The Niger Delta Mangrove Ecosystem and Its Conservation Challenges

Abstract

The Niger Delta region of Nigeria is a biodiversity hotspot, rich in fauna and flora, with robust ecological processes that drive the economy of the local communities. However, the area has not been formally recognized as a global biodiversity hotspot by international agencies such as the International Union for the Conservation of Nature (IUCN). The Niger Delta abounds in several species of mangroves, but the most dominant ones are the red (Rhizophora racemosa), black (Laguncularia racemosa), and white (Avicennia germinans) mangroves. The mangrove ecosystem in the Niger Delta is fast being replaced by Nipa palm (Nypa fruticans), yet no result-oriented action plan has been put in place to conserve the mangrove. The serious menace of oil pollution and overexploitation of fisheries and mangrove resources, in addition to environmental pressures, as well as poor recognition of brackish water bodies and aquaculture, causes serious danger to the sustainability of Nigeria’s brackish water and the various fish resources, wildlife, and, inevitably, the coastal communities of the nation. This chapter provides essential information on the rich Niger Delta mangrove ecosystem and current conservation challenges and recommends strategies for sustainable management.

1. Introduction

Nigeria is the most populous country in Africa. Since the attainment of independence in 1960, it has continuously grown in both size and influence, with an approximate population of 203 million people, accounting for 47% of the total West African population [1]. It hosts a variety of terrain and climate. The seven main ecological zones are the Mangrove Swamp and Coastal Vegetation, Freshwater Swamp Forest, Lowland Rain Forest, Derived Savanna, Guinea Savanna, Sudan Savanna, and Sahel Savanna [2]. The Niger Delta geographic basin is defined by the many tributaries of the lower River Niger, as it enters the Atlantic Ocean through its many tributaries and estuaries. It is a region with very high biodiversity as well as a dense human population. At the base of the Niger Delta, formed by a network of many creeks, is Africa’s largest mangrove forest, the third largest in the world.

The Niger Delta is located on the Atlantic coast of Southern Nigeria, which extends between latitudes 4^o 2′ and 6⁰ 2′ north of the equator and is 50 2′ east of the Greenwich meridian [3, 4] and bordered by the Atlantic Ocean in the south, and Cameroon in the East. It occupies a surface area of about 112,110 km². It represents about 12% of Nigeria’s total surface area and is almost two-thirds of the entire population of Nigeria (i.e., 200 million). The region is made up of nine of Nigeria’s constituent states (Table 1) [5]. The region has a yearly rainfall of 3000–4500 mm and an average temperature of 27°C. The wet season is relatively long, occurring from March to October, and the dry season from November to February [6, 7].

State	Land area (km^2)	Population	Capital
Abia	4,877	5,106,000	Umuahia
Akwa Ibom	6,806	5,285,000	Uyo
Bayelsa	1,107	2,703,000	Bayelsa
Cross River	21,930	4,325,000	Calabar
Delta	17,163	5,681,000	Asaba
Edo	19,698	4,871,000	Benin City
Imo	5,165	5,283,000	Owerri
Ondo	15,086	4,782,000	Akure
Rivers	10,378	7,679,000	Port Harcourt
Total	112,110	45,715,000

Table 1.

Niger Delta states of Nigeria, their land areas, population, and capitals [5].

The Niger Delta floodplain occupies 7.5% of Nigeria’s total land mass and is her largest wetland. It is the third largest drainage basin in Africa. The Delta’s environment includes four ecological zones: coastal barrier islands, mangrove swamp forests, freshwater swamps, and lowland rainforests. This vastly well-endowed ecosystem contains one of the Earth’s highest concentrations of biodiversity. Apart from supporting abundant flora and fauna, arable terrain that sustains a wide variety of crops, lumber, or agricultural trees, it also harbors more freshwater fish species than any other ecosystem in West Africa.

The biodiversity of the Niger Delta is of regional and global significance. Barrier islands, estuaries, mangroves, creeks, and freshwater swamps characterize the Niger Delta ecosystem. The Niger Delta region, considered a “Biodiversity Hotspot” by biodiversity experts, abounds in many locally and globally endangered species with robust ecological processes that drive the economy of the local communities.

Despite the importance of forest resources, there has been a rapid decrease recently due to the demand from an ever-increasing population. Drivers of forest loss in the different forest types are complex, including illegal logging from commercial loggers, due to high timber demand and communal logging of firewood for domestic cooking [8]. Mangrove ecosystems in the Niger Delta are fast being replaced by Nipa palm (Nypa fruticans). Yet no result-oriented action plan has been put in place to conserve the dwindling mangrove. The serious menace of oil pollution and overexploitation of fisheries and mangrove resources, in addition to other environmental pressures, as well as poor interest and recognition of brackish water bodies and aquaculture, cause serious danger to the sustainability of Nigeria’s brackish water and the various fish resources, wildlife, and, inevitably, the coastal communities of the nation [9].

The importance of mangroves as nurseries has been one of the reasons to support their conservation and management [10]. Many African, Latin American, and Asian countries are now estimated to have lost at least 50% of their original mangrove area [11]. The current wave of global concern for the maintenance of essential ecological processes and life support systems, preservation of genetic diversity, and the sustainable utilization of species and ecosystems has led to the intensive survey of nature reserves for their ecological descriptions, biodiversity, and conservation status [12]. According to the International Union for the Conservation of Nature and Natural Resources (IUCN) report [13] on the global status of mangrove ecosystems, 18 countries have established mangrove reserves to safeguard the habitat and associated species. The mangrove zone of the Niger Delta is said to traverse parallel to the coast and reaches between 15 and 45 km inland. This deep belt of mangrove forest protects the freshwater wetlands in the Inner Delta [14].

The Niger Delta mangrove ecosystem, a fragile ecosystem increasingly being threatened by Nipa palm (Nypa fruticans) encroachment and industrial/oil pollution, is presently receiving urgent national and international attention for sustainable management and conservation. The problem is that the area has not been formally recognized as a global biodiversity hotspot by international agencies such as the IUCN. This Book Chapter provides essential information on the rich Niger Delta mangrove ecosystem and its conservation challenges and recommends strategies for sustainable management.

2. Terrestrial ecosystems of Nigeria and mangrove ecosystems of the Niger Delta

Nigeria has many regions with rich biodiversity serving as home to critical ecosystems. Among these regions are the Cross River basin rainforest, mountains along the Cameroon border with Nigeria, and the Niger Delta coastal creeks (Figure 1) [15, 16]. The Niger Delta mangrove is Africa’s largest mangrove ecosystem and the world’s third largest after India and Indonesia, the third largest drainage basin in Africa, and Africa’s largest river delta [17, 18].

Figure 1.

Nigeria’s terrestrial ecoregions [15].

The coastal region of Nigeria extends from Benin Republic/Nigeria border in the west to the Cameroun/Nigeria border in the east, a distance of 800 km but with a coastline of 853 km [19]. This coastline has interfered with a series of estuaries that empty into the extensive lagoon system in Lagos and Ondo State. At least twenty-two (22) estuaries exist between the Benin River in Delta State coastal region and the Cross River in Akwa Ibom State. In the Delta and River State areas, the estuaries join with tributaries and distributaries of the lower Niger River/Benue system to form an anastomosing system of fresh and brackish water creeks, backwaters, and flood plains in what is better described as the Niger Delta basin swamps [20]. The creeks, floodplains, lagoons, and rivers in Lagos State account for approximately 22% of the 790 km² land mass [21].

The brackish water sector consists of estuaries, beach ridges, intertidal mangrove swamps, intersecting rivers, and winding saline creeks. The exact area of the brackish water for the entire Nigerian coastline is unknown except for the Niger Delta portion, the area of which has been estimated [22] and includes the constant saline creeks that form an area of about 1000 km² and the intertidal mangrove swamps consisting an area of about 5048 km². In addition, there are intersecting rivers and estuaries plus beach ridges with an area of about 679 km² and 688 km², respectively. The total brackish water area of the Niger Delta is therefore approximately 7415 km². The inland area of the mangrove swamp fairly delimits the brackish water area. The brackish water region of Nigeria’s coastal zone between the Benin River and Cross River has been adequately delineated [23].

Mangrove forests could be cleared for aquaculture ponds, salt pans, agricultural use, including rice fields, airport and road construction, port and industrial development, resettlement, and village development [24]. They are, however, important factors in stabilizing the shoreline (http://www.panda.org). Some endangered and potentially vulnerable species are endemic in this area. The mangrove swamps and adjoining freshwater swamps in the Niger Delta provide critical habitats for many endangered animal species, important wildlife, and migratory and resident aquatic reptiles, birds, and mammals. These include crocodiles, Nile monitor lizards, parrots, duikers and antelopes, monkeys, bush pigs, bush cow or buffalos, sitatungas, hippopotamus, diving pelicans, and deep swimming shags [25]. The subtidal prop root of mangrove habitats serves as nurseries for fishes of economic importance [26]. The interest in protecting mangroves worldwide is due to their purported importance to fisheries and a number of rare and endangered species [27]. However, because the same mangrove species can often occur under marine, estuarine, and freshwater conditions, a wide variety of fish assemblages can be found among their inundated “mangrove habitats.” As such, mangrove habitats likely play a variety of roles in the lives of associated fishes, feeding areas for some species or life stages, daytime refugia for others, and nursery and/or nesting areas for yet more. Mangroves of the Atlantic coast of Africa, including the Niger Delta, on account of their gentle gradient of sediment, are sensitive/fragile [28]. The rich biodiversity and high aquatic productivity of the West African coastal waters are constantly undergoing bio-modification due to the adverse impact of human activities [29].

2.1 Mangrove flora of the Niger Delta

The most symbolic mangrove species in Nigeria consists of six species in three families, namely: Rhizophoraceae (Rhizophora racemosa, R. harrisonii, and R. mangle), Avicenniaceae (Avicennia africana), and Combretaceae (Laguncularia racemosa and Conocarpus erectus). In terms of biodiversity, the Niger Delta mangrove ecosystem is one of the richest wetlands globally by researchers.

There are several species of mangroves in the Niger Delta, with the most dominant ones being red (Rhizophora racemosa), black (Laguncularia racemosa), and white (Avicennia germinans) mangroves. They provide highly productive habitats and ecological niches for reptiles, monkeys, birds, fishes, shrimps, mollusks, and other wildlife species, and a number of other ecological services [30]. Also prominent but less studied and uncommon around core mangrove forests are Button wood mangroves (Conocarpus erectus). They are common in inland sandy soil areas [5]. The following eight true West African mangrove species are found in Nigeria, particularly in the Niger Delta region: Avicennia germinans, Rhizophora mangle, Rhizophora racemosa, Rhizophora harrisonii, Laguncularia racemosa, Conocarpus erectus, Nypa fruticans, and Acrostichum aureum. The Families Rhizoporaceae, Avicenniaceae, and Combretaceae, such as Rhizophora racemosa, Rhizophora harrisonii, Rhizophora mangle, Avicennia africana (White mangrove), Laguncularia racemosa, and Conocarpus erectus and a few other stunted shrubs and woody species are the most dominant plant species.

Most locations in the Niger Delta have similar mangrove species composition [5]. The floristic composition of the plants and family distribution in the Mangrove Swamp Forest of Anantigha in Calabar South Local Government Area (LGA), Cross River State, Nigeria, is shown in Table 2 [30]. The plant population consisted of Rhizophora racemosa, Nypa fruticans, Avicennia germinans, Acrostichum aureum, Drepanocarpus lanatus, Lagunicularia racemosa, Hibiscus tiliaceus, Thespesia populnea, Pandanus candelabrum, Carapa procera, Raphia hookeri, Conocarpus erectus, and Chrysobalanus spp. [30]. The most abundant species in the area are Rhizophora racemosa, Nypa fruticans, Avicennia germinans, and Acrostichum aureum. Rhizophora racemosa had the highest species importance value (SIV), followed by Nypa fruticans, an exotic palm that encroaches on the mangrove and dominates the indigenous oil palm, Elaeis guineensis [30]. For the grass species, Dalbergia ecastaphyllum is reported to have the highest diversity (Table 3). Species distribution from seaward to landward areas indicates that core mangrove species were found on the seaward side, whereas the non-mangrove species were found in the landward direction [5].

S/N	Plant Species	Families	Species density (Individuals/ Ha)	Siv
1	Rhizophora racemosa	Rhizophoraceae	45	34.892
2	Nypa fruticans	Arecaceae	23	19.245
3	Avicennia germinans	Avicenniaceae	18	15.468
4	Acrostichum aureum	Pteridaceae	15	13.309
5	Drepanocarpus lanatus	Fabaceae	10	9.892
6	Laguncularia racemosa	Combretaceae	8	8.273
7	Hibiscus tiliaceus	Malvaceae	7	8.094
8	Thespesia populnea	Malvaceae	5	6.295
9	Pandanus candelabrum	Pandanaceae	4	5.935
10	Carapa procera	Meliaceae	3	3.957
11	Raphia hookeri	Palmaceae	2	3.777
12	Conocarpus erectus	Combretaceae	1	1.799
13	Chrysobalanus spp.	Chrysobalanaceae	1	1.439

Table 2.

Plants found in Anantigha mangrove swamp forest with their species density, relative density, and species importance value (SIV) [30].

S/No.	Scientific name	Common name	Growth form	Uses
1	Acrostichum aureum	Mangrove fern	Herb	Crayfish trap
2	Alchornea cordifolia	Christmas tree	Shrub	Medicinal
3	Alstonia boonei	Stool wood		Timber, medicinal
4	Allanblackia floribunda		Tree	Timber
5	Ananas comosus	Pineapple	Herb	Edible fruit
6	Anthostema aubryanum		Tree	Timber
7	Artocarpus communis	Breadfruit	Tree	Edible fruit
8	Bulbophyllum oreonastes		Epiphyte	Ornamental
9	Carapa procera		Tree	Timber
10	Calamus decratus	Rattan palm		Basket, mat, rope, fish trap
11	Cathormion altissimum		Tree	-
12	Dalbergia ecastaphyllum		Scrambler	Rope, erosion control
13	Elaeis guineensis	Oil palm	Tree	Wine, edible oil, thatch, kernel
14	Ficus trichopoda	Fig tree	Tree
15	Fimbristylis sp.		Herb	-
16	Fleurya ovalifolia	Tropical stinging nestle	Herb	Weed
17	Hallea ciliate	Abura	Tree	Timber, joinery, flooring board, canoe, pestle, mortar, pestle, barrel
18	Lycopodium cernum		Epiphyte	-
19	Mariscus ligularis	Seaside sedge	Sedge	Erosion control
20	Mucuna sloanei	Horse eye bean	Climber	Soup condiment
21	Musa sapientium	Plantain		Fruit
22	Pandanus togoensis	Screwpine		Mat, basket
23	Pentadesma butyracea		Tree	Timber, canoe, margarine, comb
24	Piptadeniastrum africanum			Timber, canoes, railway sleeper, furniture
25	Raphia hookeri	Wine palm		Wine, gin
26	Raphia vinifera	Bamboo palm		Thatch
27	Rhizophora mangle	Dwarf red mangrove	Tree	Tannin
28	Rhizophora racemosa	Tall red mangrove	Tree	Tannin, timber, fuel wood, prop, railway sleepers
29	Sacciolepis africana		Herb	-
30	Sphenoclea zeylanica		Herb	-
31	Spondias mombin	Hog plum	Tree	Edible fruit,
32	Symphonia globulifera		Tree	Timber
33	Syzygium guineensis		Tree	Timber
34	Uapaca heudelottii		Tree	Timber, edible fruit
35	Vitex grandifolioala	Black plum	Tree	Edible fruit

Table 3.

Plant species list in Tunu field in south Forcados [31].

A related study documented the common plant species in Tunu and Kanbo in South Forcados of the Niger Delta ecosystem. The predominant vegetation type within the Tunu/Kanbo field is the mangrove swamp forest type, attested to by the floral composition (Table 3) [31].

The dominant plant with the highest frequency of occurrence is the dwarf red mangrove, Rhizophora mangle. The tall red mangrove R. racemosa was confined to the edge of the creeks and rivers. Herbaceous grasses, fern, and sedges such as Acrostichum aureum, Paspalum vaginatum, and Fimbristylis spp. occurred on the forest floor below the mangrove canopy. Epiphytes were abundant on the tree trunks. The commonest species include Bulbophyllum oreonastes and the moss Ocoblepharum sp. The mangroves are admirably adapted to the brackish environment. They have stilt roots that are covered with lenticels. The latter serves as passages for gaseous exchange. Oxygen and carbon dioxide are channeled through these structures. The superficial roots are thus sensitive. The trees can easily die if the ventilating organs (lenticels) are blocked. Mangroves grow best where there is regular tidal flushing. Changes in hydrology resulting from dredging or blockage of natural tidal channels can severely reduce their growth and may result in death (Figure 2).

Figure 2.

Mangrove vegetation in Tunu field (Note: Nymphaea lotus floating around the mangrove roots) [31].

Some parts of the barrier forests, common along the Nigerian shoreline, occur within the field. They appear as small forest islands within the “sea” of mangroves. Typical plants within these forests include the breadfruit tree (Artocarpus communis), Abura (Hallea ciliata), Allanblackia floribunda, Fig tree (Ficus trichopoda), Uapaca heudelottii, and Symphonia globulifera. These species are typical of freshwater swamp forest. These forest islands within the sea of mangrove forests are analogous to oasis within the desert. They are the only places the coastal communities get their potable water from. They are thus sensitive ecosystems with a unique function. Salt-water intrusion can easily kill the vegetation within these islands as the plants inhabiting them are not salt tolerant. The coastal communities also utilize the forest islands for agricultural purposes. Breadfruit and plantain are cultivated within these forests. The forests thus allow them to get staple food in an otherwise nonarable environment (Table 4).

Scientific name	Common name	Abundance	Proportion (%)
Dalbergia ecastophylum	Corn vine	6	24
Chrysobalanus icoco	Coco plum	4	16
Paspalum sp.	Silt grass	2	8
Scleria verrucosa	Bush knife	1	4
Combretum racemosum	Christmas rose	3	12
Osbeckia tubulosa	Melastomatacea	1	4
Mariscus longibracteatus	Sedge	1	4
Acrostichum aureum	Aquatic fern	1	4
Scleria naumanniana	Bush knife	1	4
Lycopodium cernuum	Fern	1	4
Alchornea laxiflora	Christmas bush	1	4
Syzygtium guineense	Myrtaceae	3	12
Total abundance		25	100
Number of species		12
Abundance		25
Dominance index (D)		0.1296
Shannon Wiener (H^1)		2.248
Margalef index (d)		3.417
Equitability (J)		0.9046

Table 4.

Abundance and diversity of weed species commonly found around mangrove forests in the Niger Delta, Nigeria [5].

Indices based on the proportional abundance of species provide a simplified approach to diversity measurement. These heterogeneity indices take into account both evenness and species richness to produce a single value, the Shannon Index of diversity (H¹). The diversity was analyzed using the Shannon diversity index (H¹), Margalef index of species richness (d), Equitability or Evenness index (J), and Dominance index (D).

The Shannon diversity index is a measure of community stability or ecological robustness that makes any community able to withstand external perturbation with minimal disruption. Diversity values of <1.00 indicate heavily polluted, 1–3 indicate moderate pollution, while values > 3 indicate clean and unperturbed habitats. The equitability index measures how evenly distributed the individuals are among the species, while the Dominance index is the opposite of equitability, that is, when individuals are not evenly distributed among the species, but rather, only a few species have a concentrated number of individuals. The environment belongs to the category of moderate perturbation or pollution.

2.2 Mangrove fauna of the Niger Delta

The highest concentration of aquatic organisms in Africa is found in Nigeria’s Niger Delta mangrove forest zones (Figure 3). The species include from plankton (phytoplankton and zooplankton), aquatic invertebrates (bivalves, crabs, mussels, periwinkles, hermit crabs, etc.), and land insects (beetles, butterflies, mosquitoes, ants, and termites) to vertebrates (monkeys, manatee, pygmy hippopotamus, python, etc.) (Figures 4 and 5) [32]. The enigma is that the region is yet to be formally recognized as a global biodiversity hotspot by international agencies like the International Union for the Conservation of Nature (IUCN). Biodiversity hotspots in Africa have commonly grouped together without recognizing the rich biodiversity across different locations. For example, the entire forest biodiversity in West Africa was grouped as “West African forests” [34], whereas individual countries in this region have a diverse and rich supply of biodiversity.

Figure 3.

Some species found within the Niger Delta mangrove forest: (A) Senilia senilis, Anadara; (B) Crassostrea gasar, oyster; (C) Tympanotonus fuscatus, periwinkle; and (D) Uca tangeri, male crab [32].

Figure 4.

Heslops pygmy hippo, Hexaproton (syn-Choeropsis), liberiensis heslopi.

Figure 5.

Protected python seen close to a residential building in Brass Island [33].

The Nigerian mangrove swamps harbor a great diversity of macro in- and epi-faun of which crabs and mollusks form the great majority in terms of biomass. The fauna faces severe water and salt balance, siltation, desiccation, oxygen availability, weight, and temperature limitations, especially when attempting to invade the adjacent land. Presently, few quantitative estimates of Niger Delta mangrove swamp fauna exist, perhaps due to the difficulty of sampling among thickets of mangroves, deep mud banks, and semi-stagnant lagoons, in addition to the difficulty in obtaining reliable estimates of fauna, which spend much of their time in burrows or very active when on the ground. In many cases, the seasonal occurrence and distribution of most macrofauna in the swamp are related to the characteristic fluctuations in environmental parameters associated with the dry and wet seasons as well as stress from anthropogenic activities. Besides, the influence of substratum on faunal species distribution in the Nigerian mangrove swamp of Nigeria has been reported [35].

There are presently few quantitative estimates of Niger Delta mangrove swamp fauna in the literature. Studies of Buguma Creek, a mangrove creek in Rivers State, Nigeria, resulted in the documentation of Zooplankton made up of 37 taxa in the Phyla Arthropoda, which was the most dominant: Coelenterata (Cnidaria); Ctenophora; Annelida; Chaetognatha; and Chordata. Macrobenthic invertebrates comprised 68 taxa in the Phyla Arthropoda and Annelida (the most dominant), Nematoda, and Mollusca. Fish species made up of 20 taxa in the families Sciaenidae (the most dominant), Elopidae, Ariidae, Gobiidae, Serranidae, Carangidae, Lutjanidae, Haemulidae, Monodactylidae, Sphyraenidae, Polynemidae, Trichiuridae, Cynoglossidae, and Dasyatidae (these excluded members of the family Cichlidae (Tilapia guineensis and Sarotherodon melanotheron), Mugilidae (Liza facipinnis, Mugilcephalus, and Mugil curema), and Sardinella maderensis (Clupeidae), which the baited hooks and lines could not catch) [9].

Table 5 shows the macrofaunal distribution and abundances in a study area that extends from the Cross River Estuary (which is relatively less perturbed and therefore used as a control swamp) to Takwa Bay in Lagos, approximately 870 km stretch and encompassing Imo River, Bonny River, Brass River, and Forcados and Escravos river estuaries. The preponderance of polychaetes in the upper ranks can be seen with Capitella capitata, Chaetozone setosa, and Magelona filiformis dominating. Echinocardium was ranked 65th with a density of 5/m². The coefficient of variation for each species is also presented in the table. This index (standard deviation ÷ mean density) indicates the spatial evenness in the abundance of the species, with numbers much less than one indicative of a uniform distribution [35].

Rank	Specimen	No/m^2	Coefficient of variation
1	Capitella capilata	62	0.20
2	Chaetozone setosa	56	0.26
3	Megalona filiformis	45	0.32
4	Exogone lebes	41	0.46
5	Nemetina	38	0.54
6	Abra alba	25	0.42
7	Uca tangeri	21	0.85
8	Ampellisca	18	0.83
9	Nematoda	16	0.67
10	Nephthys sp.	15	0.64
11	Glycera	9	0.38
12	Scoloplos armiger	8	0.62
13	Polydora sp.	7	0.56
14	Turritella communis	6	0.63
15	Notomastus filiformis	5	1.08
16	Cirratulus cirratulus	5	1.1
17	Neries sp.	5	0.68
18	Marphysa belli	4	1.32
19	Hydrobia	4	0.52
20	Tellina sp.	3	0.86
30	Pectinaria	5	0.65
35	Pchygrapsus gracillis	5	0.72
40	Sesamaelegans	15	0.68
48	Sesarina alberti	12	0.65
55	Metagrapsus curratus	12	0.65
60	Littonia sp.	2	1.21
62	Mya arenaria	1	1.02
65	Echinocardium sp.	1	1.23

Table 5.

Rank and abundance of dominant macrobenthic Fauna in the Niger Delta mangrove ecosystem during 1985–2005 [35].

3. Ecosystem services of Niger Delta mangroves

The four categories of ecosystem services that have been identified are: regulating services, provisioning services, cultural services, and supporting services.

The Niger Delta mangroves perform almost all the services listed above, such as from atmospheric and climate regulation, flood and erosion control, wood and timber for cooking fuel and construction to benefits such as esthetic value, sacred sites, traditional medicine, and supporting services such as nutrient cycling and habitat for fish nursery [18].

In addition to the aforementioned ecosystem services, the Niger Delta mangrove provides niche types that support the existence of a diverse fauna of all developmental stages. Several burrows contain diverse species of polychaetes, crabs, shrimps, and fishes supported by the rich food sources peculiar to the mangrove. Stilt roots of Mangrove plants provide substrate for the attachment of edible epifaunal organisms such as the oysters Crassostrea spp. (Figure 6) [18].

Figure 6.

Prop roots providing attachment sites for oyster Crassostrea gazar [18].

The mangrove trees conserve water resources and serve as windbreaks in many communities. Specifically, in the Niger Delta, other uses of mangroves by the indigenous people include building materials, food baskets, fishing tools, and so on [5].

4. Threats to mangroves of the Niger Delta

The mangrove forests of the Niger Delta are undergoing gradual but steady loss, resulting from uncontrolled deforestation for sand dredging and canalization. The mangrove forest is also cut to recover stems, which are used in producing firewood and wood for the construction of houses. Numerous oil and gas exploratory activities all over the Niger Delta area also open up the forests to further exploitation of resources [34]. Invasion by foreign species, such as nipa palms (Nypa fruticans), also threatens the existence of the rich indigenous mangrove species. Habitat loss is one of the three factors responsible for the recent extinction of species. Overexploitation and the introduction of exotic species are the other two. The consequence of the loss of mangrove habitat is the loss of ecosystem services it renders to society [34].

Oil exploration and extraction activities constitute Nigeria’s most severe threat to the mangrove forest ecosystem. Nigeria’s Oil Industry is located mostly in the mangrove forest ecosystem. The activities of various oil exploration companies have resulted in fragmentation, deforestation, and degradation of the mangrove forest ecosystem. The Nigerian Oil Industry has deforested mangrove ecological zones for drilling purposes and further deteriorated the health of the surrounding mangrove areas through oil spillages. Oil spill kills plants and animals in the estuarine zone, endangers fish hatcheries in coastal waters, and contaminates the flesh of commercially valuable fish. The oil that settles on beaches decimates the inhabiting organisms, while that settling on the ocean floor smothers and kills benthic (bottom-dwelling) organisms like crabs and disrupt major food chains. Oil also submerges birds, impairing their flight or reducing the insulating property of their feathers [36].

5. Management and conservation of the Niger Delta mangrove ecosystem

5.1 Mangrove forests

The Niger Delta mangrove forest requires urgent protection due to the adverse impact of anthropogenic activities resulting in the decimation of their populations in many regions. Incessant destruction of the mangrove forest will result in the local extinction of these species. Therefore, to reduce the anthropogenic impact on mangroves, there is an urgent need to establish protection and restrict human entry into the forest. Based on the importance of the biodiversity hotspots to the environment, it is pertinent to protect them for future generations [34].

In the past, the conservation of biodiversity hotspots was mainly focused on species richness, but now it considers ecosystem integrity, water quality, climate impacts, unique adaptations, ecosystem services, intact fauna and flora, specialized/unique habitat, and ecological processes.

Mangrove forest protection should follow the principles of reserve design, which include: the protection of entire habitats (because the more protected habitat, the better it is); avoidance of reserve fragmentation by anthropogenic activities such as the construction of highways through mangrove forests; establishment of the clumped reserve, which is far better than linear, for easy migration of species within the mangrove forests; and establishment of a circular reserve to minimize edge effects.

A damaged mangrove ecosystem could also be repaired through ecological restoration, the process of repairing damage caused by humans to the diversity, and dynamics of the indigenous ecosystem. This process includes erosion control, reforestation, removal of non-native species such as N. fruticans and weeds, revegetation of disturbed areas, daylighting streams, reintroduction of native species, and range improvement for targeted species. Hydrological links to natural restoration sites are also significant in allowing saline water inflow and mangrove seeds. The inflow of water also cleans the site from oil spillage and other pollutants [37].

There is an urgent need for a long-term management plan for the sustainable use of mangrove ecosystems. Monitoring, research, and evaluation constitute vital components of a successful conservation strategy for natural, managed, and man-made mangrove forests.

Conservation of existing mangrove forests is often more effective than planting new forests. When a decision for planting has been made, there is a need to emphasize the careful selection of appropriate sites and species and an ecosystem-based approach to mangrove planting and management, which utilizes and supports natural regeneration and other natural processes. In the process of any rehabilitation intervention work, planting should be done only when absolutely necessary. Local communities must be engaged in mangrove management to effectively maintain and enhance the protective function of the mangrove forest while providing a livelihood for local people and contributing to better assessment and governance of natural resources. Continuous assessment of mangrove forest status through research, economic assessment, and valuation is important for better conservation, planning, and management [38].

As a result of the current threat to forest resources, forest reserves have been created by the Nigerian government in the swamp/mangrove ecosystems of some Niger Delta States (Table 6).

State	Swamp/Mangrove (ha)	High forest (ha)	Derived savannah (ha)	Total (ha)
Abia	2,870.00	4,949.60	1,302.50	9,122.10
Akwa Ibom	31,080.00	777.00	-	31,857.00
Delta	29,345.42	6,608.00	-	35,953.42
Edo	-	565,035.00	-	565,035.00
Imo	-	1,345.30	211.81	1,557.11
Rivers	-	121,440.00	-	121,440.00
Total forest type	63,295.42	700,154.90	1,514.31	764,964.63
%	8.274294	91.52774815	0.197958172

Table 6.

Forest reserves according to vegetation type.

Source: Federal Department of Forestry, 1998.

5.2 Game reserves/wildlife sanctuary

Most of the Protected Areas are Forest Reserves. A few have been designated at one time or the other Game reserves (Table 7). None has the legal instrument for the designation as Game Reserve. They were recognized as having rich faunal compliments but never transited legally into Game Reserves.

S/N	Name	Habitat	Area (ha)	Remarks
Abia	NA	None	-	No designated game reserves
Akwa Ibom	Stubbs Creeks	Swamp forest	21,000	Stubbs creek is not managed as a game reserve
Bayelsa	Taylor Creek	Swamp forest	30,000	PA shared between Bayelsa & Rivers States. The larger part of Bayelsa state
Delta	Kwale	Degraded	340	Cassava farming and cattle trespass
Edo	Gele-Gele	Degraded	36,200	Retained as Forest Reserve. Lack of political will to transform FR to Game Reserve.
	Ohosu	Degraded	47,100	Never transformed to Game Reserve
	Okomu	Moist forest	11,200	Most of the original Okomu forest reserve has been sold to tree crop plantation developers. The core of the reserve has been preserved as a National Park and ceded to the Federal Government of Nigeria
	Ologbo	Degraded	19,440	Converted to an oil palm plantation
	Orle River	Converted	5,440	De-reserved
Imo	NA	-	-	No game reserve
Rivers	Taylor Greek	Swamp forest	30,000	Split between Bayelsa and Rivers states. No management regime in place

Table 7.

Existing and proposed game reserves/wildlife sanctuaries in the zone.

Source: Federal Department of Forestry, 1998.

A Preliminary Assessment of the Context for Reducing Emissions from Deforestation and Forest Degradation (REDD) in Nigeria commissioned by the Federal Ministry of Environment, the Cross River State’s Forestry Commission, and UNDP Strict Nature Reserve (SNR) remarked that:

These former forest reserves have been specifically designated for conservation, management, and propagation of wild animals in addition to the protection and management of the critical habitats on which they depend. In practice, almost all the game reserves in the country (except for Yankari, which was formerly a National Park) are highly degraded due to poor management; they best be described as “paper” game reserves.

6. Surface water and sediment quality in mangrove ecosystem

The Escravos Beach within the Niger Delta Mangrove was used as a case study [39]. The mean values of all physicochemical parameters investigated during the wet and dry seasons are summarized in Table 8. The water bodies sampled in the location had slightly alkaline pH in the wet season and slightly acidic in the dry season, high conductivity and dissolved solids, slightly high turbidity and color, and moderate dissolved oxygen and Biochemical Oxygen Demand (BOD) level. The water was brackish with higher salinity values in the dry than in the wet season. The cations were dominated by sodium and potassium, followed by calcium, truly reflecting the brackish nature of the water body. The nutrient level was higher in the wet than the dry season, probably due to the influx of allochthonous organic materials and ions into the water body. The parameters were all within the normal range stipulated by the Department of Petroleum Resources (DPR) (now called Upstream Petroleum Regulatory Commission, NUPRC) and Federal Ministry of Environment (FMEnv), except in a few cases such as turbidity, color, and total dissolved solids (TDS), where the limits were exceeded. The turbidity and color were expected to be higher in the wet season due to the input of organic materials from the terrestrial environment. The high total dissolved solids followed the electrical conductivity pattern, which was greatly influenced by the high salt content. The hydrocarbon pollution indicators, like the total petroleum hydrocarbons (TPH), polyaromatic hydrocarbons (PAH), BTEX (benzene, toluene, ethylbenzene, and xylene), and phenols, were all very low in concentrations and fell within the recommended limits by DPR and FMEnv. The low concentrations of heavy metals indicate no serious pollution from petroleum and other anthropogenic activities at the study location.

Parameter	Unit	Dry season			Wet season			DPR, FMEnv limits
		Min	Max	Mean	Min	Max	Mean
Physical
Temp	0C	29.60	34.50	32.871	21.2	29.6	27.629	25,35
Color	Pt. Co	15.30	20.90	17.757	25.1	50	36.914	7
Turbidity	NTU	7.20	11.20	8.900	10.3	36	20.300	10,-
TSS	mg/L	8.00	15.00	11.000	10	36	19.286	30,30
TDS		11872.0	14687.0	13233.6	171	3780	1564.00	2000,-
pH		7.31	7.80	7.653	5.6	7.4	6.643	6.5–8.5,
EC	μS/cm	22400.0	27710.0	25174.3	362	7580	3135.3
Salinity	‰	10.13	12.53	11.386	0.2	3.4	1.429
DO		4.3	4.90	4.586	3.6	4.7	4.171	-,5
BOD5		2.8	4.40	3.471	1.7	3.9	2.500	10,10
COD		61.6	75.20	67.543	55.2	528	134.743	10,40
HCO3		171	281.00	222.429	73.2	201.3	108.057
Na		334.4	467.90	438.043	10.9	221.6	100.457	20,-
K		371.6	551.10	491.529	3.3	98.1	60.286
Ca		122	188.00	153.143	58	946	523.429
Mg		87	134.00	117.143	23	722	319.571	75,-
Hardness		213	320.00	270.286	81	2960	1285.429
Cl-		6990	8320.00	7587.143	63	198	129.429	600,200
PO4		0.46	2.37	1.520	5.3	9.2	6.800	5,-
NH4N		2.4	3.69	3.074	6.1	7.4	6.771	0.2,0.2
NO2-N		1.07	3.60	2.159	1.2	10.4	5.357
NO3-N		2.17	4.17	3.039	2.5	19	9.929	20,-
SO4		336.1	377.2	361.0	50.5	253	123.7	200,-
H2S		0	0.00		0	0		1,20
THC		0.22	0.49	0.300	0.06	0.36	0.189
TOC	%	0.59	1.20	0.804	0.39	1.19	0.860
TPH	mg/l	0.079	0.15	0.112	0.059	0.137	0.094
PAH	μg/l	0.02	0.10	0.047	0.017	0.089	0.037
Benzene		0.002	0.06	0.013	0	0.013	0.002
Toluene		0.001	0.00	0.002	0	0	0.000
Ethylbenzene		0.001	0.01	0.003	0	0.002	0.001
Xylene		0.001	0.00	0.002	0.001	0.003	0.002
Phenol		0.004	0.02	0.009	0.002	0.012	0.006
Heavy Metals
Fe	mg/L	0.061	0.25	0.123	0.02	0.17	0.066
Cu		0.051	0.16	0.112	0.04	0.14	0.107
Mn		0.016	0.10	0.059	0.01	0.09	0.056
Zn		0.012	0.19	0.066	0.01	0.05	0.026
Cd		0.002	0.00	0.003	0.01	0.05	0.021
Cr		0.044	91.00	15.225	0.04	0.09	0.073
Pb		0.026	0.05	0.039	0.02	0.08	0.049
Ni		0.03	0.05	0.044	0.01	0.06	0.026
V		0.011	0.04	0.027	0.01	0.02	0.012
Hg		0	0.00		0	0
Ba		0	0.00		0	0
As		0	0.00		0	0
CN		0	0.00		0	0

Table 8.

Summary of physicochemical characteristics of surface waters in the Escravos Beach study area [39].

Microorganisms are responsible for forming various sediment and mineral deposits and dominating secondary waste treatment. Microorganisms such as bacteria and fungi act as living catalysts enabling numerous chemical processes in water and soil. Most significant chemical reactions in water, especially those involving organic matter and oxidation-reduction processes, occur through bacterial intermediaries.

The bacterial and fungal counts of the surface water samples from the study area were low. The total heterotrophic bacteria counts ranged between 0.7 × 10³ cfu/ml and 2.4 × 10³ cfu/ml. The total heterotrophic fungal counts ranged from nil to 1.0 × 10³ cfu/ml. The hydrocarbon-utilizing bacterial and fungal counts were absent for most surface water samples.

The summary of sediment quality parameters is presented in Table 9. The sediment was moderately acidic (<7.0). The nutrient concentrations were higher in the sediment than in the surface water. The earth metals (cations) were low in concentration, with calcium dominating in the wet season. The heavy metals were low in concentration in all the stations. The higher sediment concentration is expected since the bottom sediment typically serves as a reservoir for heavy metals in the water. The recorded values are all below the intervention values. Concentrations above the intervention values corresponded to severe contamination. The target values indicated the soil quality levels ultimately aimed for [39].

Parameter	Unit	Dry season			Wet season
		Min	Max	Mean	Min	Max	Mean
pH		3.43	5.37	4.57	3.3	6.9	5.029
EC	μS/cm	2190	7650	3952.86	1070	1960	1660.00
Organic carbon	%	0.47	2.57	1.81	1.4	3.05	1.921
Total nitrogen	%	0.22	1.23	0.86	0.12	0.28	0.174
TOC	%	1.37	7.63	5.36	4.2	9.06	5.714
Na	meq/100 g	5.84	8.47	7.23	0.86	1.26	1.023
K		3.98	5.01	4.50	0.13	0.3	0.191
Ca		2.11	3.21	2.59	1.6	4.64	2.377
Mg		2.44	3.95	3.14	1.1	2.72	1.556
CEC		14.85	19.3	17.32	3.69	8.76	5.147
EA		1.6	3.1	2.31	0.9	1.6	1.214
CL-	mg/kg	48.5	132	82.39	13.6	20.2	17.357
PO4		12.2	17.5	14.20	16.2	23.7	19.557
NH4N		15.6	28.5	22.80	11	21.1	16.243
NO2-N		7.8	12.4	9.87	7.9	14.3	10.500
NO3-N		9.9	14.2	11.93	11.6	20.7	16.043
SO4		45.4	92.1	60.67	15.2	27.6	22.443
Silt	%	3.3	6.4	4.07	2.5	3.2	2.971
Clay		3.3	6.3	5.37	6.8	9.4	8.043
Sand		90.2	91.2	90.56	87.6	90.2	88.986
THC	mg/kg	15.8	66.9	45.14	23	48.8	39.200
TPH		0.1	0.357	0.19	0.1	0.357	0.214
PAH		0.015	0.079	0.04	0.179	0.343	0.265
Benzene	μg/kg	0	0.007	0.00	0	0.007	0.003
Toluene		0	0	0.00	0	0.001	0.000
Ethylbenzene		0	0	0.00	0	0	0.000
Xylene		0	0.007	0.00	0.002	0.008	0.005
Phenol		<0.001	<0.001
Heavy Metals
Fe	mg/kg	40	74	53.71	230	554	381.143
Cu		0.6	22.4	21.00	14.6	15.7	15.100
Mn		0.16	0.24	0.20	28.6	34.6	31.214
Zn		3.6	23	14.73	40.1	55.9	48.829
Cd		0.04	0.07	0.06	1.2	2.2	1.757
Cr		0.6	2.06	0.97	1.2	3.6	1.957
Pb		1.86	3.82	3.32	1.1	5.9	2.829
Ni		0.21	0.75	0.39	1	2.5	1.657
V		0.144	0.51	0.27	0.6	1.8	1.143
Hg		ND	ND		0	0
Ba		ND	ND		0.003	0.015	0.009
As		ND	ND		0.001	0.004	0.002

Table 9.

Comparison of the characteristics of sediment samples from the study area during the dry and wet seasons [39].

There was no adverse effect of anthropogenic activities on the water and sediment of the study area.

The microbial load in the sediment was low for both the heterotrophic and the hydrocarbon-utilizing bacteria and fungi, indicating no serious hydrocarbon contamination.

The mean values of physical and chemical parameters for surface water and sediment are true reflections of the typical mangrove quality unless there is gross contamination from oil exploration and production activities by oil companies.

7. Conclusion

The coastal areas of West Africa contain some of the world’s richest ecosystems, including extensive mangrove forests, lagoons, and estuaries that support wide biodiversity with significant economic benefits to the coastal populations. Wetlands are important buffers for adjacent marine ecosystems, trapping sediments, nutrients, and many anthropogenic chemical contaminants. Mangrove ecosystems, for instance, serve as sinks for heavy metals because mangrove sediments’ physical and chemical properties allow them to sequester large quantities of metals. There is wide variability in the ability of mangrove plants to absorb heavy metals because the sediment conditions vary widely. Different species of mangrove trees have different sediment-trapping root systems adapted to combating the anoxic conditions in the peat, including the pores or lenticels on the prop roots of Rhizophora spp. or on the pneumatophores (peg roots) of Avecinnia germinans.

Ineffective planning and management of coastal zone, the absence of environmental impact assessment, inadequate incorporation of environmental issues into feasibility projects, population pressures, as well as economic expansion are threatening the integrity of natural ecosystems. The main types of human activities that damage coastal ecosystems are (1) overexploitation, (2) physical alterations and habitat loss, (3) pollution, (4) introduction of alien species, and (5) global climatic change. In many coastal areas of Africa, vast areas of coastal wetlands and beaches have been cleared to make way for coastal development, industrialization, aquaculture, and agriculture. It destroys rich mangrove and estuarine fisheries and many commercially valuable species that depend on these coastal habitats as nursery grounds. Continuous examination of the relationship between man, the coastal environment, and its resources (human impacts) is clearly needed to avoid the risk of depletion and damage.

The rich ecosystem services of the Niger Delta mangrove, which is currently threatened by anthropogenic influence, require urgent protection and conservation attention. There is an urgent need for policymakers, development planners, and other stakeholders to arouse public consciousness for the restoration, rehabilitation, and conservation of mangrove ecosystems in Nigeria to achieve sustainable utilization.

Acknowledgments

This contribution contains information gathered from our Ph.D. research and environmental consultancy reports carried out for some oil and gas industries operating in the Niger Delta region of Nigeria. This is in addition to the cited literature. The authors wish to particularly acknowledge MacGill Engineering and Technical Services Limited, an environmental consultancy outfit that funded the Environmental Impact Assessment studies for South Forcados Development Project and the Escravos Beach Project carried out for SPDC.