Spatial Changes in the Wetlands of Lagos / Lekki Lagoons of Lagos , Nigeria

Lagos metropolis, the current economic capital of Nigeria is a low-lying coastal city endowed with a number of lagoons and wetland ecological assets. Lagos/Lekki Lagoons being the largest with a combined size of 646km are fringed on many sides by wetlands. Many of these wetlands have undergone severe spatial changes from rapid urbanization in the past three decades. The precise nature of these changes is largely unknown and unreported. As the area is experiencing intense development pressure, this study therefore examined the spatial changes in the wetlands fringing these lagoons using the integrated approach of remote sensing data and GIS with topographic maps providing baseline data. The objective is to quantify and establish the precise location and magnitude of these changes over the years from 1984 to 2006. Two types of wetlands are prevalent in the Lagos area namely: the swamps and mangroves. ENVI software was used along with parallelepiped supervised classification in processing the Landsat images. Results show that the mangrove wetlands decreased from 88.51km to 19.95km at -3.12km annually while swamps decreased from 344.75km to 165.37km at 8.15km annually both between 1984 and 2006. Results further show that mangroves which were widespread in seven council areas around these lagoons in 1984, have dwindled to only four councils in 2006. These decreases are attributable to urban development pressures. Some of the implications of these losses and conservation issues are briefly highlighted.


Background
As part of natural ecosystem, wetlands where they occur in the landscape are valued for their contribution to ecological balance and biodiversity.Also, they are valued for the numerous goods and functions delivered freely to the ecosystem and human habitats which include flood storage and distribution, retention of sediments and nutrients, aquifer recharge, water quality improvement, aesthetic and educational benefits among others (Kindscher et al., 1998;USEPA, 2009).Unrestrained degradation of wetlands and ecosystem will inevitably lead to a loss or diminution of some or all of these functions.
Wetlands are land areas covered with water or where water is present at or near the soil surface all year or varying periods of the year.These areas support the prevalence of hydrophytes or aquatic plants that are typically adapted to life in water saturated (hydric) conditions (USEPA, 2009).Wetlands include a variety of habitats such as marshes, peat lands, flood plains, rivers and lakes, coastal salt marshes, mangroves and sea grass beds, coral reefs and other marine areas no deeper than six meters (6m) at low tide.They also include human-made wetlands such as waste-water treatment ponds and reservoirs (Ramsar Convention Secretariat. 2007a).Urban populations and wetlands are said to have been engaged in a turbulent, somewhat symbiotic marriage since the dawn of civilization.Being essential for human well -being, wetlands have been progressively lost and degraded from human activities since then.The rate of their loss is known to be greater than for any other type of ecosystem (UN -Habitat, 2010).
Metropolitan Lagos, the current economic capital of Nigeria and some of its suburbs have developed on a coastal environment characterized by low-lying tidal flats, estuaries, wetlands and sandy barrier beaches, some of which were reclaimed haphazardly for development (Abegunde, 1988).Spurred by demand for land for rapid urbanization, Abegunde (1988) further suggested that this unplanned and extensive reclamation of wetlands, sand filling of lagoon shores, excessive dredging, encroachment on natural drainage channels and unrestrained deforestation have all been significant features of metro Lagos.Adeniyi (1980) had earlier observed that over 87% of vacant wetlands in metro Lagos between 1962 and 1974 had been converted to high density unplanned residential housing.This uncontrolled urban expansion in an unsystematic manner has had serious repercussions on the environmental quality of many parts of the metropolis.Brody et al. (2007) submitted that rising population density in coastal area is usually associated with greater amounts of impervious surfaces, alteration of watersheds, coupled with diminished capacity of these systems to naturally hold surface runoff.The Lagos/Lekki Lagoons are the largest of the coastal barrier -lagoon complex which extends 200km eastward from the Nigeria/Benin Republic border (Ibe, 1988).With a combined size of 646km 2 , they are fringed on many sides by wetlands.As the area has been and is still experiencing intense development pressure, many of these wetlands have undergone severe spatial changes from rapid urbanization in the last three decades.But like in many coastal cities of the world, the precise impacts of these human activities on coastal wetlands are poorly understood (James et al., 2007).In this case, the precise nature of these changes is largely unknown and unreported.Although Odunuga & Oyebande (2007), Taiwo & Areola (2009) provided recent useful insights into wetland conversion in parts of the Lagos coastal area, a comprehensive study which assessed quantitatively the spatial changes in the wetlands of Lagos/Lekki Lagoons and their consequences is yet unreported in the literature.Besides, any conceptualization of comprehensive wetland monitoring and conservation in the area ought to be preceded by the inventory and establishment of their changes, what remains and their locations.These were the impetus for this study.
This study therefore assessed the spatial changes in the wetlands fringing these lagoons.The objective is to quantify and establish the precise locations, magnitude and trend of these changes over the years from 1984 to 2006.In doing so, the study sought to answer these questions namely: (i) what is the extent of wetlands fringing these lagoons at the base year and how have these changed in recent times?(ii) what are the implications of these changes?

Urbanization & the Need to Assess Wetland Loss
Urbanization which is the conversion of land to uses associated with growing population and economy has been recognized as having a world -wide trend.More than 50% of the world's population currently resides in cities and urban settlements with this shift to urban living expected to continue at rates of almost 1.6% per annum worldwide.The highest urbanization rates were expected in developing and least developed countries (UN-Habitat, 2010) while 95% of the net increase in global population would be in cites of the developing world (Zhang et al., 2008) such as Lagos.As part of this trend, the coastal zones are known to be home to nearly 75% of the global population (Asangwe, 2006).
Lagos city has grown from a settlement of about 3.85km 2 in 1881 to a huge metropolis of over 1,183km 2 in 2004 (Okude & Ademiluyi, 2006).In consequence, the population which was negligible in those years had risen from about 5.7million in 1991 to about 9.1million in 2006(National Population Commission, 2006) with average population density of 20,000 persons/km 2 (Presidential Committee on the Redevelopment of Lagos mega-city Region, 2006).The nature of population growth in some of the local councils around the two lagoons between 1991 and 2006 is reflection of the urbanization pressure in the study area (Table 1).For instance, while Ikorodu council area which is rapidly developing had a population density of 7,207 persons/km 2 in 2006, Etiosa LGA had 3,423/km 2 , while densely developed Lagos Mainland, Shomolu and Lagos Island LGA's had densities of 28,154; 39,053 and 39,661persons/km 2 respectively in 2006 (Mehrotra et al., 2009).The obvious consequence of this population expansion on natural resources in a coastal terrain with about 44% of the study area being water bodies and wetlands in 1984 (Figure 1) is development on marginal lands within or fringing urbanized areas.
Rapid urbanization and urban areas are known to generate negative impacts on the environment (UN-Habitat, 2010) as they lead to changes in landscape patterns, ecosystem functions and their capacity to perform functions in support of human populations.This is especially so when rapid or unplanned growth occur in areas of highly vulnerable systems such as wetlands.The conversion of large tracts of wetlands into built -up area results in increased impervious surfaces which can lead to flooding and altered aquifer recharge (Odunuga & Oyebande, 2007).Quantifying such changes in the landscape patterns can be useful in tracking the capacity of natural ecosystems to render services in support of human systems (Flores, Olivas & Chavez, 2008).It is also necessary as inputs to environmental resource planning, management and sustainable development.
Fortunately, the development of remote sensing along with GIS provides a rich data source and a powerful analytical tool towards understanding the structure, function and dynamics of landscapes on the earth's surface.
The integration of remote sensing and GIS has made possible the systematic inventory and assessment of land resource and land degradation over space and time for intervention strategies to be instituted to safeguard the health of ecosystem (Fasona, Omojola & Onyeahialam, 2007).Also integrating temporal satellite data, GIS and historic maps provide effective monitoring tools for land usage as they provide firm portrayal of growth patterns and how development results in profound changes to the landscape (Olaleye, Abiodun & Igbokwe, 2009).Remote sensing have been widely used to observe and record the earth's land and water surfaces through the means of reflected or emitted electromagnetic energy (Jensen, 2007;Campbell, 1996).Recent advances in sensor design and data analysis are making remote sensing very practical and attractive for monitoring natural and anthropogenic wetland changes (Klemas, 2011) To inventory, monitor and model spatial locations and changes in earth's resources require a dynamic tool beyond the capabilities of the traditional paper maps and overlays.Geographic Information System (GIS) which has its roots in this traditional map and cartographic process offers that tool.It is a computer system with appropriate software for capturing, storing, querying, manipulating, analyzing and displaying geospatial or geographically referenced multi-thematic data in raster or vector formats from many sources (Chang, 2006).Geospatial or geographically referenced data are data which describe locations (where is) and characteristics (attributes) of spatial features on the earth's surface.GIS is therefore a combination of spatial and attribute data.
Wetlands as critical and sensitive land cover have variously been studied and assessed with remote sensing data.Because of repetitive coverage, satellite remote sensing has provided a tool for inventory, mapping and assessment of wetlands over large geographical areas.It has become appropriate for inventorying and monitoring wetlands in developing countries where funds are limited and scanty information is available on wetland areas and their losses over time (Ozesmi & Bauer, 2002).Almost every type of wetland has been studied with satellite imagery.But only few of these studies have been reported in a developing country like Nigeria.In these studies, multi-temporal imageries often aided classification of wetlands as well as their separation from other land cover classes.Included in the types of wetlands studied with remote sensing are marshes, swamps, lagoons, coastal tidal marshes, mangroves and other coastal wetlands, bogs and fens, inland freshwater marshes, forested wetlands or swamps, open water areas, wet meadows and submerged aquatic vegetation.Some of the recent studies to identify or monitor wetlands and their changes with remote sensing and GIS, involve the assessment of the extent and changes in the mangrove ecosystem of Niger Delta (James et al., 2007); the detection of change in the lower Ogun River flood Plain (Odunuga & Oyebande, 2007); monitoring of land degradation along Ondo Coastal Zone of Nigeria (Abbas, 2008); the monitoring of wetlands in the semi-arid west, USA (Neale et al., 2007); the mapping of Canada's wetland with optical, radar and DEM data (Li & Chen, 2005); the inventory, monitoring of temporary and permanent wetlands of Western Cape, South Africa (De Roeck et al., 2008) and the spatial -temporal analysis of wetland losses in the Lagos Coastal region (Taiwo & Areola, 2009)  by Ogun State.On the south, it is bounded by Bight of Benin/Atlantic Ocean.Both Lagos and Lekki Lagoons have a combined size of 646km 2 .The only western outlet for the two lagoons is through Commodore Channel which links Lagos Lagoon to Bight of Benin/Atlantic Ocean.With direct connection to the sea, salinity is generally higher in Lagos Lagoon and the waters are brackish while the Lekki Lagoon is of fresh water (Ibe, 1988;Oyebande et al., 2004).Two arms of Lekki Lagoon connect the Niger Delta in the east.Two major wetland regimes identified in Oyebande et al. (2004) namely the Lagos/Lekki Lagoon wetlands and the Ogun River wetlands are in the area.The climate of the area is controlled alternately by both the wet tropical maritime air mass (from the Atlantic Ocean) and dry tropical continental air mass from Sahara Desert.Of the 20 Local Government Areas (LGA's) in the state, 10 are coterminous with these lagoons and their western outlet.These include part of Amuwo -Odofin, Apapa, Epe, Etiosa, Ibeju/Lekki, Ikorodu, Kosofe, Lagos Island, Mainland and Shomolu LGA's.

Data and Image Processing
Due to lack of digital data, several hardcopy 1:25,000 topographic maps were scanned, geo-referenced and digitized for baseline data.The digitized maps were then edge -matched to produce a seamless mosaic.This vectorised mosaic was thoroughly cross-checked for errors and then input into ArcGIS for editing and area calculations.Two scenes of Landsat TM (Thematic Mapper) of 18 th December, 1984 and ETM + (Enhanced Thematic Mapper) of December 2006 were processed to extract the wetlands, mangroves and water bodies across the area of study.Bands 3, 4 and 5, were used for colour composite in order to identify the wetlands, mangroves and water bodies distinctly.The image classification technique used was the supervised classification.The supervised classification was chosen over the unsupervised classification because of the ease at which the former identifies the wetlands and/or water bodies and also because this simplifies the editing stage.In supervised classification, pixels are clustered in a dataset into classes corresponding to user-defined training samples.The parallelepiped classification technique which uses a simple decision rule to classify multispectral data was employed.The wetlands were identified as having a greenish spectral signature, while the mangroves had the dark green colour as their trademark.With field and ancillary data, these were then uniquely assigned as regions of interest or training samples.Using ENVI (Environment for Visualizing Images) software, a parallelepiped supervised classification was then carried out on the trained classes and then used to extract the wetlands from the entire multispectral imageries of the two dates independently.After the classification exercise, the output showed only the wetlands and a few other features picked as a result of similar spectral reflectance.A post classification auto vectorization of the output was executed.The vectorized wetlands were then exported to Arcmap as shapefiles for editing, change detection and calculation of areas.

Results
Results show that mangroves decreased by about 77% from 88.51km 2 to 19.92km 2 at 3.12km 2 /yr deficit while swamps decreased by 52% from 344.74km 2 to 165.37km 2 at a loss of 8.15km 2 /yr both between 1984 and 2006 (Table 2).Results further show that the mangroves which were widespread in 7 (seven) LGA's in 1984 have dwindled to only 4 LGA's in 2006 with about 4km 2 and above.Epe, Etiosa, Ikorodu and Kosofe in that order were the councils with the largest area of mangroves in 1984 of 24.76; 22.65; 19.43 and 17.94 km 2 respectively.Similarly, swamps which had sizeable presence in five (5) LGA's in 1984 had whittled to four councils with about half of their sizes converted in 2006.The relative loss of wetlands (mangrove and swamps) and gains by succeeding land cover mainly bare land and built -up areas are displayed in Figure 1.conservation, it is conceivable that the LCC/NCF may be willing to expand their coverage area if funded and encouraged to do so.Being private sector driven, they may also provide a platform for comprehensive management of the remaining wetlands if provided the enabling environment.
An area for further enquiry arising from this study involves the modelling of the contribution of wetland loss to flooding in the metropolis.Sequel to the completion of the bathymetric mapping of the Lagos Lagoon, it is imperative to inquire into the water storage capacity of these lagoons between the rainy and dry seasons along with bottom accretion rate.These information seem to be lacking in published works.

Figure
Figure 3. C

.
Table1.Population figures for some LGA's around the lagoons

Local government area 1991 Population Census 2006 Population Census
Source: National Population Commission ofNigeria, 2006.

Table 2 .
Area -wide wetland changes from 1984Area -wide wetland changes from   -2006