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# Erosion vs. accretion of sandy beaches: their impacts on nursery areas of coral reef fish

David Lecchini, Julien Tessonneau, Maguy Dulormne, Emma Gairin, Natacha Roux, Viliame Waqalevu, Léa Vignaud, Lana Minier, Océane Beaufort, Dorothée James, et al.

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## To cite this version:

David Lecchini, Julien Tessonneau, Maguy Dulormne, Emma Gairin, Natacha Roux, et al. Erosion vs. accretion of sandy beaches: their impacts on nursery areas of coral reef fish. Environmental Biology of Fishes, 2024, 107 (5), pp.593-598. 10.1007/s10641-024-01547-w. hal-04611805

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Distributed under a Creative Commons Attribution 4.0 International License
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Prepared for submission in "Environmental Biology of Fishes" as brief communications

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Erosion vs. accretion of sandy beaches: their impacts on nursery areas of coral reef fish

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David Lecchini¹,²*, Julien Tessonneau³,⁴*, Maguy Dulornne³, Emma Gairin⁵, Natacha Roux⁵, Viliame

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Waqalevu⁶, Léa Vignaud⁴, Lana Minier¹,⁷, Océane Beaufort⁸, Dorothée James⁹, Malika Trouillefou⁴,

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Charlotte.R. Dromard⁴, Frédéric Bertucci¹⁰

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¹ PSL Research University: EPHE-UPVD-CNRS, UAR 3278 CRIOBE, 98729 Moorea, French

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Polynesia

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² Laboratoire d'Excellence "CORAIL", 66000 Perpignan, France

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³ UMR Ecologie des Forêts de Guyane (EcoFoG), Université des Antilles CNRS, Cirad, INRAE,

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Université de Guyane, 97159 Pointe-à-Pitre, Guadeloupe

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⁴ Laboratoire de Biologie des Organismes et Ecosystèmes Aquatiques (BOREA), Université des

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Antilles - MNHN - CNRS 8067 - SU - IRD 207 - UCN, 97100 Guadeloupe & 97200 Martinique,

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France

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⁵ Marine Eco-Evo-Devo Unit, Okinawa Institute of Science and Technology, Okinawa, Japan

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⁶ Yas SeaWorld Research and Rescue Center, Abu Dhabi, United Arab Emirates

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⁷ Polynésienne des Eaux, Vaitape, Bora-Bora, French Polynesia

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⁸ Association Kap Natirel, 97100 Guadeloupe, France

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⁹ EPHE-PSL University, Coastal GeoEcological Lab, 35800 Dinard, France

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¹⁰ UMR MARBEC, University of Montpellier, CNRS, IFREMER, IRD, 34203 Sète, France

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* Equal first authors

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□ Corresponding author: david.lecchini@ephe.psl.eu

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ABSTRACT

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Sandy beaches have experienced substantial human modifications worldwide for centuries. Yet, sandy beaches are known as nursery areas for coral reef fishes. Due to global and local stressors, the degradation of nurseries such as the sandy beaches may have wide reaching consequences for the resilience of reef fishes. In our study, the impact of erosion vs accretion phenomena was highlighted on coral reef fish community in Guadeloupe in comparing the abundance and diversity of juvenile fish between two nearby beach sites (one that undergoes accretion and the other one that undergoes erosion since the 1950s). The main result showed that a significant retreat of sandy beach along the entire coastline could create new habitats (by bringing out an old reef) favorable to the settlement of fish juveniles. This result is surprising when considering that erosion affects negatively coral reefs through the sedimentation, deterioration of water quality, and loss of habitat.

**Keywords** Sandy Beach, Nursery area, erosion, coral reef fish
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44 **Funding:** We wish to acknowledge the Fondation de France for supporting the project "Quel littoral
demain dans l'Outre-mer Français" (2019-08602), Polynésienne des Eaux, ANR-19-CE34-0006-Manini
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46 and ANR-19-CE14-0010-SENSO, ANRT grant (CIFRE 2021/1268). We also acknowledge the BOREA
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and EcoFoG laboratories and Kap Natirel association for their precious help.

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**Conflicts of interest/Competing interests:** The authors declare that no conflict of interest or
competing interests exist.

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**Ethics approval:** No ethical approval was needed for this study.

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**Consent to participate:** All listed authors fully agree to participate in this study.

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**Consent for publication:** All listed authors gave final approval to publish the present study.

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**Availability of data and material:** All geomorphology and fish data are fully available with a private
link from the Figshare database.

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**Code availability:** Not applicable.

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**Authors' contributions:** David Lecchini, Maguy Dulornne, Océane Beaufort and Malika Trouillefou
contributed to the study conception and design. Material preparation and data collection were performed
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by Julien Tessonneau and Léa Vignaud. The data were analysed by Julien Tessonneau, Emma Gairin,
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Lana Minier and Dorothée James. The first draft of the manuscript was written by David Lecchini,
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Frédéric Bertucci and Julien Tessonneau. All authors commented on previous versions of the manuscript
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(Natacha Roux, Viliame Waqalevu, Charlotte.Dromard). All authors read and approved the final
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manuscript.
70 INTRODUCTION

As they represent a transition zone between the terrestrial and marine environments, sandy beaches are one of the most important ecotones in the world, covering 31% of the world's ice-free shoreline (Luijendijk et al. 2018). Sandy beaches are highly valuable economical, ecological, recreational, and cultural assets for societies worldwide (e.g., Barbier et al. 2011; Jordan and Frohle 2022). For example, beside buffering wave action, sand can act as a natural filter of water from the terrestrial environment, helping to retain nutrients or degrade some pollutants. This helps to protect marine ecosystems located close to the shoreline, such as seagrass beds and coral reefs. Thus, beaches and their adjacent lagoon are essential habitats, notably as breeding and feeding grounds of numerous marine and terrestrial emblematic animals such as sea turtles, coral reef fish, shorebirds, and seabirds (e.g., Defeo and McLachlan 2013; Dugan et al. 2008).

However, sandy beaches and their associated biodiversity are under increasing threat from erosion (e.g., Zhang et al. 2004; Gairin et al. 2021). Erosion can be due to global rising waters, currents and hydrodynamic conditions, runoff waters, but also anthropogenic activities, such as sand removal for constructions or coastal development (e.g., ports, marinas, resorts, dikes) that can cover or remove sandy beaches (Hubbard et al. 2014; Jaramillo et al. 2021). In Guadeloupe (Lesser Antilles), the coastline has become increasingly urbanized, rising from 4% of artificial structures on the coast in 1950 to 18% in 2017 (Giraud-Renard et al. 2022). This change is linked to fast-paced economic development in the 1980s, associated with the modification of coastal zones and beaches (landscaping, roads, parking, introduction of exotic species such as Cocos nucifera) to improve their attractiveness (Desse 2011). This, especially through tourism, increased erosion by inducing the disappearance of native vegetation and trampling. In addition to leading to a loss of terrestrial habitats, erosion impact marine biodiversity by increasing the amount of sand and potentially pollutants found in the water (Brown and McLachlan 1990; Bełdowska et al. 2016). Yet, beaches and their adjacent lagoon have long been recognised to be nurseries for many coral reef fish (Gairin et al. 2023).

Overall, due to the increasing effects of human activities on coastal habitat degradation, there is a pressing need to highlight and conserve nursery habitats in coral reefs (Beck et al. 2001; Dance et al. 2021; Gairin et al. 2023). Sand movement, which may or may not silt up adjacent habitats, can alter the
98 environmental conditions of each beach, significantly influencing habitat structure which in turn affects
99 the ecology and behavior of marine populations (e.g., Cardoso et al. 2012; Silva de Aguiar et al. 2020).
100 The present study aims to highlight the impact of erosion vs accretion phenomena on coral reef fish
101 community in Guadeloupe. To do so, the abundance and diversity of juvenile fish populations were
102 compared between two nearby beach sites (one that undergoes accretion and the other one that undergoes
103 erosion since the 1950s).
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105 MATERIAL AND METHODS

106 Sandy beaches are one of the Caribbean's most emblematic landscapes. They represent 6,256
107 km, i.e., one third of the total coastline of the region (Spencer et al. 2022). An analysis of the coastline
108 evolution between 1950 and 2022 was carried out to identify erosion and accretion sites along the 'Autre
109 Bord' beach area in the city of Le Moule, Guadeloupe (Lesser Antilles, 16°19'55"N / 61°19'56"W). This
110 tourist area is characterized by a white sand 1.3 km-long beach (Fig. 1). For over 50 years, urbanization
111 and tourism development have been increasingly causing erosion almost all along the "Autre Bord"
112 beach (Iskander 2021). The 1950 satellite image of the "Autre Bord" beach was downloaded from the
113 IGN platform, while the 2022 image was obtained via the Dinamis platform. QGIS (which version)
114 software was used to georeference these images, plot coastlines, draw polygons corresponding to erosion
115 or accretion areas, and measure coastline retreat or advance (see methods in Giraud-Renard et al., 2022).

116 Juvenile fish counts were carried out at a site identified as eroding since 1950 (erosion site) and
117 at a site that had been identified as accreting (accretion site - Fig. 1). At both sites, two 20m long transects
118 were positioned at 5m, 15m and 25 m from the coastline. A visual snorkeling census was carried out
119 along 2.5m-wide transects. Four temporal replicates were conducted between February and May 2023
120 (one count per month centered on the new moon). Fish were identified at the family level due to, in the
121 absence of an identification guide, the difficulty to recognize them at juvenile stages in Guadeloupe
122 reefs.

123 Juvenile fish abundance did not follow a Normal distribution (Shapiro-Wilk test, W = 0.63, P <
124 10⁻³) and differences between erosion and accretion sites at the three distances were compared by means
125 of Wilcoxon-Mann-Whitney tests. For erosion and accretion sites, differences between distances and
replicates were compared by means of Kruskal-Wallis test followed by a Dunn post-hoc test should there be a significant difference. All statistical analyses were performed with R Studio (version 2023.06.0) at the significance level α = 0.05.

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130 RESULTS AND DISCUSSION

131 The comparison of the coastline profile in 1950 and 2022 showed that it has mostly retreated over
132 time (95% of the coastline has undergone erosion - Fig. 1). The erosion is most marked on the eastern
133 side of the "Autre Bord" seaside, with a maximum loss of 31 meters. Only 5% of the beach underwent
134 accretion, with a maximum coastline advance of 10m. The coastline dynamic since 1950 has thus shown
135 that the "Autre Bord" seaside is mainly eroding (Fig. 1). This phenomenon can be linked to daily (waves)
136 or exceptional (storms) environmental conditions (Dolique et al. 2019; Berre 2017). Indeed, the east
137 coast of Guadeloupe (Grande Terre) is particularly exposed to waves and the Guadeloupe archipelago
138 has seen many devastating weather events, such as Hurricane Hugo in 1989. Since the 1950s, cyclonic
139 activity in the Caribbean has multiplied and intensified as a result of climate change (Stephenson and
140 Jones 2017). Sea-level rise is also one of the causes of erosion accentuated by global warming
141 (Stephenson and Jones 2017). In addition to this phenomenon, which is becoming more pronounced
142 every year, the coastline is also being modified by human activities. Elko et al. (2021) estimated that the
143 coastal land loss and beach erosion have increased the demand for sand resources, with ~28 million m3
144 of sand mined per year from U.S. waters over the past decade for the purposes of beach renourishment.
145 Often linked to tourism (Desse 2011), these modifications weaken the natural structure of beaches and
146 increase their vulnerability to erosion (Shadrin 2013). Even though the coastline dynamic of "Autre
147 Bord" seaside shows some accretion, this phenomenon, linked to the site's geomorphology and
148 sedimentary movements, appears negligible considering the proportion of coastline movement in the
149 area (95% retreat and 5% advance) between 1950 and 2022.

150 The fish surveys revealed a total of 148 juveniles at the erosion site, divided in 8 families, with
151 Labridae being the most represented family with 83 juveniles, followed by Haemulidae (20),
152 Pomacentridae (19), and Acanthuridae (16). These four families also represented the large majority of
153 the 22 juveniles observed at the accretion site (Table 1). The overall abundance of juveniles was
significantly different between sites, with more fish observed on erosion sites than accretion sites (Wilcoxon-Mann-Whitney test, W = 489, P < 10⁻³). This difference was observed on each transect regardless of distance from the shore (Wilcoxon-Mann-Whitney tests, at 5m: W = 60, P = 0.001; at 15m: W = 52.5, P = 0.025; at 25m: W = 50.5, P = 0.042). Within sites, there were no differences between the three transects for the erosion site (Kruskal-Wallis test, χ²₂ = 1.89, P = 0.39) and the accretion site (Kruskal-Wallis test, χ²₂ = 2.01, P = 0.36) (Fig. 2). While there was no difference between the four temporal replicates on the accretion site (Kruskal-Wallis test, χ²₃ = 4.26, P = 0.23), significantly more fish were observed during the fourth replicate (May) than during the second one (March) at the erosion site with a total number of respectively 77 and 6 juvenile fish (Kruskal-Wallis test, χ²₃ = 13.42, P = 0.004; Dunn test, Z = 3.47, P = 0.003). All other replicates did not significantly differ (Dunn tests, Z = 1.00 – 2.46, P > 0.05).

The spatial dynamic of fish communities along the “Autre Bord” seaside reveals a possible link between sand erosion/accretion and the presence of juveniles (Fig. 2). Erosion has a positive effect on juvenile communities (more fish and more families than the accretion site). This may seem surprising when considering that erosion affects coral reefs through the sedimentation, deterioration of water quality, and loss of habitat (e.g., Bartley et al. 2014; Feagin et al. 2019). However, the limited availability of a nursery zone such as a sandy beach in Guadeloupe could represent a bottleneck for reef fish. Indeed, if the settlement rate of juveniles is low, there will be a low renewal of adult populations and the fish stock is going to decrease (Lecchini and Galzin 2003). Johnson (2007) showed that settlement success and adult population dynamics of reef fishes are inherently linked to the availability of low-relief habitat along the coastline. França et al. (2012) showed that shallow water estuarine and coastal fish nurseries are often most vulnerable to anthropogenic impacts which adversely affect the quantity and quality of such habitats and then have a significant effect on adult fish stocks (Lecchini and Galzin 2003). Our observation of the higher number of juveniles on erosion site might be linked to the particular geomorphology of the “Autre Bord” seaside, which has a reef close to the beach that can be buried by sand or uncovered depending on sedimentary movements. However, different other environmental drivers could also explain the fish juvenile distribution along sandy beaches (e.g., Hajisamae and Chou 2003; Gairin et al. 2023). For example, Silva de Aguiar et al. (2020) showed that fish abundance was
182 mainly influenced by physicochemical variables, as it was positively correlated with salinity and
183 dissolved oxygen and negatively correlated with temperature. Moreover, Defeo (2003) showed that
184 sandy beaches are dynamic habitats, inhabited by fish and invertebrate assemblages that are often
185 structured by physical drivers (e.g., wind, waves, tide).

186 To conclude, our study, although limited to a specific site of Guadeloupe Archipelago ("Autre
187 Bord" seaside), showed that a significant retreat of sandy beach along the entire coastline can create new
188 habitats (by bringing out an old reef) favorable to the settlement of fish juveniles. Sandy beaches are
189 often inhabited by diverse assemblages of fish and marine invertebrates that use this coastal zone as
190 their only nursery area (e.g., Silva de Aguiar et al. 2020; Gairin et al. 2023). As suggested by Dance et
191 al. (2021), artificial reefs or restoration of low-relief habitat for juvenile fishes are relatively rare,
192 although they could increase early life survival (e.g., Lecchini 2004, Wetmore et al. 2020). However,
193 similarly to results from many studies around the world (e.g., Gairin et al. 2021; Giraud-Renard et al.
194 2022), the "Autre Bord" seaside is mainly affected by erosion, which is becoming more pronounced as
195 a result of global warming, threatening both human users and biodiversity. Tourism managers in
196 Guadeloupe (and elsewhere in the world) should take into consideration the relationship between nature
197 conservation and the creation of economic and social benefits along sandy beaches.

198

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285 Tables

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287 Table 1: Total number of juvenile fish of each family observed at erosion and accretion sites from

288 February to May 2023 (all transects confounded).

<table><thead><tr><th>Family</th><th>Erosion site</th><th>Accretion site</th></tr></thead><tbody><tr><td>Labridae</td><td>83</td><td>3</td></tr><tr><td>Haemulidae</td><td>20</td><td>10</td></tr><tr><td>Pomacentridae</td><td>19</td><td>3</td></tr><tr><td>Acanthuridae</td><td>16</td><td>5</td></tr><tr><td>Scaridae</td><td>5</td><td>0</td></tr><tr><td>Pomacanthidae</td><td>2</td><td>1</td></tr><tr><td>Sciaenidae</td><td>2</td><td>0</td></tr><tr><td>Chaetodontidae</td><td>1</td><td>0</td></tr><tr><td>Total</td><td>148</td><td>22</td></tr></tbody></table>

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291 **Figure captions**

292
293 **Fig. 1: Location of the “Autre Bord” seaside** in Le Moule city, Guadeloupe with some sites having
294 undergone accretion since the 1950s (in yellow on the map) and some other having undergone erosion
295 (in red). The two sites chosen for the fish counts are represented by the stars.

296
297 **Fig. 2: Abundance of juvenile fish** (mean ± SE – number of fish per 100m²) at 5m, 15m, and 25m from
298 the coastline for the erosion and accretion sites.

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300
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