Ph.D Clémentine Mutillod
email: clementine.mutillod@gmail.com

Teddy Student

Pascale Baumberger

Arne Prudent

Eric Saatkamp

Lidwine Vidal

Laurence Le-Mire-Pecheux

Affre

Clémentine Mutillod¹

Teddy Baumberger²

Pascale Prudent³

Arne Saatkamp¹

Eric Vidal

Lidwine Le-Mire-Pecheux

Laurence Affre¹

Lidwine Le-Mire-Pecheux

Lidwine Le-Mire-Pecheux

Yellow-legged gull populations (Larus michahellis) link the history of landfills to soil eutrophication and time-related vegetation changes on small Mediterranean islands

Keywords: Long-term diachronic survey, seabirds, Trace elements, Plant community, land-use history Long-term diachronic survey, seabirds, trace elements, plant community, land-use history

Yellow-legged gull populations (Larus michahellis) link the history of landfills to soil eutrophication and time-related vegetation changes on small Mediterranean islands

Introduction

Manmade global change has had a profound impact on biodiversity (Vié, Hilton-Taylor and Stuart, 2009), especially in the Mediterranean basin which is considered one of the biodiversity hotspots [START_REF] Cunningham | Changes in human population density and protected areas in terrestrial global biodiversity hotspots, 1995-2015[END_REF]. An important part of Mediterranean plant diversity is linked to the occurrence of 10,000 small islands, islands being refuges for rare and threatened mainland plant species [START_REF] Médail | The specific vulnerability of plant biodiversity and vegetation on Mediterranean islands in the face of global change[END_REF]. However, the Mediterranean basin has been subjected to an increase in human activity, particularly in coastal and island habitats (UNEP/ MAP-Plan Bleu, 2009). Human activities alter ecosystem properties and services [START_REF] Hooper | Effect of biodiversity on ecosytem functioning: a consensus of current knowledge[END_REF], especially when they affect biotic factors [START_REF] Chapin | Biotic control over the functioning of ecosystems[END_REF].

In this insular context, populations of the yellow-legged gull (Larus michahellis, hereinafter refered to as gulls) have greatly increased over the past few decades [START_REF] Thibault | Recent trends in breeding marine birds of the Mediterranean region with special reference to Corsica[END_REF][START_REF] Bellout | Population growth for yellow-legged gulls in morocco changes in the population size of yellow-legged gull larus michahellis at Essaouira and Mogador Island, West-Central Morocco[END_REF]. Indeed, the high availability and predictability of food resources [START_REF] Bosch | Dependence of yellow-legged gulls (Larus cachinnans) on food from human activity in two Western Mediterranean colonies[END_REF], provided by open compartments of landfills during their operation on the close mainland, is known to have driven and shaped gull colony distribution and demographic explosion on the studied islands [START_REF] Duhem | Influence de l'accessibilité des ressources anthropiques sur les paramètres reproducteurs de deux colonies de goélands leucophées Larus michahellis[END_REF][START_REF] Duhem | Opportunistic feeding responses of the yellow-legged gull larus michahellis to accessibility of refuse dumps[END_REF][START_REF] Duhem | Distribution of breeding sites and food constrains size and density of yellowlegged gull colonies[END_REF][START_REF] Duhem | Effects of anthropogenic food resources on yellow-legged gull colony size on Mediterranean islands[END_REF]. Gulls have therefore been considered as a superabundant bird species in the Mediterranean basin [START_REF] Vidal | Is the yellow-legged gull a superabundant bird species in the Mediterranean? Impact on fauna and ¯ora, conservation measures and research priorities[END_REF] that conflict with island biodiversity conservation programmes (e.g. [START_REF] Médail | The specific vulnerability of plant biodiversity and vegetation on Mediterranean islands in the face of global change[END_REF][START_REF] De | Effects of a yellow legged gull (Larus michahellis) colony on soils and cliff vegetation in the Atlantic Islands of Galicia National Park (NW Spain)[END_REF][START_REF] Mouga | Recent efforts to recover armeria berlengensis, an endemic species from berlengas archipelago, Portugal[END_REF]. Gulls, like other seabirds, are chemical and physical ecosystem engineers [START_REF] Ellis | Marine birds on land: A review of plant biomass, species richness, and community composition in seabird colonies[END_REF][START_REF] Grant | The influence of seabirds on their breeding, roosting and nesting grounds: A systematic review and meta-analysis[END_REF]. Due to the high spatial heterogeneity of their feeding, nesting and defecation behaviour, seabirds can have contrasting impacts on ecosystems. On the one hand, their presence can lead to an increase of plant biomass and height [START_REF] Ellis | Marine birds on land: A review of plant biomass, species richness, and community composition in seabird colonies[END_REF] and seed dispersal [START_REF] Nogales | Ecological and biogeographical implications of Yellow-Legged Gulls (Larus cachinnans Pallas) as seed dispersers of Rubia fruticosa Ait. (Rubiaceae) in the Canary Islands[END_REF][START_REF] Calvino-Cancela | Gulls (Laridae) as frugivores and seed dispersers[END_REF]. On the other hand, high seabird densities can prevent plant growth owing to the uprooting of plants for nest building, trampling and soil compaction [START_REF] Sobey | The Relationship between Herring Gulls and the Vegetation of their Breeding Colonies[END_REF]. Many studies have shown that large seabird colonies can alter soil characteristics, increasing the quantity of nitrogen, phosphorus, potassium, magnesium, total salts [START_REF] Sigurdsson | Effects of seagulls on ecosystem respiration, soil nitrogen and vegetation cover on a pristine volcanic island, Surtsey, Iceland[END_REF][START_REF] Bouyahmed | Influence of the yellow-legged gull on soil, composition and organisation of plant communities on some small Islands of Algeria[END_REF][START_REF] Aerts | Effects of sea birds and soil development on plant and soil nutritional parameters after 50 years of succession on Surtsey[END_REF], heavy metals [START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF][START_REF] Ziółek | The influence of seabirds on the concentration of selected heavy metals in organic soil on the Bellsund Coast, Western Spitsbergen[END_REF][START_REF] Otero | Trace elements in biomaterials and soils from a Yellow-legged gull (Larus michahellis) colony in the Atlantic Islands of Galicia National Park (NW Spain)[END_REF], organic matter [START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF][START_REF] Ghermaoui | Influence du Goéland leucophée Larus michahellis sur les formations végétales ouvertes du littoral de Rachgoun (Ouest Oranie, Algérie)[END_REF] and soil depth [START_REF] Ishida | Seed germination and seedling survival in a colony of the common cormorant, Phalacrocorax carbo[END_REF][START_REF] Mulder | Burrowing seabirds and reptiles: Impacts on seeds, seedlings and soils in an island forest in New Zealand[END_REF]. They also usually lead to a decrease of soil pH [START_REF] Mulder | Burrowing seabirds and reptiles: Impacts on seeds, seedlings and soils in an island forest in New Zealand[END_REF][START_REF] Sigurdsson | Effects of seagulls on ecosystem respiration, soil nitrogen and vegetation cover on a pristine volcanic island, Surtsey, Iceland[END_REF]. Other soil characteristics can also be altered, but the trend can be both upward or downward such as for calcium, carbon content, water retaining capacity and soil moisture [START_REF] Ellis | Marine birds on land: A review of plant biomass, species richness, and community composition in seabird colonies[END_REF]. These different alterations may cause changes in plant communitieswith high pressure and duration, exotic, ruderal or nitrophilous plant species may develop that may lead to a reduction of the abundance of native plant species [START_REF] Vidal | Is the yellow-legged gull a superabundant bird species in the Mediterranean? Impact on fauna and ¯ora, conservation measures and research priorities[END_REF][START_REF] Vidal | Seabirds drive plant species turnover on small Mediterranean islands at the expense of native taxa[END_REF][START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF][START_REF] Bou | Habitat changes over 45 years on the small Mediterranean archipelago of the Medes Islands (Catalonia, NE Spain)[END_REF]. The reduction of food availability following the closure of open compartments of landfills is indicated as the main driver for a long term reduction on gull population size [START_REF] Duhem | Distribution of breeding sites and food constrains size and density of yellowlegged gull colonies[END_REF][START_REF] Duhem | Effects of anthropogenic food resources on yellow-legged gull colony size on Mediterranean islands[END_REF] and the negative density-dependent breeding investment contributed to population decline [START_REF] Real | Predictable anthropogenic food subsidies, density-dependence and socio-economic factors influence breeding investment in a generalist seabird[END_REF].

Few studies [START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF][START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF][START_REF] De | Effects of a yellow legged gull (Larus michahellis) colony on soils and cliff vegetation in the Atlantic Islands of Galicia National Park (NW Spain)[END_REF] have investigated the long-term patterns of change in plant communities and soil factors after the decrease of gull populations, showing that the environmental effects of the colonies may persist for a long time. For example, high concentrations of phosphorus (P) have been observed 5 [START_REF] De | Effects of a yellow legged gull (Larus michahellis) colony on soils and cliff vegetation in the Atlantic Islands of Galicia National Park (NW Spain)[END_REF] and 14 years [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF] after the decline of gull populations. The amount of P generated by the colony did not decrease and can be considered as an irreversible impact for the plant community [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF][START_REF] De | Effects of a yellow legged gull (Larus michahellis) colony on soils and cliff vegetation in the Atlantic Islands of Galicia National Park (NW Spain)[END_REF]. The Mediterranean basin has a particular climate with low precipitation. In these semi-arid ecosystems, alterations linked to trophic shift can lead to a persisting imprint of eutrophication in the soil [START_REF] Saatkamp | Romans Shape Today's Vegetation and Soils: Two Millennia of Land-Use Legacy Dynamics in Mediterranean Grasslands[END_REF] and the impact on plant communities is linked to higher P, C:N and organic matter. The threats to the Mediterranean islands are exacerbated by climate change because of the limited habitat availability and climatic range limitations which may hamper the expansion of the species' range [START_REF] Vogiatzakis | Mediterranean island biodiversity and climate change: the last 10,000 years and the future[END_REF].

Our study focused on nine islands in the two Marseille archipelagos, Frioul and Riou (Calanques National Park, SE France), which are under the influence of breeding gulls (Larus michahellis) in comparison with the nearby mainland coastal area without gull colonies. Previous studies have shown that the high pressure of gull colonies leads to a ruderalisation of the floristic community [START_REF] Vidal | Is the yellow-legged gull a superabundant bird species in the Mediterranean? Impact on fauna and ¯ora, conservation measures and research priorities[END_REF][START_REF] Vidal | Seabirds drive plant species turnover on small Mediterranean islands at the expense of native taxa[END_REF]. This pattern has persisted [START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF] despite four open landfills having been closed in the Marseille area during the last decade, which has caused a decline in the nesting gull population (Calanques National Park monitoring). We have extended monitoring of the vegetation changes up to 24 years and added information with additional analysis of nutrients and trace elements in the soil according to the fluctuations in the nesting gull density. Because of the strong relationships within the gull/soil/plant system, we expected to demonstrate a resilience in plant communities despite the persistent effect of gull colonies on soil chemistry.

Material and Methods

Study area

The study was conducted on two archipelagos and on the mainland area near Marseille (SE France) (Fig. 1). The sampling took place on two archipelagos of Marseille: the Riou archipelago (43°11'N, 5°21'E; five islands and two islets), and the Frioul archipelago (43°16'N, 103 5°18'E; four islands) (Fig. 1). The Riou archipelago islands studied are from 50 to 3525m distant from the mainland (Table 1).

Concerning the Frioul archipelago, the two islands studied are 1800 and 2800m away from the mainland.

The mainland area was considered as a habitat not subjected to gull pressure (no gulls nesting on the studied mainland area). These islands and the mainland area are included in natural protected areas managed by the Calanques National Park (CNP). Situated within the thermo-Mediterranean bioclimatic belt [START_REF] Quézel | Ecologie et biogéographie des forêts du bassin méditerranéen[END_REF], these islands and the mainland area have semi-arid climatic conditions, with lower precipitations on the islands (e.g., Planier, an island located 9km from Marseille: 342mm) compared to the mainland (Marseille 524mm), however little reliable data exists (Météo France). The studied sites are composed of compact limestone on the surface of which soils are limited to thin protorendzina patches, and are calcareous, skeletal and oligotrophic (Vidal, 1998).

The plants on the islands and the mainland coastal area are subjected to stresses both hydric (low precipitation and frequent strong wind episodes) and haline (with salt input from the sea and the toxicity of the salt within the tissues). The islands are mainly covered by low vegetation from halophyte plants to matorral shrub species [START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF]. Vegetation on the mainland shows a similar composition, with however higher species richness compared to islands of similar size [START_REF] Martín-Queller | Islands, mainland, and terrestrial fragments: How isolation shapes plant diversity[END_REF].

Yellow-legged gull status is considered as Least Concern in the IUCN European Red List, it is a protected specie in France and a determining specie of natural areas of ecological, faunistic and floristic interest [START_REF] Inpn Mnhn | Statuts d'Evaluation[END_REF]. Gull colonies are nesting in the Mediterranean basin and the Atlantic coast from Morocco to the southern Brittany [START_REF] Duhem | Goélands surabondants et ressources alimentaires anthropiques: cas des colonies insulaires de Goélands leucophées du littoral provençal[END_REF]. Gull colonies can be found on all the islands close to Marseille. However, nesting gull abundance and densities are not homogenous between the archipelagos, and depend on local habitats. The gull population increased in the Riou and Frioul archipelagoes between 1997 and 2005, with a growth rate ranging from 6% for the Riou archipelago to 177% for the Frioul archipelago [START_REF] Duhem | Effects of anthropogenic food resources on yellow-legged gull colony size on Mediterranean islands[END_REF]. The fluctuations of gull population size in the two archipelagos are presented in figure 2, between 1923 and 2021. Data were compiled by the CNP.

Gulls' census on the islands of Marseille is part of a CNP program taking place every five years as well as the national census every ten years. Gull estimation was carried out by counting the number of nests (empty and with eggs) exhaustively on each island until 2021.

In 2021, exhaustive counting was done on only Grand Congloué and Tiboulen de Maïre. For the rest of the islands, estimation was done by distance sampling, an effective method for many gull species [START_REF] Barbraud | A comparison of direct and distance sampling methods to estimate abundance of nesting gulls[END_REF].

The main available anthropogenic food resources, four landfills (with open compatments during their operation) located within a radius of 60 kms, were closed in 2008, 2010 and 2013 and led to a decline in gull population size (Calanques National Park monitoring). The decrease of easily accessible food resources led to the decline of the nesting population by 63% in the Riou archipelago and 37% in the Frioul archipelago between 2005 and 2015 (Fig. 2). However, an increase was recorded in 2021 with the difference being about 1100 nesting couples.

Sampling design

The vegetation was surveyed on the 56 island plots and on the 22 mainland plots in 1997, 2008and in 2021 (Tab. 1) (Tab. 1). The number of plots was proportional to the size of the island in order to take into account the heterogeneity of the zones studied and the different vegetation types. A plot consisted of two concentric circles, a first one of 500m² where environmental variables and gull density were measured, and, a second of 100m² where floristic inventories and soil sampling took place (Vidal, 1998).

Mesological parameters

Environmental variables were measured in the 500 m² circles for each plot to better qualify and quantify the environmental effects (see table 2 for further details). Measures and estimations were made during the floristic inventories from April to May in 1997May in , 2008May in and 2021. The environmental variables measured were the elevation above sea level, the distance from the coastline, the slope, the exposure, and the cover rate of each substrate type: rock, block, stone, bare ground, the vegetation cover of the different layers.

Gull nest density (number of nests) was estimated and we also considered the cover of the resting places per plot being an indication of gull pressure.

Floristic inventories

Three plant species inventories (1997, 2008 and 2021) were carried out within the same 78 100m² concentric plots from April to May using Tison et al. flora (2014). The plant cover in the different layers was estimated (2008,2021) for each species present on each plot using the 6 cover abundance categories (0: cover = 0%, 1: 0% < cover <10%, 2: 10% ≤ cover < 25%, 3: 25% ≤ cover < 50%, 4: 50% ≤ cover < 75%, 5: cover ≥ 75 %). The following easily accessible plant functional types, i.e., Raunkiaer growth form [START_REF] Raunkiaer | The life forms of plants and statistical plant geography; being the collected papers of C. Raunkiaer. The life forms of plants and statistical plant geography; being the collected papers of C. Raunkiaer[END_REF] and Grime life strategy [START_REF] Grime | Evidence for the existence of three primary strategies in plants and its relevance to ecological and.pdf[END_REF], for each plant species, were extracted from the BASECO data base [START_REF] Gachet | BASECO: A floristic and ecological database of Mediterranean French flora[END_REF].

Soil, droppings sampling and chemical analysis

The soil was sampled in 1997 (56 plots only on the islands) and in 2021 (56 plots on the islands and 22 on the mainland). In 1997, organic carbon, total nitrogen, and pH were measured within the plots.

In 2021, soil parameters included organic carbon, total nitrogen, pH, conductivity, total phosphorus, potassium, sodium, copper, lead, zinc and stable isotopes (δ15N and δ13C).

The soil samplings were performed once between March and April, when most of the gulls were nesting and after the winter rainfall. Within each plot, five soil subsamples of 200mL each were collected (one in the center and four on each side, approximately 50cm from the 100m² radius boundary) and pooled in a composite soil sample, in order to make a composite sample representative of the plot. Soil subsamples were collected from the top 15cm layer after litter removal and sieved at 2mm directly on site. The soil samples were dried before analysis (at 40°C up to constant weight). Dry gull droppings were sampled on nine different plots in the Frioul and Riou archipelagos in 2021. They were collected on rocks to avoid contact with soil.

To assess the soil organic matter percentage (SOM), the method of [START_REF] Mathieu | Analyse chimique des sols Méthodes choisies[END_REF] was used: the samples were first weighed and then heated several times at high temperature. The samples were heated first at 220°C for 16 hours (bound water elimination), then heated again at least three times at 450°C for four hours (loss of organic matter by combustion, and SOM was estimated by mass loss between 220 and 450°C). The organic carbon was estimated using the values of the organic matter multiplied by 0.50, based on the assumption that the soil organic matter contains 50% carbon [START_REF] Pribyl | A critical review of the conventional SOC to SOM conversion factor[END_REF].

To determine the pH and the conductivity, the samples were extracted with osmosed water (ratio 1:5 V/V), then agitated for at least one hour, decanted for one more hour and finally the conductimeter (WTW Multi 3420) and the pH-meter (Orion 2 Star Thermo Scientific SM30B) could be used (according to norm ISO 11265 (1995) andISO 10390 (2005) respectively; Baize andGirard, 1992).

The total nitrogen, carbon, δ15N and δ13C content in ground soil samples (ground to less than 0.2mm with RETSH 22, agate mortar) and droppings samples (n = 9, not composites) were performed on an elemental analyzer (Flash HT Plus, Thermo Fisher Scientific) coupled with a mass spectrometer (Delta V Advantage, Thermo Fisher Scientific). Elemental analysis of carbon and nitrogen were performed in a combustion reactor (1,000°C) under a carrier gas (Helium: 100mL/min) with samples weighing approximately 800µg sealed in tin capsules, introduced via an automatic injector. The gases were separated on a chromatographic column in an oven at 45°C before being quantified via the thermal conductivity detector (TCD) of the elemental analyzer. The gases were injected into the Delta V spectrometer for isotopic analysis. The calibration of the catharometric detector and the ratio-isotopic mass spectrometer was performed with a secondary standard, EDTA (tetra-acetic ethylene diamine acid). The analyses (samples or standards) of the ratio 13C/12C were calculated in delta value which represents the deviation from an international standard and is calculated as follows :

δ 13 𝐶 = ( 𝑅 𝑆𝑎𝑚𝑝𝑙𝑒 (𝑅 𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑 ) -1) × 1000 (‰)
The international standard for δ13C is Vienna PeeDee Belemnite (VPDB), Rstandard = 0.011237. The δ15N is expressed in the same way in relation to atmospheric nitrogen with a conventional value of 0‰.

The results of δ13C and δ15N analyses are produced with an uncertainty of +/-0.5 ‰. The δ15N and the δ13C were analysed because they can give an indication of the trophic transfer, being robust and effective in tracing the path of the organic matter and the trophic relationships within the ecosystems [START_REF] Post | Using stable isotopes to estimate trophic position: Models, methods, and assumptions[END_REF].

The analysis of the other elements (Cu, K, Na, P, Pb and Zn) in the soil and the droppings samples was carried out after acid mineralization. Samples were digested in a microwave mineralizer (Milestone Start D) using aqua regia (1/3 HNO3 (Fisher chemical, Trace metal grade 67-69%) and 2/3 HCl (Fisher chemical, Trace metal grade 35-38%)). After mineralization, the samples were filtered through a 0.45μm cellulose ester membrane filter and stored at 4°C until analysis (mineralization protocol for pseudo-total concentration determination in soil, adapted from norm ISO 11466, 1995). Then element concentrations in samples were carried out by ICP-OES (Inductive Coupled Plasma, Optical Emission Spectrometry, Jobin Yvon Horiba Spectra 2000). Quality controls and accuracy were checked using standard soil reference materials (CRM 049-050 from RTC-USA), with accuracies within 100±10%).

Concerning Cu, Pb and Zn, considered as potentially toxic elements (PTE), contamination factors in soil (CF) were calculated as followed: CFPTE=

Statistical analysis

Since plant names have been revised between different study periods, we followed the taxonomy of Gargominy and collaborators [START_REF] Gargominy | TAXREF v14.0, référentiel taxonomique pour la France : méthodologie, mise en oeuvre et diffusion[END_REF]. In order to understand patterns of change of soil parameters, we used boxplots in pairs for contrasting years, and separated to compare Riou, Frioul and the mainland. We used Student tests for variables with normal distribution and homosedasticity or pairwise Wilcoxon in the other cases. To measure the potential effects of the gulls on the soil parameters, linear models were used (stats package). We pooled all the plots from the two archipelagos and considered the number of counted nests or the cover of resting places as explaining variables for the soil parameters measured in 2021. We excluded mainland plots from this analysis to avoid confounding effects. When conditions were not met for model residuals (normality, homoscedasticity and independency), outliers were removed and/or the variables were log transformed. To compare Na and conductivity mean values properly without the confounding effect of the sea shore distance, we selected a subsample of plots from the mainland area. We selected only plots within 188m distance from the sea shore, which is the maximum distance from the sea shore of island plots.

With the aim of describing the flora trajectory and to know whether there was a flora niche alteration due to climate change or only to gulls pressure, different analyses were carried out. Non-metrical multidimensional scaling (NMDS) was performed to view the alterations in the flora on the islands between the three dates. Using the coordinates of groups in the NMDS, we used Wilcoxon paired tests to verify whether the changes are significatively different. To quantify the changes, mean comparisons were made between years with paired Wilcoxon tests, using the number of species, Grime life strategies and Raunkiaer growth forms.

In order to compare the vegetation cover between 2008 and 2021, we performed an Ascending Hierarchical Classification (AHC) (stats package) with the data of 2021 (mesological and soil parameters) in order to have different groups of plots and to carry out paired Wilcoxon tests between them.

The observations made on the islands could be explained by more global alterations such as climate change [START_REF] Heilmeier | Functional traits explaining plant responses to past and future climate changes[END_REF][START_REF] Ahrens | Plant functional traits differ in adaptability and are predicted to be differentially affected by climate change[END_REF]. To comprehend the flora ecological requirement changes All statistical analyses were performed with R software (R version 4.2.2: R Core Team, 2022). Null hypotheses were rejected when the p-value was below 0.05.

Results

Nesting gulls and dynamic of the soil parameters

The number of nests has decreased since 1997 (Fig. 3. A). The cover of resting places decreased less on the Frioul archipelago compared to the Riou archipelago between 2008 and 2021 (Fig. 3. B).

Soil acidity decreased during the study period (see Supplementary Materials Table S1 for a summary of all means values ± SD), with mean pH in 2021 significantly higher than in 1997 (Fig. 4. A., p < 0.001).

Soil pH was lower on Riou islands (mean ± standard deviation, 7.89±0.47, n = 30) compared to the mainland (8.18±0.28, n = 22) in 2021 (p < 0.05). There was no significant difference between soil pH values on Riou and Frioul islands whatever the year considered.

Organic carbon in soils of the archipelagos decreased during the study period (Fig. 4. B, p < 0.01), but

was not significantly different between islands in 1997 (p=0.332) and 2021 (p=0.091), respectively, and between islands and the mainland in 2021 (64.07±27.01, 53.06±20.70 and 56.06±31.12 mg/g for Riou,

Frioul (n = 26) and the mainland respectively; mainland vs Frioul p=0.70, mainland vs Riou p=0.34,

Frioul vs Riou p=0.09).

Soil total nitrogen content of Frioul and Riou samples was significantly higher in 2021 than in 1997 (Fig. 4. C, p < 0.01). Soil total nitrogen on Riou islands (9.14±3.63 mg/g) was significantly higher than on the mainland (5.86±2.45 mg/g) and also than on Frioul islands (7.18±2.84 mg/g) in 2021 (p < 0.001).

Total nitrogen values for Riou and Frioul in 1997 were not significantly different from the mainland in 2021 (p=0.161 and p=0.088, respectively). Total nitrogen content of droppings (44.50±8.34 mg/g) was significantly higher than in any soils (p < 0.001).

The C/N ratio was significantly lower for Riou (13.57±6.

Additional chemical characterisation of soils and impacts of gull presence on edaphic parameters

No difference was shown for the soil conductivity between the archipelagos (p=0.96), nor for islands versus mainland (mainland vs Frioul p=0.99, mainland vs Riou p=0.23), mean values from 0.64 to 1.07 mS/cm, not presented here, Table 3).

Regarding nutrient concentrations in soils, the mean amount of P in soils of the mainland (Table 3) was significantly lower than in the soils of the archipelagos (Fig. 5. A, (p < 0.001), and total P concentrations were significantly higher in the soils of Riou compared to Frioul (Fig. 5. A, p < 0.01). The concentration of P appeared to be higher in droppings than in the soils. The K values were significantly higher in the soils of the Riou archipelago than on the mainland (Fig. 5. B, p <0.05), but there was no significant difference between the soils of the archipelagos (p=0.58). The droppings appeared to be less concentrated in K than the soils. No significant difference was observed for Na concentration whatever the soils on the islands or on the mainland (Fig. 5. C, mainland vs Frioul p=0.33, mainland vs Riou p=0.08, Frioul vs Riou p=0.87), and Na contents were higher in droppings.

Regarding potentially toxic elements (PTE), Pb soil concentrations were higher on the mainland than on the archipelagos (Fig. 5. D, p < 0.01 between the mainland and Riou and p < 0.001 between the mainland and Frioul It was noticeable that the mean amounts of Cu, Zn, Na and P showed a tendency to be higher in the droppings than in soils (Fig. 5. E, D, F). The contrary was observed for K and Pb (Fig. 5. B, E).

The analysis of the stable isotopes showed an enriched signature of the δ15N for soils of the two archipelagos compared to the reference plots of the mainland (Fig. 6.), with higher values for Riou (> 5‰). Concerning the δ13C, values did not show a clear pattern.

Analyses of potential impacts of a colony of gulls on physical and chemical parameters of the islands' soil in 2021 showed that the concentrations of K were significantly and positively correlated with the number of nests (p < 0.01, Adjusted R² = 0.1123), P concentrations (p < 0.05, Adjusted R² = 0.08533)

and Na concentrations (p < 0.05, Adjusted R² = 0.08361). These results also showed that the pH values were significantly and negatively correlated with the number of nests (p < 0.05, Adjusted R² = 0.09594).

Moreover, the resting places were positively correlated with conductivity (p < 0.05, Adjusted R² = 0.1728).

Concerning the other parameters, no significant correlations were observed neither with the number of

Changes in plant communities

NMDS drew a pattern of the flora community based on the three successive surveys conducted on the islands (Fig. Results of mean comparisons showed that the total number of species was not different between years on the islands (1997 vs 2008: p=0.669, 1997 vs 2021: p=0.2446, 2008 vs 2021: p=0.5376). However, The vegetation cover was significantly higher on two groups of plots in 2021 than in 2008 (p < 0.01 for the first group and p < 0.05 for the second group) (for more details and photographs, see Supplementary Materials Fig. S1 andS2).

for

Discussion

Dynamic of the soil parameters and characterisation

Comparing the fluctuation of soil parameters between 1997 and 2021, we recorded an increase of pH values. This means that the soil was less acidic in 2021 than 24 years ago. Considering the values of pH, it was still approaching neutrality so little impact on the flora was expected. Conversely, the amount of N had increased because of droppings accumulation and limited leaching. When decomposing, the organic matter increases total N concentration in soil. Moreover, organic matter decomposition explains its decrease and the decrease of C/N values. These quantities of N could help plant development as seen on a recently emerged volcanic island [START_REF] Sigurdsson | Effects of seagulls on ecosystem respiration, soil nitrogen and vegetation cover on a pristine volcanic island, Surtsey, Iceland[END_REF][START_REF] Aerts | Effects of sea birds and soil development on plant and soil nutritional parameters after 50 years of succession on Surtsey[END_REF]. However, in the case of the Marseille archipelagos, where a flora community was already established, this could trigger a lasting alteration of plant composition. The C/N ratio plays a role in the balance of the bacterial and fungal communities of soil, with an increase of the bacterial community when C/N decreases [START_REF] Grosso | Bacterial and fungal growth on different plant litter in Mediterranean soils: Effects of C/N ratio and soil pH[END_REF]. Moreover, soil organic matter is considered as a good indicator of soil quality and high levels of organic matter are associated with nutrient availability and increased water retention capacity [START_REF] Nyiraneza | Changes in soil organic matter over 18 yr in Prince Edward Island, Canada[END_REF].

The values of P measured in 2021 showed that the soils in the archipelagos are still enriched with, and this observation was corroborated by other studies which showed that P enrichment by seabirds is an irreversible process [START_REF] Myrcha | Ecological role of the current and abandoned penguin rookeries in the land environment of the maritime Antarctic[END_REF][START_REF] Holdaway | Soil indicators of pre-European seabird breeding in New Zealand at sites identified by predator deposits[END_REF][START_REF] Sun | A 3,000-year record of penguin populations[END_REF][START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF][START_REF] De | Effects of a yellow legged gull (Larus michahellis) colony on soils and cliff vegetation in the Atlantic Islands of Galicia National Park (NW Spain)[END_REF]. Moreover, the values in both reference and colonies plots were comparable to the study of [START_REF] De La Peña-Lastra | Soil nutrient dynamics in colonies of the yellow-legged seagull (Larus michahellis) in different biogeographical zones[END_REF]. The lower coverage of plants when the gull colony was at its demographic peak, implying a decline in nutrient retention [START_REF] Hanrahan | Extending vegetative cover with cover crops influenced phosphorus loss from an agricultural watershed[END_REF], might also explain the high values of P and its accumulation in the soils. The soils of the archipelago of Riou showed the highest levels of P. This observation could be explained by the fact that the colony was first established on Riou before expanding to Frioul [START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF]. Hence, its impact on the soil and the flora lasted longer and today the colony on Riou is still larger.

The values of conductivity and Na concentrations measured in soils in 2021 showed no differences between the mainland and the islands, which means gull presence today has no impact with regards to these characteristics. Usually, conductivity and Na concentrations have been shown to increase with the presence of seabird colonies [START_REF] Ellis | Marine birds on land: A review of plant biomass, species richness, and community composition in seabird colonies[END_REF][START_REF] Zwolicki | Seabird colony effects on soil properties and vegetation zonation patterns on King George Island, Maritime Antarctic[END_REF]. However, another study also did not register salinization [START_REF] De | Effects of a yellow legged gull (Larus michahellis) colony on soils and cliff vegetation in the Atlantic Islands of Galicia National Park (NW Spain)[END_REF], probably due in their case to high precipitations and leaching [START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF].

In 2021, we also added to the analysis trace elements (or PTE) in soils, and the determination of pollution load index (PLI), integrating Cu, Pb and Zn, reveals relatively low multi-contamination levels on the mainland and the archipelagos, with values around 2.2. These PLI values correspond to those calculated for soils in the Calanques hills (mean value of 2.3) some distance away from the urbanized area of Marseille and from current and former industrial activities [START_REF] Affholder | As, Pb, Sb and Zn transfer from soil to root of wild rosemary: do native symbionts matter?[END_REF]. We observed higher values for the amount of Pb in soils of the mainland than in those of the archipelagos. We can argue that the origin is mostly anthropic, notably with the use of leaded gasoline, coal burning and waste incineration [START_REF] Erel | Tracing the transport of anthropogenic lead in the atmosphere and in soils using isotopic ratios[END_REF][START_REF] Komárek | Lead isotopes in environmental sciences: A review[END_REF], but mainly with the former lead smelter sites located on the south-east coast of Marseille [START_REF] Daumalin | Les calanques industrielles de Marseille et leurs pollutions -Une histoire au présent[END_REF][START_REF] Heckenroth | Selection of native plants with phytoremediation potential for highly contaminated Mediterranean soil restoration: Tools for a non-destructive and integrative approach[END_REF][START_REF] Heckenroth | Coronilla juncea, a native candidate for phytostabilization of potentially toxic elements and restoration of Mediterranean soils[END_REF][START_REF] Affholder | As, Pb, Sb and Zn transfer from soil to root of wild rosemary: do native symbionts matter?[END_REF]). The values of Cu and Pb are comparable to the results of a previous study on the impact of waterbird colonies in the Mediterranean semiarid floodplain [START_REF] Laguna | Nutrient enrichment and trace element accumulation in sediments caused by waterbird colonies at a Mediterranean semiarid floodplain[END_REF], but we cannot argue for a direct effect of the gulls on these PLI. However, the literature shows the importance of pollutants transport, such as Cu, Pb, Zn, by seabirds [START_REF] Shoji | Biotransport of metallic trace elements from marine to terrestrial ecosystems by seabirds[END_REF][START_REF] Castro | Seabirds enrich Antarctic soil with trace metals in organic fractions[END_REF][START_REF] Grant | The influence of seabirds on their breeding, roosting and nesting grounds: A systematic review and meta-analysis[END_REF].

Moreover, the values of K, Na, P and conductivity on the islands showed greater amplitudes, which led to a greater spatial variation for the chemical composition of soils, plant richness and assemblages [START_REF] Wait | Seabird guano influences on desert islands: Soil chemistry and herbaceous species richness and productivity[END_REF].

The results of stable isotopes δ15N reflected higher values for the two archipelagos and the droppings, with values around 10‰ for the droppings and even higher than 15‰ for the Riou archipelago. The values of previous studies were on the same scale [START_REF] Ellis | Marine birds on land: A review of plant biomass, species richness, and community composition in seabird colonies[END_REF]. High δ15N derived from the droppings, and hence explained the values of the archipelagos, the seabirds being positioned high in the trophic chain [START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF][START_REF] Wait | Seabird guano influences on desert islands: Soil chemistry and herbaceous species richness and productivity[END_REF][START_REF] Laguna | Nutrient enrichment and trace element accumulation in sediments caused by waterbird colonies at a Mediterranean semiarid floodplain[END_REF]. The values of the mainland higher than 0‰, derived from the matter of the vegetation [START_REF] Post | Using stable isotopes to estimate trophic position: Models, methods, and assumptions[END_REF]. There was no significant difference concerning the δ13C, which resulted mostly from the autochthonous organic matter even on the island [START_REF] Torres | Stable isotope (δ 13C and δ 15N) values of sediment organic matter in subtropical lakes of different trophic status[END_REF].

An increase of pH was observed but there was still a negative link with gull density, as in other studies (Vidal, 1998;[START_REF] Ellis | Marine birds on land: A review of plant biomass, species richness, and community composition in seabird colonies[END_REF]. Moreover, cover of the resting places had a positive link with conductivity.

Except for pH, K, P, Na, we did not find the same links as in 1997 with positive correlations between nest density and the organic matter nor the nitrogen content (Vidal, 1998). Moreover, the results also indicated that the amounts of Na, P and Zn were higher in the droppings than in soils. According to these observations, we could expect existing links between gull densities and today's values in the islands' soils. However, with the absence of links as shown before, we believe that the levels of nutrients and other elements found today could still be linked with the density of nests of the last decades. Actual observations have been shown to retrace land-use history and be linked to it even millennia after in dry ecosystem (Mizutani et al.,1991;[START_REF] Saatkamp | Romans Shape Today's Vegetation and Soils: Two Millennia of Land-Use Legacy Dynamics in Mediterranean Grasslands[END_REF]. As previously seen, changes in soil P driven by seabird colonies have persistent effects [START_REF] Myrcha | Ecological role of the current and abandoned penguin rookeries in the land environment of the maritime Antarctic[END_REF][START_REF] Holdaway | Soil indicators of pre-European seabird breeding in New Zealand at sites identified by predator deposits[END_REF][START_REF] Sun | A 3,000-year record of penguin populations[END_REF][START_REF] Otero | Phosphorus in seagull colonies and the effect on the habitats. The case of yellow-legged gulls (Larus michahellis) in the Atlantic Islands National Park (Galicia-NW Spain)[END_REF][START_REF] De | Effects of a yellow legged gull (Larus michahellis) colony on soils and cliff vegetation in the Atlantic Islands of Galicia National Park (NW Spain)[END_REF]. The remanence in soil is also the case for other nutrients and contaminants such as N or Selenium [START_REF] Hawke | Soil selenium in a forested seabird colony: distribution, sources, uptake by plants, and comparison with non-seabird sites[END_REF][START_REF] Jaunatre | Can ecological engineering restore Mediterranean rangeland after intensive cultivation? A large-scale experiment in southern France[END_REF][START_REF] De | Effects of a yellow legged gull (Larus michahellis) colony on soils and cliff vegetation in the Atlantic Islands of Galicia National Park (NW Spain)[END_REF]. The retention and so the remanence time for nutrients or contaminants can vary with several factors, such as successional state and ecosystem type, demand or retention capacity, land-use history, type of soils, topography, climate and the rate, timing and type of deposition as was shown for N inputs [START_REF] Matson | The globalization of nitrogen deposition: consequences for terrestrial ecosystems[END_REF].

Vegetation dynamics Lag dynamics between soil and vegetation

The previous hypothesis, according to which the current observations would be explained by previous gull pressure, is reinforced by the study of the flora community because we highlighted that the plant community on the island is still different from 1997, being as ruderal as in 2008. This observation could find an echo in a previous study [START_REF] Purschke | Linking landscape history and dispersal traits in grassland plant communities[END_REF] which found that current factors alone could not explain today's flora pattern. This finding is similar to those of a recent study [START_REF] Saatkamp | Romans Shape Today's Vegetation and Soils: Two Millennia of Land-Use Legacy Dynamics in Mediterranean Grasslands[END_REF] where it was demonstrated that the use of land two thousand years ago by the Romans is still modelling the vegetation and the soil today. Furthermore, we can also surmise that the plant community can be firstly influenced by the mesological parameters such as the distance from the mainland, the surface area, the topography, the perimeter and the maximum altitude of the island [START_REF] Macarthur | An Equilibrium Theory of Insular Zoogeography[END_REF][START_REF] Whittaker | Island biogeography: Taking the long view of natures laboratories[END_REF], the direct effects of the seabird colonies [START_REF] Bancroft | Burrowing seabirds drive decreased diversity and structural complexity, and increased productivity in insular-vegetation communities[END_REF][START_REF] Wait | Seabird guano influences on desert islands: Soil chemistry and herbaceous species richness and productivity[END_REF], and secondly by the induced soil alterations [START_REF] Duda | Long-term changes in terrestrial vegetation linked to shifts in a colonial seabird population[END_REF]. It appears that at the level of the species and the functional traits, there have been no significant changes since 2008. Indeed, there are still more ruderal species strategies (with the most represented R: Sonchus tenerrimus, RC: Parietaria judaica, RS: Lobularia maritima, SR: Euphorbia linifolia) and more geophytes, hemicryptophytes, and therophytes in 2021 than in 1997 on the islands. The differences observed on the islands means that the plant species which are highly tolerant to disturbances in harsh environment are still favoured even with the decline of the pressure induced by the presence of the gull [START_REF] Baumberger | Plant community changes as ecological indicator of seabird colonies' impacts on Mediterranean Islands[END_REF].

The decrease of the chamaephytes recorded in 2008 compared to 1997 is no longer occurring in 2021 on the island. This is noticeable because certain of the heritage/endangered plant species of the archipelagos are chamaephytes and specialists of this environment (e.g., Astragalus tragacantha, Limonium pseudominutum, Frankenia hirsuta, Euphorbia pithyusa, Plantago subulata). However, further analyses and studies are needed to conclude more precisely on the dynamics of each of the species mentioned above. Nevertheless, this observation still permit to underline the vulnerability of these species [START_REF] Bou | Habitat changes over 45 years on the small Mediterranean archipelago of the Medes Islands (Catalonia, NE Spain)[END_REF] being not ruderal and jeopardized by the eutrophication of the soil. These observations also raise questions regarding their protection and the support of the viable populations until the habitat returns, or not, to a previous state (Knoerr, 1960 ;Molinier 1936, 1939[START_REF] Vidal | Is the yellow-legged gull a superabundant bird species in the Mediterranean? Impact on fauna and ¯ora, conservation measures and research priorities[END_REF]. On this time scale, we may inquire about the expression of the seed banks, which are considered essential for plant communities because of their contribution to ecological processes, in terms of resilience, the persistence of the community and the restoration of the ecosystems [START_REF] Del Cacho | Resilience of Mediterranean shrubland to a severe drought episode: The role of seed bank and seedling emergence[END_REF][START_REF] Kiss | Grassland seed bank and community resilience in a changing climate[END_REF].

An increase in the vegetation cover was registered between 2008 and 2021 especially on island plots.

As supported by the previous results on functional types, it was not a sign of the resilience of the functional flora community such as there was in 1960 (Knoerr 1960[START_REF] Vidal | Is the yellow-legged gull a superabundant bird species in the Mediterranean? Impact on fauna and ¯ora, conservation measures and research priorities[END_REF] -with more stress-tolerant species. However, it was a sign of the resilience of the flora developed since then, being more ruderal. Concerning the mainland plots where vegetation cover increased, we can assume that in 24 years and without major disturbances (fire, rockslide), the plants followed a natural dynamic and so the aboveground vegetation cover increased.

Interactive effect of gulls and climate change

In the context of the alteration of the biota (e.g., the yellow-legged gull colony and so the flora) linked to the land use (e.g., landfills), it was interesting to explore whether the alteration of the disturbance is modulated by climate change [START_REF] Perring | Global environmental change effects on ecosystems: The importance of land-use legacies[END_REF], or whether they may act in synergy. This could also offer us a basis for deciding on future management actions [START_REF] Perring | Advances in restoration ecology: Rising to the challenges of the coming decades[END_REF]. Using the bioindicators of Pignatti [START_REF] Schwabe | Plant indicator values of a high-phytodiversity country (Italy) and their evidence, exemplified for model areas with climatic gradients in the southern inner Alps[END_REF][START_REF] Saatkamp | Calibrating ecological indicator values and niche width for a Mediterranean flora[END_REF] helped us to characterise the requirements of plant species between the three dates and underline the results obtained from the soil analyses. In fact, we could highlight that since 1997 there has been a decline of the nutrient factor on the islands, not correlated with the gull density in 2021. This observation is corroborated by the chemical results. [START_REF] Schaffers | Reliability of Ellenberg indicator values for moisture, nitrogen and soil reaction: a comparison with field measurements[END_REF] found that the nutrient factor is not directly linked to the nitrogen but is correlated with the productivity. Previous studies have shown that gull colonies can lead to an increase in soil moisture, which can be explained by greater water retention due to higher amounts of organic matter [START_REF] Ishida | Seed germination and seedling survival in a colony of the common cormorant, Phalacrocorax carbo[END_REF][START_REF] García | Seagull influence on soil properties, chenopod shrub distribution, and leaf nutrient status in semi-arid Mediterranean islands[END_REF]. The decrease of the productivity, soil moisture and light all imply a diminution of the vegetation cover before 2021. It will be interesting to see in the future whether the increase of the cover observed today will trigger this factor.

We could also observe differences on the mainland concerning the temperature, with plants indicating a decrease in 2021 compared to 2008. The fact that there were no significant differences between 2021 and 1997 on the mainland, regarding the temperature, could perhaps point to a fluctuation being due to the meteorology, with dryer or wetter years. However, we could also note that there were fewer CSR strategies in 2008 than in 1997 on the islands and fewer in 2021 compared to 1997 on the mainland.

This could be explained by a natural fluctuation, with the renewal rate of these plants, which here in the Calanques National Park are Cistus albidus, Cistus monspeliensis, Cistus salvifolius. Euphorbia characias, Halimione portulacoides, and Lotus cytisoides (mostly nanophanerophytes). On the mainland, we also observed the increase of the therophytes. These observations could also be linked with the increase of more generalist and fewer thermophilous species, the closure of the vegetation or the pattern with less rainfall [START_REF] Kimball | Contemporary climate change in the Sonoran Desert favors cold-adapted species[END_REF].

Climate change in the region and its impacts on flora have been synthesized in details in a recently accepted work (Saatkamp et al., 2023 in press), showing that there is an overall warming and drying in lowland areas, with a stronger effect inland compared to coastal areas. It seems however that climate change effects are overridden by trophic changes and long-term succession in the present data-set, since there is no clear effect of thermophilization and xerophytisation. Moreover, higher temperatures during the cold season could increase the presence of plants from higher altitudes or latitudes but not thermophilous species because on the islands this type of plant is already present. Further investigations should be undertaken linked with the precipitation and the temperature as measured and not only bio-indicated.

Conclusions

We highlight that seabird populations provide a link between landfills history and soil eutrophication and time-related vegetation changes on small Mediterranean islands. We observed a remanence of the effects on soil parameters. However, we were expecting more changes in plant community composition but apart from the cover, the community remained similar to 11 years ago, with more ruderal plants.

It is important to follow the patterns of change in gull populations, the flora and the physical and chemical parameters of the soil to better understand the resistance and resilience of the ecosystem postdisturbance. Furthermore, gull populations showed a slight increase in 2021. It is necessary to determine whether this a natural fluctuation between years, a stochastic event, or whether the gulls found another food resource (such as the rubbish bins in coastal cities) leading to a new demographic explosion in the coming years.

  with the environmental alterations due to gulls pressure or/and climate change between 1997, 2008 and 2021, we compared Ellenberg-Pignatti factors (Organic Matter, Nutrient, Soil Acidity, Humidity, Light, Salinity, Temperature) with paired pairwise Wilcoxon tests and Generalised Additive Models (GAM) (mgcv package). The variables included were the different Pignatti factors: organic matter, nutrient, soil acidity, humidity, light, salinity, temperature; and the explained variable was the number of nests. As variables responses are expected to be non-linear, we used Generalized Additive Models with gaussian family to evaluate the stastistical link between Pignatti factors and the number of nests. The indicators were adapted for the Mediterranean species by[START_REF] Saatkamp | Calibrating ecological indicator values and niche width for a Mediterranean flora[END_REF], so that the bioindications given by plants fit better than the initial values.

  86) and Frioul (17.95±6.88) in 2021 than the values for Riou (21.97±13.22) and Frioul (18.24±2.88) in 1997, and than the mean value for the mainland (19.03±6.15) in 2021 (Fig. 4. D, p < 0.001).

  7.). The first axis separated the saline environment with salt-adapted plant species (Camphorosma monspeliaca, Crithmum maritimum, Limonium echioides, Pancratium maritimum) on the left, to the less saline environment represented by other plant species (Avena barbata, Lonicera implexa, Rosmarinus officinalis, Urospermum picroides) on the right. The second axis showed a gradient from the ruderal plant community (top: Hordeum murinum, Malva arborea, Senecio leucanthemifolius, Sonchus asper subsp. glaucescens) to the more stress-tolerant plant community (bottom: Asteriscus aquaticus, Brachypodium retusum, Sedum littoreum, Teucrium flavum). Ruderal plants increased in the vegetation over time as highlighted by increased scores of recent compared to ancient vegetation data on the second axis. The flora in 1997 was less ruderal than in 2008 and in 2021 (p < 0.01). The flora in 2008 and 2021 was also different (p < 0.01) with more ruderal plant species in 2021.No pattern of change was detected on the salinity axis (axis 1) when using the coordinates of the NMDS,(1997 vs 2008 p=0.11, 1997 vs 2021 p=0.44, 2008 vs 2021 p=0.19).The Pignatti nutrient ecological indicator indicated by the flora community on the islands was significantly lower in 2021 and 2008 compared to 1997 (p < 0.05). The humidity factor decreased in 2021 compared to 1997 on the islands (p < 0.05). The light factor decreased in 2021 compared to 1997 and 2008 on the islands (p < 0.05). The soil acidity indicated by the flora community was not significantly different between years for islands plots(1997 vs 2008 p=0.48, 1997 vs 2021 p=0.16, 2008 vs 2021 p=0.057). Generalized Additive Models (GAM) showed no relation with the number of nests counted in 2021 (nutrient p=0.72, humidity p=0.64, light p=0.06, soil acidity p=0.83).The modification of the temperature indicated by the flora community on the islands showed a diminution in 2021 compared to 1997 and 2008 (p < 0.01). In addition, there was a positive correlation with the number of nests (p < 0.05). However, a diminution of the temperature in 2008 and 2021 (p < 0.05) was registered with the mainland plots, where there were no gull nests.The fluctuation of the salinity factor indicated by the flora community was significantly lower in 2008 than in 1997 on the islands (p < 0.05). The GAM showed a positive correlation with the number of nests (p < 0.01). Concerning the mainland, no other difference has been observed (p > 0.15).

  the mainland, differences were observed between 1997 and 2008 (p < 0.05) and between 2008 and 2021 (p < 0.001) with the lowest value in 2008. When analysing Grime plant strategies, ruderals (the most strongly represented Ruderals (R): Sonchus tenerrimus, Ruderal-Competitives (RC): Parietaria judaica, Ruderal-Stress tolerants (RS): Lobularia maritima, Stress tolerant-Ruderals (SR): Euphorbia linifolia) increased between 1997 and 2008 (p < 0.001) and this pattern remained in 2021 (p < 0.001). Moreover, intermediate plant strategies (Competitive-Stress tolerant-Ruderals (CSR), such as Cistus albidus, Euphorbia characias, Lotus cytisoides) decreased significantly on the islands from 1997 to 2008 (p < 0.05) and slightly increased again to 2021, with no significant difference between 1997/2021 (p=0.06277) and 2008/2021 (p=0.8016). Island vegetation showed a significant increase of geophytes, hemicryptophytes and therophytes (p < 0.001) in 2008, remaining constant to 2021. Chamaephytes on the islands decreased from 1997 to 2008 (p < 0.001) and afterwards remained stable (p > 0.005). Chamaephytes on the mainland continuously decreased from 1997 through 2008 (p < 0.01) to 2021 (p < 0.001), and annual plants increased continuously. No significant pattern of change was detected for geophytes (1997 vs 2008 p=0.38, 1997 vs 2021 p=0.59, 2008 vs 2021 p=0.85) and hemicryptophytes (1997 vs 2008 p=0.77, 1997 vs 2021 p=0.037, 2008 vs 2021 p=0.99) on the mainland. No pattern of change was detected for the nanophanerophytes and the phanerophytes at any site.

Fig. 1 :

 1 Fig. 1: Location of the islands in the Frioul archipelago and the Riou Archipelago (Marseille, south-

Fig. 2 :

 2 Fig. 2: Gull (Larus michahellis) population dynamics in the Riou and Frioul archipelagos from 1923 to

Fig. 3 .Fig. 4 .Fig. 5 .

 345 Fig. 3. A. Mean number of nests (mean + SD) at plots in 1997, 2008 and 2021 on Frioul (n = 26) and

Fig. 6 .

 6 Fig. 6. The Delta (δ) of 13C versus δ15N (means ± SD) of stable isotopic composition in soil samples

Fig. 7 .

 7 Fig. 7. Non-metrical multidimensional scaling (NMDS) of plant community composition of the three

  [PTE]pseudo-total in soil/[PTE]local background value where[PTE]local background value are 43, 66 and 7.5 mg.kg -1 for Pb, Zn and Cu respectively[START_REF] Affholder | As, Pb, Sb and Zn transfer from soil to root of wild rosemary: do native symbionts matter?[END_REF]. The potential multi-contamination level of soils was estimated by calculating an integrative index

i.e. pollution load index (PLI) according to the formula

[START_REF] Affholder | As, Pb, Sb and Zn transfer from soil to root of wild rosemary: do native symbionts matter?[END_REF][START_REF] Rashed | Monitoring of contaminated toxic and heavy metals from mine tailings through age accumulation in soil and some wild plants at Southeast Egypt[END_REF]

: PLI= √CFPb x CFZn x CFCu 3

  Frioul vs Riou p=0.61) (Fig.5. E, F). Cu and Zn contents were higher in droppings than in soils. In order to estimate the potential soil contamination by PTE, contamination factors (CF) were calculated for Cu, Pb and Zn, in relation to the local contamination background proposed for the Calanques hills[START_REF] Affholder | As, Pb, Sb and Zn transfer from soil to root of wild rosemary: do native symbionts matter?[END_REF]. The highest CF values (Table4) were measured for Pb in mainland soils (8.23±6.90) and for Cu in soils of the Frioul archipelago (6.87±12.94), and these values reflected a high contamination (CF > 5). CF values for Pb in soils of the archipelagos reflected a low contamination rate (low contamination estimated when CF < 3, 3.22±1.83 at Riou and 2.24±2.28 at Frioul), and those for Cu in soils of Riou and the mainland reflected a medium contamination rate (medium when 3 < CF < 5, 4.26±1.48 at Riou and 3.68±1.57 on the mainland). Concerning Zn, CF values reflected a low contamination rate whatever the sites (values between 2.39 and 3.22). The pollution load index (PLI) for Cu, Pb and Zn estimates the potential multi-

	contamination level of soils. This index, revealed a relatively low multi-contamination level whatever
	the sites studied, with PLI values around 2 (2.10±0.30 at Riou, 2.10±0.54 at Frioul, and 2.33±0.57 on
	the mainland).

). Pb concentrations were lower in droppings (mean of 4.34 mg/kg). With regards to Cu and Zn soil concentrations, no significant difference was observed whatever the soils on the islands or on the mainland (Cu: mainland vs Frioul p=0.99; mainland vs Riou p=0.44; Frioul vs Riou p=0.77; and Zn: mainland vs Frioul p=0.15; mainland vs Riou p=0.56;

Table 3 :

 3 Complementary soil parameters measured on archipelagos and mainland plots, and on gulls' droppings (mean ± SD) in 2021.

	Area	Conductivity	P (mg/g	K
		(mS/cm)	dry	(mg/
			matter
			(DM))

gDM) Na (mg/gDM) Cu (mg/gDM) Pb (mg/gDM) Zn (mg/gDM) Number of plots

	Frioul	1.07 ± 1.78	1671.42	8897.83 ±	1467.50 ±	48.12	±	96.50	±	150.18 ±	26
			± 939.93	2663.84	1531.58	90.62		98.26		82.03
	Riou	0.75 ± 0.35	3300.35	9705.64 ±	1339.63 ±	29.85	±	133.48 ±	157.64 ±	30
			±	2597.42	714.84	10.37		81.75		58.31
			2020.55
	Mainland	0.64 ± 1.19	472.26 ±	8165.27 ±	1125.71 ±	25.74	±	337.90 ±	212.70 ±	22
			205.22	2692.98	2246.57	10.99		299.36		138.27
	Droppings NA	6767.72	4217.13 ±	3665.08 ±	71.53	±	4.34	±	396.13 ±	9
			±	3671.31	1401.69	72.36		14.03		146.16
			5106.96

Table 4 :

 4 Contamination factors calculated for Cu, Pb, Zn and the Pollution Load Index for Frioul and Riou archipelagos and the mainland area (mean ± SD).

	Area	CF Cu	CF Pb	CF Zn	PLI
	Frioul	6.87 ± 12.94	2.24 ± 2.28	2.28 ± 1.24	2.10 ± 0.54
	Riou	4.26 ± 1.48	3.22 ± 1.83	2.39 ± 0.88	2.10 ± 0.30
	Mainland	3.68 ± 1.57	8.23 ± 6.90	3.22 ± 2.09	2.33 ± 0.56

Acknowledgements

eutrophication and time-related vegetation changes on small Mediterranean islands

Clémentine Mutillod¹*, Teddy Baumberger², Pascale Prudent³, Arne Saatkamp¹, Eric Vidal 1,4 , Lidwine Le-Mire-Pecheux 5 , Laurence Affre¹ (1) Aix Marseille Univ, Avignon Université, CNRS, IRD, IMBE Institut Méditerranéen de Biodiversité et d'Écologie marine et continentale, Marseille, France (2) Société EcoStat, Baudinard-sur-Verdon, France (3) Aix Marseille Univ, CNRS, LCE Laboratoire de Chimie de l'Environnement, Marseille, France (4) UMR ENTROPIE (IRD, Université de la Réunion, IFREMER, Université de la Nouvelle-Calédonie, CNRS), BP A5, Nouméa CEDEX 98848, New Caledonia (5) Distance from the coastline Reflects the amount of salt deposited on the ground [START_REF] Barbour | Salt spray as a microenvironmental factor in the distribution of beach plants at point reyes, California[END_REF].

Slope

Slope was estimated visually using 6 categories: 0: flat, 1: 0% < slope < 10%, 2: 0% ≤ slope < 25%, 3: 25% ≤ slope < 50%, 4: 50% ≤ slope < 75%, 5: slope ≥ 75 %. Cover rate of each substrate type: rock, block i.e. Ø > 20 cm, stone i.e. Ø < 20 cm, bare ground Cover was estimated visually using respectively 6 categories: 0: cover = 0%, 1: 0% < cover <10%, 2: 10% ≤ cover < 25%, 3: 25% ≤ cover < 50%, 4: 50% ≤ cover < 75%, 5: cover ≥ 75 %. Vegetation cover and the different layers (High arborescent h (height) > 10 m, Arborescent 10 > h > 4 m, High shrub 4 > h > 2 m, Shrub 2 > h > 0.5 m, Herbaceous h < 0.5 m) Gull nest density (number of nests)

Is considered as an indicator of gull pressure intensity. Following [START_REF] Thompson | A census of the black-browed albatross Diomedea melanophrys population on Steeple Jason Island, Falkland Islands[END_REF], gull nest density was measured by counting the number of both occupiedwith eggs -and empty nests during the egg-laying period in the 78 plots for the three years 1997, 2008 and 2021.

Cover of the resting places per plot

Measured on a semi-quantitative basis using the 6 cover abundance categories previously described, used as a measure of pressure by gulls.

eutrophication and time-related vegetation changes on small Mediterranean islands

Clémentine Mutillod¹*, Teddy Baumberger², Pascale Prudent³, Arne Saatkamp¹, Eric Vidal 1,4 , Lidwine Le-Mire-Pecheux 5 , Laurence Affre¹ (1) Aix Marseille Univ, Avignon Université, CNRS, IRD, IMBE Institut Méditerranéen de Biodiversité et d'Écologie marine et continentale, Marseille, France (2) Société EcoStat, Baudinard-sur-Verdon, France

(3) Aix Marseille Univ, CNRS, LCE Laboratoire de Chimie de l'Environnement, Marseille, France ,4 , Lidwine Le-Mire-Pecheux 5 , Laurence Affre¹ (1) Aix Marseille Univ, Avignon Université, CNRS, IRD, IMBE Institut Méditerranéen de Biodiversité et d'Écologie marine et continentale, Marseille, France (2) Société EcoStat, Baudinard-sur-Verdon, France

(3) Aix Marseille Univ, CNRS, LCE Laboratoire de Chimie de l'Environnement, Marseille, France

Declaration of interests

☒ 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.

☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Declaration of Interest Statement

Title: Yellow-legged gull populations (Larus michahellis) link the history of landfills to soil eutrophication and time-related vegetation changes on small Mediterranean islands Clémentine Mutillod¹*, Teddy Baumberger², Pascale Prudent³, Arne Saatkamp¹, Eric Vidal 1,4 , Lidwine Le-Mire-Pecheux 5 , Laurence Affre¹ (1) Aix Marseille Univ, Avignon Université, CNRS, IRD, IMBE Institut Méditerranéen de Biodiversité et d'Écologie marine et continentale, Marseille, France (2) Société EcoStat, Baudinard-sur-Verdon, France

(3) Aix Marseille Univ, CNRS, LCE Laboratoire de Chimie de l'Environnement, Marseille, France


domain: math spi

text:
Mahmoud Elsawy

Alexis Gobé

Stéphane Lanteri
email: stephane.lanteri@inria.fr-

Guillaume Leroy

Claire Scheid

ADVANCED COMPUTATIONAL DESIGN OF COMPLEX NANOSTRUCTURED PHOTONIC DEVICES USING HIGH ORDER DISCONTINUOUS GALERKIN METHODS AND STATISTICAL LEARNING GLOBAL OPTIMIZATION

ABSTRACT

Nanostructuring of materials has paved the way for manipulating and enhancing light-matter interactions, thereby opening the door for the full control of these interactions at the nanoscale. In particular, the interaction of light waves (or more general optical waves) with matter is a subject of rapidly increasing scientific importance and technological relevance. Indeed, the corresponding science, referred to as nanophotonics, aims at using nanoscale light-matter interactions to achieve an unprecedented level of control on light. Nanophotonics encompasses a wide variety of topics, including metamaterials, plasmonics, high resolution imaging, quantum nanophotonics and functional photonic materials. Previously viewed as a largely academic field, nanophotonics is now entering the mainstream, and will play a major role in the development of exciting new products, ranging from high efficiency solar cells to personalized health monitoring devices able to detect the chemical composition of molecules at ultralow concentrations. In this talk, we will present our recent efforts and achievements toward the development of innovative numerical methodologies for the design of nanoscale photonic devices. Numerical modeling plays a crucial role in this context, in particular for discovering non-intuitive nanostructures or material nanostructuring for harvesting and tailoring the interaction of light with matter at the nanoscale. In our works, we combine two main numerical ingredients: (1) high order Discontinuous Galerkin (DG) methods [START_REF] Lanteri | Analysis of a generalized dispersive model coupled to a DGTD method with application to nanophotonics[END_REF] for solving the system of time-domain [START_REF] Lanteri | Analysis of a generalized dispersive model coupled to a DGTD method with application to nanophotonics[END_REF] or frequency-domain [START_REF] Agullo | High order HDG method and domain decomposition solvers for frequency-domain electromagnetics[END_REF] Maxwell equations in 3D coupled to appropriate differential models of physical dispersion in photonic materials and (2) one of the most advanced optimization techniques that belongs to the class of Bayesian optimization and is known as Efficient Global Optimization (EGO) [START_REF] Jones | Efficient global optimization of expensive black-box functions[END_REF]. The capabilities of the resulting numerical methodology has been first demonstrated for the inverse design of phase-gradient metasurfaces [START_REF] Elsawy | Global optimization of metasurface designs using statistical learning methods[END_REF]- [START_REF] Elsawy | Optimization of metasurfaces under geometrical uncertainty using statistical learning[END_REF] and RGB metalenses [START_REF] Elsawy | Multiobjective statistical learning optimization of RGB metalens[END_REF] (see Fig. 2). In the present talk, we will also report on recent extensions and applications of this methodology to deal with improving the light absorption in ultra-thin solar cells and nanostructured CMOS imager pixels (see Fig. 1). In the context of the Atlantis research group at Inria, we are developing an innovative software suite dedicated to computational nanophotonics that integrates the above-mentioned numerical ingredients (see Fig. 3).

Fig 1 .

 1 Fig 1. Optimization of nanostructuring for improving light trapping in ultra-thin solar cells (left) and light absorption in nanostructured CMOS imager pixel.

Fig 2 .

 2 Fig 2. Optimization of phase gradient metasurface (left) and of a RGB metalens (middle) for the design of planar photonic devices (right). Architecture of the DIOGENeS software suite. DGTD and HDGFD are the high order DG-based fullwave solvers for time-domain and frequency-domain modeling settings. GFactory is the geometrical modeling component that exploits the Python API of the GMSH mesh generation tool. Observer is the base component for developing post-processing scripts of simulation results. Optim is the base component for developing inverse design workflows in Python by using statistical learning global optimization algorithms [DIOGENeS: https://diogenes.inria.fr/].