JCBPS; Section D; August 2020 –October 2020, Vol. 10, No. 4; 362-376. E- ISSN: 2249 –1929 [DOI: 10.24214/jcbps.D.10.4.36276]

Journal of Chemical, Biological and Physical Sciences

An International Peer Review E-3 Journal of Sciences Available online atwww.jcbsc.org

Section D: Environmental Sciences CODEN (USA): JCBPAT Research Article

Physical factors limiting the success of mangrove restoration policies in the Biosphere Reserve of the Delta,

1,2 1 2 El Hadji SOW , Taibou Ba , Boubou Aldiouma SY

1Centre de Suivi Ecologique, Rue Léon Gontran Damas, BP 15532, Fann Résidence-Dakar, Sénégal, 2Laboratoire Leïdi «dynamiques des territoires et développement», Université Gaston Berger, Saint- Louis, Sénégal,

Received: 18 August 2020; Revised: 30 August 2020; Accepted: 06 September 2020

Abstract: The mangrove ecosystem of the Biosphere Reserve of Saloum Delta (RBDS) presents a variety of ecological and socio-economic interests. However, this ecosystem continues to be continuously degraded despite numerous restoration programs that have been underway for more than two decades. Several actions have not produced satisfactory results. Therefore, in addition to socio-economic factors, it is also necessary to evaluate and analyze the physical factors that hinder the success of these mangrove restoration policies in the RBDS. To this end, the scientific approach adopted integrates a literature review and field work that involves in situ measurements of water salinity and acidity, and sedimentation. The results show that high salinity levels in several reforestation plots hamper the survival of seedlings. In addition, the silting noted in some plots transforms the soil texture, making mangrove regeneration difficult. Keywords: mangrove, degradation, politics, restoration, physical, Saloum delta

INTRODUCTION

Senegal's mangroves are the most northerly in West and constitute a valuable ecosystem for biodiversity conservation. Most of the lagoons, estuaries and delta are populated by mangroves which extend over an area of about 200,000 hectares [1]. These mangrove populations, sources of many natural resources (firewood and services, oysters, arches, fish, etc.) but also an important breeding

362 J. Chem. Bio. Phy. Sci. Sec. D ; August 2020 –October 2020, Vol. 10, No. 4; 362-376. DOI:10.24214/jcbps.D.10.4.36276.]

The Limiting … El Hadji SOW et al. ground for many fish species, play a socio-economic role (food, shellfish exploitation, fishing, etc.) and ecological (windbreaks, species reproduction and refuge area, sediment fixation, etc.) for the Sine Saloum natural region [2,3]. The mangrove therefore occupies an important place in the natural environment and biodiversity. It is also essential for the life of the local populations who maintain a close relationship with it. However, the degradation and decrease of the Senegalese mangroves is accentuating due to the decrease in rainfall recorded since the 1970s, illegal exploitation by local populations, and other factors having a negative effect on the resources needed to the life and economic activities of local populations (forest, fishery and tourism resources) as well as environmental degradation. The Saloum Delta Biosphere Reserve (RBDS) is facing this degradation process. It is located in the center-west of Senegal in the natural region of Sine Saloum, between 13 ° 35 and 14 ° 15 North latitude and between 16 ° 03 and 16 ° 50 West longitude. It covers an area of 234,000 ha and combines the characteristics of a marine, estuarine, lacustrine and marsh wetland, and its various sites fulfill the classic functions of a wetland. It is located on the Senegalese coast about 150 km south of Dakar (Figure 1). The Saloum, which functions as a reverse estuary [4], is a veritable arm of the sea [5] bordered by vegetation characteristic of coastal areas, in this case the mangrove. The Saloum is home to the northern margin of the Southern Rivières mangrove [6, 2,3]. Along the banks of the Saloum delta stretch leashes and shallow seas. The delta's countless tree channels that are sensitive to the tide, grow into islands, and are bordered by mangroves on both shores. Vast mudflats and dense and varied vegetation form an ecosystem ensuring biodiversity. Mangroves also green on the lowlands of the continental parts of the delta and behind these green parts develop sulfuric soils called "tanne". The RBDS is placed in the bounded zone of the isohyet lines between 700 and 900 mm. According to data recorded from 1918 to 1993, annual precipitation shows a decreasing trend from 1968. On the other hand, especially for the decade from 1980 to 1990, annual precipitation is low and remains at the very low level of 400 at 600 mm. However, they have resumed a rise to the normal level since 1999 [1]. The altitudes of the region do not exceed 40 m and are marked by a weakness of the slopes which explains the deep rise of the sea and a predominant marine influence. The tides are sensitive up to more than 100 km inland. The salinity there is very high (greater than seawater estimated at 35g / l) and is a limiting factor for the development of vegetation [7]. In this area, the mangrove ecosystem constitutes the main source of income for the populations. However, a state of continuous degradation of this mangrove in the RBDS is observed despite its restoration policies initiated and adopted since the end of the 1990s and the beginning of the 2000s. Various mangrove preservation and restoration programs have been undertaken. , initiated by local populations, the State, various donors and NGOs. Among these initiatives, we can mention those of the West African Association for Marine Environment (WAAME), the International Union for the Conservation of Nature (l’IUCN)[8], the Oceanium, the World Wide Fund for Nature (WWF) , Wetlands International Africa (WIA), Aide au Développement Gembloux[9], etc. To these programs are added those coordinated by the Directorate of National Parks (DPN), the Directorate of Community Marine Protected Areas (DAMPC) and the Directorate of Water and Forests, Hunting and Soil Conservation (DEFCCS) [2, 3, 10] In this context, a number of political development orientations have been recommended and implemented. Among the latter is the establishment of biological rest and rotation in the collection of marine invertebrates. Added to this is the establishment of an institutional framework to ensure sustainable management of renewable resources and ecosystem restoration (reforestation and / or

363 J. Chem. Bio. Phy. Sci. Sec. D , August 2020 –October 2020, Vol. 10, No. 4; 362-376. DOI:10.24214/jcbps.D.10.4.36276.]

The Limiting … El Hadji SOW et al. assisted natural regeneration) which aims to restore degraded environments (for example) man and / or nature) in their initial natural state. Despite everything, mangrove degradation is still a thorny issue in the RBDS. In this context, the objective of this contribution is to study the physical factors limiting the success of mangrove restoration policies and programs undertaken in the RBDS. The study focused on the analysis of the environmental conditions that hinder the success of mangrove restoration actions in the area.

Figure 1: location of the Saloum delta biosphere reserve

MATERIALS AND METHODS

1. Data: In order to have data on the situation of sediment dynamics but also on the salinity and acidity of the water at the mangrove reforestation sites of the RBDS, a documentary review was carried out. In addition, series of field measurements were carried out in 14 reforestation plots. The data collected relate to in situ measurements of the following parameters: change in the level of siltation, electrical conductivity (EC) and hydrogen potential (pH). The objective of these measurements is to understand the level of these parameters and their impact on young plants because it must be understood that the level of acceptability of mangroves for these physicochemical parameters is limited to certain thresholds. 2. Methods

2.1. The choice of experimental plots: There are 14 selected plots spread over 09 sites, 06 of which are partly land and 03 partly insular. The choice of sites is justified by their geographical distribution at the RBDS level, the level of intervention of projects and / or NGOs at said sites and the presence of reforested plots less than 7 years old. Thus, the plots are located in the villages of Djirnda, Baout,

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Moundé and Dionewar (island part), Bangalère, Keur Mbar, Néma Bah, Gagué Mody and Mbassis (mainland). 2.2. Measurement campaigns: Two measurement campaigns were carried out: in the dry season (from 01 to 05 May 2018) and in the rainy season (from 20 to 25 September 2018). These two campaigns made it possible to collect the targeted data during two different seasons. The period covering the duration of monitoring of these plots is one (01) year. The aim here is to follow the evolution of these parameters between two seasons in order to better understand their effects. 2.2.1. In situ measurements of the intensity of sediment dynamics: In the framework of this study, the stake method (sighting stations) was used. This network of stakes was used to quantify soil inputs / losses (deposition / erosion) expressed in height and determined from the natural change in the level of the soil surface. This quantification is calculated on the basis of the balance sheet of fluctuations in ground level from an initial state taken as the origin. This measurement method was applied in 10 experimental plots reforested with mangrove (Djirnda, Baout and Moundé (island part), Gagué Mody and Mbassis (land part). In each plot the 1 m stakes are half-driven (50 cm). Thus, 02 series of lifting four months apart were operated (May and September 2018). The evolution of the environment is evaluated here according to the decrease or increase in cm of the remaining height of the stake indicating respectively the accumulative dynamics or the erosive dynamics (silting up or erosion). The result obtained after checking each stake and in each plot is listed on a log sheet. The result of the sediment dynamics in each plot is based on the average of the measurement results of the 03 stakes of said plot; This gave us an idea of the plots with stronger sediment dynamics and having an impact on mangrove development.

2.2.2. In situ measurements of the conductivity and hydrogen potential of water: EC (Too high a salinity negatively influences the development of mangroves, especially Rhizophora, and does not promote its regeneration.) and pH (Strong acidification of the soil due to the prolonged contact of the soil and the air leads to a reduction in the mangrove area, the mangroves not supporting too much acidity, therefore contributing to the creation of tan.) are measured to determine the level of salinity and acidity of the water in the 14 experimental plots. At the level of these plots, measuring points 100 m apart are chosen. The number of points chosen within each plot is determined according to its area. Thus, for a one hectare plot, 04 points were taken. On the other hand, for the two hectare plots, 06 points were taken, ie a total of 64 measurement points in the 14 plots for each campaign. The in situ measurements were carried out using a HI 9829 Multiparameter device (photo 1). This made it possible to assess the level of mineralization and the acidity of the waters of the mangrove reforestation areas.

Photo 1: HI 9829 GPS multiparameter Garmin Montana 680t model

All the sampling points are georeferenced using a Garmin Montana 680t GPS. This resulted in a mapping of the salinity and acidity level of the area concerned (photo 2).

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Photo 2: Georeferencing and measurement of a sampling point and reading of a water sample [2,3] 2.2.3. Analysis of the relationship between physico-chemical parameters and the state of mangrove reforestation: This part establishes the correlation between the distribution of these aforementioned parameters (EC, pH and erosion / deposition) and the survival rate of the reforestation campaigns during the last years after the evaluation in the 14 experimental plots. As part of this research, this survival rate is calculated by the method recommended by the FAO which is as follows:

T = N2 / Ntot X 100

T = rate; Ntot = the number of propagules planted (Rhizophora) / plants planted (Avicennia) in year 0; N2 = the number of propagules / plants still alive after 01 years of planting (year 2). Knowing that the reforestation of one hectare requires 5000 propagules at the rate of 2m2 per plant (with a spacing of 1 m and a spacing of 2 m) and 10,000 propagules at a rate of 1m2 per plant (with a spacing of 1 m and a spacing of 2 m) of 1 m), it is then easy to know the number of plants in year 0 depending on the area of the plot. To know the number of plants that survived in year 2, the number of living plants in an area of 100 m2 was counted and an interpolation for the total area of the plot was made. This calculation made it possible to know the variation in the survival rates of reforestation and was correlated with the variation in the physicochemical parameters mentioned above in order to see if the plots with low survival rates correspond to those affected by salinity, acidity and silting up.

RESULTS

1. Surface water measurements in the dry season:

1.1. Salinity: The results of the measurements reveal variations in the insular and continental domains.  In the continental domain: the electrical conductivity varies between 63.10 to 116.9 ms / cm. The highest values of this EC are measured at the level of the 06 plots of Gagué Bocar, Mbassis, Mbam Bangalère and Keur Mbar (eastern part of the RBDS) and vary between 70 and 116 ms / cm while the lowest values are collected. In the Néma Bah plot (southern part of the RBDS) and do not exceed 64 ms / cm.  In the island domain: The electrical conductivity varies between 69.39 and 90.40 ms / cm at the level of the 07 reforestation plots. Significantly high values are measured in the Djirnda plots and vary between 73 and 90ms / cm. At the level of plots of Moundé, Baout and Dionewar, the mores reveal almost identical values which revolve between 69 and 71ms / cm. Apart from the reforestation plots, the EC measurements carried out at the level of the bolons 366 J. Chem. Bio. Phy. Sci. Sec. D , August 2020 –October 2020, Vol. 10, No. 4; 362-376. DOI:10.24214/jcbps.D.10.4.36276.]

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and the main arm of the Saloum give values that vary between 70.78 and 81.75 ms / cm. The salinity distribution map (Figure 2) made it possible to distinguish 04 sectors: 1) a sector with low salinity located in the large part of the commune of Toubacouta; 2) a medium salinity sector located to the north and west of the commune of Toubacouta and in the communes of Dionewar, Bassoul and Djirnda; 3) a high salinity sector located in the commune of Nioro Alassane Tall and to the east of the communes of Diossong, Djilor and Mbam and 4) a sector with very high salinity located in the center of the communes of Diossong and Djilor and in the west of Mbam commune.

Figure 2: Distribution of the EC of water in the dry season in the RBDS

1.2. Acidity: The pH values vary from 7.78 to 9.12 in the 07 reforestation plots on the mainland: Bangalère, Keur Mbar, Néma Bah, Gagué Mody, Mbassis and Mbam. These values turn between 7.47 and 9.82 in the 07 reforestation plots of the island part (Djirnda, Baout, Moundé and Dionewar). In general, the pH did not vary too much in all the plots (Figure 3).

Figure 3: pH values in RBDS during the dry season

The measurements carried out in the dry season in the reforestation plots reveal very high conductivity values, especially in the continental part and quite remarkable pH values. In other words, there is a difference between the maximum and minimum values of these different measured parameters. 2. Surface water measurements in the rainy season: 2.1. Salinity: The results of salinity measurements show variations both on the continent and in the island space.

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 In the continental domain: the electrical conductivity varies from 40 to 64 ms / cm in the 07 mangrove reforestation plots in the continental part. The highest values are found at the level of the 03 plots of the localities of Mbassis and Gagué with more than 60ms / cm. On the other hand, they are too low at the level of the plot of the locality of Néma Bah (southern part and downstream of the RBDS) where they do not exceed 44 ms / cm.  In the island domain: the values of electrical conductivity vary between 51 and 57 ms / cm at the level of the 07 mangrove reforestation plots. However, these measurements go up to 62.8 ms / cm at the level of certain parts of the main arm of the Saloum at Foundiougne. The salinity distribution map (figure 4) made it possible to distinguish 03 sectors: 1) a sector with very low salinity located in the large part of the commune of Toubacouta; 2) a low salinity sector located to the north-west of the commune of Toubacouta and in the communes of Dionewar, Bassoul, Djirnda, Nioro Alassane Tall; in most of the communes of Mbam and Djilor and Diossong; 3) an area with medium salinity located in the center of the communes of Bassoul and Djirnda, to the west and the center of the commune of Diossong, in the center of the commune of Djilor and to the north of that of Mbam, of Nioro Alassane Tall and to the east of the communes of Diossong, Djilor, and Mbam.

Figure 4: Distribution of the EC of water in the rainy season in the RBDS

2.2. Acidity: The hydrogen potential varies between 8.12 and 10.01. The values vary between 8.4 to 9.96 at the level of the 07 reforestation plots of the mainland (Bangalère, keur Mbar, Néma Bah, Gagué Mody, Mbassis, and Mbam). In the reforestation plots of the island part (Djirnda, Baout, Moundé and Dionewar), these values vary between 8.12 and 10.01. In the bolons and the main arm of the Saloum, this pH varies between 8.25 and 9.19. As in the dry season, the pH did not vary too much in the RBDS during the rainy season (Figure 5).

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Figure 5: pH values of RBDS during the rainy season

The measurements carried out in the rainy season at the level of the reforestation plots reveal rather high values of conductivity and remarkable pH values, especially in the island part. In other words, there is a difference between the maximum and minimum values of these different measured parameters. 3. Analysis of the variation in salinity and pH of water at the RBDS level : In view of these various results, several observations are made: 1) electrical conductivity is, in general, higher in the mainland than in the island part; 2) electrical conductivity is also higher in the dry season than in the rainy season; 3) the hydrogen potential is more remarkable in the rainy season than in the dry season; 4) the variability of the hydrogen potential between the two sectors is not too remarkable and 5) the electrical conductivity generally increases from the downstream part of the river to the upstream part, especially in the dry season. These findings are linked to interdependent natural factors. Indeed, the conductivity values lower than those of sea water during the rainy season, generally measured in certain sectors of the mangrove swamps in Moundé, Djirnda and Nema Bah indicate dilution phenomena due to runoff from the rain. On the other hand, the highest values (up to 116 ms / cm) are characteristic of the waters of young mangrove shoots which have been reforested in areas which, in the past, were colonized by live tannes. These values are generally noted in the continental sector (Gagué Mody 2 and 3, etc.). 4. Measurements of sediment dynamics: The results of measurements of sediment dynamics carried out at the level of the 10 experimental plots show variations in terms of space (continental sector and island sector) and time (dry season and rainy season). 4.1. Sediment dynamics in the dry season: The sedimentary dynamics in the mangrove marshes of the RBDS are marked by accumulation sectors and erosion sectors.  In the continental sector, the measurements of the stations show the phenomena of sand deposit with an average height ranging from 1 to 3 cm compared to the reference situation for the plots of Mbassis, Gagué Mody 2 and 3. Erosion is only noted at the level of the plot of Gagué Mody 1 with a value of - 02 cm compared to the reference (figure 6).  In the island sector, five (05) of the six (06) plots are confronted with the deposit of sand with average heights varying between 01 and 04 cm (depending on the plot) compared to the initial situation. The plot most affected by this phenomenon is that of Baout 2. It is only at the level of the plots of Djirnda 1 and Baout 1 where sedimentary erosion was observed (figure 6).

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Figure 6: Sediment dynamics in the mangrove reforestation sectors in the dry season

4.2. Sediment dynamics in the rainy season: During the rainy season, the results of the measurements are as follows:  In the continental sector, the deposit of sand persists in all 04 plots. The sediment deposition heights vary on average between 01 and 03 cm. This phenomenon is more remarkable in the plots of Gagué Mody 2 and 3 (figure 7).  In the insular sector, if the parcels of Djirnda 2 and Moundé 2 are stable between the two measurement periods, we note an evolution for those of Moundé 1, Baout 1 and Baout 2. This evolution is manifested by a respective erosion of - 1, -2 and -3 cm compared to the previous measurement. As for the Djirnda 1 plot, it is marked by an accretion situation of up to 04 cm from the reference (figure 7).

Figure 7: Sediment dynamics in the mangrove reforestation sectors in the rainy season

4.3. Analysis of sediment dynamics in mangrove reforestation areas : The results of the two measurement campaigns thus reveal several scenarios: 1) the deposit (accumulation) is more remarkable than the erosion, both in the dry season and in the rainy season; 2) the deposit is more noted in the mainland than in the island part; 3) erosion is more visible in the island part than in the mainland; 4) the deposit is more evident in the rainy season than in the dry season in the continental sector and 5) the erosion is more remarkable in the rainy season than in the dry season in the island sector. In view of these results, it is important to emphasize that the hydrodynamic regime is remarkable. It largely conditions the sedimentological evolution of the estuary. Water erosion leads to a qualitative degradation of the soil but also to silting up of mudflats linked to the transport and deposition of sediments. This situation observed at the level of certain plots such as those of Baout 1, Moundé 1 and 2 and Djirnda 2 would be linked to this erosion which has been accentuated since the opening of the Sangomar breach in February 1987). 5. Analysis of the relationship between physico-chemical parameters / state of mangrove reforestation: The aim here is to analyze the relationship between the variation of chemical parameters (EC, pH), sediment dynamics and the state of mangrove reforestation.

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5.1. Assessment of reforestation survival rates: The survival rate evaluated on the basis of the FAO method is summarized in Table 1. Table 1 show the survival rates of mangrove reforestation at the level of the experimental plots. This table shows an average survival rate of 34.12%. This also makes it possible to observe a disparity in these rates depending on the said plots. In view of this disparity, several observations emerge: 1) only two plots have survival rates greater than or equal to 50%; 2) the plots in the mainland generally have lower survival rates than those in the island part; 3) the survival rates in the plots of Néma Bah (90%) and Gagué Mody 3 (0.9%) are very remarkable extremes. It would be interesting to determine the explanatory factors of these phenomena. For this, acidity, salinity and sand deposition are targeted in the plots in order to assess their effects. As a reminder, the development of mangroves is subject to certain requirements. In other words, a high rate of these aforementioned physicochemical parameters constitutes a factor of degradation of this ecosystem. Hence the interest in making this correlation.

Table 1: Survival rate of reforested plants in the experimental plots

Name of the plot Area Distance/ Number of Number Survival Longitude Latitude (ha) line space reforested ofplants rate (%) plants alive Mbassis 1 1/1 10 000 3 900 39 343796 1555324 Gagué Mody 1 2 2/1 10 000 3 800 38 345616 1564149 Gagué Mody 2 1 2/1 5 000 1 800 36 346737 1564678 Gagué Mody 3 1 1/1 10 000 9 00 0,9 352672 1562499 Djirnda 1 1 2/1 5 000 2 100 42 326875 1545043 Djirnda 2 2 2/1 10 000 3 200 32 327683 1544950 Moundé 1 2 2/1 10 000 5 000 50 322493 1544827 Moundé 2 1 2/1 5 000 1 900 38 322921 1545158 Baout 1 1 2/1 5 000 1 800 36 332612 1549176 Baout 2 2 1/1 20 000 5 400 27 334287 1551579 Bangalère 1 1/1 10 000 1 500 15 342201 1551646 Keur Mbar 1 1/1 10 000 1 700 17 345670 1542957 Nema Bah 2 2/1 10 000 9 000 90 339405 1519257 Dionewar 1 1/1 10 000 2 400 24 313414 1535830 Total 19 ha 130 000 44 400 TM1 : 34,12 %

1Average rate

5.2. Correlation of physico-chemical parameters / mangrove survival rate: To verify this relationship, according to the two sectors (continental and insular) the variations in the survival rates of mangrove reforestation were correlated with the variations in the annual means of the EC, of the pH and of the sedimentation. Figure 8 highlights this correlation in the mainland and showed the parameter that most influences the survival rate.

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Figure 8: Correlation of annual means of EC, pH, sedimentation with the survival rates of reforestation in the continental sector

Figure 8 shows pH which is basic in all the plots and therefore has not changed too much. Therefore, this pH would not have had an impact on the survival rate of reforestation, which has experienced remarkable variations depending on the plot. When it comes to sedimentation, there is a disparity in values. This disparity would have had an impact on the survival rate. In fact, in plot 4 (Gagué Mody 3) at which an average accretion level of 03 cm is observed, the survival rate is very low at 0.9%. In addition, in plot 2 (Gagué Mody 1), even if erosion exists there, it is too low (0.5 cm / year); the survival rate thus reaches almost 40%. It should be noted, however, that the level of sand deposition is not the only factor explaining the variation in the survival rate. In other words, another phenomenon which influences the state of reforestation is the level of the EC. Indeed, with regard to Fig. 8, it can be seen that the higher the EC, the lower the survival rate. At the level of plot 7 (Néma Bah) which has the highest survival rate (90%), the EC is only 52.88 ms / cm or 32.25 g / l of salt. On the other hand, at the level of plot 4 (Gagué Mody 3) the survival rate is only 0.9% while the degree of EC is 89.21 ms / cm or 54.41 g / l of salt. From these observations, we can say that salinity is the most influential parameter on the mangrove reforestation rate in the mainland. Figure 9 confirms the influence of EC and sedimentation on the state of reforestation in the 07 plots of the island sector.

Figure 9: Correlation of annual means of EC, pH and sedimentation with the survival rates of reforestation in the island sector

With regard to this figure 9, we see that the plots with low levels of sand accumulation have in return high survival rates because a low accretion rate reflects a low degradation of the composition of the mudflat: plots 1, 3 and 4. However, this variation in the level of silting is not the only explanatory factor for the disparity in the survival rate of reforestation because, despite the low level of accumulation of plot 6 (Baout 2), the rate of survival at this plot is less than 30%. In the case of EC, a strong correlation between the latter and the variation in the survival rate was observed. Indeed, the lowest survival rate (24%) was found at the level of the plot which keeps the highest average level of EC (69.57 ms / cm or 42 g / l). On the other hand, plot 3 (Moundé 1) with the highest survival rate

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(50%) has the lowest average salinity level (59.36 ms / cm or 36.20 g / l of salt). As for the pH, it has hardly changed and remains basic at the level of all these 07 plots of the island part. These results, obtained at the level of the 14 experimental plots, show that both in the mainland and in the island part, sedimentation and especially salinity are two physical factors which currently determine the state of mangrove reforestation in the RBDS. The survival rates of reforestation largely depend on these factors. With regard to the survival rates of the 14 plots, only 02 have rates greater than or equal to 50%. In addition, 11 plots have survival rates of less than 40%. In return, 9 of the 11 plots have salinity levels of over 40 g / l. This high salinity rate, which is higher than that of seawater, makes it difficult for young mangrove plants to develop. In addition, since Avicennia is more resistant to salt, and is more adapted to exposed parts, the plots of the mainland and especially those of Gagué Mody (north of the estuary) are more suitable for this species. In addition, the low survival rate can be accentuated because of the reduction in the submersion time of plots covered in sand; which promotes a chemical transformation of the environment (transformation of pyrite into jarosite) and leads to negative impacts on young plants. If we rely on the distribution of reforestation in recent years, we see that the latter is more concentrated in the communes of Mbam (continental sector), Djirnda and Bassoul (island sector). These 03 Communes are located in the northern and north-western parts which are areas very affected by salinity and silting up. As a result, the majority of reforestation campaigns are doomed to failure due to the high mortality rates of the plantations.

DISCUSSION

Results on EC, pH and sedimentation values vary by area and time period. This is due to several factors. Indeed, the maximum EC values found in the island part could be related to the proximity of the maritime opening leading to communication between the arm of the sea and the ocean. The rupture of the point of Sangomar stated could be an aggravating factor of salinization of the river Saloum. Seawater naturally invades the channels, floods the mud flats and foreshore following the flow and ebb and by ricochet, floods the mangrove marshes around the said watercourse. The work of Dieye et al.[11], attests that the rupture of the Sangomar point leads to an increase in the salinity of the Saloum and to many other impacts. In addition, the presence of the is considered as another explanatory factor; this because of the contamination by the water which can be hyper salty at the bottom of the bolons.[12,.13]. As a result, the reforestation plots near the Saloum watercourse like those in Djirnda have EC values of up to 90 ms / cm in the dry season. In addition, several factors come into play in the supply of freshwater to rivers: rainfall, the nature of the topography, the climate, etc. In the Saloum delta, the variation in rainfall marked by frequent deficits, the fairly flat topography of the river bed, strong evaporation and high temperature lead to a deficit in freshwater supply. This contribution should contribute to the dilution of the water and consequently to the favorable conditions for the development of the mangrove. Its deficit is compensated by sea water which causes high salinity. The disparity in EC values depending on the season suggests the weight of fresh water on the drop in the salinity of river water. In other words, the timid return of rainfall noted since the beginning of the 2000s [14] could change the trend in terms of reduction of water salinity values; which, consequently, should influence the regeneration of the mangrove swamp of the Saloum delta even if this is not the case if we refer to the above-mentioned results. The work of Olivry [15] attests that in the Saloum, as in the other estuarine and deltaic systems of Senegal, the flow deficit of the river systems in the 1970s and 1980s led to a high salinization state of

373 J. Chem. Bio. Phy. Sci. Sec. D , August 2020 –October 2020, Vol. 10, No. 4; 362-376. DOI:10.24214/jcbps.D.10.4.36276.]

The Limiting … El Hadji SOW et al. hydro- systems (surface or groundwater). According to Diop and Barusseau[16], this very episodic nature of river flow in the wet season was accentuated by the rainfall deficit in the 1970s. Compensation by sea water thus conditions the salinity of the Saloum, which leads one to say that like the Casamance estuary, the salinization of the Sine Saloum hydrographic network is subject to the influence of the tides [17]. The variation in this water can be explained by the interplay of the tides which daily generate pressure differences in the coastal aquifers [18].

This salinity rises, according to Diop[6], (Diop and Barusseau highlighted particular patterns of inverse estuaries in the northern domain, especially in the Saloum and Casamance where the salinity gradient increases from downstream to upstream and where tidal phenomena exert a predominant influence. on freshwater inputs; whereas, as a rule, salinity decreases from downstream to upstream in the rivers of and both Guinea.) From downstream to upstream. With often very high values, one notes an almost impossible life of the mangrove at the level of this upstream part. During the rainy season the contributions of surface runoff and precipitation promote a reduction in salinity. The measurements taken during the rainy season reveal that the salt concentration values do not exceed 57 ms / cm in the insular sector and 69 ms / cm in the continental sector. In addition, the lowest values found at the level of the plots of Nema Bah could be linked to the proximity of the course of the Bandiala which receives the tributaries of runoff water and therefore which is less salty than that of the Saloum around which, we find the highest salt values during this same period. If we refer to the ADG report [8], mangrove development is subject to certain salinity thresholds which should not exceed 60 g / l of salt for Rhizophora and 70 g / l for Avicennia. The closer the salinity level is to these thresholds, the more difficult regeneration is. In fact, the absorption capacity of salt by mangroves is limited. In other words, too high a salinity negatively influences the development of mangroves, especially the Rhizophora species, and does not promote its regeneration. Finally, regarding the pH, its variation in water is linked to the loss of carbon dioxide. It should also be noted that the pH value is a function of the refractory organic matter dissolved or not in the water [13]. The pH is linked to the nature of the soil, it is generally acidic in the water of the sandy or granitic or alkaline aquifers in limestones. Measured pH values show variability and does not correlate well with the increase and dilution of water mineralization. If the pH values are relatively high in the dry season, especially partly insular, in the rainy season, they have almost all increased. With regard to sediment dynamics, the evolution of the movement of sedimentary stocks over the past decades in the Pointe de Sangomar sector is a determining factor in the extension of the breach and therefore a determining factor increase in sedimentary dynamics which is felt as far as the channels, and by extension to the mangrove mudflats. Also, the storms that appear in December and January on the Atlantic, also cause the formation of strong swells and high waves compared to normal. These swells, combined with the surges caused by sea winds and the tide, are factors of erosion [19]. In addition, erosion causes destruction of the vegetation cover which, consequently, causes intense wind erosion phenomena, which causes sand deposits. This sedimentary dynamics observed on the part of the coast, made up of islands, causes short-term morphological modifications which are observed on a scale of the order of one year [19]. This accumulation of sand is also very remarkable in the plots of the continental sector, both in the dry season and in the rainy season. The exposure of this sector to the winds favors the transport and deposit of sand at the level of said plots. According to a report by DEEC [20] on the vulnerability of biodiversity in the coastal zone of the Saloum Delta National Park, wind erosion is a serious problem that presents the risk of considerable

374 J. Chem. Bio. Phy. Sci. Sec. D , August 2020 –October 2020, Vol. 10, No. 4; 362-376. DOI:10.24214/jcbps.D.10.4.36276.]

The Limiting … El Hadji SOW et al. loss of land, silting up of mud flats and other socio-economic impacts. It should be noted that the sand accumulated at the level of mangrove mud flats transforms the texture of this environment by the significant modification of the granulometric composition of the mud, which handicaps the development of the mangrove which cannot resist a certain silting threshold (Mechanical degradation).

CONCLUSION

Restoring the RBDS mangrove ecosystem remains a thorny issue. Despite the restoration policies undertaken, this ecosystem continues to deteriorate. Measures undertaken over the past two decades have been hampered by socioeconomic factors. In addition, there are physical factors such as silting up of mud flats, and especially the salinity of the water. These two parameters have a negative influence on the development of plants, resulting in a high mortality rate in mangrove reforestation. Even if this species is par excellence, resistant to salinity, this resistance is limited to a threshold of 60 g / l for the Rhizophora species and 70 g / l for the Avicennia species. The results showed that out of the 14 targeted plots, only 02 plots had survival rates of more than 50%. However, the seedlings are only 02 years old. In other words, this death rate could be higher in the future

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1,2 * Corresponding author: Dr.El Hadji SOW ;

1Centre de Suivi Ecologique, Rue Léon Gontran Damas, BP 15532, Fann Résidence-Dakar, Sénégal,2Laboratoire Leïdi «dynamiques des territoires et développement», Université Gaston Berger, Saint-Louis, Sénégal, :

Online publication Date: 06.09.2020

376 J. Chem. Bio. Phy. Sci. Sec. D , August 2020 –October 2020, Vol. 10, No. 4; 362-376. DOI:10.24214/jcbps.D.10.4.36276.]