Key Concepts and Questions: This Chapter Will Explain

  • How a few species of flowering plants have returned to life in the sea.

  • Plant adaptations to fluctuating salinity, oxygen and light conditions.

  • What specialised animals live in the mangroves and seagrass meadows of Angola.

Context: A Return to the Sea: Mangroves and Seagrasses

The early ancestors of vascular plants migrated from the sea to the land in the Silurian (444–419 Ma). By the Triassic (252–201 Ma), advanced land plants (the seed-bearing Spermatophytes) had evolved, which in turn were succeeded by the first flowering plants (Angiosperms) by the beginning of the Cretaceous (145 Ma). Flowering plants radiated across terrestrial biomes which they soon came to dominate. However, during the late Cretaceous (100–66 Ma), a small group of angiosperm members recolonised the coastal and marine environments from which their vascular plant ancestors had migrated. These plants now occupy very specialised habitats known as mangroves and seagrass meadows.

The challenges faced by flowering plants in colonizing marine environments are reflected in the fact that out of the over 350,000 species of living flowering plants known to science, only 55 species have succeeded as mangroves, and 70 species as seagrasses. Representatives of these extraordinary plants, and the ecosystems that they have created, are found in Angola.

Angola’s Mangroves and Seagrass Meadows (Ecoregion 16)

1 Definition and Distribution

Mangroves have acquired the unfortunate image of mud, methane and mosquitoes (Hogarth, 2007). Floristically, they are the most species-poor biome in Africa and they are among the least-well documented ecosystems of Angola. Yet despite their limited biodiversity and uncomfortable milieu for humans, mangroves provide outstanding examples of adaptations by angiosperm plants to inhospitable physical environments. They are currently enjoying increased study by Angolan ecologists.

Mangroves are recognised as a distinctive biome because of the remarkable habitat which they occupy—at the interface between terrestrial and marine environments. They are exposed to inundation by sea and river water of widely varying salt concentrations (salinity), low oxygen concentrations and often anoxic conditions. They are subjected to repeated wave erosion and occasional storm surges, daily fluctuations in water levels due to tides, and seasonal fluctuations in water levels and salinity according to rainfall and riverflow patterns. They have remarkable adaptations to life in tidal zones, with specialised stilt roots for physical support (Fig. 17.1) and breathing roots (pneumatophores) for gas exchange.

Fig. 17.1
A photograph of mangroves with their roots spread on the bank of a river.

A dense web of stilt roots support mangroves at Foz da Longa lagoon, Cuanza-Sul

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Globally, mangroves occupy 180,000 km2 of tropical river mouths, deltas and tidal mudflats. They occur as isolated ‘ecological islands’ along tropical coasts, at river mouths and in coastal lagoons, often separated by hundreds of kilometres from one another. The stabilizing effect of dense mangrove forests are important to many other organisms, and play a key role as natural buffers in reducing impacts of human-mitigated disturbances such as floods, and of natural disasters such as tsunamis.

The global distribution of mangroves corresponds to tropical coastlines where the mean monthly air temperature of the coldest month exceeds 20 °C. In Africa, mangroves are found along the East African coast from Somalia to South Africa, and on the West African coast, from Mauritania to Angola. The Indian Ocean distribution stretches as far south as East London in South Africa at 32° S, and the Atlantic Ocean distribution extends as far south as Lobito at 12° S. The considerable difference in southern limits on east and west coasts results from the influence on local climates of the warm Mozambique Current of the Indian Ocean, and the cold Benguela Current of the Atlantic Ocean. The most extensive mangrove communities in Angola are in Cabinda and Zaire provinces, at the mouths of the Massabe and Congo rivers (Figs. 3.40, 3.41 and 3.42), and in Bengo province at the mouth of the Cuanza. The Congo mangroves cover some 500 km2, those at the Cuanza approximately 40 km2. The salinity of the water in which mangroves grow at the river mouths range from 0.5 to 35 parts per thousand, with a tidal range from 0.5 to 1.8 m.

Associated with mangroves is another specialised ecosystem—seagrass meadows. In Angola, these are restricted to the Baía do Mussulo, just south of Luanda, and deserve special mention because of their ecological adaptations and conservation importance.

2 Floristic Composition and Physiognomy

The global mangrove flora of 55 mangrove species (including 35 tree species and 20 associated species comprising shrubs, climbers, ferns and epiphytes) represent 16 families and 20 genera. The taxonomic diversity of the mangrove flora reflects independent but convergent evolution of their specialised physiological adaptations for survival, growth and reproduction in 16 distinct families. Two families, Avicenniaceae and Rhizophoraceae, contribute 30 species to the flora. In Angola, three species of red mangroves—Rhizophora racemosa, R. mangle and R. harrisonii, and two species of white mangroves—Avicennia germinans and Laguncularia racemosa have been recorded out of a total of six mangrove species known from West Africa.

The low species diversity in mangrove ecosystems and the limited topographic diversity of their coastal riverine and estuarine habitats result in relatively simple patterns of community structure. Most mangroves have monospecific stands of species distributed in zones according to multiple interacting factors. These include the ability of their propagules to disperse and survive in a site determined by shore morphology, the influence of tides and river flow, and the variations in salinity and sedimentation (Hogarth, 2007).

Due to the steep gradient of the Angolan coastal margin, large deltas dominated by mangrove forests, such as are found at the mouths of the Amazon, Niger, Zambesi, Brahmaputra and Mekong rivers, are not found in Angola. At the mouths of the Cuanza and Congo, strong river currents flow directly into the sea, with a narrow belt of mangroves on their banks. The mangroves extend from the margins of the rivers into adjacent floodplains and swamp forests, wetlands and mudflats. The physiognomic and floristic structure of the communities varies with edaphic conditions. The mangroves of the Congo extend 40 km, and of the Cuanza 15 km, up river, succeeded by thickets of Raphia matombe palms in marginal swamps (Fig. 17.2). Mixed riverine forests occur on higher ground where palms such as Elaeis guineensis and Phoenix reclinata and deciduous trees such as Albizia glaberrima, Pterocarpus tinctorius, Lonchocarpus sericeus and Hibiscus tiliaceus are common.

Fig. 17.2
A photograph of dense Raphia palm trees on the bank of a river.

Raphia palms on the river margin upstream of mangrove forest, Cuanza River

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The mangrove forests differ from other forests in Angola in their extreme paucity of species and a virtual absence of understorey vegetation. Exceptions occur through disturbance factors. Gaps in the forest canopy are occasionally caused by tree fall, while the forests at Barra da Cuanza were exploited for construction timber during the colonial era, and the raphia thickets continue to be harvested for poles. Disturbed areas of the Cuanza mangroves have a dense undergrowth of shrubs and climbers, including Dalbergia ecastaphyllum, Machaerium lunatum and Sarcocephalus pobeguini and the mangrove fern Acrostychyum aureum.

Rhizophora mangle dominates the mangroves of Cabinda and with R. racemosa reaches 25 m in height. The diversity and height of mangroves decreases southwards to the Longa and Queve estuaries. Avicennia germinans seldom exceeds 5 m height, with stunted R. mangle and Laguncularia racemosa at their southern limit of Angolan mangroves in Lobito, where most mangroves have been destroyed by urban development.

2.1 Mangrove Adaptations to Waterlogged Soil

The roots of most terrestrial plants access oxygen for respiration through gas diffused through pores in the soil. The soil of mangrove communities has the pores filled with water, and the little oxygen present is depleted by anaerobic respiration of soil bacteria, creating anoxic conditions. The complex biochemical reactions involved in these stagnant waterlogged soils results in the production of methane, which accounts for the unpleasant smell of mangrove ecosystems.

A key adaptation to these soil conditions is illustrated by various forms of aerial roots. Unlike dryland plants, which have their roots branching below ground from the base of their trunks, mangrove trees have roots descending from up to two metres above the ground, radiating out around the tree as buttresses. Known as stilt roots, these provide physical support to the trees, which lack tap roots. In Rhizophora racemosa the primary roots give off looping horizontal roots that radiate outwards, crossing those of adjoining trees and forming an impenetrable network (Figs. 17.1 and 17.3).

Fig. 17.3
A photograph of trees with their roots in the river.

Mangrove forest on margins of Cuanza River. Note stilt roots exposed at base of trees by an out-going tide

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The aerial roots have specialised absorptive pores—lenticels—which absorb air while exposed above the water surface, but which close as soon as the incoming tide reaches them. The roots contain very porous aerenchyma tissue, providing channels for the transfer of oxygen to the deeper roots which lie in the anoxic, waterlogged soil. Aerenchyma comprises up to 70% of the root volume. In Avicennia germinans, which covers the mudflats around Ilha dos Pássaros at Mussulo, the main roots radiate outwards from the tree base, just below the mud surface, and give rise to hundreds of short vertical roots that are called pneumatophores which serve as conduits of atmospheric oxygen to the root tissues. A single Avicennia may have more than 10,000 pneumatophores (Hogarth, 2007).

2.2 Adaptations to Changes in Salinity

Mangroves are found at the mouths of large tropical rivers, on the mudflats adjoining river estuaries, or in sheltered lagoons. These sites experience salinities ranging from that of fresh water (zero parts per thousand salt), to that of sea water (35 parts per thousand salt), to far higher salt concentrations where evaporation has taken place on the surfaces of mudfats exposed to the sun during low tide. The high negative osmotic potential of the soil water is addressed by several mechanisms. These include the exclusion of salt by the roots, tolerance of high tissue salt concentrations, and elimination of excess salt by excretion. Some species of Avicennia have salt glands on their leaves, excreting salt crystals which are visible to the naked eye. The mechanisms must deal not only with high levels of salinity, but also with large fluctuations in salinity, due to the variable impacts of tidal fluxes, river flows and evaporation. In addition to the challenges of waterlogging and salinity, soils in which mangroves grow are low in nutrients. Like the trees of the mesic/dystrophic savannas, they withdraw a high proportion of nitrogen and phosphorus before dropping their leaves. They also retrieve nutrients from the soil, from decaying roots of other trees or from the leaves decomposed by crabs.

2.3 Reproductive Adaptations

Pollination is the first step needed for successful reproduction in plants. Mangrove trees have not developed many elaborate floral morphologies to attract pollinators, other than rather simple mechanisms of scent and large flowers that attract bats, with smaller flowers attracting butterflies, bees, flies and other small insects. Fertilization success is low, from 3 to 7% in flowers studied.

While their flowers are unsophisticated, mangrove propagules are not. All mangroves disperse their unusually large fruit by water. Unlike most land plants, mangroves do not produce dry, dormant seeds; instead, they produce large actively growing seedlings, where the embryo remains on the tree until the seedling is well developed. This reproductive trait is known as vivipary, evolved independently in 16 mangrove families. The propagule inherits the physiological adaptations to salinity and water, begins photosynthesis while still attached to the adult tree, and develops an extended embryonic axis or hypocotyl, which in Rhizophora mangle might be 25 cm length. Such fruiting behaviour comes at a cost to the adult, and reproductive effort in typical mangrove species might be as much as 10–40% of the tree’s investment in growth (Hogarth, 2007).

The purpose of this great investment in large, actively growing propagules seems related to dispersal by floating on the water currents of streamflow, sea tides, and storm surges. The propagules of Rhizophora harrisonii can remain viable for a year or more. Once stranded on a mudbank, the propagule will send out roots to anchor in the mud and shoots and leaves to establish themselves as seedlings. The advantages of vivipary are poorly understood, but it provides a diversity of dispersal and colonization mechanisms that vary in space and time—ideal for patchy and challenging environments.

2.4 Seagrass Meadows

Along the Angolan coast, sediments carried from the mouths of the Catumbela, Queve, Longa and Cuanza rivers are transported northwards by the coastal current driven by southwesterly winds. Some of these sediments are deposited on the coast, and form long sandspits (restingas) to the north of the river mouths. The sandspits often have lagoons on their landward margin. In the case of the 30 km-long Restinga das Palmeirinhas, both mangroves and seagrass meadows have developed on the sandy and muddy sediments within the Baía do Mussulo. These ecosystems have been studied by Portuguese and Angolan biologists for several decades (Costa et al., 1994; Santos, 2007).

Seagrasses belong to a small group of monocotyledons which, like mangroves, are adapted to challenging environments at the terrestrial/marine interface. Although they might have the general appearance of grasses, they are not members of the Poaceae. Globally, about 70 species of seagrass have been described from tropical to temperate regions, of which six species occur along the west coast of Africa. Only two species, Halodule wrightii and Cymodocea nodosa (both of the family Cymodoceaceae), are known from Angola.

In the Mussulo lagoon, seagrass meadows are found in basins of shallow water, adjacent to mudflats. The challenges to establishment, growth and reproduction are great. The sandy and muddy sediments in which the seagrasses grow are not only unstable, but like mangroves, seagrasses have to transfer gases from submerged organs—leaves and stems—to belowground roots, requiring complex physiological adaptations. Their photosynthetic leaves have no stomata, instead, they have a thin cuticle layer which allows gases and nutrients to be exchanged directly with the surrounding water. For effective photosynthesis, the permanently submerged leaves of seagrasses must absorb adequate light. Most seagrasses are found in shallow waters of 1–3 m depth with high levels of light. Like mangroves, they also have to tolerate high and variable levels of salinity at or above that of seawater. Finally, for reproduction, they have to overcome the challenges of successful pollination and seed dispersal while their flowers are permanently submerged. Given the complexity of these adaptations, it is not surprising that only 70 species of seagrasses have evolved in the 100 million years of the group’s evolution.

Seagrass meadows and mudflats are exposed to inflowing and receding tides, with widely fluctuating salinity levels. The mudflats are in constant flux, due to tidal and wave action. As alluvium is gradually deposited, developing new areas of mudflats, they are colonised by mangrove pioneers, and on the drier salt flats such as the Saco dos Flamingos at the head of Baía do Mussulo, salt tolerant plants (halophytes) such as Sesuvium crithmoides, S. portulacastrum, Salicornia sp. and Arthrocnemum sp., form a short carpet of succulents, with grasses such as Sporobolus virginicus on higher ground above tidal waters.

3 Faunal Composition of Mangrove and Seagrass Meadow Ecosystems

The mosaic of habitats created by rivers, estuaries, sandspits, mangroves, mudflats, seagrass meadows and salt marshes support a great diversity of animal life, even though the diversity of flowering plants living in these physical environments is very limited when compared with terrestrial communities. Vertebrate species breeding in the mangrove forests, seagrass meadows, rivers and coastal shores are noted in Table 17.1.

Table 17.1 Vertebrate Species Breeding in Mangrove Forests, Rivers and Coastal Shores
Full size table

The Massabe and Congo river mouths have populations of West African Slender-snouted Crocodile, African Dwarf Crocodile and Nile Crocodile. The Cuanza has only the latter species. The West African Manatee occurs in the Cuanza, Congo and Massabe rivers, the African Softshell Turtle occurs in most rivers from Cabinda to the Cunene, while five species of marine turtle (Loggerhead, Green, Olive Ridley, Hawksbill and Leatherback) are known to breed on the sandy beaches and sandspits of Angola, especially northwards from the Restinga dos Palmeirinhas. Green and Olive Ridley turtles have also been reported from the Mussulo lagoon, possibly grazing on the seagrass meadows.

The dense matrices of mangrove roots, especially the pneumatophores of Avicennia germinans, create traps for sand and mud, thus stabilizing the sediments as they are transported across the root mats with the ebb and flow of tides. These repeated inundations of the mudflats and interactions with the seagrass Halodule wrightii and the epiphytic brown alga Acanthophora spicifera, and nutrient, water and oxygen cycles create diverse habitats for many specialised animals, especially crabs, barnacles, oysters, mussels, and mudskipper fishes. Angolan marine biologist Carmen Santos made a detailed study of the macroinvertebrates and fishes associated with the seagrass meadows of the Mussulo lagoon and recorded 163 taxa belonging to 10 phyla from this habitat (Santos, 2007). The fish fauna of the seagrass meadows included 17 species, while a survey of the more diverse mangrove habitat (Costa et al., 1994) recorded 36 species. The mudflats are also important feeding grounds for migratory shorebirds, which motivated proposals for the declaration of Ilha dos Pássaros as a Ramsar site (Morais, 2004).

The fauna of Angola’s mangrove forests at the mouths of the Cuanza and Congo includes populations of three monkeys (Malbrouch, Northern Talapoin and Blue Monkey) while birds characteristic of the habitat include: Palm-nut Vulture, African Pygmy-kingfisher, Grey Parrot, Swamp Boubou, Mangrove Sunbird and Loango Weaver. The fauna of these forests awaits detailed study.