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Biodiversity Patterns of the Marine Benthic Fauna on the Atlantic Coast of Tropical Africa in Relation to Hydroclimatic Conditions and Paleogeographic Events

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Biodiversity Patterns of the Marine Benthic Fauna on the Atlantic Coast of Tropical Africa in Relation to Hydroclimatic Conditions and Paleogeographic Events J I Acta Oecologica 19 (3) (1998) 309-321/ O Elsevier, Paris Biodiversity patterns of the marine benthic fauna on the Atlantic coast of tropical Africa in relation to hydroclimatic conditions and paleogeographic events Pierre Le Lœuff '*, Rudo von Cosel Çmire Orstom de Brest, BP 70, 29280 Plouzané, Frunce. Laboratoire de biologie des iwertébrk marins et malacologie du Muséum national d'histoire nalurelle, 55 me Buffon, 75005 Puns, Fraiace. ;I Correspoiidiiig author Received September 17,1996; revised February 24,1997; accepted September 1,1997 Abstract - Five different hydroclimatic regions are recognized in the tropical eastern Atlantic: the northern alternance region (Cape Blanc - Cape Verga), the atypical tropical region (Cape Palmas - border BeninlNigeiia), the southern alternance region (Cape Lopez - Cape Frio), all with periodical upwelling of colder water, and two intercalated typical tropical regions with warm water and reduced salinity. The faunal rich- ness in the regions with upwelling is higher than in the typical tiopical regions because many benthic species avoid warm and reduced salinity water, Faunistic exchange and affinity are greater between the upwelling zones and the bordering temperate zones. The cold regions are also more similar in faunal composition. Benthic communities in both tropical and temperate eastern Atlantic are not fundamentally different. Species diver- sity of benthic invertebrates in tropical West Africa is about the same order of magnitude as in Europe and the Mediterranean. Hydroclimatic con- ditions (upwelling, salinity reduction and internal waves with drastic temperature changes) and absence of coral reef formations do not favour the establishment and thriving of a warm stenohaline and stenotherm fauna in West Africa. Paleozoogeographic events, such as the formation of a large Euro-West African tropical province during the Miocene after the breakup of the Tethys, repeated climate deteriorations during Pliocene and Pleistocene with reductions of the tropical zone, sea level changes and large-scale extinction of warm-tropical species, are also factors responsible for the current low biodiversity in the tropical eastern Atlantic. Some species survived in two major relict pockets in Senegal and southern Angola with particular ecological conditions. O Elsevier, Paris Tropical West Africa / marine benthic fauna / biodiversity / hydroclimate / paleozoogeography 1. INTRODUCTION and by Cosel [14] in a study on the zoogeography of bivalve molluscs of the tropical Atlantic. The first biological studies of the marine fauna of The purpose of this study is to extend and confirm the Atlantic coast of Africa have revealed the existence these observations by correlating distribution patterns of drastic faunistic changes at Cape Blanc in Mauri- of benthic species belonging to the best known zoo- tania (20'59' N) and Cape Frio in Northem Namibia, logical groups with the recognized hydroclimatic divi- near the Angolan border (18'30' S), that were corre- sions. lated with biogeographic barriers [30, 34, 37, 47, 67, 68, 69, 781. The tropical West African region is con- 2. HYDROCLIMATIC DIVISIONS ALONG THE sidered today to be situated between these two limits; WEST AFRICAN COAST it is characterized by a specific marine fauna. Within this part of the West African coast, hydroclimatology On the West African coast, cold waters from upwell- is quite complex, with zones of periodical upwelling ings are permanently present in the north of Cape of cooler water, but now well known [4, 12,48,49,61, Blanc (Canary region) and in the south of Cape Frio 70, 79, 921. Regional hydroclimatic divisions can thus (Benguela region). Between these points lies a tropical be identified. The importance of these hydroclimatic zone delimited by two thermal fronts with steep gra- divisions within the trop_ :al zone for the marine fauna dients that are often associated with salinity changes. 1- €ïas bëeñ-ñoETgy Intbs and Le Loeuff [52] in their The fronts are marked by a close set of surface iso- , work on the distribution If West African polychaetes, theiins between 23 and 27 'C. In the north, the current Fonds Documentaire ORSTQ 310 P. Le Lœuff, R. von Cosel regime (Canary Current) seems to favour some fau- upwelling disperse along the Gulf of Guinea coast, nistic exchanges in spite of the thermic barrier; but this from Ivory Coast to Benin, causing the formation of tfoes not appear to be the case in the south where the two additional thermic fronts [61]. The first front, well direction of the Benguela Current south of Cape Frio marked, occurs at Cape Palmas on the border between moves away from the continent in the direction of St. Liberia and Ivory Coast, whereas the second, more Helena Island [l, 851. As a result, the meroplancton of diffuse, occurs at the border between Benin and species capable of establishing themselves in the trop- Nigeria. Between these fronts, the so-called central ical zone is transported offshore. atypical tropical region takes place. Thus one can distinguish two types of hydroclimatic regions within The two thermic fronts move seasonally to the south the tropical zone: contrasted regions and uniform (Cape Blanc Front) and north (Cape Frio Front) to rel- regions ($gum I). The provinces with contrasted atively fixed positions [4, 581 that delimit the regions hydroclimate include the northern alternance region dominated by strong seasonal contrasts, called the (Mauritania, Senegal, Gambia, Guinea-Bissau, alternance regions. In northern winter, the thermic extreme north of Guinea), the southern alternance fronts reach the southmost points of Cape Verga region (Gabon, Congo, Angola) and the central atyp- (Guinea) in the north and Cape Frio in the south. In ical tropical region (Ivory Coast, Ghana, Togo, Benin). northern summer, they ascend to the northmost points The latter region forms a climatic enclave surrounded of Cape Blanc and Cape Lopez (Gabon). During this by the two typical tropical regions, the northern, or season (July to September), cold waters from rather western region (greater southern part of Guinea, WW 100 00 100 E I 1 ZOON 100 00 100 S Figure 1. Hydroclimatic regions on the coast of tropical West Africa (2-6). External regions: 1. Atlantic coast of Northwest Africa and Europe, Mediterranean; 7. Namibia and Atlantic coast of South Africa. The zones of permanent and periodic upwelling are hatched. Acta Oecologica J West African marine benthic fauna biodiversity 311 Sierra Leone, Liberia) and the southern, or rather [39], ophiurids [64], asterids [lo, 11,72,73] and echi- eastern region (Nigeria, Cameroon, Fernando Poo noids [8, 711. However, literature on holothurians is [Bioko], Equatorial Guinea, extreme north of Gabon). very dispersed across the end of the 19th century to the These typical tropical regions are characterized by late 80s (see [9]).The same situation exists for poly- waters with little seasonal temperature variation but chaete annelids, although a number of important fau- important salinity fluctuations near the coast. They are nistic studies was published, notably Augener [2], subject to the largest precipitations of the West African Fauvel [25-271, Fauvel and Rullier [28-321, Tebble coast. The thermic characteristics of each region [87], Kirkegaad [54, 551, Rullier [81-831, Zibrowius (figure 2) are taken from Hastenrath and Lamb [46]. It [95], Intès and Le Loeuff [50,51]. Gastropod molluscs should be noted that the Cape Verde Islands are in the were not included in this study owing to the frequent northern alternance region (see also [13]), whereas the uncertainties of locations and the numerous doubtful islands of Annobon, São Tomé and Principe, of which or obviously incorrect identifications in the literature. at least the first two are affected by equatorial The West African marine gastropods are in need of a upwelling, are included in the southern alternance thorough revision. region. Most of the systematic works used for this study are based on biological material collected either by ocea- 3. MATERIAL AND METHODS nographic expeditions along the coast of West Africa I (Belgian expedition on the coasts of the southern Atlantic; Danish expedition of the RN 'Atlantide' all All benthic invertebrates identified in our study were along the West African coast; expeditions of the FUV recorded from tropical West Africa between Cape 'Calypso' to the Gulf of Guinea and the Cape Verde Blanc in the north, and Cape Frio (or southern Angola) Islands; expedition of the American R/V 'Pillsbury' in the south, from the continental shelf and slope down to the Gulf of Guinea), or during other cruises in more to 200m. The studied species belong to the best or less limited parts of the coast on board smaller known zoological groups, that include species for research vessels of marine research institutes during which the distributional gaps are not likely due to sam- both colonial and independence periods. The most pling deficiencies. Some of these were subjects of sys- important of these institutes are Ifan (Institut Fonda- tematic and biogeographic surveys across West Africa: mental d'Afrique Noire) which worked in Mauritania, gorgonians [43, 441, brachyurans [65], bivalve mol- Senegal and Guinea, Wafri (West African Fisheries luscs [15, 741. Biological studies were also conducted Research Institute) which covered the shelf of Sierra on other benthic invertebrates, namely pagurid crusta- Leone, Ghana and Nigeria, and Orstom (now Institut ceans [35, 36, 381, and echinoderms, such as
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