The Tale of a Disappearing Lagoon in Ghana: A GIS -Based Habitat Mapping and Ecological Assessment.
Updated: Apr 19, 2020
Since its introduction in the 1970’s, there has been an increase in recognition of geospatial techniques for several purposes such as mapping and management of natural resources. In 2017 ZGRS leveraged geospatial techniques for habitat mapping and ecological assessment of Fosu Lagoon. Lagoons with their associated marsh vegetation are very important components of coastal ecosystems. Generally, they support coastal livelihood, serve as habitat for aquatic plants and animals, carbon sequestration and flood protection unit, and sometimes religious centers. Despite the numerous ecosystem services they provide, a significant number of coastal lagoons are being lost through anthropogenic activities such as encroachment, pollution and the spread of invasive species and weeds.
Lagoon with associated marsh / mangrove vegetation
It has become necessary to demonstrate how geospatial technology can be used to take inventory and assess the current status of Fosu lagoon in Ghana which is considered to be one of the lagoons under serious threat. A painstaking review of literature revealed that a major conservation challenge is the lack of inventory on Fosu Lagoon. While few qualitative studies have been done on the status of fishes in the Fosu Lagoon and the rate of pollution, none has been tailored to identifying and taking inventory of the current extent and the vegetation surrounding. We cannot assess or conserve what we have not measured, the lack of ecological inventory ( baseline data) can therefore pose a challenge to conservation efforts and hinder further research or analysis.
ZGRS embarked on this project to mainly
Map the lagoon’s habitat and identify extent of change over the years
Classify the vegetation surrounding the lagoon
Create a Geo-database for Fosu Lagoon
Th lagoon is located in Cape Coast , the southern part of Ghana with a surface area of 97.3 acres and an average water depth of 1.6 metres. It is bounded by the sea to the south and surrounded by various anthropogenic activities. Currently, there are five fish species found in the lagoon and they include the Sarotherodon melanotheron, Heterobranchus bidorsalis, Pellonula leonensis, Liza falcipinnis, and Clarias gariepinus. Click here to view the location of Fosu Lagoon
Reconnaissance Survey : The team embarked on reconnaissance survey to gather relevant information and perform all necessary rite ( pouring libation to the ancestors) prior to the commencement of this project. The Fosu Lagoon is accorded with a religious importance. During the cape coast Fetu Afehye festival, a vigil is observed at Fosu Lagoon near its shrine on every last Monday of August. As part of traditional requirement we performed various rite before we were allowed to embark on this project.
Field Interviews and Visualization : Due to the difficulty we faced in getting a reliable aerial photo showing the extent of the lagoon in 1970, we relied on field interview and visualization approach which is also known as sketch map approach to acquire historic data to describe the condition ( initial extent and boundary) of the lake in the 1970s. A structured historic data condition form was deployed for this purpose.
Sketch Map Result 1970
Acquisition of High Resolution Aerial Photo: In order to capture existing situation on ground as at 2017, we relied on high resolution drone aerial imagery and also imagery from the Town and Country Planning Department. The imagery were further geo-referenced using Ground Control Points ( GCPs) to improve their accuracy.
A ZGRS team member picking GCPs
The use of satellite imagery would have appeared to be the satisfactory option to survey and map a large area, but satellite imagery are mostly not suitable for distinguishing the full range of small fragmented vegetation. Hence a high resolution imagery from a flying sensor was preferable.
Aerial imagery of Fosu Lagoon obtain from flying sensors
This is the process of categorizing all the features in an image or raw remotely sensed data to obtain a given set of labels or land cover themes. In order to accurately classify the 2017 imagery , the object-based classification technique was used. This method offers a promising and a better option for classifying fragmented vegetation composition, water resources and invasive species than the pixel based method. Among the many software that can perform object-based classiﬁcation, the eCognition was used because of its commercial maturity.
Object - Based Classification of Fosu Lagoon in eCognition
The aerial photo was classified into seven classes, namely, standing water, mangrove vegetation, semi-natural mangrove, coastal salt marsh, semi-natural salt marsh, and introduced shrub and weeds. The handbook for phase 1 habitat survey prepared by Joint Nature Conservation Committee served as a guide for defining the classes
Classification Result 2017
Accuracy Assessment: In order to ensure that the accuracy of the classiﬁcation was acceptable, a ﬁeld veriﬁcation of the classiﬁcation was done before accuracy assessment. In this approach, random GPS co- ordinates were picked on the ﬁeld and its corresponding classiﬁcation using RTK GNSS receiver. A total of 20 coordinate points were picked. The coordinates points were retrieved from the RTK GNSS machine and using the compute confusion matrix tool in ArcGIS software, the overall accuracy and Kappa coeﬃcient was determined. The classification was 90% accurate with a Kappa coefficient of 0.8251.
Geo-database of Fosu Lagoon:
A Geo-database of Fosu Lagoon has been created to host all the spatial and attributes data used during project. Additionally, the classification maps and descriptive photos are also available in the database. The purpose of creating this Geo-database was to leverage web - GIS utilities to create a platform where the public can access the project output as baseline data for educational, research and conservation purposes.
There has been a drastic decline in the size of the lagoon’s habitat: The rate of decline of the different landcover is presented below. The mangrove vegetation which is very useful for carbon sequestration has reduced 81% from 1970 to 2017. The lagoon is gradually disappearing. The rapid urbanization of the Cape Coast Metropolis and weak land use regulation is a major cause. Particularly, the siting of important land uses such as the hospital and stadium in the catchment of the Fosu lagoon’s habitat has caused the area to develop rapidly and also raised the interest of land acquisition. High demand for land has triggered urban dwellers to acquire environmental sensitive areas
There has also been an increased level of plastic waste pollution. As a matter of fact, the metropolitan drain that enters the lagoon has no trap net to stop plastics from entering the lagoon. The communities surrounding the lagoon are also engaged in open refuse disposal. Consequently, if not all, almost all their refuse are washed away by runoff water into the lagoon when it rains
In the light of the aforementioned problems we identified, ZGRS embarked on several conservation actions such as
Cleaning up Exercise : You can click here to watch videos
Educational campaign : We engaged students ( youth) in this project to enable them appreciate environmental issues of critical concern.
ZGRS was able to organized meetings with people in communities surrounding the lagoon. The purposed was to educate them on the ecological importance of the lagoons.
Acknowledgments: We acknowledge the Oguaa ( Cape Coast) Traditional Council for the permission they granted to work on the site and their great contribution to the project. We also acknowledge the Rufford Foundation for funding the project activity with grant number 21775-1