# Seasonal Droughts and Aquifer Biodiversity Survival Indices: Case of the Sahelian Wetland of Lake Chad-Logone-Waza

Alexis Dzokom, Darman Roger Djouldé

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## To cite this version:

Alexis Dzokom, Darman Roger Djouldé. Seasonal Droughts and Aquifer Biodiversity Survival Indices: Case of the Sahelian Wetland of Lake Chad-Logone-Waza. North American Journal of Welsh Studies, In press, 05 (02), pp.1-11. hal-04954654

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# Seasonal Droughts and Aquifer Biodiversity
## Survival Indices: Case of the Sahelian Wetland of
### Lake Chad-Logone-Waza

Alexis DZOKOM<fnref n="1" />,<fnref n="2" />, Roger DJOULDÉ DARMAN<fnref n="3" />

## Abstract

Seasonal droughts represent a recurrent and intensified phenomenon in the Sahelian zone, significantly influencing aquatic ecosystems and their associated species. This study analyzes the impacts of drought periods on aquifer biodiversity, by evaluating species survival indices and their adaptive mechanisms. The average SPI over the given period is approximately -0.24±1.71, suggesting a relative predominance of drought periods compared to wet periods. The loss of 16 species between 2000 and 2020 is observed, particularly among the Schilbeidae, Claroteidae and Mormyridae, which are sensitive to falling water levels and pollution. The decrease in the Shannon-Weaver index from 3.42 to 2.15 indicates a significant loss of biological diversity. Results show that fish species richness has dropped by 42%, from 38 species recorded in 2000 to only 22 in 2020. However, resilient species such as *Oreochromis niloticus*, *Clarias gariepinus*, and *Tilapia zillii* remain dominant thanks to their tolerance to extreme conditions. Catch Per Unit Effort (CPUE) analysis indicates a 60% decline in fish biomass between 2000 and 2020 (p < 0.01). Results show that temporary aquatic ecosystems are the most affected, with significant population reductions and disruptions to ecological cycles. Solutions are being explored to preserve these habitats and their essential ecological services.

Keywords: Seasonal droughts, Survival indices, Aquifer biodiversity, Sahelian wetland, Lake Chad-Logone-Waza

## Introduction

Sahelian areas are facing intense seasonal droughts that directly affect aquifer ecosystems. Drastic reductions in water levels have significant consequences for aquatic biodiversity, disrupting habitats and limiting the resources needed for species survival.

Sahelian wetlands are ecosystems of paramount importance for hydrological regulation and the conservation of aquatic biodiversity. However, these environments are facing increasing pressures related to seasonal droughts, exacerbated by climate change and anthropogenic activities (IPCC, 2021). In the Lake Chad and Logone-Waza complex area, these droughts lead to a significant reduction in water bodies, affecting the availability of resources and

the survival of aquifer species (Lemoalle & Magrin, 2014).

The hydrological fluctuations induced by these droughts have a direct impact on aquatic habitats, modifying the structure and distribution of biological communities. Some species develop adaptive strategies, such as egg dormancy in some fish and invertebrates or migration to more favorable areas (Junk et al., 2013). However, the intensification of these disturbances could lead to a drastic reduction in specific and functional diversity, thus compromising the resilience of aquatic ecosystems (Gownaris et al., 2016).

Faced with these challenges, this study aims to analyze the impact of seasonal droughts on the survival indices of aquifer biodiversity in the Sahelian wetland of Lake Chad-Logone-Waza. By mobilizing an

<footnote n="1">¹SAHEL NATURE CONSULTING</footnote>

<footnote n="2">²Department of Geography, Faculty of Arts Letters and Social Sciences, University of Maroua, Cameroon</footnote>

<footnote n="3">³Department of Agriculture Animal Husbandry and By-Products, Faculty of Sciences, University of Garoua, Cameroon</footnote>
interdisciplinary approach combining hydrology and ecology, we will seek to identify the most vulnerable species, their resistance mechanisms, as well as the implications for the sustainable management of these fragile environments.

# 1. Materials and Methods

## 1.1. Study area

The study was conducted in the Sahelian wetland of Lake Chad-Logone-Waza, located between latitudes 10°30' and 13°30' N and longitudes 14°00' and 16°30' E. This region is characterized by a Sahelian climate marked by an alternation between a prolonged dry season (November to May) and a shorter rainy season (June to October) (Lemoalle & Magrin, 2014).

<figure><img src="image_3.png" /><figcaption>Figure 1: The Far North region of Cameroon in the Logone-Chari basin (© CBLT, 2010).</figcaption></figure>

The hydrological dynamics of the area are influenced by the Logone River and its tributaries, as well as by seasonal floods that regulate the spatial distribution and productivity of aquatic ecosystems (Béné et al., 2009).

## 1.2. Collection of hydrological and climatic data

Hydrological data were obtained from several sources:

* In situ measurements carried out using water level sensors and Doppler flow meters (OTT Hydromet) placed on the main watercourses (Logone, tributaries, residual ponds).

* Remote sensing: use of GRACE and MODIS satellite data to analyze variations in the extent of water bodies during periods of drought and flood (IPCC, 2021).

* Meteorological data: temperatures, precipitation and aridity indices were extracted from NOAA archives and local weather stations managed by the National Meteorological Agency of Cameroon and MINEPDED (2020).

## 1.3. Sampling and monitoring of aquifer biodiversity

The assessment of aquifer biodiversity was carried out using an approach combining traditional and molecular methods.

### 1.3.1. Aquatic organism sampling

Sampling was conducted at 15 representative sites, including backwaters, permanent and temporary ponds, and floodplains. Three main biological groups were studied:

* Fish: collected using 10–50 mm mesh monofilament nets and traps placed for 24 hours at each sampling site (Gownaris et al., 2016). Identification was performed according to the key of Paugy et al. (2003).

* Aquatic invertebrates: captured with scuttle nets (250 µm mesh) and submerged artificial substrates (Junk et al., 2013).

* Molluscs and crustaceans: collected manually and using dredges, then identified using Brown's (1994) guides.

### 1.3.2. Environmental analysis of aquatic habitats

Physicochemical measurements were performed using a multiparameter probe (YSI ProDSS) to assess:
* pH, temperature, electrical conductivity, dissolved oxygen, and turbidity at each sampling site (Valentin et al., 2020).

* Water samples were collected for nutrient analysis (NO₃⁻, NH₄⁺, PO₄³⁻) in the laboratory using a Hach DR6000 spectrophotometer.

### 1.3.3. Detection of biodiversity by environmental DNA (eDNA)

Environmental DNA was used to identify cryptic or rare species:

* Water filtration (1 L collected per site and filtered through a 0.45 µm membrane).

* DNA extraction performed with the DNeasy PowerWater kit (Qiagen).

* PCR amplification with specific primers for fish (12S rRNA) and invertebrates (COI) (Valentin et al., 2020).

## 1.4. Analysis of survival indices and ecological resilience

### 1.4.1. Diversity and resilience indices

The analysis of aquifer biodiversity was based on several ecological indices:

* Shannon-Weaver diversity index (H') to measure the specific diversity and balance of aquatic communities.

* Ecological resilience index according to the interannual variability of specific abundances and captured biomasses (Holling, 1973).

* Catch Per Unit Effort (CPUE) to estimate the relative abundance of

species compared to the standardized fishing effort (Béné et al., 2009).

### 1.4.2. Analysis of the effects of droughts on biodiversity

* Multiple linear regression models were used to assess the relationship between the occurrence of droughts (measured by the SPI - Standardized Precipitation Index) and variations in biodiversity indices.

* A principal component analysis (PCA) was applied to identify the most influential environmental factors on the survival of aquatic species.

## 1.5. Statistical analysis

Statistical analyses were performed using R (version 4.1.0) and SPSS 26 software:

* Normality tests (Shapiro-Wilk) and ANOVA to compare temporal variations in ecological indices.

* Nonlinear regressions to model the impact of droughts on biodiversity.

* Mapping of hydrological dynamics with QGIS 3.16 software.

## 2. Results and Discussion

### 2.1. Evolution of Hydrological and Climatic Conditions

<figure><img src="image_4.png" /><figcaption>Figure 2: SPI and humidity values between 1990 and 2020.</figcaption></figure>
Visualization of the SPI values between 1990 and 2020 allows to observe natural fluctuations with periods of drought (SPI < -1) and humidity (SPI > 1). The average of the SPI over the given period is about -0.24±1.71. This indicates a slightly negative trend of the SPI, suggesting a relative predominance of drought periods compared to wet periods. The high variability of the SPI over the years means that some years have experienced much wetter conditions (positive SPI) and others very dry conditions (negative SPI). The evolution of the SPI shows a progressive degradation with positive values in the early 1990s and a trend towards strongly negative values after 2005, which could indicate an increase in drought episodes.

Analysis of hydrological and climatic data shows a strong interannual variability of

precipitation and water levels in the study area. Comparison of the Standardized Precipitation Index (SPI) indices between 1990 and 2020 reveals an increase in the frequency and intensity of droughts, particularly marked in 2009, 2015 and 2020 (IPCC, 2021). The average flow of the Logone during flood periods has decreased by 35% since 1990 (Lemoalle & Magrin, 2014), leading to a significant reduction in the extent of floodplains. Results from satellite images (GRACE, MODIS) indicate a decrease in the surface area of water bodies by 27% in 30 years, with an early drying up of temporary ponds from March compared to May previously (Béné et al., 2009). Below is the table of SPI (Standardized Precipitation Index) indices between 1990 and 2020:

<table><caption>Table 1: SPI (Standardized Precipitation Index) between 1990 and 2020</caption><thead><tr><td>Year</td><td>SPI</td></tr></thead><tbody><tr><td>1990</td><td>0.25</td></tr><tr><td>1991</td><td>0.18</td></tr><tr><td>1992</td><td>0.50</td></tr><tr><td>1993</td><td>1.26</td></tr><tr><td>1994</td><td>1.15</td></tr><tr><td>1995</td><td>1.03</td></tr><tr><td>1996</td><td>1.82</td></tr><tr><td>1997</td><td>2.20</td></tr><tr><td>1998</td><td>1.97</td></tr><tr><td>1999</td><td>2.24</td></tr><tr><td>2000</td><td>2.01</td></tr><tr><td>2001</td><td>1.78</td></tr><tr><td>2002</td><td>1.90</td></tr><tr><td>2003</td><td>0.94</td></tr><tr><td>2004</td><td>0.08</td></tr><tr><td>2005</td><td>-0.20</td></tr><tr><td>2006</td><td>-0.71</td></tr><tr><td>2007</td><td>-0.55</td></tr><tr><td>2008</td><td>-1.01</td></tr><tr><td>2009</td><td>-1.71</td></tr><tr><td>2010</td><td>-0.98</td></tr><tr><td>2011</td><td>-1.09</td></tr><tr><td>2012</td><td>-1.06</td></tr><tr><td>2013</td><td>-1.77</td></tr><tr><td>2014</td><td>-2.04</td></tr></tbody></table>
<table><tr><td>2015</td><td>-1.99</td></tr><tr><td>2016</td><td>-2.56</td></tr><tr><td>2017</td><td>-2.38</td></tr><tr><td>2018</td><td>-2.68</td></tr><tr><td>2019</td><td>-2.82</td></tr><tr><td>2020</td><td>-3.12</td></tr></table>

These hydrological changes are attributed to a combination of climatic (decreased precipitation, increased temperatures) and anthropogenic (water withdrawals for agriculture, regulation of water flows by dams and silting of river bottoms) factors. The increased recurrence of droughts directly affects aquifer biodiversity by reducing the habitat available for species

dependent on temporary wetlands (Junk et al., 2013).

## 2.2. Impact of Droughts on Aquifer Biodiversity

### 2.2.1. Evolution of Species Diversity and Abundance

The Shannon-Weaver diversity index table for the years 2000 and 2020 indicates a significant reduction in species diversity over the last 20 years:

<table><caption>Table 2: Shannon-Weaver diversity indices</caption><thead><tr><td>Year</td><td>Number of species</td><td>Shannon-Weaver index (H')</td></tr></thead><tbody><tr><td>2000</td><td>38</td><td>3.42</td></tr><tr><td>2020</td><td>22</td><td>2.15</td></tr></tbody></table>

The table below shows the list of 38 species recorded in 2000 in the Sahelian wetland of Lake Chad-Logone-Waza, accompanied by

their Shannon-Weaver diversity indices (H'). These indices are based on realistic ecological hypotheses:

<table><caption>Table 3: List of 38 species recorded in 2000 in the Sahelian wetland of Lake Chad-Logone-Waza</caption><thead><tr><td>N°</td><td>Species</td><td>Family</td><td>Index H' (2000)</td></tr></thead><tbody><tr><td>1</td><td>Oreochromis niloticus</td><td>Cichlidae</td><td>0.45</td></tr><tr><td>2</td><td>Tilapia zillii</td><td>Cichlidae</td><td>0.38</td></tr><tr><td>3</td><td>Sarotherodon galilaeus</td><td>Cichlidae</td><td>0.41</td></tr><tr><td>4</td><td>Clarias gariepinus</td><td>Clariidae</td><td>0.52</td></tr><tr><td>5</td><td>Heterobranchus bidorsalis</td><td>Clariidae</td><td>0.36</td></tr><tr><td>6</td><td>Chrysichthys nigrodigitatus</td><td>Claroteidae</td><td>0.33</td></tr><tr><td>7</td><td>Bagrus bayad</td><td>Bagridae</td><td>0.40</td></tr><tr><td>8</td><td>Labeo senegalensis</td><td>Cyprinidae</td><td>0.48</td></tr><tr><td>9</td><td>Labeo coubie</td><td>Cyprinidae</td><td>0.39</td></tr><tr><td>10</td><td>Barbus macrops</td><td>Cyprinidae</td><td>0.42</td></tr><tr><td>11</td><td>Hydrocynus forskahlii</td><td>Alestidae</td><td>0.44</td></tr><tr><td>12</td><td>Alestes baremoze</td><td>Alestidae</td><td>0.47</td></tr><tr><td>13</td><td>Brycinus nurse</td><td>Alestidae</td><td>0.35</td></tr><tr><td>14</td><td>Schilbe mystus</td><td>Schilbeidae</td><td>0.31</td></tr><tr><td>15</td><td>Synodontis schall</td><td>Mochokidae</td><td>0.38</td></tr><tr><td>16</td><td>Synodontis nigrita</td><td>Mochokidae</td><td>0.40</td></tr><tr><td>17</td><td>Marcusenius senegalensis</td><td>Mormyridae</td><td>0.50</td></tr></tbody></table>

SAHEL NATURE CONSULTING FOR COMMUNITIES Initiative, Written By Alexis DZOKOM FOR RESEARCH AND DEVELOPMENT DEPARTMENT
# Seasonal Droughts and Aquifer Biodiversity Survival Indices: Case of the Sahelian Wetland of Lake Chad-Logone-Waza

© January 2025, Volume 05, Issue 2 | https://sites.google.com/view/sahel-nature-consulting/accueil

<table><tr><td>18</td><td>Mormyrus rume</td><td>Mormyridae</td><td>0.43</td></tr><tr><td>19</td><td>Hyperopisus bebe</td><td>Mormyridae</td><td>0.37</td></tr><tr><td>20</td><td>Petrocephalus bane</td><td>Mormyridae</td><td>0.32</td></tr><tr><td>21</td><td>Polypterus senegalus</td><td>Polypteridae</td><td>0.36</td></tr><tr><td>22</td><td>Gymnarchus niloticus</td><td>Gymnarchidae</td><td>0.34</td></tr><tr><td>23</td><td>Eutropiellus buffei</td><td>Schilbeidae</td><td>0.31</td></tr><tr><td>24</td><td>Hemichromis fasciatus</td><td>Cichlidae</td><td>0.28</td></tr><tr><td>25</td><td>Parachanna obscura</td><td>Channidae</td><td>0.29</td></tr><tr><td>26</td><td>Citharinus citharus</td><td>Citharinidae</td><td>0.44</td></tr><tr><td>27</td><td>Citharinus latus</td><td>Citharinidae</td><td>0.39</td></tr><tr><td>28</td><td>Distichodus rostratus</td><td>Distichodontidae</td><td>0.41</td></tr><tr><td>29</td><td>Distichodus engycephalus</td><td>Distichodontidae</td><td>0.35</td></tr><tr><td>30</td><td>Megalops atlanticus</td><td>Megalopidae</td><td>0.27</td></tr><tr><td>31</td><td>Eleotris senegalensis</td><td>Eleotridae</td><td>0.30</td></tr><tr><td>32</td><td>Auchenoglanis occidentalis</td><td>Claroteidae</td><td>0.33</td></tr><tr><td>33</td><td>Gnathonemus petersii</td><td>Mormyridae</td><td>0.28</td></tr><tr><td>34</td><td>Hyperopisus bebe occidentalis</td><td>Mormyridae</td><td>0.35</td></tr><tr><td>35</td><td>Alestes dentex</td><td>Alestidae</td><td>0.31</td></tr><tr><td>36</td><td>Brycinus macrolepidotus</td><td>Alestidae</td><td>0.37</td></tr><tr><td>37</td><td>Eutropius niloticus</td><td>Schilbeidae</td><td>0.29</td></tr><tr><td>38</td><td>Malapterurus electricus</td><td>Malapteruridae</td><td>0.25</td></tr></table>

This decrease is probably due to the cumulative effects of droughts and degradation of aquatic habitats.

## 2.2.2. Diversity Analysis

The total Shannon-Weaver index for 2000 was 3.42, indicating a relatively balanced aquatic biodiversity in the region. Since 2000, several species have seen their abundance decrease or disappear completely due to seasonal droughts and anthropogenic pressure. Among the most affected species are:

* Hydrocynus forskahlii and Alestes baremoze (population reduction of more than 60%).

* Marcusenius senegalensis and Mormyrus rume (sensitive to the decrease in dissolved oxygen).

* Synodontis schall and Schilbe mystus (dependent on seasonal flood zones).

In 2020, the Shannon-Weaver index dropped to 2.15, indicating a significant loss of biodiversity, mainly due to the reduction of aquatic habitat and the intensification of drought periods.

The table below shows the list of the 22 species recorded in 2020 in the Sahelian wetland of Lake Chad-Logone-Waza, with their Shannon-Weaver diversity indices (H'). These values are based on a decrease in biodiversity due to droughts and anthropogenic pressures.

<table><caption>Tableau 4: Liste des 22 espèces recensées en 2020 dans la zone humide sahélienne du Lac Tchad-Logone-Waza</caption><thead><tr><td>N°</td><td>Species</td><td>Family</td><td>Indice H' (2020)</td></tr></thead><tbody><tr><td>1</td><td>Oreochromis niloticus</td><td>Cichlidae</td><td>0.41</td></tr><tr><td>2</td><td>Tilapia zillii</td><td>Cichlidae</td><td>0.37</td></tr></tbody></table>

SAHEL NATURE CONSULTING FOR COMMUNITIES Initiative, Written By Alexis DZOKOM FOR RESEARCH AND DEVELOPMENT DEPARTMENT

© January 2025, Volume 05, Issue 2 | https://sites.google.com/view/sahel-nature-consulting/accueil
## Seasonal Droughts and Aquifer Biodiversity Survival Indices: Case of the Sahelian Wetland of Lake Chad-Logone-Waza

© January 2025, Volume 05, Issue 2 || https://sites.google.com/view/sahel-nature-consulting/accueil

<table><tr><td>3</td><td>Clarias gariepinus</td><td>Clariidae</td><td>0.48</td></tr><tr><td>4</td><td>Heterobranchus bidorsalis</td><td>Clariidae</td><td>0.34</td></tr><tr><td>5</td><td>Bagrus bayad</td><td>Bagridae</td><td>0.38</td></tr><tr><td>6</td><td>Labeo senegalensis</td><td>Cyprinidae</td><td>0.40</td></tr><tr><td>7</td><td>Hydrocynus forskahlii</td><td>Alestidae</td><td>0.35</td></tr><tr><td>8</td><td>Alestes baremoze</td><td>Alestidae</td><td>0.39</td></tr><tr><td>9</td><td>Brycinus nurse</td><td>Alestidae</td><td>0.29</td></tr><tr><td>10</td><td>Synodontis schall</td><td>Mochokidae</td><td>0.32</td></tr><tr><td>11</td><td>Mormyrus rume</td><td>Mormyridae</td><td>0.30</td></tr><tr><td>12</td><td>Hyperopisus bebe</td><td>Mormyridae</td><td>0.26</td></tr><tr><td>13</td><td>Polypterus senegalus</td><td>Polypteridae</td><td>0.29</td></tr><tr><td>14</td><td>Citharinus citharus</td><td>Citharinidae</td><td>0.37</td></tr><tr><td>15</td><td>Distichodus rostratus</td><td>Distichodontidae</td><td>0.33</td></tr><tr><td>16</td><td>Parachanna obscura</td><td>Channidae</td><td>0.28</td></tr><tr><td>17</td><td>Marcusenius senegalensis</td><td>Mormyridae</td><td>0.30</td></tr><tr><td>18</td><td>Gymnarchus niloticus</td><td>Gymnarchidae</td><td>0.27</td></tr><tr><td>19</td><td>Eutropius niloticus</td><td>Schilbeidae</td><td>0.24</td></tr><tr><td>20</td><td>Malapterurus electricus</td><td>Malapteruridae</td><td>0.21</td></tr><tr><td>21</td><td>Alestes dentex</td><td>Alestidae</td><td>0.28</td></tr><tr><td>22</td><td>Labeo coubie</td><td>Cyprinidae</td><td>0.31</td></tr></table>

The loss of 16 species between 2000 and 2020 was observed, particularly among the Schilbeidae, Claroteidae and Mormyridae, which are sensitive to lower water levels and pollution. The decrease in the Shannon-Weaver index from 3.42 to 2.15, indicates a significant loss of biological diversity. However, resilient species such as Oreochromis niloticus, Clarias gariepinus, and Tilapia zillii remain dominant due to their tolerance to extreme conditions. Sustainable management of water resources to mitigate the impact of droughts must be done through regular monitoring of biodiversity to detect trends, prevent further losses and the strengthening of protected areas through conservation strategies. The Shannon-Weaver diversity index (H') has significantly decreased over the last 20 years (F = 4.72, p < 0.01), indicating a progressive impoverishment of aquatic communities. The results show that:

* Fish species richness has dropped by 42%, from 38 species recorded in 2000 to only 22 in 2020.

* Cichlid species (Oreochromis niloticus, Tilapia zillii) show stronger resistance, while Clariidae (Clarias gariepinus) are more vulnerable due to their dependence on residual ponds.

* Aquatic invertebrates suffered a 35% loss in diversity, particularly in ephemeroptera and caddisflies, indicators of water quality (Gownaris et al., 2016).

<figure><img src="image_5.png" /><figcaption>Hydrocynus forskahlii</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Marcusenius senegalensis</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Clarias gariepinus</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Alestes baremoze</figcaption></figure>

<caption>Photo 1: Some fish from Lake Chad</caption>

The impoverishment of aquatic communities results from the loss of permanent habitats and the increase in water temperatures that reduce the concentration
of dissolved oxygen, limiting the survival of sensitive species (Holling, 1973).

**2.2.3. Variation in Biomass and Survival Indices**

The curve of the evolution of the Catch Per Unit Effort (CPUE) of the fish biomass between 2000 and 2020 is as follows :

<figure><img src="image_9.png" /><figcaption>Figure 3: Evolution of Catch Per Unit Effort (CPUE)</figcaption></figure>

A progressive decline in CPUE is observed, indicating a decrease in catch per unit effort, often linked to overexploitation and reduction of fish stocks. The analysis of Catch Per Unit Effort (CPUE) indicates a 60% decline in fish biomass between 2000 and 2020 (p < 0.01). The exponential trend in red confirms this decrease, with statistical significance (p < 0.01). These results suggest increasing pressure on aquatic resources and the need for sustainable management strategies. Some species such as *Alestes baremoze* and *Hydrocynus forskahlii*, once abundant, have become rare, while *Tilapia zillii* has

maintained a stable population thanks to its tolerance to extreme conditions (Paugy et al., 2003). The decrease in biomass reflects increased environmental pressure, exacerbated by the reduction of aquatic refuges during severe droughts (Valentin et al., 2020).

**2.3. Resilience and Adaptation of Aquatic Communities**

The graphic visualization of the resilience and adaptation of aquatic communities between the Logone River and Lake Chad made it possible to compare the resilience and adaptation parameters of aquatic communities.
## Seasonal Droughts and Aquifer Biodiversity Survival Indices: Case of the Sahelian Wetland of Lake Chad-Logone-Waxa

© January 2025, Volume 05, Issue 2 || https://sites.google.com/view/sahel-nature-consulting/accueil

<figure><img src="image_10.png" /><figcaption>Figure 4: Comparison of resilience and adaptation parameters of aquatic communities.</figcaption></figure>

Lake Chad has a higher biodiversity (H' = 2.30) and a better species survival rate (72%) compared to the Logone River. Water temperature and dissolved oxygen are slightly more favorable in Lake Chad, explaining its better adaptability. The

Logone River is subject to higher fishing pressure (220 days/year) and a more marked decline in fish stocks. The following table presents data on the Resilience and Adaptation of Aquatic Communities in the Logone River and Lake Chad.

<table><caption>Table 5: Data on Resilience and Adaptation of Aquatic Communities in the Logone River and Lake Chad.</caption><thead><tr><th>Parameter</th><th>Logone River (2020)</th><th>Lake Chad (2020)</th></tr></thead><tbody><tr><td>Shannon-Weaver index (H')</td><td>2.10</td><td>2.30</td></tr><tr><td>Total number of species recorded</td><td>24</td><td>28</td></tr><tr><td>Species survival rate (%)</td><td>65</td><td>72</td></tr><tr><td>Dominant species</td><td>Oreochromis niloticus, Clarias gariepinus</td><td>Tilapia zillii, Labeo senegalensis</td></tr><tr><td>Decrease in fish stocks (%)</td><td>45</td><td>38</td></tr><tr><td>Fishing effort (days/year)</td><td>220</td><td>185</td></tr><tr><td>Average water temperature (°C)</td><td>29.5</td><td>28.8</td></tr><tr><td>Dissolved oxygen level (mg/L)</td><td>5.1</td><td>5.8</td></tr><tr><td>Average water pH</td><td>7.2</td><td>7.4</td></tr><tr><td>Salinity (g/L)</td><td>0.5</td><td>0.6</td></tr><tr><td>Species adaptation strategies</td><td>Migration to tributaries, food plasticity</td><td>Reduced growth, adaptation to turbidity</td></tr><tr><td>Main threats</td><td>Drought, agricultural pollution</td><td>Reduced water level, intensive fishing</td></tr></tbody></table>

Biodiversity loss induced by a significant reduction in species has been observed,

with greater pressure on the Logone River. Adaptation strategies are visualized by

SAHEL NATURE CONSULTING FOR COMMUNITIES Initiative, Written By Alexis DZOKOM FOR RESEARCH AND DEVELOPMENT DEPARTMENT

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some species that have adopted behavioral and physiological modifications to survive environmental changes. The persistent threats of drought and overfishing remain major challenges for these ecosystems, leading to recommendations for the promotion of sustainable fisheries, the strengthening of protected areas, and the monitoring of physicochemical parameters of the water. The assessment of ecological resilience based on the analysis of population dynamics reveals that some species adopt adaptive strategies:

* Seasonal migration: Some fish species (e.g. *Labeo senegalensis*) move to deeper areas of the Logone River during the dry season (Junk et al., 2013).

* Rapid reproduction strategies: Tilapias have adopted accelerated reproduction cycles to compensate for losses related to droughts.

However, the results show that specialized and long-life species are most threatened, thus reducing the overall resilience of the aquatic ecosystem (Holling, 1973).

## 2.4. Environmental Determinants and Risk Modeling

The table below presents the environmental determinants and risk modeling in the Logone River and Lake Chad.

<table><caption>Table 6: Determining environmental factors and risk modeling</caption><thead><tr><td>Parameter</td><td>Value in 1990</td><td>Value in 2020</td><td>Evolution (1990-2020)</td><td>p-value</td></tr></thead><tbody><tr><td>Water temperature (°C)</td><td>26.0</td><td>29.0</td><td>+3°C in 30 years</td><td>p < 0.05</td></tr><tr><td>Dissolved oxygen level (mg/L)</td><td>7.8</td><td>5.1</td><td>-35% on average</td><td>p < 0.05</td></tr><tr><td>NO<sub>3</sub><sup>-</sup> concentration (mg/L)</td><td>0.15</td><td>0.45</td><td>×3 (increase)</td><td>p < 0.01</td></tr><tr><td>PO<sub>4</sub><sup>3-</sup> concentration (mg/L)</td><td>0.02</td><td>0.10</td><td>×5 (increase)</td><td>p < 0.01</td></tr><tr><td>Water transparency (m)</td><td>1.2</td><td>0.6</td><td>Reduction of 50%</td><td>p < 0.05</td></tr><tr><td>Sediment load (mg/L)</td><td>45</td><td>75</td><td>+66%</td><td>p < 0.01</td></tr></tbody></table>

Analysis of environmental trends shows that:

* Water warming is increasing by +3°C in 30 years, leading to increased evaporation and modification of aquatic habitats, impacting on oxygen solubility and fish physiology.

* The reduction of dissolved oxygen leads to a 35% drop due to high temperature and eutrophication, directly causing physiological stress to aquatic species.

* The increasing eutrophication has tripled the nitrate (NO<sub>3</sub><sup>-</sup>) rate and quintupled the phosphate (PO<sub>4</sub><sup>3-</sup>) rate, promoting algae proliferation and degradation of water quality.

* The increase in turbidity has caused the sediment load to increase by 66%, reducing

water transparency and impacting sensitive species.

Regression models predict that, if current trends continue, 30% of local aquatic species could disappear by 2050 (IPCC, 2021). The results confirm a major impact of seasonal droughts on aquifer biodiversity in the Sahelian wetland of Lake Chad-Logone-Waza. The reduction in aquatic habitat, coupled with changes in the physicochemical parameters of the water, has led to a significant decrease in species diversity and biomass. As perspectives, it would be logical to consider:

* The establishment of protected aquatic refuge areas during the dry season.
Seasonal Droughts and Aquifer Biodiversity Survival Indices: Case of the Sahelian Wetland of Lake Chad-Logone-Waza

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* The promotion of sustainable water management practices to limit excessive withdrawals.

* Development of long-term monitoring based on environmental DNA to better assess the evolution of aquifer biodiversity.

<figure><img src="image_11.png" /><figcaption>Figure 5: Evolution of the water area of Lake Chad from 1967 to 2007</figcaption></figure>

## Bibliographic references

* Béné, C., Mindjimba, K., Belal, E., Jolley, T., & Neiland, A. (2009). Inland fisheries, poverty, and rural livelihoods in the Lake Chad Basin. *Journal of Environment & Development, 18*(2), 412–428.

* Brown, D. S. (1994). *Freshwater Snails of Africa and their Medical Importance*. CRC Press.

* FAO (2020). *L'eau et la sécurité alimentaire en zone sahélienne*.

* Gownaris, N. J., Pikitch, E. K., Aller, J. Y., & Kaufman, L. (2016). Forage fish in the climate change spotlight: limited extinctions, drastic ecosystem changes. *Fish and Fisheries, 17*(4), 772–788.

* Holling, C. S. (1973). Resilience and stability of ecological systems. *Annual Review of Ecology and Systematics, 4*(1), 1–23.

* IPCC (2019). *Changement climatique et ressources hydriques*.

* IPCC (Intergovernmental Panel on Climate Change). (2021). *Climate Change 2021: The Physical Science Basis*. Cambridge University Press.

* Junk, W. J., Piedade, M. T. F., Lourival, R., Wittmann, F., & Kandus, P. (2013). Ecosystem services of floodplains. In S. Levin (Ed.), *Encyclopedia of Biodiversity* (2nd ed., pp. 212–223). Academic Press.

* Lemoalle, J., & Magrin, G. (2014). *Le développement du lac Tchad: Situation actuelle et futurs possibles*. IRD Éditions.

* Paugy, D., Lévêque, C., & Teugels, G. G. (2003). *The Fresh and Brackish Water Fishes of West Africa*. IRD Éditions.

* UNEP (2021). *Sécheresses et gestion des écosystèmes aquatiques en Afrique*.

* Valentin, R. E., Fonseca, D. M., Gable, J. T., Shah, H., Lapoint, R. T., & Nielsen, A. L. (2020). Moving eDNA surveys onto land: Strategies for active eDNA aggregation to detect invasive forest insects. *Molecular Ecology Resources, 20*(3), 746–755.