island. Rodrigues (1974) synthesized the information on climate and soils presented by Lains e Silva (1958) and Cardoso and Garcia (1962). Jones et al. (1991) provide a useful synthesis of background information available at the time. More recently, Diniz and Matos (2002) added to our understanding of the climate and soils of São Tomé and Príncipe islands, providing an updated and detailed map of the ecosystems and land-use types of the islands. A series of geological studies conducted by Munhá et al. (2002), Caldeira et al. (2003), Caldeira (2006), Munhá et al. (2006a, b, c, d, 2007), and Barfod and Fitton (2014) have provided important updates to our knowledge of the geology of São Tomé. Chou et al. (2020) provided the first modern analysis of the climate of São Tomé and Príncipe, downscaling global projections of climate change to these islands. For Annobón, the information is scarcer with initial geological works by Schultze (1913), petrological studies by Fuster Casas (1954) and Cornen and Maury (1980), work on volcanic geochemistry by Liotard et al. (1982), and a review by De Castro and De la Calle (1985), with subsequent additions by Fa (1991) and Velayos et al. (2014). Besides these island specific studies, several reviews summarize the main geophysical characteristics of the Gulf of Guinea islands (e.g., Lee et al. 1994; Jones 1994; Jones and Tye 2006; Juste and Fa 1994; Schlüter 2008).
Location, Extent, and Political Boundaries
The Gulf of Guinea island system (sensu lato) includes the ecological or “sky” island of Mount Cameroon, the land-bridge island of Bioko, and the three oceanic islands of Príncipe, São Tomé, and Annobón (Fig. 2.1). They are, from northeast to southwest:
Mount Cameroon, with an approximate area of 1750 km2 (50 × 35 km), is an ecological island in the southwest province of the Republic of Cameroon. Mount Cameroon is the highest mountain in West Africa, with a peak elevation of 4095 m above sea level.
Bioko Island is a land-bridge island with an area of 2027 km2 (roughly 35 km × 72 km). Bioko sits upon the continental shelf 32 km from the coast of Cameroon from which it is presently separated by a sea 60 m deep. During recent glacial periods, however, Bioko experienced recurring cycles of isolation and connectivity (Ali 2018), and was most recently connected c. 11,000 years ago (Einsentraut 1965; Lambert and Chappel 2001). Rising to an impressive 3011 m above sea level, Pico Basilé is the highest point of the island and one of its main landmarks.
The three oceanic islands that are the focus of this book have never been connected to the continent, and they are:
Príncipe Island (Fig. 2.2(1)) with a total area of 139 km2 (c. 17 km × 8 km) is located 210 km SSW of Bioko and 220 km west of continental Africa. The island has six main satellite islets: Pedra da Galé, Mosteiros, and Bom-Bom in the north, Caroço (also known as the Jockey’s Cap; Fig. 2.2(3)) in the southeast, and Tinhosa Grande and Tinhosa Pequena (Fig. 2.2(2)), which are about 20 km to the south. The highest point, Pico do Príncipe, is 942 m above sea level.
(1) Pico Agulhas, Príncipe Island; (2) Tinhosa islets; (3) Jockey Bonet; (4) Pico São Tomé, São Tomé Island; (5) Pico Cão Grande, São Tomé Island; (6) Rolas islet; (7) Lagoa Amélia, São Tomé Island; (8) Lake A Pot, Annobón Island. Photo credits: (1 and 8) Martim Melo, (2, 3, 5–7) Luis M. P. CerÚaco; (4) Ricardo Lima
São Tomé Island with a total area of 857 km2 (47 km × 28 km) lies 150 km SSW of Príncipe and 255 km west of Gabon. The island has several islets, of which Cabras to the north, Santana in the east, and Sete Pedras and Rolas (Fig. 2.2(6)) in the south are the largest. The Equator passes through the center of Rolas Islet. The highest point, Pico de São Tomé (Fig. 2.2(4)), is 2024 m above sea level.
Annobón Island has an area of 17 km2 (6 km × 3 km) and is the smallest and remotest of the Gulf of Guinea islands. It sits 180 km to the SSW of São Tomé and is about 340 km from the continent. The highest peak is Santa Mina, which rises 610 m above sea level.
Politically, the Gulf of Guinea oceanic islands belong to two countries: the Democratic Republic of São Tomé e Príncipe and the Republic of Equatorial Guinea. São Tomé e Príncipe is a nation state made up of Príncipe and São Tomé islands and the surrounding islets. It was once a colonial province of Portugal, from which it gained independence in 1975. It is one of the smallest countries in the world, with an approximate area of 1001 km2. The country is internally organized into different levels of political and administrative divisions. São Tomé Island hosts the capital, the city of São Tomé, and is divided into six districts (Água Grande, Cantagalo, Caué, Lembá, Lobata, and Mé-Zóchi); Príncipe Island is an Autonomous Region, and is comprised of a single district, Pagué (Fig. 2.3).
Administrative divisions of Príncipe (a), São Tomé (b), and Annobón (c)
Annobón (formerly known as Pagalu; Fig. 2.3), the smallest and most southwestern of the Gulf of Guinea oceanic islands, is one of eight provinces of Equatorial Guinea. This geographically disjunct country was a Spanish colony from 1778 to 1968. Equatorial Guinea is composed of a territory in continental Africa, Rio Muni, bordered by Cameroon to the north and Gabon in the east and south, the surroundings islets of Corisco, Elobey Chico, and Elobey Grande, the land-bridge island of Bioko (formerly known as Fernando Pó), where the country’s capital is (Malabo), and finally the small oceanic island of Annobón. Whereas the mainland territory and Bioko have a long history of human occupation, Annobón was not peopled at the time of its discovery by the Portuguese, in 1473.
Geological History
The Gulf of Guinea islands form the southern part of the Cameroon Volcanic Line, a 1000-km line of volcanoes that has been active since the Cenozoic, and that extends from the Mandara Mountains on the Nigeria-Cameroon border to Annobón Island (Burke 2001). This line runs in a NE-SW direction and includes four islands and two seamounts (Fig. 2.4). Onshore, there are four continental massifs (Mount Cameroon, Mount Manengouba, Mount Bambouto, and Mount Oku), all of which are in the Republic of Cameroon. Often, the Ngaoundéré and Biu swells, also in Cameroon, are considered part of the line, in which case the line becomes Y-shaped and 1600 km long (Fitton 1987; Lee et al. 1994; Fig. 2.5). Volcanic activity in the continental and oceanic sector has been more or less continuous since the Cretaceous (Fitton 1987; Lee et al. 1994; Burke 2001). There is no age progression in the line, except in the offshore section—with the oldest sub-aerial origins estimated at about 31 Ma for Príncipe, 15 Ma for São Tomé, and 6 Ma for Annobón (Lopes 2020).
The Cameroon line of volcanoes. Note the similarity in shape between the volcanic line (black) and the Benue Rift (grey). This was likely due to the rotation of the African plate c. 30–35 Ma that displaced the asthenospheric hot zone underlying the Benue Rift to its current position—resulting in a volcanic line without a rift and a rift without volcanoes, a unique feature on Earth. After decades of debate, the alignment of the volcanic centers is now thought to be controlled by the geometry of the northwest edge of the Congo Craton. Adapted from Lee et al. (1994)
The age of the oldest lava flows only provides estimates of the minimum age when each island was sub-aerial because older rocks may be buried under the most recent ones. For example, all the exposed lavas on Mount Cameroon are less than one million years old, but the mountain is built upon much older lava flows (Fitton 1987). Furthermore, volcanic activity persisted until recently on all the islands, and is still ongoing in Mount Cameroon and to a lesser extent in Bioko. This dynamic aspect of the islands is well illustrated in São Tomé, where the oldest rocks, at about 15.7 Ma, are from the small Cabras Islet, while the surface rocks of more than half of the island, including its highest peak, date between 1.5 and 0.4 Ma (Caldeira et al. 2003; Barfod and Fitton 2014). Although still poorly understood, the volcanic history of the Gulf of Guinea islands has no doubt played a major role in the assembly of their current biological communities. For instance, landslides or lava flows can split species ranges or cause extinctions, and distinct islands and islets may fuse and split over time (Milá et al. 2010; Gillespie and Roderick 2014; Ramalho et al. 2015).
Quaternary Sea-Level Fluctuations
Across the globe, glacial-interglacial sea-level fluctuations have shaped insular biodiversity and diversification by repeatedly connecting and isolating populations on coastal landmasses (e.g., Ali and Aitchison 2014; Rijsdijk et al. 2014; Fernández-Palacios 2016; Weigelt et al. 2016; Norder et al. 2018, 2019). Ceríaco et al. (2020) modeled the area of the islands throughout the last glacial period to the present day and demonstrated that the Gulf of Guinea islands show marked changes in area in response to eustatic sea-level fluctuations. During the exceptionally low sea level of the Last Glacial Maximum, as much as 134 m lower than present day (Lambeck et al. 2014), Bioko was connected to continental Africa, Annobón was five times its present size, Príncipe was about six times its present size, and São Tomé was approximately 50% larger than present day (Ceríaco et al. 2020; Fig. 2.6).
Paleogeographic reconstructions of the Gulf of Guinea: (a) Area change curves of Príncipe, São Tomé, and Annobón islands; (b) area of islands today (dark green), and extreme area at the last glacial maximum (LGM, approximately 21 ka; light green). Adapted from Ceríaco et al. (2020)
Topography and Hydrography
Due to recent volcanic activity, Príncipe, São Tomé, and Annobón are old islands that have the topography of young islands, including rugged mountains with steep slopes, deep valleys, volcanic chimneys, table mountains, and huge waterfalls (Figs. 2.7). The topography varies between islands. São Tomé is dominated by steep slopes and mountains across the majority of the island, with the exception of the flatter areas in the northeast (Figs. 2.7e, f). The maximum elevation reaches 2024 m at Pico de São Tomé, and several other mountain and peaks areas in the center of the island are well above 1000 m (Figs. 2.2(5), 2.7e). Príncipe has a plateau in the north but is mountainous in the south, where several peaks rise above 500 m, including Pico do Príncipe at 942 m (Figs. 2.7b). Annobón is small and steep, except for a small portion in the north, where most of the human population resides. The elevation rises considerably in the center and south, reaching 613 m at Santa Mina (Fig. 2.7h).
Overview of the hydrography and topography of (a, b, c) Príncipe, (d, e, f) São Tomé, and (g, h, i) Annobón. For each island, elevation is presented in meters above sea level and steepness in degrees. Main rivers and waterbodies: São Tomé Rivers (d): 1—Provaz; 2—Lembá; 3—Xufexufe; 4—Quija; 5—Mussacavú; 6—Pedras; 7—Gumbela; 8—Malanza; 9—Gogô; 10—Caué; 11—Martim Mendes; 12—Miranda Guedes; 13—João Nunes; 14—Ana Chaves; 15—Ió Grande; 16—Angobó; 17—Angra Toldo; 18—Pedra Furada; 19—Ribeira Afonso; 20—Abade; 22—Ouro. São Tomé Crater Lake: 21—Lagoa Amélia. Príncipe Rivers (a): 1—Ribeira Banzú; 2—Ribeira São Tomé; 3—Ribeira Porco; 4—Chibala; 6—Papagaio. Annobón (g) has no significant rivers; the crater lake Lago A Pot is shown in red
The available data on the terrestrial hydrography of the islands are limited. Both São Tomé and Príncipe are mostly covered by the hydrographic basin of a few large rivers in a dense network and also include several small coastal rivers (Fig. 2.7a, d). São Tomé Island has many small lagoons, estuaries, and mangroves, including the Malanza river estuary in the south, which forms the most extensive mangrove in the country. São Tomé also has a unique freshwater palustrine system in the crater of Lagoa Amélia (Fig. 2.2(7)), which is the source of the largest rivers in the north of the island (Fig. 2.7d). Annobón only has a few small streams, but Lago A Pot crater lake (Fig. 2.7g, shown in red; Fig. 2.2(8)) is a dominant feature of the island with a diameter of approximately 700 m at 150 m above sea level.
Geology and Soils
The geology of São Tomé and Príncipe has been well studied since the early twentieth century. This is partly due to the importance of geology and soils for agriculture, which has been the major driver of the local economy for centuries (Lains e Silva 1958; Lains e Silva and Cardoso 1958; Rodrigues 1974). The first overview of the geology of São Tomé Island was provided by Carvalho in Henriques (1917), followed by a more detailed study on the microscopic characteristics of its rocks (Carvalho 1921). Teixeira (1948–1949, 1949) provided a more complete overview of the geology of the islands, followed by a petrological work by Pereira (1943). The most extensive and complete contributions to the geology of the islands were provided by the Portuguese geologist João Manuel Cotelo Neiva (1917–2015), whose work was fundamental to understanding the geochemistry and geomorphology (Neiva 1946, 1954, 1955a, b, 1956a, b, c; Neiva and Pureza 1956; Neiva and Neves 1956). Assunção (1956, 1957) and Barros (1960) also contributed to our understanding of the geochemistry. In the twenty-first century, new research on the geology of São Tomé (Munhá et al. 2002; Caldeira et al. 2003; Caldeira 2006) has resulted in updated geological maps (Munhá et al. 2006a, b, c, d, 2007). By contrast, the geology of Annobón has received far less attention. The first information on its geological history and composition was provided by Schultze (1913), followed by studies by Tyrrell (1934), Fuster Casas (1954), Cornen and Maury (1980), and Liotard et al. (1982). More recently, De Castro and De la Calle (1985) and Fa (1991) provided an overview of the geology of Annobón. On Príncipe, basaltic rocks predominate in the north and phonolites and tephrites in the south, whereas São Tomé and Annobón are mostly built by basaltic lavas (Fig. 2.8). A more detailed description of the geology of the islands is provided by Schlüter (2008).
Geological overview of the islands of Príncipe, São Tomé, and Annobón. Adapted with permission from Springer Nature Geological Atlas of Africa by Schlüter © Springer-Verlag Berlin Heidelberg 2006 (2008)
Regarding the soils of Príncipe and São Tomé, Lains e Silva (1958) and Cardoso (1958) drafted the first maps, with a more comprehensive map and revision by Cardoso and Garcia (1962). Other works were done by Pissarra and Rocha (1963) and Pissarra et al. (1965). The dominant soil types of Príncipe and São Tomé are highly weathered, such as Ferralsols and Lixisols (Lains e Silva 1958; Cardoso and Garcia 1962; Diniz and Matos 2002), which are typical of tropical climates. Vertisols are restricted to the dry north and northeast of São Tomé, while Lithosols can be found everywhere on the island, often associated with ridges, steep slopes, and cliffs near the coast (Diniz and Matos 2002). Fluvisols, as expected, are mostly associated with riparian areas. Very little is known about the soils of Annobón, other than that they are ultrabasic with low silica and high proportions of ferromagnesian elements (De Castro and De la Calle 1985; Fa 1991).
The only reported fossils are from Príncipe and date to the Miocene (Teixeira 1949; Silva 1956a, b, 1958a, b; Serralheiro 1957). These include marine organisms such as gastropods, bivalve mollusks, coelenterates, echinoderms, and fishes’ teeth, but also calcareous algae, radiolarians, and foraminifera. Modern foraminifera are known from both Príncipe and São Tomé beaches (Moura 1961). A palaeoecological study is currently taking place on Príncipe and São Tomé collecting data from pollen, spores, charcoal, and sedimentology to reconstruct ecosystem changes associated with glacial cycles and the impacts of human activities (unpublished data by Alvaro Castilla-Beltrán).
Climate
The Gulf of Guinea oceanic islands have an oceanic equatorial climate. Mean temperatures are above 25 °C at sea level but decrease with altitude (Fig. 2.9). The year is divided into rainy and dry seasons, which are determined by the Intertropical Convergence Zone, and by the interaction between the southern monsoon winds from the Atlantic Ocean and the northern dry harmattan winds from the Sahara. Seasons differ between the continental and the oceanic sectors. On Mount Cameroon and Bioko, the main dry season is from December to March, and a shorter dry season occurs from July to August (Juste and Fa 1994). In Príncipe and São Tomé, the long dry season, locally known as gravana, extends from June to mid-September, while a shorter dry season, the gravanito, lasts for a few weeks that may fall anywhere between mid-December and mid-March (Lains e Silva 1958). Annobón, south of the Equator, has a single extended dry season from mid-May to the end of October (Jones and Tye 2006).
Annual average temperature (in Celsius) for the islands of Príncipe, São Tomé, and Annobón. Data obtained from the Global Solar Atlas 2.0, provided by the World Bank Group
Due to their small area and heterogeneity, modern rainfall and climate measurements based on remote sensing likely do not accurately describe the climate of Príncipe, São Tomé, and Annobón. To the best of our knowledge, until recently Annobón had no functional meteorological station, while there was only one on Príncipe and five on São Tomé, of which only one had been collecting long-term data systematically (Chou et al. 2020). This network was greatly improved over the last decade (https://www.thegef.org/project/strengthening-climate-information-and-early-warning-systems-sao-tome-and-Príncipe-climate), but detailed long-term information on the climate of the islands is still lacking.
The topography of Príncipe and São Tomé islands is similar, resulting in a similar distribution of climatic zones (Diniz and Matos 2002). The high relief areas of the south and center intercept the predominant warm and moist south-westerly winds, creating a striking north–south divide in precipitation (the Foehn effect). The southern-facing regions are “Super Humid,” with annual precipitation above 3000 mm, and often much higher (c. 5000 mm on Príncipe, above 7000 mm on São Tomé—Diniz and Matos 2002), enhanced by extremely high humidity levels and low sun exposure (Fig. 2.10). The north, under the rain-shadow effect, has climatic belts associated with the decreasing levels of humidity with decreasing altitude. The higher slopes benefit from the monsoon winds that pass over the peaks, and have precipitation levels between 1500 and 3000 mm, making the “Humid” belt (Fig. 2.10). Lower down, from the coast to about 400 to 550 m, moderate slopes (below 15%) receive between 1000 and 1500 mm of rain per year, making up the “Sub-Humid” belt, which has a well-defined rainy season (Fig. 2.10). Finally, only on São Tomé, the littoral area in the flatter N-NE platform, below 1000 m, has a “Semi-Arid” belt that has annual precipitation levels between 600 and 1000 mm (Fig. 2.10). This general zonation, with more humid climates in the south and drier climates in the north, also seems to apply to Annobón and to the continental islands. Annual precipitation on the southwestern slopes of Mount Cameroon may be over 10,000 mm, and between 1500 and 2000 mm in the northern slopes. In the south of Bioko Island, annual precipitation can be over 11,000 mm (Juste and Fa 1994) while the capital Malabo, in the north, receives <2000 mm/year.
Climatic zones for the islands of São Tomé (left) and Príncipe (right). Adapted from Diniz and Matos (2002)
Wind and Ocean Currents
Understanding the wind and ocean currents is fundamental to infer potential colonization pathways for island fauna and flora. The prevailing winds in the Gulf of Guinea are the southwestern monsoon winds and the northern dry harmattan winds. The southwestern monsoon winds are unlikely to have dispersed colonizers from continental Africa but may have played a role in southwest-to-northeast dispersal between islands. During glacial cycles, the northern dry harmattan winds extended their influence southward, displacing the meteorological equator further south (Lézine et al. 1994), likely having a more important role in bringing colonizers to the island during those periods.
Data on sea surface currents originate from a combination of historical ship drifts, hydrographical data, surface-drifting buoy trajectories, and Argo floats surface drifts (Richardson and Walsh 1986; Arnault 1987; Stramma and Schott 1999; Renner 2004; Lumpkin and Garzoli 2005; Ollitrault and Rannou 2013). The Gulf of Guinea is dominated by two currents (northward and eastward) that follow the shoreline. In the north, the eastward current, also known as Guinea Current (GC), moves west to east along the southern coast of West Africa, and, when reaching the Biafra Bay, converges with the northward current and becomes more diffuse, turning back westward around the Equator (Feiler 1988; Haft 1993; Dupont et al. 2000; Fig. 2.11). In the southeast, the northward current, known as the Benguela Current (BC), moves along the northern coast of South Africa, the coast of Namibia, and is diverted west to the Atlantic around the mouth of the Cunene River, the natural and political border between Namibia and Angola. It also feeds the South Equatorial Current (SEC), which represents the northern limb of the South Atlantic Ocean subtropical gyre (Philander 2001; Rodrigues et al. 2007). Moving north, the coast of Angola is dominated by two different currents, the Benguela Coastal Current (BCC), a cold and less-saline northward flow, and the Angola Current (AC), a fast and narrow southward geostrophic flow of warm and saline water found between the equatorial band and about 15°S. The intersection of these two currents, around 15°S, is known as the Angola Benguela Front (Hopkins et al. 2013; Lass and Mohrholz 2008, their Fig. 1; Houndegnotono et al. 2021, their Fig. 1b). The discharge of the Congo River, which extends offshore as the Congo River plume, is a thin layer (3 m thick at the river mouth) of fresher/lower salinity water. This freshwater plume is entrained mostly westward, through Ekman-driven circulation in which the wind deflects surface water to the left of its direction in the southern hemisphere.
Main ocean currents in the Gulf of Guinea. Angolan Current (AC), Benguela Current (AC), Benguela Coastal Current (BCC), Guinea Current (GC), and South Equatorial Current (SEC). Warm currents in red; cold currents in dark blue
Ocean currents are important for understanding the biogeographic history of aquatic organisms and terrestrial organisms that disperse in water—such as some seed plants—but, in the Gulf of Guinea, they may also hold the key to understanding how many non-volant, non-swimming, or salt-intolerant species made their way to the islands (Melo et al. 2022). The prevailing hypothesis to explain the origin of such unlikely oceanic island taxa proposes that they came as passengers of natural rafts that drifted to the islands along “freshwater pathways” on the ocean surface (Measey et al. 2007). Such rafts would reach the islands along “freshwater pathways” created by the large input of freshwater plumes and precipitation into the Gulf of Guinea (Fig. 2.11; Dessier and Donguy 1994; Large and Yeager 2009; Hopkins et al. 2013; Berger et al. 2014). Because saltwater is denser than freshwater, during the rainy season the ocean surface in the Gulf of Guinea exhibits reduced salinity—a well-known phenomenon by local fishermen (Measey et al. 2007; Hopkins et al. 2013). These “freshwater pathways” would give the rafts some protection against saltwater as they cross the sea.
The Gulf of Guinea receives the freshwater discharge of three major rivers that originate from different regions: the Niger in West Africa, the Congo in East-Central Africa, and the Ogooué in West-Central Africa (Fig. 2.12). The Congo River is only second to the Amazon in terms of discharge, having an average discharge of 40×103 m3s−1 (Mahé and Olivry 1999), while the Niger River has about 7×103 m3s−1 (Dai and Trenberth 2002). When reaching the ocean, these waters are directed toward the islands by the surface currents of the Atlantic Ocean. Although the mouth of the Ogooué River is the closest to the islands (approximately 250 km), the currents in the Gulf of Guinea direct the freshwater plumes from the Niger and Congo rivers toward the islands (Richardson and Walsh 1986), such that vegetation rafts originating in the more distant West and East African drainages may also reach the islands.
Monthly average sea surface salinity for the months of February (left) and July (right) 2021. Values displayed with a range of 20 and 37 using the Practical Salinity Scale (PSS), roughly equivalent to parts per thousand of salt. Data from the Remote Sensing Systems SMAP Ocean Surface Salinities [Level 3 Monthly] Dataset, Version 4.0 validated release (Meissner et al. 2019)
Conclusions
Despite their small area, the oceanic islands of the Gulf of Guinea include a wealth of geological substrates and topographical features that underlie the development of diverse soils and micro-climates (Fig. 2.2). This diversity of geological features has been recognized by ten formations being proposed as geosites on São Tomé Island, which have a wide range of cultural, scientific, and scenic values (Henriques and Neto 2015). These landscapes have also promoted the appearance of distinct ecosystems (Dauby et al. 2022) and species (Melo et al. 2022). The location of the islands, at moderate distances from the mainland and at the crossroads of freshwater plumes from three large rivers, has likely further contributed toward the assembly of their rich biological communities. These rivers are thought to have been the source of natural rafts bringing species that would otherwise be unable to cross saltwater barriers.
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Acknowledgments
The authors thank Jason Ali and Bernard Bourles for suggestions on the original draft of this manuscript, and Branca Moriés, from the library of the Museu de História Natural e da Ciência, Universidade de Lisboa, for her support with historical bibliography. Data for map of Fig. 2.9 were obtained from the “Global Solar Atlas 2.0,” a free, web-based application, developed and operated by the company Solargis s.r.o. on behalf of the World Bank Group, utilizing Solargis data, with funding provided by the Energy Sector Management Assistance Program (ESMAP). For additional information: https://globalsolaratlas.info. SMAP salinity data are produced by Remote Sensing Systems and sponsored by the NASA Ocean Salinity Science Team. They are available at www.remss.com. MM was supported via the European Union’s Horizon 2020 Research and Innovation program under grant agreement 854248. “Fundação para a Ciência e a Tecnologia” (Portugal) funded BSS (2021.06659.BD), cE3c (UID/BIA/00329/2021; to RFL), and (UIDB/50027/2021; to MM). SJN was supported by the European Research Council under the EU H2020 and Research and Innovation program (SAPPHIRE grant 818854).
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Ceríaco, L.M.P., Santos, B.S., de Lima, R.F., Bell, R.C., Norder, S.J., Melo, M. (2022). Physical Geography of the Gulf of Guinea Oceanic Islands. In: Ceríaco, L.M.P., de Lima, R.F., Melo, M., Bell, R.C. (eds) Biodiversity of the Gulf of Guinea Oceanic Islands. Springer, Cham. https://doi.org/10.1007/978-3-031-06153-0_2
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