This thesis is subject to the evaluation for getting a government fellowship (bourse de these ministeriel).

This doctoral project aims at understanding the ecological role of Posidonia wrack as a support for biodiversity and for the trophic ecology of the marine environment and the beach. In the project the candidate will study how invertebrates, fish and their trophic relationships are affected by the wrack in the shallow areas facing the beach.

Scientific Background
Sandy shores, with their dune system, the sandy beach and the surf zone are socio-ecological system and provide exceptionally high ecosystem services: over $100,000/ha/year, 4 times more than seagrass beds and almost 30 times more than terrestrial forests (McLachlan and Defeo 2018). In the Mediterranean, they also represent a significant share of the GNP of coastal countries. One of the original features of Mediterranean beaches, on a global scale, is the presence, permanent or not, of dead leaves of the endemic magnoliophyte (flowering plant) Posidonia oceanica (Boudouresque et al 2017). Leaves that detach from the plant accumulate on the beach where they are called wrack and sometimes compact with the sand, forming berms called “banquettes” (Boudouresque and Meinesz 1982; Mateo et al. 2003). These structures can have a direct effect on erosion as a barrier against waves and they constitute a reserve of sand. Posidonia accumulations can also play an important ecological and trophic role. First, they constitute a carbon sink, just like the plant, and therefore contribute to global ecosystem services. Their disappearance is just as important as that of seagrasses (Waychott et al 2019). In addition, they constitute a fertilizer for the dune vegetation (Cardona and García, 2008) ,for the macroalgae and the seagrass itself when they stay or return to the sea (Fig. 1; Abadie et al 2018). When waves move them back and forth from the beach to the surf zone or from the seagrass to other habitats, this necromass (or leaf litter or wrack) becomes a habitat and a food source for many organisms. During decomposition the litter can be consumed by many invertebrates that inhabit the beach and marine shallow areas (small crustaceans, gastropods, bivalves); Simeone et al., 2013; Colombini et al., 2009; Mateo et al., 2003; Simeone and De Falco, 2012; Michel et al. 2015, Vacchi et al., 2017). The necromass exported to adjacent marine habitats can potentially be of great importance to local and artisanal fisheries (Boudouresque et al 2016; Jackson et al. 2001). Invertebrates that feed on decaying leaves are prey for many coastal fishes that spend life history stages near beaches. This is the case, for example, for some commercial species such as seabreams or red mullets (Garcia-Rubies and Macpherson 1995; Bussotti and Guidetti 2011). In a preliminary study done in Sardinia (Italy) in 2020, we found more juvenile fish in leaf litter patches than in the adjacent sand (Bussotti, Guidetti, Rossi, in preparation). Although according to Boudouresque (personal communication), one ton of bank turned over or returned to the sea represents 35 kg (wet mass) of fish for fishermen, currently there are no scientific publications addressing this subject. The scientific community recognizes that additional studies are needed to understand the ecological and functional role of Posidonia accumulations (Vacchi et al., 2017).
We will sample in beaches with and beaches with removed wrack (by grooming to enhance tourism). We will sample wrack patches that are in front of the beach and in corresponding areas without litter, during the spring-summer recruitment period in 2022. We will sample juvenile fish using visual census methodology and will collect individuals with a beach seine. We will sample invertebrates with a fishing net and cores. In addition, with the stable isotope method, we will test whether litter provides shelter or rather food for fish, indirectly through invertebrates that hide and feed on litter and associated microorganisms. Stable isotopes will be measured in the different organisms sampled and in the litter. The material will be separated into species, dehydrated and prepared for analysis in the LOV laboratory (to which UCA and ECOSEAS have in-house access). The results will be analyzed with the R software, MixSIAR package. The Southern Region is financing part of this work.
We require an expertise in ecology and an interest in marine environment. A good level of writing and speaking English is appreciated. Some statistical knowledge is also required. Diving is not a necessary condition, but diving licence will be appreciated. THe PhD student will be also encouraged to participate to education /communication activities related to the project

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