Senegal River Estuary Research Articles

Calibration of HEC-RAS Model for One Dimensional Steady Flow Analysis—A Case of Senegal River Estuary Downstream Diama Dam

The Sahelian regions have experienced a drought that has made them vulnerable to hydro-climatic conditions. Strategies have been developed to reduce this vulnerability. The governments of Senegal, Mauritania, Mali and Guinea have created the Organization for the development of the Senegal River (OMVS in french) with the aim of realizing large hydraulic installations. This resulted in the construction of the Diama and Manantali dams in the Senegal River Basin. The first aims to stop the saline intrusion, the second to regulate the flow of the river, to allow the irrigation of agricultural perimeters, and to produce electrical energy. The impoundment of the Diama dam has modified the hydraulic behavior of the estuary. The purpose of this study is to carry out the hydraulic modeling of the estuary of Senegal river downstream of the Diama Dam in transient mode by the HEC-RAS software. Two geometric models were constructed on the basis of a digital terrain model (DTM) using the Arc-GIS and HEC GeoRAS soft wares after processing the collected topographic data. The first geometric model, of which the areas of Senegal river downstream Diama Dam have been represented by cross-section, is one-dimensional. The second one is also one dimensional; in this model, the area of the Senegal River estuary downstream Diama Dam is introduced as water storage zones. The components of these models are the stream sections, lateral links, and storage areas. The flood hydrograph downstream Diama Dam is introduced as conditions at the upstream limits of the models while the tidal is introduced as a downstream condition. After the stability and calibration, the results given by HEC-RAS simulations are the variations of the water levels, the temporal variations of the flow rates for each section, the maximum flow velocities and the propagation times of the flood waves. The analysis and comparisons of these results strongly suggest using HEC-RAS issues as a decision-making tool helping to manage floods during times of crisis.

Stocks of Mugil bananensis (Pellegrin, 1927) in the Estuary of Senegal River by Determining Yields Per Recruit and Biomass Per Recruit

Mullet occupy an important place in artisanal fishing in the region of Saint-Louis. They provide substantial income to the various actors in the sector. Overfishing would adversely affect the recovery capacity of the Mugilidea stock in the Senegal River Estuary. A biological collapse is then to be feared. This risk is all the more so since fishermen from Saint-Louis go as far as Mauritania, Guinea Bissau and Sierra Leone to search for mules that have reached sexual maturity for their gonads. Works on the assessment of the stock status of M. bananensis have not been done in Senegal, although it has been carried out in other countries. The objective of the study is to contribute to the assessment of banana mullet stocks in the Senegal River estuary by determining yields per recruit and biomass per recruit. Data were collected on total length, total weight and the sex. FiSAT II and ViT4 softwares enabled analytical models based on virtual populations’ analysis, yield per recruit (Y / R) and biomass per recruit (B / R). The results obtained showed overexploitation of M. bananensis in the estuary of the Senegal River. The stock of M. bananensis is overexploited in sea and the coefficients of fishing mortality and total mortality are remarkably high to the class mastses fish of III+ age, IV + and V + . In sum, the study allowed us to know the dynamics of exploitation of mules in the estuary of the Senegal River and to propose a management plan and management of banana mullet for a sustainable management of this species.

Clay Fraction Distribution in the Sediments of the Senegal River Estuary after the Diama (Senegal) Dam Construction

Clay fraction contents distribution in the Senegal River estuary after the building of the anti-salt Diama dam has shown that upstream sediments are very muddy. Clay fraction content is as high as 60 % of sediments in the middle of the channel with a pronounced downstream decreasing trend on the banks comparing to the contents in the middle of the channel. Towards the river mouth, the fine fraction tends to disappear leading to a shelly sand sediment type essentially. During low water period, clay fraction contents increase particularly at the vicinity of the dam reservoir. In the estuarine reach, downstream the dam, this increasing trend during the low water stage is particularly marked in the middle of the channel. The clay mineral assemblage shows that kaolinite and smectite are the main components, whereas interlayered illite-smectite and illite are minor constituents. The upstream-downstream profile shows a decrease in kaolinite content although it remains the dominant constituent. Floating clay minerals (smectite and illite-smectite) concentrate rather in the middle of the channel and on the right bank where the water depth is higher. According to seasonal variations, the distribution of these clay minerals often follows the concentration of the entire clay fraction; their contents increase during low water stage period. This hydrodynamics sorting of floating minerals, evident in the up-dam zone, tends to disappear in the eastern reach between Diama dam and the mouth. Clay minerals assemblage shows, through the time, a rather homogeneous composition which does not seem to be altered by the seasonal fluctuations.

Effects of the sandbar breaching on hydrobiological parameters and zooplankton communities in the Senegal River Estuary (West Africa)

This study describes the changes in hydrology, zooplankton communities and abundance in the Senegal River Estuary (SRE) before and after the breaching of the sandbar in October 2003. Samples were taken in 2003 at 3 stations located upstream (DI), in mid estuary (HY) and downstream (RM), and in 2005 at the same stations (RM becoming Old River Mouth: ORM), plus the new river mouth (NRM) resulting from the morphological evolution of the SRE.The study showed marked seasonal variations that affected the structure and distribution of zooplankton as well as major changes caused by the sandbar opening:–increased marine influence throughout the whole SRE,–changes in the horizontal gradients,–arrival of euryhaline species and increase in meroplankton, in particular decapod larvae,–transformation of the ORM area into a slackwater area with limited exchanges and the highest zooplankton numbers during high waters.

The salinity responses of tropical estuaries to changes in freshwater discharge, tidal mixing and geomorphology: case study of the man-affected Senegal River Estuary (West Africa)

Salinity in estuaries is influenced by a variety of processes including tidal advection and diffusion and river discharge. The effect of hydrodynamic features on salinity was studied in the Senegal River Estuary (SRE). This estuary is of strategic importance for large populations, but it has been greatly affected by human action (regulation of the freshwater inflow, change in the location of the river mouth ...), which has caused major changes in salinity and ecological functioning. To analyse the impacts of these changes and to determine the spatial and temporal variations in salinity in the SRE, we used a combination of three-dimensional modelling and field measurements. The overall salinity depends on freshwater inflow and tidal fluctuations. Salinity variation is mainly driven by ebb and flood near the mouth and by fortnightly cycles upstream. The enlargement of the mouth increases salinity, whereas its shifting location changes the location of the salinity front and creates a slack water zone downstream. Connection and disconnection of the tributaries also affects the salinity. The present study explains how the recent modifications have increased spatial variation and reduced seasonal differences of salinity and provides a tool for managing the water in the estuary.

A study of plankton dynamics under osmotic stress in the Senegal River Estuary, West Africa, using a 3D mechanistic model

Abstract This paper presents the outline of an estuary modeling study encompassing a study of the biogeochemical model on its own. It includes a preliminary validation of the new mechanistic features using mesocosm data, and a study of a coupled physical and biogeochemical model of the Senegal River Estuary (SRE) from flood to low-water conditions. The originality of this work is that it distinguishes the marine and freshwater origins of some of the organisms modeled and includes additional mortality terms owing to the osmotic stress experienced by the freshwater organisms. The mechanistic model implemented in the Eco3M modelling tool, and partially designed for this study, succeeded in representing the dynamics of the planktonic food web in two mesocosm experiments and in the SRE under very different conditions using a single set of model parameters. Among the important results, it reveals that distinguishing between marine and freshwater origins provides substantial additional information on the distribution of organisms along the estuary. Finally, failure to take account of the osmotic stress and the origin of the organisms can give rise to substantial errors in the calculation of the biomass in the SRE.

Large Dams and Uncertainties: The Case of the Senegal River (West Africa)

This article examines the environmental and socioeconomic consequences of hydraulic infrastructural changes along the Senegal River valley and estuary. During the 1980s, two dams were built along the valley floor to facilitate hydroelectrical production and regular water supply for crop irrigation. In 2003, a breach was dug across Barbary Spit to alleviate flooding in the nearby Saint-Louis city. Although these structures helped regulate the river flow, they also resulted in a series of unforeseen impacts. The study examines these, after on-site measurements (topometry, water salinity, piezometry), analysis of hydrological data, field observations, and surveys with dam managers, state regional directions, and local stakeholders. It stresses the urgency of elaborating models of hydraulic management to limit the negative consequences of the hydraulic constructions. These models should consider the variety of water uses in the middle valley and delta, and also the water safety and quality (salinity) downriver from Diama Dam.

Estuarine microbial community characteristics as indicators of human-induced changes (Senegal River, West Africa)

Understanding how human-induced management interacts with and affects the structure and functioning of large estuarine ecosystems is a major research challenge. In West Africa, human intervention on the Senegal River Estuary was intended to reduce the impact of major flooding by opening a new mouth in October 2003, 25 km to the north of the existing mouth. This study describes the effects of the new environmental conditions on the physical and biochemical characteristics of the water column and on microbial communities (bacteria, phytoplankton by size class and heterotrophic nanoflagellates (HNF)) in comparison with the situation in 2002. In 2006, seventeen stations were sampled, during both neap and spring tides, at a depth of 0.5 m along a salinity gradient from freshwater to marine conditions. Inorganic nutrient levels were often low but there were high levels of chlorophyll a in the estuarine area (mean of 13.7–20.7 μg L −1 in spring and neap tide conditions, respectively) producing a eutrophic status in this estuary. Average HNF abundances were lower (mean of 108 and 174 cells l −1 during neap and spring tides, respectively) compared to the situation in 2002 (mean between 2.5 and 6.7 × 10 4 cells l −1). Three biological indicators for assessing environmental changes are discussed: ratio of bacteria to heterotrophic flagellate abundances, ratio of picophytoplankton to nanophytoplankton, and the density of thermo-tolerant coliforms (TTC) and faecal streptococci. It is demonstrated that man-made alteration of the hydrologic regime can modify the microbial community structure and cause the health status of the estuary to deteriorate.

Phytoplankton growth control and risk of cyanobacterial blooms in the lower Senegal River delta region

Eutrophication, and the resulting proliferation of phytoplankton can affect water quality. Although environmental and ecological processes that lead to phytoplankton proliferations in temperate areas have received considerable attention, scientific background regarding this topic in tropical areas, especially West Africa, are scarce. In this study, bioassays in batch cultures were carried out in order to identify factors that may be stimulating or limiting the biomass of phytoplankton at various sites in the lower Senegal River delta region. Complete factorial designs were used to test several factors both alone and combined (nitrogen, phosphorus, sulfate, EDTA and trace elements, stirring). The effects of these factors on the growth of the whole phytoplankton communities were followed by the daily measurement of in vivo fluorescence. At all but two of the sites, enrichment with certain nutrients significantly stimulated phytoplankton biomass growth, with nitrogen and/or phosphorus appearing to be the primary limiting factor(s). Nitrogen was the primary limiting factor in coastal ocean waters and in the Senegal River estuary. In the freshwater ecosystems studied, nitrogen and phosphorus acted as alternating or concomitant limiting factors. We assessed the changes in the phytoplankton composition induced by nutrient enrichments in Lake Guiers, a drinking water reservoir for the population of Dakar. Our findings lead us to define the conditions of a possible proliferation of the potentially toxic cyanobacterium Cylindrospermopsis raciborskii mainly in response to phosphorus inputs.

Relationships between meso- and macro-zooplankton communities and hydrology in the Senegal River Estuary

This project studied how meso- and macro-zooplankton communities (composition, abundance, and biomass) in the Senegal River Estuary varied in time and space depending on hydro-biological factors and human action. Samples were taken approximately every 45 days, from January to December 2003, at 3 sampling stations where the estuary was subject to different marine and continental influences. Temperature, salinity, fluorescence and oxygen were measured using a Seabird probe (SBE 19). Meso- and macro-zooplankton were collected using standard nets (200 μm and 500 μm mesh sizes respectively), towed horizontally at the surface and at a depth of 3 m–5 m. The study showed a marked seasonal variation which clearly influenced the structure and distribution of meso-zooplankton and macro-zooplankton communities. During the low water period (November–June) the marine influence prevailed, decreasing from the river mouth to the upstream part of the estuary near the Diama anti-salt dam. Zooplankton was abundant, containing mainly marine organisms and a few euryhaline species (i.e. Acartia clausi) which colonized the estuary. During the high water period (July–October) the estuary was mainly under the continental influence and the impact of the annual flood. Zooplankton was less abundant but more diversified than during the low water season and was dominated by freshwater species which entered the estuary through the Diama Dam. By comparison with previous works, this study showed significant changes in the abundance of zooplankton and the types of community since the Diama Dam came into operation in 1985. The results also showed the initial effects on the pelagic ecosystem of a channel opened in October 2003 in the middle part of the estuary.

Use of Satellite Images to map Flood Extension around the city of Saint Louis in the Senegal River Estuary

The traditional method of landsat satellite data combination and the gathering of important information made it possible to produce a Geographical Information System to monitor floods in the lower estuary of the Senegal River valley (Sandholt,I., Fog, B. & Fensholt, R., 2001). This technical approach is a powerful tool for combining important information for a better comprehension of the floods and the characterization of surface qualities on the estuary. By way of a multi-temporal approach, the study team established the qualitative and quantitative impact of floods on the various geographical objects, a detailed cartography of the land use and the surfaces flooded in 1998 and 1999. The study undertaken in Saint Louis made it possible to consider surfaces flooded in 1999, and to understand the extent of these floods compared to those of 1998. The constitution of a tool of decision-making aid makes it possible to have information relating to the extent of the flood, the scope of flooded surfaces and to detect the more exposed zones in order to establish a hierarchical map according to the percentage of exposure to the risk of the geographical objects affected by the floods (populations), road infrastructures and tracks, medical and social infrastructures and perimeters of cultures (agriculture).

Herbivorous and microbial grazing pathways of metazooplankton in the Senegal River Estuary (West Africa)

The trophic relationships between metazooplankton and natural particles were studied in May 2002 in the Senegal River Estuary (16°N, 16°W) in low water conditions (dry season). Environmental factors, micro-organism and metazooplankton were analyzed through sampling at a fixed station. Gut fluorescence measurements of and field experiments on zooplankton metabolism were also performed. Chlorophyll a concentrations ranged from 6.5 to 10.2 μg l −1. The phytoplankton was dominated by picoplanktonic cells (83–94% of total numbers). The particulate organic carbon (1.2–2.7 mg l −1) originated for a large amount from organic detritus (20–70%). The zooplankton biomass was dominated by Cirripedia larvae and calanoid copepods ( Acartia clausi, Temora stylifera and Paracalanus spp.). These taxa showed diel vertical migrations and maximal gut fluorescence at night, independently of tidal effects. Metabolic budgets show that their daily ingestion rates on phytoplankton (27–55% of body carbon weight) were insufficient to balance their respiration needs (40–51% of body carbon) and suggest that a selective feeding upon micro-heterotrophs (Heterotrophic NanoFlagellates, HNF) and/or detritus would be necessary to complete their energetic needs. The daily grazing pressure of metazooplankton represented only 5% of the in situ chlorophyll a and 14% of the primary production, but this pressure was mainly orientated towards nanophytoplankton. The daily recycling of nutrients by the metazooplankton excretion was rather high (83 and 46% of the in situ NH 4-N and PO 4-P concentrations, respectively). Therefore, the impact of metazooplankton on phytoplankton through top-down (grazing) and bottom-up (nutrient recycling) processes seemed substantial in this tropical estuary.

Functional structure of microbial food web in the Senegal River Estuary (West Africa): impact of metazooplankton

We studied the impact of various metazooplanktonic predators (two calanoid copepods and barnacle larvae) on microbial plankton isolated from the Senegal River Estuary. Experiments performed in microcosms were based on size-class fractionation (3, 12, 20 and 60 µm size classes) of the microbial community in order to obtain different assemblages, including bacteria, heterotrophic nanoflagellates (HNF), ciliates, pico- and nanophytoplankton. Removal of bacterial predators >3 µm increased the net growth rate of heterotrophic bacteria from 0.437 day−1 in the presence of all predators (fraction <60 µm) to 0.682 day−1 (fraction <3 µm). Removal of protozoa >12 µm caused an increased net growth rate of flagellates from 0.102 day−1 (fraction <60 µm) to 0.332 day−1 (fraction <12 µm). The same results were also observed on protozoa dynamics in the presence of Temora stylifera (0.498 day−1), suggesting a trophic control of the protozoa >12 µm by this copepod, allowing an increase in HNF growth rates through indirect effect. The highest ingestion rate over 60 h of experiment was recorded for the copepod Temora (61.1 µm3 ngC−1 h−1) and the lowest value for barnacle larvae (25.6 µm3 ngC−1 h−1). An intermediate value (47.5 µm3 ngC−1 h−1) was recorded for the copepod Acartia clausi. All these ingestion rates were mainly channeled in the size classes between <3 and 15–18 µm equivalent spherical diameter (ESD) (6 and 2435 µm3, respectively), with the highest values noted between <3 and 6–9 µm classes. In the presence of Temora, the cascading effects can explain both the decreases in mean volume and in growth rate for bacteria, due to the filtration of ciliates by the zooplanktonic predator, allowing the development of the HNF. The presence of a large size class of bacteria (among the five classes identified by flow cytometry) and a high abundance of HNF suggest an efficient heterotrophic pathway within the microbial loop of this sub-saharan estuary.

Water quality and health status of the Senegal River estuary

The Senegal River estuary was sampled in May 2002 to get the first data on both the trophic and sanitary status of the water of the main river of the northwest African coast. Several physical, chemical and microbiological variables were measured twice along a transect. Inorganic nutrient concentrations were low while phytoplanktonic abundances (0.58–1.8 × 10 5 cells ml −1), bacterial abundances (0.27–8.1 × 10 7 cells ml −1), activity (22–474 pmol l −1 h −1), were among the highest recorded in such ecosystems. Microbiological variables revealed a eutrophicated status for this estuary. Largest abundances of fecal contamination bacterial indicators were only detected in localized areas (Saint-Louis city and surrounding areas). The apparent good survival of fecal indicator bacteria in the estuarine waters despite a long residence time (4–5 days) has been evaluated by complementary survival experiments. Exposed to a salinity gradient, a local Escherichia coli strain showed a significantly better survival than those of an E. coli reference strain.

Morphological and sedimentological changes in the Senegal River estuary after the constuction of the Diama dam

The aim of this paper is to study the behaviour of a complex and unstable coastal system (the Senegal River delta) since the construction of the Diama dam. The Senegal River mouth is located on a sandy microtidal coast. The equilibrium of the coastline (the so-called “Langue de Barbarie” sand spit in particular) and the inner part of the river mouth is highly dependent on the longshore drift and the transported sandy material. The role of the Diama dam could therefore be very important according to the seasonal rainfall. For example, between 1986, after the Diama dam began to function, and 1990, the Senegal River mouth moved southwards by more than 2.8 km and is now located in its southernmost position. As a whole, the high accretion has helped strenghen the maritime segment of the “Langue de Barbarie” by a decreasing of the fluvial water discharge and the Senegal River mouth will be maintained for a while at the southern part of the delta. Field measurements, including topography profiles, sounding transects and sedimentological analysis, Spot-imagery processing and comparisons of recent topographical maps have been used to evaluate the functioning and evolution of the Senegal distributary and coastal zone.