Grain-size trend and ecological state of southern mediterranean coastal lagoons: Case of the Gulf of Tunis

Highlights

• Mediterranean coastal lagoons are rich ecosystems as they offer ecological niches for several endemic species.

• Active sediment dynamic of the sandbank barrier-lagoon.

• Biodiversity indexes and EXCLAME indicator are adopted to assess the lagoon ecological state.

• Anthropogenic drivers impact dramatically the ecological state of the coastal lagoon.

Abstract

Coastal lagoons are among the highest biological productive ecosystems; they offer a variety of habitats and feeding grounds for several species. These ecosystems are besmirched by human activities which entail the degradation to their environmental aspects. 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. The methodology is based on the sedimentological analysis and the microfauna identification. The study of the environmental quality of the lagoon was built on an unprecedented identification of the macrophyte communities and the use of diversity indexes and the EXCLAME indicator; the latter combines the taxonomic composition and the species abundance. The sedimentological investigations showed that the northern and southern areas of the lagoon, as well as its channel area are all dominated by a sandy facies provided by the continental shelf i.e. the nearshore zone. The central area of the lagoon is characterized by the finest grain size facies (<0.063 mm) mostly supplied by the aeolian terrestrial fluxes and the delta flood yields. The harvested pelagic and benthic microfauna indicated that the lagoon is supplied by sediments of mixed continental-fluvial and marine origins. The inventory of macrophytes demonstrates the presence of 19 species, five of which are reference species. The dominant taxa are those of Green algae (53%) with several opportunistic and invasive species with a high outbreak altering the ecosystem's natural balance, then Brown algae (21%), Red algae (16%) and angiosperms (10%). Those results, along with the biodiversity indexes outcomes, revealed the quality loss of the ecosystem and the water eutrophication. The lagoon floristic communities are slightly unbalanced, which might be due to the natural disturbance and the anthropogenic activities. Those results conform to the EXCLAME indicator (EQRMAC = 0.24), illustrating the poor ecological state of the Kalâat Andalous lagoon water body.

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.