Climate Change and Anthropogenic Impacts on the Ecosystem of the Transgressive Mud Coastal Region of Bight of Benin, Nigeria

Abstract

The transgressive mud coastal area of Bight of Benin is a muddy coastal complex that lies east of the Barrier/lagoon coast and stretches to the Benin River in the northwestern flank of the Niger Delta Nigeria. It constitutes a fragile buffer zone between the tranquil waters of the swamps and the menacing waves of the Atlantic Ocean. Extensive breaching of this narrow coastal plain results in massive incursion of the sea into the inland swamps with serious implications for national security and the economy. Climate change impacts from the results of meteorological information of the regions shows a gradual degradation in the past 30 years. Temperature, rainfall and humidity increase annually depict climate change, resulting from uncontrolled exploitation of natural resources is rapidly pushing the region towards ecological disasters. The ecosystem is very unique being the only transgressive mud coastal area of the Gulf of Guinea. The chapter describes the geomorphology, tidal hydrology, relief/drainage, topography, climate/meteorology, vegetation, economic characteristics, anthropogenic activities and their impacts on the ecosystem.

1. Introduction

The transgressive mud coastal zone is a fragile ecosystem, full of vast resources of minerals, foods, and energy but not without scenes of often conflicting uses. Presently, the uncontrolled and overexploitation of natural resources, vis a vis the climate change impacts pose a great threat to the economic prosperity and thereby resulting in environmental nightmares portending a lot of dangers for the generations to come [1]. This environment has witnessed a lot of degradation resulting from several years of oil exploitations and explorations. People’s livelihood and their social well-being have often been affected adversely resulting from changes in the environment. Meanwhile, government over the years has dashed the hope of the inhabitants for protection. Bursting the petroleum pipeline by the people in an attempt to fight back, thereby disrupting the activities of the oil companies ended up compounding the challenges in their environments which eventually leads to oil pollution. Fishing is the major occupation of the predominant group of people (the Ilajes) in the coastal zone. Their settling pattern just like the Ijaws along the coast is in such a way that their houses were constructed with wood and were suspended on water.

This fragile ecosystem poses a delicate balance with the inhabitants in the area. The rate of environmental degradation in this region, as a result of anthropogenic pollution and climate change, is rapidly pushing the region towards ecological disasters. Uncontrolled reclamation of lands has been the last hope of the people due to lack of land to build settlements. Rivers, which are the people’s only means of transportation, have been a nightmare, as a result of flood continual modifications of the rivers, thereby rendering them useless. Economically, human activities have been crippled. More also, shortage of land for development, flooding, siltation, occlusion and other environmental problems are associated with the hydrology and natural terrain of the area. Mangrove swamp forest (vegetation) reduces the impacts of floods, exacerbated by land subsidence, coastal erosion, and rising sea level. Also, the diurnal tidal movement modifies the floods which continuously impairs the river courses with significant impacts on the economy and human life patterns [2].

The transgressive Mud inter-tidal zone typically has a slope of 1:50, while beach elevation averages 3 m above the mean low water level. The coastline lies between Ajumo and the Benin river-estuary on the northwestern flank of the Niger delta [3]. The transgressive mud coastline runs from the northwest to southeast in the Ondo State coastline. About 90% of Nigeria’s foreign exchange is derived from crude oil and gas, which is one of the major natural resources in this region. Other natural resources domiciled in this region include: fisheries, touristic resources, mangroves and forest.

2. Coastline geomorphology of the region

The Nigerian coastline is separated into four physiographic zones, as described by Fabiyi [4]. The sections include: the Strand Coast, the Niger Delta Coast, the Barrier-Lagoon Coast and the Transgressive Mud Coast. Each of the sections is associated with differing erosive activities, resulting from anthropogenic and natural factors as explained thus;

2.1 The Barrier-Lagoon coast

This complex is located in the Lagos State axis of Nigeria’s coastline. It is predominantly made up of coarse sand beach, which allows easy drains of excess flood water and allows it to percolate. This easily flows back into the sea in few days.

2.2 The Niger Delta coast

The Niger Delta coast is predominated by fine beach sand and Mangrove forest. It is characterized by intense flooding around the communities and in some elevated areas, the vegetation is rain-fed deltaic. This is found along Delta, Akwa Ibom and Rivers States of Nigeria coastline.

2.3 The strand coast

It is the most eastern section of Nigeria coastline with a lot of vegetation (mostly Nypa Palms’- Nypa fruticans) which holds the fragile beach and acts as a barrier to ocean flooding. The beach is fronted by flat beaches and changes into a beach ridge plain. It is located at the Cross River axis of Nigeria coastline.

2.4 The transgressive mud beach or Mahin mud coast

This is located at the Ondo State axis of Nigeria coastline. It is composed of vegetated bluff, mud and flat marsh forming a transgressive mud beach. The area is characterized by a lot of floods trapped in the mud creeks, remaining there for days. Making it vulnerable to coastal flooding, thereby paralyzing activities in the affected communities. During the summer, it is left with mud cracks, formed from dried saturated mud (Figure 1).

Figure 1.

Mud beach (A) and Mud crack, formed from dried saturated mud (B).

Ondo State coastal plain (Transgressive mud coast) is narrow (about 700 m wide), unlike other coastal plains that have extensive mangrove swamps, lagoons and raised beaches. Freshwater swamp, lacustrine marshes and an intricate network of interconnected creeks backed the coastline by about 30–60 km wide freshwater marshes [5]. This, therefore, constitutes a fragile buffer zone between the Atlantic Ocean menacing wave and the tranquil freshwater swamp. Massive incursion of the sea into the inland swamps occurs as a result of extensive breaching into the coastal plain, leading to economic loss and threat to national security. One of the most important implications of massive incursion of the Atlantic Ocean is the coastline recession in Ayetoro and Awoye communities (Figure 2).

Figure 2.

Coastline recession in Ayetoro (A) and Awoye (B) communities resulting from massive incursion of the Atlantic Ocean.

3. Geology of the region

It is believed that the mud beach coast evolved from the growth of the Niger delta into the Gulf of Guinea following the gradual retreat of the sea after a short-lived Paleocene transgression [1]. The major geological formations in the area include general alluvium, lagoonal marshes, abandoned beach ridges and coastal plains sand. The general alluvium comprises coarse, clayey, unsorted sands with clay lenses and occasional pebble beds which are lithologically indistinguishable from typical coastal plains sand strata [6]. These formations produce generally swampy soils on the nearly level coastal plains sand on alluvium, and very deep, well-drained soil, with very dark brown to dark brown surface sands from the nearly level coastal plains on coastal plain sand [7]. Elevation rises from about 1 m along the coastline to between 35 m (Igbokoda town) and 55 m (Okitipupa town) in the upland [8].

4. Soil of the region

The base of the sedimentary fill in Nigeria coastal area consists of unfossiliferrous sandstones and gravel weathered from the underlying pre-Cambrian basement [9]. Above the course materials are marine shales, sandstones and limestones of Santonian age, whose deposition was ended in parts of the Nigerian basin by folding, faulting, and basic igneous intrusion during the Santonian age. The next cycle of deposition began with the transgression that lasted into the Maestrichtian. The present Niger delta was initiated during regression that began in the early Eocene [10]. The soils underlying the Niger delta are generally characterized as soft, highly compressible, organic and inorganic silty clays overlying fine sands at great depths.

5. Vegetation of the region

The freshwater swamp forest is found in the inland freshwater areas between high forests and the mangrove swamp forest. The vegetation consists initially of species of reed (Phyragnites spp) and Papyrus (Cyperus papyrus). Their roots are submerged while the shoots stand above water and the trees which later cover the reed swamps form swamp forests which are poorer species than the forest on dryland. Palm-like raffia (Raphia hookerii) are characteristically present, so also are species of cane. The trees show layering and are still rooted with dense undergrowth of shrubs, and lianas where canopy is exposed to light. The eutrophic water (water that is fairly rich in mineral matter) in the area encourages silt soil to be formed which results from the accumulation of predominantly inorganic sediments in which the water is near the surface. There are floating grasses and other creeping plants along the edges of the creeks and rivers which are invaded either by water lettuce (Pistia stratiotes) or water hyacinths (Eichhornia crassipes).

The Mangrove Swamp Forest is characterized by evergreen trees and shrubs. The plants cover the sheltered muddy areas where land is rapidly encroaching on the sea in the estuarine and deltaic environments. Mangroves play an active role in building upland from the sea, obstructing currents, thus adding humus and raising the ground level seawards. The mangrove trees are essentially halophytes (adapted to saline habitats) receiving low saline water from rivers and higher saline water from the sea at different times of the day and seasonally. Hence, the predominant species in the brackish water zone are the red mangroves Rhizopora racemosa while other species of the red mangroves are found in the freshwater swamps inland. At the eastern margin of Awoye estuary, the vegetation consists mainly of red mangroves (Rhizopora racemosa) with white mangrove (Avecinnia africana) lining the inner edges of the creeks and along the rivers. Towards the seaside, Paspalum veginatum which after replacing felled mangroves becomes more dominant, sometimes excluding other species on the inner sand banks, with dense undergrowth of scrambling shrubs and trees, such as Hibiscus tiliaceus, Chrsobalanus orticulatis, etc. At the back swamp are raffia palms growing in the marshy areas, freshwater creek and along the rivers. The raffia palm is predominant in the area, it is readily available for construction purposes.

However, with active forest removal going on in the northern axis and extensive areas of marsh and mangrove forests being decimated in the southern parts, a large stretch of land along the coastline is now permanently inundated, especially in Ayetoro and Awoye areas. Thousands of peasant populations in so many rural communities are at the risk of a shortfall in their food security and means of livelihood [11]. Research and literature on the extent of degradation on the transgressive mud coast are lacking. Coastal erosion, canalization, inundation, the emergence of coastal grassland and rapid sedimentation of lagoons are some of the degradation processes identified. The increase in offshore exploration activities in the 70s led to more pronouncement of these degradation activities [12].

6. Mangroves ecosystem of the region

The most globally significant ecosystem between tropical rainforest and marine is mangrove. It represents one of the most productive natural ecosystems from the biological perspective. It is home to some unique endangered animal species. Economically, mangrove woods are used for construction works, useful chemical extraction (wood tar, tannin, alcohol, etc.) and furniture making, High quality charcoal and firewoods which are a good source of fuel are also sourced from mangroves. They also serve as nursery areas for marine animals and fish breeding arena.

In the environment, mangroves protect against storms (natural barrier against coastal erosion and tsunami), toxic substances, roots screening out debris, CO₂ absorption thereby reducing greenhouse gas and deposition of suspended sediments in water, creating mudflat for more mangrove. From the sociocultural perspective, it serves as a good source of food and medications for communities. Also, they are good environmental indicators for our climate.

The mangrove ecosystem in Nigeria is unique, being part of the Gulf of Guinea’s large marine ecosystem, which is the largest in Africa and the third-largest in the world. It covers Lagos State, Ondo State and Niger Delta areas and occupies areas of about 10,000 km² in a 30–40 km wide belt [13]. Mangrove forests in Nigeria are found on the coast and stretch into the rivers and complex lagoons in several places. Spalding et al. [14] estimate the Nigerian mangrove to be about 10,500 km².

Plants that are usually associated with mangroves include buttonwood tree (Conocarpus), leather fern (Acrosticum aureum), hibiscus (H. tiliaceus), etc. Nypa fructicans from Singapore was introduced. Mangroves are harvested for consumption to meet the needs of the local communities [15] and are also being utilized as raw materials for industry [16]. Even though the demand has drastically declined, mangrove wood is still extensively used in Nigeria and the extraction of much higher volumes of wood is undertaken exclusively as selection harvesting under license agreements with the competent authorities [13].

Mangroves help protect the coastline from storm damage, wave action and erosion. They stabilize the elevation of land by accretion of sediment and also protect from damaging siltation seagrass beds and coral reefs. If the present level of mangroves depletion in Nigeria is not reduced, its coastal cities like Warri, Port Harcourt and Lagos will be drowned in the next couple of decades [17].

The area is generally inhospitable and difficult to develop. The area is inhabited mainly by fishermen/women and small farmers. The dense vegetation of mangrove forest found in this area has become a source of income generation, a reliable small-scale food processing and fuel wood for domestic usage [18].

7. Meteorology of the region

The meteorological station with long-time data record closest to the region is at Ondo town. Ondo town is about 85 km (north-south direction) from Ayetoro. Ondo town and the coastal communities fall under the same weather influence. Therefore the 30 years of meteorological data (1984–2014) used to characterize the area were based on Ondo town meteorological station and obtained through the central office of the Nigeria Meteorological Agency (NIMET) Lagos. A new meteorological station is now situated at Ayetoro, one of the communities but does not have long time data. In this station, air temperature and humidity, wind direction and rainfall are typically measured at 2 m above the surface on the muddy beach. Wind speeds are measured at 10 m height above the ground surface. Nigeria Meteorological Agency (NIMET) were contacted for information on the climatic data collected at Ondo from 1984 through 2014. The meteorological variables on which records were available were considered individually below;

7.1 Air temperature

The mean monthly temperature values at Ondo for the period of 30 years are presented in Table 1. The highest mean temperature was recorded in February (26.7 ± 0.5°C) while the lowest value was recorded in August (23.95 ± 0.4°C). With an annual mean air temperature of 26.86 ± 1.3°C, the year 2013 was the warmest year of the 30 years covered in the available record. The coldest year was 1991 with 25.2 ± 1.4°C annual mean value (Table 2). On the whole, a short mean air temperature range of 1.66°C was recorded.

Months	Rainfall (cm)	Relative humidity (%)	Mean Temp (oC)	Wind speed (m/s)	Wind class (Beaufort)
January	12.79 ± 20.1	68.16 ± 10	26.7 ± 0.5	1.09 ± 0.5	Light air
February	35.6 ± 35.3	70.6 ± 9.24	27.7 ± 1.1	1.45 ± 0.7	Light air
March	102.35 ± 53.2	78.58 ± 5.38	27.5 ± 0.8	1.9 ± 0.8	Light Breeze
April	165.15 ± 85.4	83.6 ± 1.5	27.1 ± 0.7	1.8 ± 0.7	Light breeze
May	180.49 ± 63.1	84.8 ± 4.7	26.5 ± 0.6	1.45 ± 0.6	Light air
June	229.88 ± 65.7	87.7 ± 2.3	25.45 ± 0.7	1.8 ± 1.0	Light breeze
July	257.5 ± 108.2	90.5 ± 1.7	24.29 ± 0.4	2.16 ± 0.9	Light breeze
August	183.6 ± 107.3	90.6 ± 2.7	23.95 ± 0.4	2.24 ± 0.9	Light breeze
September	280.1 ± 90.1	90.29 ± 1.2	24.6 ± 0.7	1.7 ± 0.7	Light breeze
October	179.1 ± 59.4	88.5 ± 1.9	25.38 ± 0.6	1.47 ± 0.9	Light air
November	54.8 ± 41.1	81.2 ± 3.5	26.8 ± 0.6	0.89 ± 0.5	Light air
December	10.4 ± 17.9	73.1 ± 6.6	26.8 ± 0.6	0.9 ± 0.7	Light air

Table 1.

Monthly meteorological data of the study area.

	Temperature (°C)	Relative humidity (%)	Wind speed (m/s)	Rain fall (cm)	Vapor pressure	Wind direction
Year	Annual mean	A. mean	A. mean	A. mean	A. mean	Dominant wind
1984	25.90 ± 1.63	79.25 ± 9.96	2.07 ± 0.62	123.17 ± 97.6	27.17 ± 2.17	SW
1985	25.78 ± 1.69	80.5 ± 10.72	2.22 ± 0.51	211.86 ± 171.4	27.26 ± 2.52	SW
1986	25.43 ± 1.4	83.08 ± 7.25	2.99 ± 0.9	130.55 ± 91.98	27.47 ± 2.69	SW
1987	26.38 ± 1.32	82.17 ± 6.58	2.18 ± 0.83	133.06 ± 118.2	28.77 ± 1.7	SW
1988	25.79 ± 1.39	82.92 ± 7.86	2.16 ± 1.02	141.22 ± 98.3	27.94 ± 2.36	SW
1989	25.69 ± 1.26	81.00 ± 13.79	1.63 ± 0.8	134.03 ± 107.3	26.65 ± 4.4	SW
1990	26.03 ± 1.7	83.00 ± 9.17	1.68 ± 0.67	113.12 ± 88.36	28.10 ± 1.57	SW
1991	25.20 ± 1.38	84.08 ± 6.93	1.56 ± 0.75	192.47 ± 161.96	27.98 ± 1.78	SW
1992	25.68 ± 1.68	80.25 ± 12.74	1.76 ± 0.62	126.68 ± 137.52	26.43 ± 3.72	SW
1993	25.78 ± 1.22	79.83 ± 10.95	1.73 ± 0.66	121.13 ± 98.7	26.64 ± 3.2	SW
1994	25.65 ± 1.15	81.08 ± 11.21	2.1 ± 0.87	150.00 ± 106.33	27.08 ± 2.94	SW
1995	25.98 ± 1.25	81.42 ± 10.04	1.92 ± 0.68	137.36 ± 123.9	27.67 ± 2.74	SW
1996	26.16 ± 1.26	84.17 ± 5.92	2.05 ± 0.77	146.69 ± 101.05	28.46 ± 1.12	SW
1997	26.01 ± 1.19	82.08 ± 11.04	1.49 ± 0.71	120.99 ± 94.4	27.56 ± 2.98	SW
1998	26.58 ± 1.6	79.58 ± 11.02	1.86 ± 1.1	125.75 ± 121	28.17 ± 2.9	SW
1999	25.89 ± 1.36	83.25 ± 7	1.88 ± 0.95	139.19 ± 112.3	28.05 ± 1.34	SW
2000	26.03 ± 1.56	80.42 ± 12.25	1.28 ± 0.48	129.61 ± 115.6	27.07 ± 3.22	SW
2001	25.98 ± 1.52	82.17 ± 9.25	0.88 ± 0.45	132.37 ± 117.8	27.85 ± 1.94	SW
2002	26.04 ± 1.36	80.83 ± 11.89	0.77 ± 0.39	139.89 ± 103	27.36 ± 3.22	SW
2003	26.14 ± 1.55	82.75 ± 6.72	0.96 ± 0.53	140.90 ± 157.4	28.42 ± 1.78	SW
2004	26.33 ± 1.52	82.08 ± 7.39	0.98 ± 0.78	134.68 ± 103.54	28.11 ± 1.49	SW
2005	26.3 ± 1.42	81.42 ± 9.98	1.17 ± 1.14	137.53 ± 121.13	28.32 ± 3.3	SW
2006	26.43 ± 1.35	83.25 ± 7.52	1.19 ± 1.64	131.15 ± 86.23	28.80 ± 1.97	SW
2007	26.03 ± 1.26	80.58 ± 12.12	1.20 ± 0.46	133.08 ± 109.9	27.48 ± 3.69	SW
2008	26.17 ± 1.17	80.08 ± 12.2	0.98 ± 0.3	160.91 ± 139.52	27.24 ± 3.8	SW
2009	26.28 ± 1.17	84.25 ± 5.91	1.07 ± 0.22	135.11 ± 113.08	29.25 ± 1.42	SW
2010	26.68 ± 1.46	84.33 ± 6.5	1.54 ± 0.31	156.08 ± 133.78	30.08 ± 1.25	W
2011	26.41 ± 1.26	83.00 ± 9.38	1.36 ± 0.27	141.38 ± 119.33	28.88 ± 2.84	SW
2012	26.18 ± 1.25	85.92 ± 6.04	1.46 ± 00.28	132.63 ± 88.17	29.28 ± 2.01	W
2013	26.86 ± 1.34	86.00 ± 6.54	1.98 ± 0.31	143.83 ± 89.81	30.20 ± 2.06	W
2014	26.54 ± 1.47	87.42 ± 5.74	1.53 ± 0.26	173.97 ± 129.3	30.38 ± 1.48	S

Table 2.

Annual meteorological data of the study area.

7.2 Relative humidity

The monthly percentage relative humidity (% RH) from 1984 through 2014 was presented in Table 1. Relative humidity followed the same pattern as temperature. The lowest relative humidity for the months was recorded in January (68.16 ± 10.0%) while the highest value was recorded in August (90.6 ± 2.7%). The lowest annual relative humidity was recorded in 1984 (79.25 ± 10.0%), while the highest annual mean value of 87.42 ± 5.74% was recorded in 2014 (Table 2).

The high humidity experienced in this region makes the air to be close to saturation and thus with less capacity to store additional water. This tends to reduce the rate of evaporation despite high temperature and high energy input in the region. For humid conditions, the wind can only replace saturated air with slightly less saturated air and remove heat energy [19]. It is noteworthy that the relative humidity in the area in 1984 (about 75%) has increased steadily over a period of 30 years to 90% in 2014.

7.3 Rainfall

The mean monthly rainfall from 1984 through 2014 is shown in Table 1. This area experiences a double maxima rainfall regime characterized by two high rainfall peaks. The rainy season begins around March (102.35 ± 53.3 cm) and attains a peak in June or July.

This first peak is followed by a short dry break in August (183.6 ± 107.3 cm), known as the August break usually lasting for about two to 3 weeks in August. This break is usually followed by the resumption of the rainy season and lasts to mid-October with a second peak usually in September. The period from late October through early March constitutes the dry season. The annual highest mean rainfall was recorded in 1985 (211.86 ± 171.39 cm) and the annual lowest mean was in 1990 (113.12 ± 88.36 cm) (Table 2). This coastal zone is characterized by high rainfall, with rainfall all the months of the year and annual variability of approximately 113–211 cm as indicated in the 30-year data record.

7.4 Wind speed over the region

The mean monthly wind speed, which is shown in Table 1, revealed a lower wind speed in the dry season (October to February) and which was classified as Light air with the range from 0.5 to 1.6 m/s (according to Beaufort wind speed classification). Also, the rainy season (March to September) experienced a higher wind speed (1.7–2.24 m/s) which was classified as Light breeze according to Beaufort wind speed classification. Table 1 showed a downward progression of wind speed from 1984 through 2002 and an eventual steady increase from 2002 through 2014. Light breeze (2.99 ± 0.9 m/s) was recorded as the highest annual wind speed in 1986 while 0.77 ± 0.39 m/s (Light air) was recorded in 2002 as the lowest annual wind speed.

7.5 Wind direction

It is observed that the predominant wind in the study area during the period of study was the southwestern trade wind which originate from the Atlantic Ocean. The southwestern trade wind was predominant for 26 years of the annual record while western and southern trade winds were predominant only in 3 years and a year respectively. Moderate wind was also predominant in the study area.

8. Tidal hydrology

The daily tide data in Escravos for one annual cycle (2004) comprising about 1400 specific tide measurements (four readings per day for 1 year) were used to characterize the tidal hydrology of the area. The separation of tide phases into neap and spring tides was with regards to the moon phases.

Highlights of the tidal hydrology of the area based on tide measurements at Escravos bar (the closest tide station to the area which is about 10 km east of Awoye is presented in Table 3. The highest (or extreme) high water individual spring tide (EHWS) recorded over the one-year period was 1.77 m, and it was recorded 17th of April of the study year, a day after the full moon i.e. during the spring tide when the sun was passing the equator.

Tide description	Abbreviation n		Value ± s.d.
Extreme high water of spring tides	EHWS	1	1.77 m
Mean high water of spring tides	MHWS	370	1.50 ± 0.12 m
Average high tide level	AHTL	705	1.44 ± 0.15 m
Mean high water level of neap tides	MHWLN	335	1.37 ± 0.15 m
Mid tide level	MTL	1409	0.97 ± 0.49 m
Mean low water level of neap tides	MLWN	359	0.57 ± 0.16 m
Average low water level	ALWL	704	0.51 ± 0.18 m
Mean low water level of spring tide	MLWS	345	0.44 ± 0.17 m
Extreme low water level of spring tide	ELWS	1	0.14 m

Table 3.

Summary of the Tidal Hydrology of the region.

Mean high water of springs (MHWS) and mean low water of springs (MLWS) were 1.50 ± 0.12 m and 0.44 ± 0.17 m respectively, thus giving a mean range of spring tides values of 1.06 m (which is about 90% of the observed extreme range of springs tide). On the other hand, the mean high-water level of neap tides (MHWN) and low water level of neap tides (MLWN) were 1.37 ± 0.15 m and 0.57 ± 0.16 m, respectively. Giving a mean range of neap tides values of 0.8 m, which is about 55% of the observed extreme range of spring tides and about 80% of mean range of spring tides. Average high tide level (AHTL) and average low tide level (ALTL) were 1.44 ± 0.15 m and 0.51 ± 0.18 m respectively, while mid-tide level (MTL) for all recorded tides (n = 1409) was 0.97 ± 0.49 m (Table 3 and Figure 3). The mean range of spring tides could be divided broadly into three commonly observed shore zones: the upper shore zone (the portion above the mean range of neap tides), the middle shore zone (corresponding to the mean range of neap tides), and the lower shore zone (corresponding to the portion below the mean range of neap tides). Two harmonic tide waves influence the tidal variation along the coastline of the area, one with a period of 12.5 hours and the other with a period of 25 hours [20]. The combination of these two harmonic tide waves usually produces two low tides and two high tides each day. The twice-daily (semidiurnal) tide of 12.5 hours predominates over the daily (diurnal) tide of 25 hours, generating a diurnal inequality, or mixed semidiurnal tides. This causes a difference in height between successive high and low waters. The result is two high waters and two low waters each day [21]. The tidal characteristics of the area have many features in common with those of the typical Atlantic coast with the domination of two unequal high water and two low waters occurring within 24 hours [22]. The mean neap range of 0.49 m recorded in the area is comparable with the British coasts amounting to between 0.45 and 0.55 m of the mean spring range [22]. This small neap range is typical of locations where tidal ranges are large. However, the overall mean range of about 1.1 m (MHWS-MLWS) in this area is lower than 1.7 m for Bonny bar and 2.0 m for Calabar in Nigeria.

Figure 3.

Variations of the Tidal Hydrology of study region.

9. Relief and drainage

The area falls within the Atlantic system where most of the rivers are short, north-south flowing coastal rivers which follow more or less regular courses [23] and which drain into the sea. Rivers are being divided in a fairly simple line by western plains and ridges. The two major rivers, River Oluwa and River Ominla, display a drainage pattern which is dendritic but each river is parallel to the other and having a different tributaries. In the coastal plain area, river valley gradient is very low as it discharges its loads, leading to braided channel formation. Awoye and Abereke estuaries are the two major estuaries in the region. Abereke estuary located in the northern part of the area receives drainage from the Oluwa River and other major surrounding creeks. On the other hand, Awoye estuary located in the southernmost part of the area receives drainage from Ominla River, surrounding creeks and Benin River from Delta State.

10. Economic characteristics

The economy of the region is centred mainly on fishing, lumbering, farming, palm wine tapping, mat-weaving and petty trading. Fishing is the economic mainstay of the people in the region. The economic activities and the resultant occupation of the people have traditionally been related to the natural environment. This provides an opportunity for people in the riverine to be involved primarily in fishing, while people in the upland areas combine both farming and fishing with lumbering and carving of boats among other activities. A considerable number of people engage in traditional craft and modern processing industries. About 80% of the people in the area engage in fishing and that creates employment and generates a substantial income of about 90% of local GDP. Fishing activities in the area are carried out in two ways—freshwater fishing and ocean fishing.

11. Anthropogenic activities

11.1 Agriculture

Agriculture is the main source of livelihood for the people before the advent of crude-oil exploitation. Crude-oil exploitation results in conflicts and competition in the use of natural resources, which eventually leads to degradation of the environment. This affects the natural resources/livelihood (agriculture) of the host communities. Agriculture becomes less important in the polluted areas as it could not generate a reliable source of income for the communities. Fishing, Animal husbandry and Crop production are the major agricultural activities in the area. Agricultural wastes are discharged directly into the aquatic system [2].

11.2 Solid waste and sewage

The Ilajes (a dominant tribe in this region) are very enterprising people and one of the most dynamic in Nigeria. They are very good in aquatic skills and are able to adapt by conquering a harsh geographical environment, turning it to their advantage. Consequently, they were able to build communities like Ugbonla, Ayetoro, Zion Pepe, Awoye, Abereke, Araromi, Atijere, Ebute Ipare, Idiogba, Igbobi, Igbokoda, Igbolomi, Mahin, Mahintedo, Odun, Jirinwo, Odofado, Ago Nati, Ogogoro, Oloja, Ugbo, etc. The environment is dissected by several networks of river systems, which specifically make the coastal areas unfavorable for the development of road infrastructure that could serve as an engine for economic developments in the area. Many camps in the coastal area lack access to the hinterland except through hand-paddled canoes and some motorized boats. Some of these camps are directly on the Atlantic, especially where the tidal wave is relatively gentle. Therefore, all the communities in the area discharge their waste directly into the coast and the creeks.

11.3 Oil exploration

Oil exploration and exploitation started around 1977 by the then Gulf oil Company, presently called Chevron. This has metamorphosed into so many oil servicing and oil companies (such as; Express Petroleum and Gas Company/Conoco Energy Nigeria limited, Chevron-Texaco Nigeria limited, Global Pipeline, Consolidated Oil and Allied Energy, Agip Oil Nigeria Limited, Shell Petroleum Development Companies, etc) spreading vast installations and exploitation activities across the region [24]. The area is credited with about 14 oil fields, it contributes 12% of the country’s crude oil production and reserves, with about 3.5 billion barrels of crude oil reserves.

The exploration, exploitation, and transportation of oil and gas in this region bring a serious problem with little or no economic development to the host communities by contributing a lot of pollutants to the ocean and the coastal zone. Some of these pollutants include spilling of hydrocarbons directly on the ocean and also those oil leakages from corroded pipelines, valves, production water effluents and ballast water discharges. Toxic chemicals from drilling fluids containing vessels and heavy metals (Vanadium, Plead, and Nickel) and other pollutants are being introduced from oil-field operations. All these pollutants are known to affect life forms [2].

12. Impacts of anthropogenic activities

12.1 Coastline recession

The major cause of coastline recession in this region is the wave attack of the clay ridge sediment, because of the absence of longshore current and weak nearshore littoral Guinea current. Tides and waves energy concentrated on the bare surface of the clay ridge, exposing it to direct wave impact because of the absence of the protective force of mangroves. A network of rills is developed on the plain due to wave backwash as it overruns the plain when the tide is high. Subsequent backwash and uprush of currents progressively widened and deepened the rills forming vertical heads U-shaped gullies. The walls of the gullies are terraced and are at the sub-tidal platforms level, becoming wave penetration avenue as the gully rapidly advancing into the coastal plain. The rate of gully head retreats measured the ranged from 5.7 m to 15.8 m annually [17].

Ground surface lowering and coastline recession result from the accumulation of ocean water in the depressions across the coastal plain. The energy uprush and backwash currents of the tides and waves affect the flood pools by increasing the soil water content. This relaxes the coherence of the unconsolidated soil, weakening the soil physicochemical interparticle bonds, thereby exposing the flood pool coastal plain to sheet wash erosion [5]. The coastal plains were eventually destroyed as the depressions gradually widened and deepened until the adjacent ones become incorporated and coalesced into an expanded subtidal platform.

Ebisemiju [17] reported coastline recession in the region (specifically Awoye) by about 3.31 km between 1974 and 1996 with annual rates varying between 31 m to 19 m in 1981. About 487 hectares of the coastline have been claimed into the Atlantic Ocean as a result of coastline recession within a short period of two decades (1973–1991) [17]. This has astronomically led to the loss of about 3000 hectares by 1996, reducing about 62% of the coastal plain and leading to the loss of arable land for animal husbandry and land for settlement in this narrow coastal plain. Presently, about 35 m of arable land is being loss into the Atlantic Ocean annually from this narrow coastal plain of the region.

12.2 Coastal erosion

Large-scale destruction of mangroves, canals and buildings are some of the direct impacts of erosion disasters in the region. These are evident in the ironwood stakes of abandoned houses found within the nearshore zones and intertidal platforms, indicating previous settlement.

A lot of inhabitants have been forced to abandon their houses and migrated inland to a safe location due to tidal floods and accelerated coastline recession. This makes the people relocate their communities at least once every 4 years. Today, the Awoye community (one of the settlements in the region) was originally about 3 km on the Atlantic Ocean from the present shoreline. Permanent structures could not be erected in the region because of the constant need to dismantle their houses and relocated them to a safer area due to the continuous threat of coastal erosion and tidal floods. Massive breaching of the coastline in one of the communities in the region (Ayetoro) may undoubtedly lead to a catastrophic event that will wipe out the entire community.

12.3 Seismic investigation

Seismic investigations (which involve explosive charges detonation below the ocean floor) were believed to have induced local subsidence. Ayetoro community claimed that it was Seismic investigations conducted in the area by an oil prospecting company, that generated shock waves which caused extensive damage to the major structures in the town, particularly the King’s concrete palace. The claim was a memorandum sent by the community in 1981 to the NNPC (Nigeria National Petroleum Cooperation) but was refuted by the corporation. It is of note that explosion forces like that could result in the consolidation of subsurface sediment and liquefaction of surficial sediment. It could also lead to an increase in water depth and erodibility of sediment. Such activities could induce potential disturbance of the nearshore bottom leading to necessary conditions that promote coastal erosion. Increased water depth within the nearshore zone would enable larger waves to penetrate further inland than otherwise experienced. This would result in coastal erosion and the inundation of inland forests by seawater.

12.4 Oil spillage

Oil spillage has detrimental effects on both plants and animals. It is reported that oil spillage has caused constant threat to farmlands, crop plants, forest tree species and other vegetations in oil-producing areas in Nigeria [25]. There have been over 4000 oil spills in the Niger-Delta area of Nigeria since 1960. Toxicity of crude oil depends on its physical and chemical composition, the amount of the oil, the plant species and time of application as well as other environmental conditions [26].

Liver damages, infertility, disabilities, blindness, damages to fur and feather of birds and accidental poisoning are some of the direct effects of oil spillage in our ecosystem. It causes alterations in soil microbiological and physiochemical properties and affects soil fertility adversely, thereby having detrimental effects on the aquatic and terrestrial ecosystems.

13. Conclusion

The transgressive mud coastal ecosystem is full of vast resources of minerals, foods, and energy which has witnessed a lot of degradations as a result of several years of anthropogenic pollution (oil exploitations and explorations) and climate change (coastal recession). Meteorological information of the regions shows a gradual degradation in the past 30 years. Temperature, rainfall and humidity increase annually depict climate change, resulting from uncontrolled exploitation of natural resources is rapidly pushing the region towards ecological disasters.

Acknowledgments

The author is grateful to Prof. I. F. Adeniyi, Dr. A. O. Ajibare and Dr. L. T. Ogundele for the help rendered during the sampling period, laboratory analysis, statistical analysis and review of this paper.