Aquatic Ecosystem Health and Trophic Status Classification of Lake Ossa and the Potential Impacts on its Population of African Manatees (Trichechus senegalensis)
Lake Ossa is known as a refuge for the African manatees in Cameroon and a source of living for about 300 fishermen.1 Lake Ossa is a protected area, however, more than half of its watershed landscape is used predominantly by an agro-industry and marginally by local artisanal agriculture. Yet, there is little information on lake health and water quality as reflected by its chemical and biological characteristics and their impact on the African manatee. The present study aims at investigating the past and current trophic status of Lake Ossa and evaluating its potential impact on African manatee health. Water biotic and abiotic parameters including Secchi depth (Sd), total nitrogen (TN), phosphorous (TP) and chlorophyll (TChl) concentrations were measured monthly during four months at each of 18-water sampling stations evenly distributed across the lake. These parameters were then compared with historical values obtained from the literature to examine the dynamic trophic state of Lake Ossa based on the Carlson (1977) trophic state classifications.2 Results indicate that Lake Ossa trophic states parameters doubled in only three decades (from 1985 to 2016), moving from a mesotrophic (averaged values: TN=157 µg l−1, TP=12.9 µg l−1, TChl=8.4 µg l−1, Sd=1.17 m) to a eutrophic state (averaged values: TN=348.9 µg l−1, TP=28.15 µg l−1, TChl=19.8 µg l−1, Sd=1.17 m). This demonstrates that the lake is undergoing rapid eutrophication that is already evidenced by the drastic proliferation of invasive floating vegetative species, such as the water moss (Salvinia sp).3 The proliferation of this species not only negatively impacts local fisheries but also gradually outcompetes the antelope grass (Echinochloa pyramidalis), a major food plant for the African manatee in the lake. The decreasing nutrient gradient (TP and TN) moving from the mouth of the lake (in the south) to the north indicates that the flow of the adjacent Sanaga River (with a drainage basin close to 140,000 Km2) provides most of the nutrients for the lake.4 Further analysis suggests that the poor transparency of the lake is not associated with chlorophyll concentrations but rather with the suspended sediments brought-in by the Sanaga River flow. Consequently, our model and empirical data demonstrated that despite the nutrient enrichment, less than 5% of the lake bottom surface sustained submerged aquatic vegetation; resulting in shoreline emergent vegetation being the only food option for the local manatee population. During the dry season, water recedes drastically and disconnects from the dominant shoreline emergent vegetation, making it hardly accessible to the manatees.1 The current study unveiled major environmental concerns (nutrient and sediments enrichment) that may negatively impact manatee habitats. We recommend the establishment of efficient land use and water management strategies and their strong implementation across the entire Sanaga River watershed.
We thank the Florida LAKEWATCH program for providing for water samples collection tools and the analysis of the water samples. We also thank BioBase for processing the sonar data to generate the bathymetry and the SAV data. ESRI and Conservation GIS provided us with the latest ArcGIS software and training to run the geospatial analysis of this study. Thanks to AMMCO (African Marine Mammal Conservation Organization) and its staff who provided the fieldwork facility for this study. Special thanks to Fulbright Scholarship Program, Wildlife Conservation Society, Wildlife Conservation Network, and the Aquatic Animal Health program of the University of Florida for sponsoring this research.
1. Takoukam A.K. 2012. Activity center, habitat use and conservation of the West African manatee (Trichechus senegalensis Link, 1795) in the Douala-Edea and Lake Ossa wildlife reserves. Master’s Thesis, University of Dschang, Cameroon.
2. Carlson, R. E. 1977. A trophic state index for lakes1. Limnology and Oceanography, 22(2), 361–369.
3. Wirrmann, D. (1992). Le lac Ossa: une monographie préliminaire. Revue de Géographie du Cameroon, Vol. x1, n.1
4. Van der Waarde, J. 2007. Integrated River Basin Management of the Sanaga River, Cameroon. Benefits and challenges of decentralised water management. Unpublished, UNESCO Institute of Hydraulic Engineering, Delft, The Netherlands. https://www.internationalrivers.org/sites/default/files/attached-files/irbm_sanaga.pdf [Accessed 29 November 2018].