Carbon cycle in the Ivory Coast lagoons

 

Lagoons are difficult to define and there are no generally accepted criteria which unambiguously separate lagoons from bays, estuaries, marshes, and other parts of coastal landscape, but can comprise up to 13% of the world's coastline (Nixon, 1982). They are highly dynamic and productive shallow ecosystems, with intense net primary production rates and among the highest for near-shore coastal environments (Conde et al., 1999). A relative large fraction of this high primary production is deposited at the sediment surface. Moreover, lagoons trap allochtonous organic material from the adjacent sea, from the surrounding wetlands, and upstream terrestrial environments (Martens, 1982; Manini et al., 2003). These processes lead to benthic respiration that exceeds planktonic respiration (Boynton et al., 1996).

The convergence of multiple economic interests in lagoons and their surrounding wetlands, such as agriculture, fisheries, tourism, ports, and industrial activities make these ecosystems vulnerable and affect their productivity. These activities, in addition to barriers between lagoons and the open ocean, damming of rivers that drain into lagoons, exploitation of the basin of the rivers entering lagoons (deforestation, agriculture, …) modify sediment, organic matter and nutrient inputs into these systems. As result of such modifications, eutrophication is common in lagoons, where air-water exchanges of CO2 are expected to be intense. Dissolved inorganic carbon (DIC) dynamics in lagoons has received little attention compared to other coastal ecosystems. Moreover, data of air-water fluxes CO2 are scarce in coastal environments at subtropical and tropical latitudes, that receive about 60% of the global freshwater discharge and equivalent fraction of organic carbon inputs (Ludwig et al. 1996).

In the present project, we investigate DIC and ancillary data from two lagoons in Ivory Coast (Ebrié and Aby). They differ considerably in the density of riparian human populations (3.5 inhabitants km-2 and above 100 inhabitants km-2 around Aby and Ebrié lagoons, respectively), hence, the degree of human pressure (eutrophication) is very different in both systems.

Ebrié lagoon with a surface area of 566 km2, a length of 125 and a maximum width of 7 km, is the largest lagoon in West Africa. It receives freshwater from the Comoé, Agneby and La Mé rivers (Scheren et al., 2004). This lagoon is connected to the ocean through the Vridi channel close to Abidjan and periodically through the Grand Bassam inlet in the eastern part. This lagoon is highly eutrophicated due mainly to untreated waste loads (Torréton et al., 1989; Scheren et al., 2004). In 2000, annual nitrogen loads to the lagoon were estimated at 33 kt (kilotons), of which 45% from urban sources, 42% from land runoff and 13% from atmospheric deposition; annual phosphorous loads were estimated at 2.5 kt, of which 39% from urban sources, 48% from land runoff and 13% from atmospheric deposition (Scheren et al., 2004). Mangrove forests surrounding this lagoon have been largely destroyed and only remain in the western part.

Aby lagoon forms the natural border between Ivory Coast and Ghana and is the second largest lagoon in Ivory Coast. It extends over 30 km of the coastline, has a surface area of 424 km2 and is less impacted by human activities than Ebrié lagoon. The principal activity exerted in this lagoon is the fishing. Freshwater inputs are from the Bia and Toumanguié rivers in the northwest and the Tanoé river in the East. In its southern part, the lagoon is connected to the ocean through several channels (e.g. main Aby channel, Ehotilé Island channel), and surrounded by mangrove forests. The two main species mangroves along the Ivorian coast are Rizophora racemosa and Avicennia Africana.

This research is funded by the Agence Universitaire de la Francophonie (AUF), Projet de coopération scientifique inter-universitaire (6313PS657), a collaboration between the the University of Liège (Chemical Oceanography Unit), the Universiy of Bordeaux 1 (Environnements et Paléo environnements Océaniques) and the University of Abobo-Adjamé (Laboratoire d'Environnement et de Biologie Aquatique).

References

Boynton W. R., J. D. Hagy, L. Murray, C. Stokes and W. M. Kemp (1996) A comparative analysis of eutrophication patterns in a temperate coastal lagoon. Estuaries, 19, 408-421.
Conde D., S. Bonilla, L. Aubriot, R. de Léon and W. Pintos (1999) Comparison of the areal amount of chlorophyll a of planktonic and attached microalgae in a shallow coastal lagoon. Hydrobiologia, 408/409, 285-291.
Ludwig, W., J. L. Probst, and S. Kempe. 1996. Predicting the oceanic input of organic carbon by continental erosion. Global Biogeochemical Cycles 10(1):23-41.
Manini E., C. Fiordelmondo, C. Gambi, A. Pusceddu and R. Danovaro (2003) Benthic microbial loop functioning in coastal lagoons: a comparative approach. Oceanologica Acta, 26, 27-38.
Martens C. S.(1982) Biogeochemistry of organic-rich coastal lagoon sediments. Oceanologica Acta, 161-167.
Nixon S. W. (1982) Nutrients dynamics, primary production and fisheries yields of lagoons. Oceanologica Acta, 357-371.
Scheren P. A. G. M., C. Kroeze, F. J. J. Janssen, L. Hordijk and K. J. Ptasinski (2004) Integrated water pollution assessment of the Ebrié Lagoon, Ivory Coast, West Africa. Journal of Marine Systems, 44, 1-17.
Torréton J-P, D. Guiral and R Arfi (1989) Bacterioplankton biomass and production during destratification in a monomictic eutrophic bay of a tropical lagoon. Marine Ecology Progress Series, 57, 53-67.

Data-set of dissolved inorganic carbon and ancillary data in the Ebrié and Aby lagoons, and respective rivers (password protected)