2 Institut de Recherche pour le Développement; UR CYROCO, Marseille. 3 CNRS-GDR 2476 ; Ecosystèmes Lagunaires UMR 5119, Montpellier. This study ...
Modelling the Senegal River Estuary ecosystem M. Baklouti1 , R. Ar 2 , C. Chevalier2 , M. Bouvy2 , M. Pagano2 , M. Troussellier3 Aix-Marseille Université; CNRS ; LOPB-UMR 6535, Laboratoire d'Océanographie Physique et Biogéochimique, OSU/Centre d'Océanologie de Marseille 1
2
Institut de Recherche pour le Développement; UR CYROCO, Marseille
3
CNRS-GDR 2476 ; Ecosystèmes Lagunaires UMR 5119, Montpellier
This study belongs to the DelSen (Delta du euve Sénegal) project and has then been partially nanced by the french national Ecosphère Continentale (ECCO) program
Context Due to its geographic situation in the coastal Sahelian zone, the Senegal River (SR) plays an essential role, crossing four countries in West-Africa, among which two have very limited water resources. It has a local and regional strategic importance for large populations through its roles in agriculture, sheries and drinking water supply. As a response to a severe drought in the 70' [L'hôte et al., 2002, Mahé and Olivry, 1999], several equipments have been built in the last twenty- ve years on the SR. Diama salt-wedge dam (1986), Manantali hydroelectric dam (1988), river dykies, irrigated areas, have considerably transformed what was recently an unmanned system in a largely regulated one. In addition to these anthropogenic developments, a major change occurred in October 2003, when a breach has been made in the sandy bar separating the river and the ocean (Langue de Barbarie) in order to prevent the inundation of Saint-Louis City (SLC) (Figure 1). This temporary solution turned out to be irreversible since the shallow river mouth (located 30 km south to the breach) progressively lled with sediments. Finally, the last change is the continuous increase of the Saint-Louis population (more than 171000 inhabitants in 2007); the urban sewage is untreated, and a large part of this raw material reaches directly or indirectly the SRE [Troussellier et al., 2004]. Until the increase of human pressure in the late 80' [Gac and Kane, 1986, Cecchi, 1993], the estuarine ecosystem was oligotrophic to mesotrophic and picophytoplancton dominated the phytoplankton. Although phytoplankton biomasses were low, microzooplankton and micronecton showed high abundances suggesting that the microbial loop was likely to play an important role. This situation continued until the hydroelectric plant was put into service (in 2003), regularly draining freshwater from Manantali to the SRE. Now, the system shifted to a more mesotrophic state, and phytoplankton communities are dominated by nanoplankton. Therefore, the structure as well as the function of the SRE ecosystem has been widely changed [Troussellier et al., 2005, Bouvy et al., 2006] ; Pagano et al., submitted). This change is probably still in progress as suggested by our eld works performed in 2002 (before the main changes linked to the shortening of the SRE) and 2006. 1
Atlantic Ocean
MAURITANIA
NOUAKCHOTT
Diama SAINT LOUIS
Senegal River DAKAR
SENEGAL Bakoy Manantali
BAMAKO
Falémé MALI
Bafing 100 km
GUINEA
MAURITANIA
Atlantic Ocean
Diama dam
Tiallakt
Djeuss
16˚10 N
Senegal River Tiong
Lampsar Dakar Bango
Saint Louis
Ngalam 16˚00
Hydrobase new river mouth
Langue de Barbarie
SENEGAL
15˚50
old river mouth 5 km 16˚35 W
16˚25
Figure 1: Cartography of the SRE
Objectives In this context of evolving conditions, it is important both for scientists and public authority to gain access to a tool allowing the test of predictable scenarios. Therefore, possible consequences of various changes (low vs high oods / displacement south of the river mouth vs perpetuation of its present location / low vs high upwelling intensity / low vs high increase in the demographic rate in Saint Louis) could be anticipated. The present work can be considered as a rst step to achieve this goal in the frame of the DelSen project which aimed at studying the response of phytoplankton communities to natural and anthropogenic forcing in the SRE. This project combines modelling and eld measurements. The present component of this joint study is dedicated to the numerical modelling of the biogeochemical functioning of this ecosystem.
Description The 3D hydrodynamical model MOBEEHDYCS coupled with the Eco3M tool [Baklouti et al., 2006b] has been used to simulate the SRE hydrodynamical and hydrological and biogeochemical functioning from Diama dam up to the Ocean. The biogeochemical model describes the main biogeochemical processes and trophic relationships for the plankton elements of the pelagic food web, from bacteria to microzooplankton. The formulations relative to phytoplankton are similar to those used in Baklouti et al. [2006a]; they are based as far as possible on mechanistic representations. Bacterial and Heterotrophic NanoFlagellates (HNF) compartments have been added to this previous version of the model. The present study also aims at testing original formulations used for mineralization and zooplankton excretion; these new processes have been added in the numerical library of the Eco3M tool. Finally, and in addition to simulating the response of the microbial communities to light, tide and river ow cycles, we aim at studying how the dierent communities overcome the salinity 2
gradient encountered along the estuary. Speci c experiments have been performed during the eld campain in order to address this issue. Corresponding results have helped us in de ning the organisms for which marine or freshwater origins should be distinguished and allowed us to quantify the additional mortality due to the salinity increase.
Results and Publications Corresponding results and discussion should be later available in the following paper: Baklouti,M., R. Ar , C. Chevalier, M. Bouvy, M. Pagano, M. Troussellier, Modelling the Senegal River Estuary ecosystem with Eco3M, in prep.
References M. Baklouti, F. Diaz, C. Pinazo, V. Faure, and B. Queguiner. Investigation of mechanistic formulations depicting phytoplankton dynamics for models of marine pelagic ecosystems. Progress in Oceanography, 71(1):133, 2006a. M. Baklouti, V. Faure, L. Pawlowski, and A. Sciandra. Investigation and sensitivity analysis of a mechanistic phytoplankton model implemented in a new modular numerical tool ( co3 ) dedicated to biogeochemical modelling. Progress in Oceanography, 71(1):3458, 2006b.
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