Grain-size trend and ecological state of southern mediterranean coastal lagoons: Case of the Gulf of Tunis
Introduction
Coastal lagoons are valuable economic and ecological resources; they are among the highest biological productive marine ecosystems (Anthony el al., 2009). They provide a variety of habitats, nursery areas and feeding grounds (Anthony el al., 2009) namely salt marshes, seagrasses, and mangroves (Basset et al., 2013) for different species such as macrophytes, fish, shellfish and migrating birds (Anthony el al., 2009; Pérez-Ruzafa et al., 2010; FAO, 2015). These ecosystems have always been a great interest to humans supplying harbor and navigations facilities (Pérez-Ruzafa et al., 2011).
Coastal lagoons are highly impacted by agriculture, fishing, aquaculture, mining, tourism, pollution, urbanization, dredging, diverse forms of modification in their watersheds and general developmental activities (Vicente and Miracle, 1992; Pérez-Ruzafa et al., 2011; Moreno-González et al., 2013; Christia et al., 2014; Newton et al., 2018; Sheppard et al., 2019). Pressures on these environments are set to increase during the twenty-first century as human populations and demands for water and production of wastes continue to grow (Maanan et al., 2013; Pérez-Ruzafa et al., 2019) affecting water quality and leading to water level alteration (Sheppard, 2019).
Hence, macrophytes and microfauna presence and assemblage in the aquatic ecosystems are related to the changing environmental characteristics (Abou-Hamdan et al., 2005; El Asri et al., 2015). Those organisms have structural and physiological stress responses (Pérez-Ruzafa et al., 2011; El Asri et al., 2015). Their distribution, abundance, destruction and alteration are very sensitive to different kinds of disturbances like the increase in turbidity, sedimentary shortage and water temperature modification (El Asri et al., 2018). The dominance of opportunistic species that tolerate high levels of pollutants (organic and/or inorganic) reveals the degradation of the environment (Breugnot et al., 2004; Affian et al., 2009; Santos et al., 2011). Regarding the coastal lagoons' importance, their biodiversity and its fluctuations, it is important to study those ecosystems’ geological, biological and environmental characteristics and to understand their evolution.
Identifying the different threats and pressures, their sources, origins and impacts on the functioning of these ecosystems and their components, makes it possible to design and implement suitable strategies for suitable management. To assess quality of coastal lagoons and their vulnerability, different proxies have been used, such as biological indicators (foraminifera, macrophytes), geochemical approaches and indexes, as well as interactions between biocenosis and biotope (Pérez-Ruzafa et al., 1991; Madsen et al., 2001; Selvaraj et al., 2004; Maanan et al., 2004; Viaroli et al., 2004; Marin Guirao et al., 2005; Coops et al., 2007; Frontalini et al., 2009; Martins et al., 2013).
In Tunisia, coastal lagoons are the subject of diverse studies for the decay which they undergo (Ruiz et al., 2005; Chouba et al., 2007; Martins et al., 2015; Jouili et al., 2016; Khsiba, 2018). The purpose of this study is the identification of the grain size characteristics and the unprecedented recognition of the vegetation cover, so as to assess the ecological state and the water body quality of Kalâat Andalous lagoon: a fast changing ecosystem under its dynamic sandy spit and human pressures.
Section snippets
Study area
Kalâat Andalous (KA lagoon) is located in the western bay of the Gulf of Tunis between 37°05′22″N and 10°11′49′E coordinates, North Africa, Mediterranean Sea. It is an early-formed sedimentary lagoon bordered by the old mouth of the Medjerda River northward and the new artificial mouth southward (Fig. 1 (a)).
The lagoon is located downstream of the old course of the Medjerda River deltaic plain. The depression is filled by quaternary alluvial deposits, mostly clay-sandy (Oueslati et al., 2006).
Material and methods
We attempt to determine the grain size trend and the quality of the ecosystem of the lagoon by the sedimentological and ecological approaches. We subdivided the study site into three areas on the basis of the available information and prediction of the marine, the continental and terrigenous influences; area “A” is the northern area of the lagoon, area “B” is the central area, divided into B1 in the harbor mole zone subjected to the anthropogenic effects and B2 in the channel access zone
Sedimentological characterization
The grain size analysis results of the surface sediments collected from the KA lagoon are illustrated in Table 2. Grain size parameters indicate that 60% of the subsurface of the KA lagoon sediments are dominated by sandy facies (∼46–99% of sand). They are particularly located at the northern part of the lagoon (Area A) (i.e. R1S1, R1S10, R4S1 and R4S10) at the channel area (area B) (i.e. R3S1 and R3S10) and at the south (i.e. R5S1 and R5S10) (Area C). The muddy distribution represents 40% of
Discussion
The grain size characterization of the North-eastern area of the lagoon indicates that it is a calm area with low energy, similar to the western zone of the Korba lagoon (Tunisia) (Bouden et al., 2009). This area is also occasionally subject to storm waves and tidal actions causing the deposit of small fractions of coarse sand. It corresponds to the development area of the sandy spit, which depends on the presence of sufficient sand-sized material to form a barrier ridge (Reinson, 1979). The
Conclusion
Sedimentological analyses of the KA lagoon reveal the presence of three heterogeneous areas; the northern part of the lagoon, the access channel area and the southern part of the study area which are dominated by sandy facies indicating a dynamic wave-dominated progressive lagoon sandy barrier. The deposition and settling of the fine fraction are both explained by the natural supply of the finest grain size (i.e. aeolian fluxes and delta load) and by the presence of the harbor moles, which are
Funding
This study was undertaken by the Tunisian-French research project funded by the Mixed Committee of University Cooperation, PHC-UTIQUE- CMCU –RYSCMED n°16G1005. The fieldwork was carried out by the National Institute of Marine Sciences and Technologies in Carthage Salammbô, Laboratory of Marine Environment (LR02INSTM04), University of Tunis Carthage, Tunisia and the Laboratory of Hydrosciences, Montpellier, France.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank Dr. Mohamed MAANAN from the University of Nantes, France, for his valuable discussions that helped improve this manuscript. Thanks also to translator Ahmed GADDEH for proofreading and enhancing the language use.
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