Ester Cecere

Giorgio Alabiso

Roberto Carlucci

Antonella Petrocelli
email: antonella.petrocelli@iamc.cnr.it

Marc Verlaque

Fate of two invasive or potentially invasive alien seaweeds in a central Mediterranean transitional water system: failure and success

Keywords: alien species, Hypnea cornuta, Mediterranean Sea, transitional water systems (TWS), Undaria pinnatifida

Over the past 15 years, populations of two invasive or potentially invasive alien seaweeds have been studied by means of both quantitative and qualitative methods in the Mar Piccolo basin in Taranto (Southern Italy, Mediterranean Sea). The dynamics of the cold-temperate brown seaweed Undaria pinnatifida (Alariaceae, Laminariales) showed a boom-and-bust path, ending in apparent local extinction. In contrast, the tropical red seaweed Hypnea cornuta (Cystocloniaceae, Gigartinales) has shown a steadily invasive path since its introduction. The respective failure and success of these marine alien species were analysed in relation to their biological and ecological traits and the environmental characteristics of the Mar Piccolo in comparison with other Mediterranean coastal lagoons. The study confirms that temperature and salinity monitoring in the recipient region could provide a first estimation of the invasive potential of an alien species soon after its introduction.

Introduction

Human activities are responsible for species introduction, which is one of the major aspects of globalisation, and the increase in international maritime trade has considerably facilitated this phenomenon [START_REF] Galil | Waterways as invasion highways -Impact of climate change and globalization[END_REF][START_REF] Hewitt | Introductions of seaweeds: accidental transfer pathways and mechanisms[END_REF][START_REF] Perrings | The problem of biological invasions[END_REF]. Alien species are considered as a major threat to marine biodiversity [START_REF] Katsanevakis | Impacts of invasive alien marine species on ecosystem services and biodiversity: a pan-European review[END_REF]. Moreover, owing to their capacity to alter ecosystem composition and processes [START_REF] Olenin | Assessment of biopollution in aquatic ecosystems[END_REF][START_REF] Suárez Jiménez | Do native subtidal grazers eat the invasive kelp Undaria pinnatifida?[END_REF] as well as the negative consequences of carrying parasites and pathogens [START_REF] Diamant | Parasite and disease transfer between cultured and wild coastal marine fish[END_REF][START_REF] Perrings | The problem of biological invasions[END_REF], their economic impact should not be underestimated [START_REF] Katsanevakis | Impacts of invasive alien marine species on ecosystem services and biodiversity: a pan-European review[END_REF].

With about 1000 alien species recorded, excluding microalgae, the Mediterranean Sea is one of the geographical areas most severely affected by this phenomenon [START_REF] Zenetos | Alien species in the Mediterranean Sea by 2012. A contribution to the application of European Union's Marine Strategy Framework Directive (MSFD). Part 2. Introduction trends and pathways[END_REF]. Soon the situation could become significantly worse due to the recent enlargement of the Suez Canal [START_REF] Galil | Double trou-ble': the expansion of the Suez Canal and marine bioinvasions in the Mediterranean Sea[END_REF], which is considered one of the main vectors of introduction of alien species [START_REF] Galil | Taking stock: inventory of alien species in the Mediterranean Sea[END_REF][START_REF] Zenetos | Alien species in the Mediterranean Sea by 2010. A contribution to the application of European Union's Marine Strategy Framework Directive (MSFD). Part I. Spatial distribution[END_REF]. However, aquaculture also has a considerable responsibility as a vector of introduction. Starting in the 1970s, there were massive importations of spat and adults of the Pacific oyster Crassostrea gigas (Thunberg, 1793) for aquaculture purposes in the Mediterranean, mainly from Japan (but also from British Columbia and Korea), without any effective system of decontamination and quarantining [START_REF] Boudouresque | Biological invasion: the Thau Lagoon, a Japanese biological island in the Mediterranean Sea[END_REF]. Since this date, many species originating from cold to temperate regions of the North Pacific Ocean have been introduced [START_REF] Verlaque | Oyster transfers as a vector for marine species introductions: a realistic approach based on the macrophytes[END_REF][START_REF] Zenetos | Alien species in the Mediterranean Sea by 2010. A contribution to the application of European Union's Marine Strategy Framework Directive (MSFD). Part I. Spatial distribution[END_REF]. With regard to the seaweeds, those introductions mainly occurred in transitional water systems (TWS), where aquaculture activities are commonly carried out, such as Thau Lagoon, France [START_REF] Boudouresque | Biological invasion: the Thau Lagoon, a Japanese biological island in the Mediterranean Sea[END_REF], the Venice Lagoon, Northern Adriatic Sea [START_REF] Marchini | Current status and trends of biological invasions in the Lagoon of Venice, a hotspot of marine NIS introductions in the Mediterranean Sea[END_REF] and the Mar Piccolo of Taranto, Northern Ionian Sea (Cecere et al. 2016a). These sites represent three hotspots of species introduction in the Mediterranean, and are therefore continuously monitored for early detection of any new introduction.

Notwithstanding the impact of species introductions, few methods are as yet available to assess the fate of an introduced species and the risk of invasion [START_REF] Roy | Horizon scanning for inva-sive alien species with the potential to threaten biodiversity in Great Britain[END_REF], or to manage and control the development of their populations after introduction [START_REF] Simberloff | We can eliminate invasions or live with them: successful management projects[END_REF][START_REF] Ojaveer | Ten recommendations for advancing the assessment and management of non-indigenous species in marine ecosystems[END_REF]. For this purpose, the study of the dynamics of alien species once introduced is of great interest, especially [START_REF] Nyberg | Can species traits be used to predict marine macroalgal introductions?[END_REF]. A good knowledge of the ecology and life history traits of alien species (eco-physiology, reproductive pattern, growth rate, vulnerability to diseases, predation and competition) can offer a basis for identifying the threshold values of environmental triggers of biological invasions [START_REF] Knoepffler-Péguy | Sargassum muticum begins to invade the Mediterranean[END_REF][START_REF] Meinesz | The introduced green alga Caulerpa taxifolia continues to spread in the Mediterranean[END_REF][START_REF] Gollasch | Hazard analysis of aquatic species invasions[END_REF][START_REF] Sagerman | Divergent ecological strategies determine different impacts on community production by two successful non-native seaweeds[END_REF]. This knowledge can be useful to anticipate the outcome of a potentially invasive alien species in the short, medium or long term, providing a basis for the choice of suitable management procedures in order to minimise its impact on the ecosystems and on human activities [START_REF] Meinesz | Methods for identifying and tracking seaweed invasions[END_REF][START_REF] Rilov | Marine bioinvasions in the Mediterranean Sea-History, distribution and ecology[END_REF][START_REF] James | Using satellite-derived sea surface temperature to predict the potential global range and phenology of the invasive kelp Undaria pinnatifida[END_REF].

Today, there is a serious concern about global warming, and there is ongoing and lively debate in the scientific community about its putative role in facilitating the success of species introductions [START_REF] Boudouresque | Is global warming involved in the success of seaweed introductions in the Mediterranean Sea?[END_REF]. Recently, an extensive study carried out in the Ligurian Sea showed that, in this cold sector of the Mediterranean, the spread of alien seaweeds seems to be mostly due to their "large native geographical ranges" rather than to a considerable increase in local seawater temperatures (Parravicini et al. 2015a). However, some authors consider that the success of an alien species might depend on both its native range, which could be cold, temperate, tropical or cosmopolitan, and the environmental features of the recipient area (mainly temperature) (Boudouresque and Verlaque 2010 and references therein, [START_REF] James | Using satellite-derived sea surface temperature to predict the potential global range and phenology of the invasive kelp Undaria pinnatifida[END_REF]. In this respect, global warming might affect the success of alien species more at a local scale than at a wider scale, causing local peaks of variation in sheltered, shallow and enclosed environments, such as TWS, which are exposed to wider ranges of temperature variation than the open ocean. In the Mediterranean, this might be the case for coastal lagoons such as the Mar Piccolo of Taranto, where a slight trend of temperature increase was recently recorded (G. Alabiso, unpublished data). The aim of this study was to carry out a multiyear monitoring survey of populations of two potentially invasive alien species introduced in the Mar Piccolo of Taranto, and having contrasting native ranges: the Japanese cold-temperate brown seaweed Undaria pinnatifida (Harvey) Suringar, found for the first time in the Mediterranean Sea in 1971 (Thau Lagoon;[START_REF] Anonymous | Activité de l'Institut des Pêches en 1979. Principales actions en matière de cultures marines[END_REF], and the pantropical red seaweed Hypnea cornuta (Kützing) J. Agardh, found for the first time in the Mediterranean in 1894 (Rhodes Island; Reinbold 1898), although incorrectly identified as H. valentiae (Turner) Montagne.

We focused on salinity and seawater temperature, mainly with regard to annual variation and extreme values, as factors that could have affected the fate of both alien species, even though all the environmental variables, and their synergistic effect, are recognised as important [START_REF] Dukes | Does global change increase the success of biological invaders?[END_REF][START_REF] Báez | Macro-environmental modelling of the current distribution of Undaria pinnatifida (Laminariales, Ochrophyta) in northern Iberia[END_REF][START_REF] Cook | Impacts of climate change on non-native species[END_REF].

Materials and methods

Study site

The Mar Piccolo of Taranto is a coastal nearly enclosed basin located to the north of the town of Taranto. Its surface area is ca. 20 km 2 and it is divided by two promontories of land into two smaller basins, which are called the First and the Second Inlets (Figure 1). The maximum depths in the central part of the First and Second Inlet are 12 and 8 m, respectively. Most of the hard substrata are artificial. The soft substrata are sandy near the shore and muddy in the central zones. [START_REF] Cecere | The Mar Piccolo of Taranto[END_REF] present a detailed description of the site with the variations of the main physico-chemical variables, and highlight the presence of several possible activities and vectors favouring the introduction of alien seaweeds (e.g. mussel importation, military ships, fishing boats). To date, a total of 35 non-indigenous marine species (15 macroalgae, 8 protists and 12 invertebrate metazoans) have been found in the Mar Piccolo (E. Cecere, unpublished results). Some of these, presumably introduced by shipping, were observed in the 1920s, giving rise to transitory populations [START_REF] Pierpaoli | Prima contribuzione allo studio delle alghe del Golfo di Taranto[END_REF]), but most have been detected since the late 1980s, and many of them were probably associated with shellfish importations.

Alien species

Undaria pinnatifida (Alariaceae, Laminariales, Phaeophyceae) has an annual heteromorphic diplohaplontic life history, with the alternation of a macroscopic sporophyte generation and a microscopic dioecious gametophyte generation. Sporophytes have a short stipe and a blade with shallow pinnate divisions. Reproduction occurs when the size of individuals exceeds 20 cm in height. Unilocular meiosporangia with paraphyses are produced on two opposite jabot-shaped wings (sporophylls) arranged along the stipe [START_REF] Castric-Fey | The distribution of Undaria pinnatifida on the coast of St Malo (Brittany, France)[END_REF], NIMPIS 2016 and references therein). The survey of the introduced population was initiated at Cariati in 1998. Monthly quantitative sampling was carried out during the growth period of sporophytes in 2000-2001, 2001-2002 and 2006-2007. This monitoring survey lasted up to May 2015 but, since sporophytes became more difficult to find, quantitative sampling was not possible after 2007. Each month, 30 quadrats (50 × 50 cm) were haphazardly sampled by SCUBA diving. The total number of sporophytes and their distribution per size class on the basis of the total length (TL in cm) were determined in each quadrat. The density of the population was expressed as the mean number of sporophytes per m 2 (No m -2 ) and the recruitment was estimated as the number of young sporophytes (TL < 10 cm) per m 2 . Hypnea cornuta (Cystocloniaceae, Gigartinales, Florideophyceae) is a species characterised by the presence of stellate branchlets that act as vegetative propagules [START_REF] Yamagishi | Taxonomic notes on marine algae from Malaysia. XI. Four species of Rhodophyta[END_REF][START_REF] Cecere | Morphology and vegetative reproduction of the introduced species Hypnea cornuta (Rhodophyta, Gigartinales) in the Mar Piccolo of Taranto (Italy, Mediterranean Sea)[END_REF]). Like all the species of the genus, it has a Polysiphonia-type life history with the alternation of an isomorphic diploid tetrasporophyte and haploid dioecious gametophytes, with a diploid carposporophyte developing on the female gametophyte [START_REF] Masuda | Overview of Hypnea (Rhodophyta, Hypneaceae)[END_REF].

A monitoring survey of introduced populations was initiated in December 2000 at Battendieri and in October 2007 at Cimino (Figure 1). Monthly quantitative sampling was carried out from 2007 to 2010, and in 2014 and 2015 at both Battendieri and Cimino. Each time, three quadrats (50 × 50 cm) were sampled at 1-m depth and the collected thalli were preserved in 5% formalin-seawater. For each quadrat, the blotted fresh weight of H. cornuta was measured on a triple beam balance. The biomass of H. cornuta was expressed as the mean fresh weight per m 2 (g fw m -2 ).

Seawater temperature

The seawater temperature (in °C) was continuously recorded at 1-m depth in the sampling stations by means of an underwater probe (IDROPROBE Idromar, Genoa, Italy) from 1996 to 2004, and by temperature loggers S2T (NKE Instrumentation, Hennebont, France) from 2005 to 2015. The mean annual values were calculated as well as the mean temperatures of the coldest and warmest weeks of each year, which were considered a proxy of minimum and maximum mean values.

Statistical analysis

In order to analyse the occurrence of significant changes by season in the density and recruitment of Undaria pinnatifida and biomass of Hypnea cornuta during the investigated period, parametric analysis of variance (ANOVA) was performed using the SPSS version 17.0 (Chicago, IL, USA) (SPSS Inc. 2009). Seawater temperature data were analysed by non-parametric regression.

Results

Undaria pinnatifida

In April 1998, sporophytes of Undaria pinnatifida were found for the first time in the First Inlet of the Mar Piccolo of Taranto attached to the mooring wharf of the small local fishing fleet, close to the numerous shellfish shops at Cariati in the old town of Taranto (Figure 1; [START_REF] Cecere | Undaria pinnatifida (Fucophyceae, Laminariales) spread in the central Mediterranean: its occurrence in the Mar Piccolo of Taranto (Ionian Sea, southern Italy). Cryptogam[END_REF]. In 1998 and 1999, sporophytes were restricted to the wharf at Cariati, and ropes and pebbles lying on the nearby muddy bottom, from the sea surface to 1.5-m depth. Later, some sporophytes were also present on the floating pontoons facing the wharf. The longest specimen observed was about 1-m long and the individuals exhibited great morphological variability. Examination of the sporangial sori showed complete maturation of the sporangia. The density and the size structure of sporophytes varied widely through the year (Figures 2 and3). New sporophytes were detected from December to April, with a peak in February 2002. They grew up to late spring (May-June) and disappeared completely in summer (July). In May, the smallest sporophytes (i.e. 0-10 cm) consisted entirely of residual stipes of old specimens (Figures 2 and3). The largest individuals were observed from March to May (Figure 3). The densities observed in winter 2002 were much higher than those recorded in 2001. Temperature (°C) 2015, and was significantly higher at Battendieri than at Cimino (Figure 4). The highest biomasses were measured in August 2015 at both stations (5860 and 3477 g fw m -2 at Battendieri and Cimino, respectively; Figure 4). Seasonal variations of biomass were statistically significant (p < 0.05) among years and between stations.

Seawater temperature

For the study period (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015), in the Mar Piccolo, the annual range of seawater temperature was wide (about 15-24°C). The minimum and maximum mean temperatures of though the introduction of a potential invasive species does not always foreshadow establishment and invasion [START_REF] Simberloff | We can eliminate invasions or live with them: successful management projects[END_REF]Gibbons 2004, Walther et al. 2009), less is known about introductions that fail.

In the Mar Piccolo of Taranto, the dynamics of two potential invaders were monitored, on the basis of both quantitative and qualitative observations. Two different demographic paths were identified: the first (Undaria pinnatifida) corresponds to the boom-and-bust model, which occurs when the alien species reaches a peak of development before declining, sometimes to local extinction [START_REF] Williamson | Biological invasions[END_REF]; the invasive path of the second species (Hypnea cornuta) corresponds to the natural fluctuation model characterised by a plateau-like persistence phase with fluctuations (see [START_REF] Boudouresque | An overview of species introduction and invasion processes in marine and coastal lagoon habitats[END_REF].

Undaria pinnatifida is a cold-temperate species described from Honshu (Japan) and widely cultivated in the NW Pacific Ocean (China, Japan and Korea), and on the NE Atlantic Coast, where it has been deliberately introduced in France [START_REF] Pérez | La culture des algues marines dans le Monde[END_REF][START_REF] Floc'h | Undaria pinnatifida (Laminariales, Phaeophyta) 12 years after its introduction into the Atlantic Ocean[END_REF]. As an alien species, it occurs in all the continents, except Africa and Antarctica [START_REF] Wallentinus | Alien species alert: Undaria pinnatifida (wakame or Japanese kelp)[END_REF]) and it is considered as one of the "World's 100" and "Mediterranean's 100 Worst Invasive Alien Species" (Streftaris andZenetos 2006, GISD 2016). In the Mediterranean Sea, it has been reported only from the north-western basin (France, mainly in Thau Lagoon), the North Adriatic Sea (Venice Lagoon, Italy) and the Mar Piccolo of Taranto. One year after its discovery in the Mar Piccolo, the mean density of U. pinnatifida sporophytes (18 individuals m -2 ) was higher than that recorded in the Venice Lagoon (about 10 individuals m -2 ; [START_REF] Curiel | The introduced alga Undaria pinnatifida (Laminariales, Alariaceae) in the Lagoon of Venice[END_REF]) and lower than that in the Thau Lagoon (250 individuals m -2 ; [START_REF] Boudouresque | L'algue japonaise Undaria pinnatifida (Phaeophyceae, Laminariales) en Méditerranée[END_REF]. The first detection of new sporophytes was later in the Mar Piccolo of Taranto than in the two other coastal lagoons (December-February vs. November), and both a shortening of the recruitment period and a lowering of its intensity were observed throughout the study period, although the growth of sporophytes went as far as complete sporangial maturation. Although U. pinnatifida is flourishing in the Thau Lagoon and the Venice Lagoon [START_REF] Verlaque | Checklist of the macroalgae of Thau Lagoon (Hérault, France), a hot spot of marine species introduction in Europe[END_REF] and unpublished data, Sfriso and Facca 2013), the species quickly declined in Mar Piccolo to apparent collapse 12 years after its first sighting.

In terms of disturbance and turbidity, these three large TWS are similar, whereas salinity and temperature are usually higher in the Mar Piccolo (Table 1). As far as water temperature is concerned, optima for the growth of the sporophyte and the gametophyte of U. pinnatifida are 3.5-20°C and 10-24°C [START_REF] Sanderson | A preliminary survey of the distribution of the introduced macroalga, Undaria pinnatifida (Harvey) Suringar on the east coast of Tasmania, Australia[END_REF]), while the upper lethal temperatures are > 22-25°C and > 30°C, respectively (Table 1).

In Brittany, under optimal seawater temperatures, sporophyte recruitment started at the end of autumn and extended to late spring [START_REF] Castric-Fey | The introduced alga Undaria pinnatifida (Laminariales, Alariaceae) in the rocky shore ecosystem of the St Malo area: growth rate and longevity of the sporophyte[END_REF]. In regions where temperatures do not exceed 19°C, sporophytes persist all the year; elsewhere they disappear in summer and early autumn when only microscopic gametophytes are present [START_REF] Akiyama | Cultivation of Undaria pinnatifida (Harvey) Suringar. The decrease in crops from natural plants following crop increase from cultivation[END_REF]Kurogi 1982, Stuart et al. 1999). Observations in the Japanese native area showed that, at 28°C, less than 15% of gametophytes survive and the extinction of U. pinnatifida is forecast if local temperature rises [START_REF] Watanabe | The effect of irradiance and temperature responses and the phenology of a native alga, Undaria pinnatifida (Laminariales), at the southern limit of its natural distribution in Japan[END_REF]).

As a result, the winter-spring temperature ranges of the Mar Piccolo of Taranto allowed growth and reproduction of sporophytes. The "weak point" of the life cycle of U. pinnatifida concerns the sexual generation: summer seawater temperatures in the Mar Piccolo were frequently higher than the lethal temperature range, probably inducing high mortality and failure of reproduction of gametophytes, leading to the collapse of the population. Other environmental factors, in combination with temperature, may have also contributed to the failure of the naturalisation of U. pinnatifida. Firstly, there is the salinity of the Mar Piccolo that was close to or higher than the lethal threshold both for gametophytes and sporophytes (Table 1). Similarly, too high a salinity (37-38;[START_REF] Hopkins | Physics of the Sea[END_REF] along the north-western Mediterranean coasts might explain the failure of the naturalisation of U. pinnatifida in open sea, while the species is flourishing in the less salty waters of the Thau Lagoon [START_REF] Boudouresque | L'algue japonaise Undaria pinnatifida (Phaeophyceae, Laminariales) en Méditerranée[END_REF][START_REF] Knoepffler | Phytobenthos des Pyrénées-Orientales: complément à l'inventaire: présence d'espèces non-indigènes (Sargassum et Undaria)[END_REF][START_REF] Verlaque | Hypnea cornuta[END_REF]. Overgrazing by the sea-urchin Paracentrotus lividus (Lamarck, 1816), already observed in Thau Lagoon (M. Verlaque, unpublished data), could hardly have occurred in the Mar Piccolo because the sea urchin density never exceeded 1-2 individuals m -2 (E. Cecere and A. Petrocelli, unpublished data). Metal contamination could have also played a negative role. Experiments showed strong inhibition of both meiospore germination and gametophyte development by copper [START_REF] Leal | Exposure to chronic and high dissolved copper concentrations impedes meiospore development of the kelps Macrocystis pyrifera and Undaria pinnatifida (Ochrophyta)[END_REF], a metal strongly present in the superficial sediments of the Mar Piccolo, especially in the First Inlet [START_REF] Petronio | Spatial and temporal heavy metal concentration (Cu, Pb, Zn, Hg, Fe, Mn, Hg) in sediments of the Mar Piccolo in Taranto (Ionian Sea, Italy)[END_REF].

Hypnea cornuta is a warm-water species mentioned as widespread in tropical and sub-tropical zones, although the worldwide distribution of Hypnea species must be considered with caution because of multiple confusion with identification [START_REF] Mshigeni | Hypnea (Gigartinales, Rhodophyta)[END_REF]. In the Mediterranean Sea, where the species is considered as an alien seaweed from the Red Sea [START_REF] Verlaque | Hypnea cornuta[END_REF], it was reported from the eastern Mediterranean basin (Greece, The Middle East and Egypt), the Mar Piccolo of Taranto and, more recently, north-eastern Sicily (Manghisi [START_REF] Sghaier | Review of alien marine macrophytes in Tunisia[END_REF]) and the Tyrrhenian Sea (Cecere et al. 2016b). With regard to the TWS of the north-western Mediterranean basin (Thau) and the Northern Adriatic Sea (Venice), we note the absence of this tropical species. In the Red Sea, the temperature and salinity ranges are 18-32°C and 36.5-40.5, respectively [START_REF] Edwards | Climate and Oceanography[END_REF]. In Israel, experimental cultures of H. cornuta have succeeded with temperature and salinity ranges of 14.5-30.5°C and 30-36, respectively [START_REF] Friedlander | Rearing of agarophytes and carrageenophytes under field conditions in the eastern Mediterranean[END_REF], which are close to values recorded in the Mar Piccolo, at least in summer for temperature. Monthly observations carried out in the Mar Piccolo highlighted a seasonal cycle of development, with a growth period from May to August and a resting period from January to April, during which the species was only represented by over-wintering organs (basal parts, stellate propagules and pigmented unattached fragments; [START_REF] Cecere | Morphology and vegetative reproduction of the introduced species Hypnea cornuta (Rhodophyta, Gigartinales) in the Mar Piccolo of Taranto (Italy, Mediterranean Sea)[END_REF][START_REF] Cecere | Vegetative reproduction by multicellular propagules in Rhodophyta: an overview[END_REF]. Fertile gametophytes were never observed, suggesting that the rare tetrasporangia are apomeiotic or that they fail to germinate. Multi-year monitoring confirmed the invasive behaviour of the species, with a steady increase in biomass throughout the study period.

Although H. cornuta became the dominant component of the summer free-living seaweed assemblages, no conspicuous noxious effects on benthic assemblages or human activities have so far been detected. In Israel and Brazil, experiments on H. cornuta showed a positive correlation between seawater temperature and growth rate, with the fastest growth (up to a doubling of initial weight, i.e. 100 g in Israel, after 3 days cultivation), when seawater temperature reached the highest values (about 30°C) (Friedlander andZelikovitch 1984, Yokoya and[START_REF] Yokoya | Temperature responses of economically important red algae and their potential for mariculture in Brazilian waters[END_REF]. Therefore, high seawater temperature could explain the success of H. cornuta in the Mar Piccolo of Taranto. Frequently close to 30°C in summer, temperature was most probably responsible for the invasive path of the species. The analysis of temperature trends shows yearly fluctuations, with on-going warming observed since 2002, which might have contributed to the steady increase of biomass.

In addition to the role played by high temperatures, the high rate of vegetative multiplication of H. cornuta (pigmented fragments, propagules and possible apomeiotic sporangia) might also have favoured its establishment in the Mar Piccolo, as already observed in the case of some other marine and terrestrial introductions [START_REF] Nyberg | Can species traits be used to predict marine macroalgal introductions?[END_REF]Wallentinus 2005, Walther et al. 2009).

The comparison of the dynamics of U. pinnatida and H. cornuta in the Mar Piccolo of Taranto showed that salinity and temperature could be important environmental variables affecting the fate of introduced species. The Mar Piccolo seems to be too salty and too warm for U. pinnatifida, and not too cold and favourable for H. cornuta. In Norway, the alien seaweed Heterosiphonia japonica, which is able to survive between 1°C and 28°C, did not spread along the coasts, where seawater temperatures normally go down to 0°C [START_REF] Bjaerke | Effects of temperature and salinity on growth, reproduction and survival in the introduced red alga Heterosiphonia japonica (Ceramiales, Rhodophyta)[END_REF]. The invasive strain of Caulerpa taxifolia failed to settle in Japanese seawaters because of temperatures lower than 10°C [START_REF] Komatsu | But next time?: unsuccessful establishment of the Mediterranean strain of the green seaweed Caulerpa taxifolia in the Sea of Japan[END_REF]). In the Mar Piccolo of Taranto, this might be also the case for the highly invasive cold-temperate species Codium fragile var. fragile. It was found for the first time in the basin in 2000, and was not observed again until 2009 when a new introduction occurred [START_REF] Petrocelli | Alien marine macrophytes in transitional water systems: new entries and reappearances in a Mediterranean coastal basin[END_REF]; nowadays, it survives with only a few scattered individuals at the site of the second introduction [START_REF] Petrocelli | Alien marine macrophytes in transitional water systems: new entries and reappearances in a Mediterranean coastal basin[END_REF].

Once the ecological requirements, and especially the lethal limits, of an alien species are known, the continuous recording of salinity and seawater temperature in the introduction zone, to determine not only the trends but also their fluctuations throughout the year, could offer a highly informative basis for predicting the fate of alien species at a local scale [START_REF] Dukes | Does global change increase the success of biological invaders?[END_REF]Mooney 1999, James et al. 2015). However, it would be advisable to consider also the shift from the theoretical niche (based upon e.g. temperature and salinity) towards the actual, realised niche, taking into account biotic interactions within the ecosystem, which is still undervalued in marine ecosystems (Parravicini et al. 2015b).

It is well known that maximum and minimum temperature values can be more important than annual mean values in affecting the population dynamics of a species [START_REF] Stachowicz | Linking climate change and biological invasions: Ocean warming facilitates nonindigenous species invasions[END_REF][START_REF] James | Using satellite-derived sea surface temperature to predict the potential global range and phenology of the invasive kelp Undaria pinnatifida[END_REF]. Such values will be all the more extreme where water bodies are restricted and shallow, like the TWS.

While an increase in temperature of 1°C-1.4°C over the last 30 years was reported for the Mediterranean Sea (see [START_REF] Lejeusne | Climate change effects on a miniature ocean: the highly diverse, highly impacted Mediterranean Sea[END_REF] and references therein), a similar warming of mean annual temperature (ca. 1°C) was recorded in the Mar Piccolo of Taranto since 1996 (G. Alabiso, unpublished data). Moreover, during the last decade, some anomalous peaks of temperature have been recorded throughout the Mediterranean Sea [START_REF] Lejeusne | Climate change effects on a miniature ocean: the highly diverse, highly impacted Mediterranean Sea[END_REF]. Such climatic events can induce dramatic changes in the latitudinal distribution and development of native and alien species. In the Mar Piccolo, the extreme values for the water surface temperature seem to have played a more crucial role with regard to the fate of species invasion than the recent warming trend observed.

In the future, a warmer Mediterranean Sea should be more welcoming for tropical than for cold-temperate alien species, and make easier their establishment, spread and eventually, invasiveness (Occhipinti-Ambrogi and Galil 2010). Simultaneously, cold-temperate alien species should be disadvantaged [START_REF] Boudouresque | Is global warming involved in the success of seaweed introductions in the Mediterranean Sea?[END_REF]. Thus the Mar Piccolo of Taranto could become more unfavourable for cold-temperate alien species such as U. pinnatifida and more welcoming for tropical species such as H. cornuta, all the more so as it is located on the westward spread route of Red Sea species coming into the Mediterranean Sea via the Suez Canal.

Among the alien marine macrophytes of the Mar Piccolo, only the tropical H. cornuta has displayed an invasive trend, but without any apparent noxious effects. However, an alien species can spend a transitional period of several years before giving rise to a successful invasion [START_REF] Simberloff | We can eliminate invasions or live with them: successful management projects[END_REF][START_REF] Simberloff | Now you see them, now you don't!population crashes of established introduced species[END_REF]references therein, Walther et al. 2009). For instance, the red seaweed Gracilaria salicornia became invasive almost 30 years after its intentional introduction into Hawaii [START_REF] Smith | Ecology of the invasive red alga Gracilaria salicornia (Rhodophyta) on O'ahu, Hawai'i[END_REF]). Therefore, the continuous monitoring of H. cornuta in the Mar Piccolo of Taranto is strongly recommended. Furthermore, occasional very low seawater temperatures could cause the transitory spreading of cold-temperate aliens that could damage the recipient ecosystem as well, before undergoing a definitive collapse [START_REF] Simberloff | Now you see them, now you don't!population crashes of established introduced species[END_REF]. Therefore, the monitoring of these alien species is also advisable.

Acknowledgments:

The support of the National Research Projects V.E.C.T.O.R. and RITMARE (www.ritmare.it) is acknowledged. G. Portacci is thanked for his help during sampling in the Mar Piccolo of Taranto. We are indebted to Michèle Perret-Boudouresque for bibliographical assistance and to Michael Paul for correcting the English. We thank Prof. M. Dring and two referees for improving the manuscript with their precious suggestions. The study was performed within the framework of LTER activities.

Bionotes Ester Cecere

Institute for the Marine Coastal Environment (IAMC) -CNR, via Roma 3, 74123 Taranto, Italy Ester Cecere is Senior Researcher and head of the Phycology Laboratory. The main topics of her studies are: biodiversity of marine benthic macroalgae; floristic and vegetational studies of marine benthic communities, in particular in Transitional Water Systems; cultivation of economically important marine benthic macroalgae; biology of marine benthic macroalgae, with particular reference to the vegetative propagation; bioremediation with marine benthic macroalgae (CO 2 sequestration, nutrient abatement); evaluation of anthropic impact; taxonomy, biology and physio-ecology of marine benthic macroalgae, in particular non-indigenous species.

Giorgio Alabiso

Institute for the Marine Coastal Environment (IAMC) -CNR, via Roma 3, 74123 Taranto, Italy Giorgio Alabiso is Researcher (retired) and head of the Oceanography Laboratory. His research activity is focused on: studies on the interference of marine biofilm on corrosion resistance of stainless steels in polar sea water; studies on chemical-physical conditions of seawater and on biochemical composition of particulate matter; planning of cells for carbon dioxide sequestration by seaweeds; planning of a photobioreactor for CO 2 sequestration by marine biomasses for biodiesel production; planning pilot plants for interdisciplinary studies (biological and corrosionistic); bioremediation by seaweeds; studies on nutrient uptake by seaweeds.

Roberto Carlucci

Department of Biology, University of Bari, via Orabona 4, 70125 Bari, Italy Roberto Carlucci is Assistant Professor in Ecology. His main research interest is focused on the application of biological, statistical and mathematical models to marine ecology and population dynamics. In particular, his research is oriented to provide scientific advice to regional, national and EU administrations for the setting of the best practice in the Mediterranean for a sustainable exploitation of marine resources. He uses the statistical and geostatistical packages (SPSS, Statistica, PRIMER V, ISATIS -Geovariance) and ARCVIEW GIS. He has (co)-authored more than 70 scientific publications in congress proceedings, national and international ISI journals. RC is a reviewer for international ISI journals.

Antonella Petrocelli

Institute for the Marine Coastal Environment (IAMC) -CNR, via Roma 3, 74123 Taranto, Italy, Fax: +39 0994542215, antonella.petrocelli@iamc.cnr.it Antonella Petrocelli is a researcher at the Phycology Laboratory. Her research activity deals with marine biology, phycology and ecology with regard to: biodiversity; habitat destruction; bioremediation; biotechnologies; ecophysiology; alien seaweeds; cultivation in little and meso scale of marine macroalgae; floristic and vegetational studies in marine benthic communities; biology of seaweeds with particular reference to vegetative propagation; active metabolites from marine macroalgae; ecophysiology of seaweeds with particular reference to photosynthesis; seagrasses.

Marc Verlaque

Figure 1 :

 1 Figure 1: Map of Mar Grande and Mar Piccolo of Taranto with sampling stations for each species. �: Undaria pinnatifida, Cariati (First Inlet); ■: Hypnea cornuta, Battendieri and Cimino (Second Inlet); left, location of Taranto in Italy.

  In 2003 and 2004, U. pinnatifida stopped spreading in the basin. In 2005 and 2006, a slight reduction in the population was observed. In 2007, the first new sporophytes were detected later (in February) and, during this year, the mean density of sporophytes never exceeded two individuals m -2 (Figure 3). Very few sporophytes (total < 10) were observed in 2008 and 2009 and none since 2010. The inter-annual differences in the density of sporophytes were statistically significant (p < 0.05).

Figure 3 :Figure 2 :

 32 Figure 3: Undaria pinnatifida: densities of sporophytes in different length classes, expressed as means (± SD) of data from 30 quadrats in[START_REF] Cecere | Undaria pinnatifida (Fucophyceae, Laminariales) spread in the central Mediterranean: its occurrence in the Mar Piccolo of Taranto (Ionian Sea, southern Italy). Cryptogam[END_REF][START_REF] Verlaque | Checklist of the macroalgae of Thau Lagoon (Hérault, France), a hot spot of marine species introduction in Europe[END_REF][START_REF] Verlaque | Checklist of the macroalgae of Thau Lagoon (Hérault, France), a hot spot of marine species introduction in Europe[END_REF][START_REF] Gollasch | Hazard analysis of aquatic species invasions[END_REF] and 2006-2007 at Cariati (First Inlet). Sporophytes were absent in quadrats from July to November.

  warmest weeks were registered in January 2002 (7.51 ± 0.85°C) and June 2003 (32.29 ± 0.93°C), respectively. An increasing trend of ca. 1°C was observed in the mean annual temperature from 1996 to 2015 (p < 0.01), but no significant trend was observed in the mean values of coldest and warmest weeks of each year (Figure5).

DiscussionAFigure 5 :

 5 Figure 5: Annual mean temperatures (�) and mean temperatures of coldest (■) and warmest ( ) weeks from 1996 to 2015 in the Mar Piccolo. Data are means ± SD (n = 8736).

Figure 4 :

 4 Figure 4: Hypnea cornuta: biomass (g fw m -2 ) from 2007 to 2010, and in 2014 and 2015 at Battendieri and Cimino (Second Inlet). Data are means ± SD (n = 3).

  Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, GIS Posidonie, 13288 Marseille, France Marc Verlaque is a retired researcher who continues his research activities in the GIS Posidonie, a non-profit organization hosted by the Mediterranean Institute of Oceanography (Aix-Marseille University). His study fields deal with marine macrophytes (algae and sea grasses), biology of conservation and species invasions, marine ecology, taxonomy, biogeography and phylogeny.

Table 1 :

 1 Salinity and seawater temperature ranges in the Thau Lagoon, the Lagoon of Venice and the Mar Piccolo of Taranto, and salinity and temperature tolerances for the sporophytic and gametophytic generations of Undaria pinnatifida.

		Salinity	Temperature	References
	Thau Lagoon	27-40	(4-) 5-27 (- 29)°C	Jouffre and Amanieu 1991, Plus et al. 2003
	Lagoon of Venice	(20-) 27-34 (- 35.9) (- 1) 5-26 (- 33)°C	Curiel et al. 2001, Sfriso et al. 2009, Sfriso and Facca 2013
	Mar Piccolo of Taranto -First Inlet 33.04-37.81	7.5-32.29°C	Cecere and Petrocelli 2009
	Lethal values for gametophytes	< 15 and > 34	< - 1°C and > 30°C	Saito 1975, Bardach et al. 1982, Sanderson 1990, Pérez et al.
				1992, O'Loughlin et al. 2006, Peteiro and Sánchez 2012
	Lethal values for sporophytes	< 15 and > 34	< 0°C and > 22-25°C


A Do

M Briat

S D Baton

M Krumrey

L Lecherbourg

B Loupias

F Pérez

P Renaudin

C Rubbelynck

Ph Troussel

Two-channel high-resolution quasi-monochromatic X-ray imager for Al and Ti plasma

High-resolution, high-sensitivity X-ray imaging is a real challenge in High-Energy Density Plasma (HEDP) experiments. We present an improved design of the FUHRI (Fresnel Ultra High-Resolution Imager) instrument). Using an Ultra-High-Intensity (UHI) laser to generate hot and dense plasma in a small volume of an Al-Ti mixed target provides simultaneous imaging of both Al and Ti X-ray emission. Specifically, the Al Heβ (or Lyβ) and the Ti Heα lines are imaged with a resolution of (2.7±0.3) µm and (5.5±0.3) µm, respectively. It features two transmission Fresnel phase zone plates fabricated on the same substrate, each associated with a multilayer mirror for spectral selection. Their spatial resolution has been measured on the PTB synchrotron radiation facility laboratory at BESSY II and on the EQUINOX laser facility. Results obtained on an UHI experiment highlight the difference of emission zone sizes between Al and Ti lines and the versatility of this instrument.

I. Introduction

Ultra-short laser pulses offer plasma conditions relevant to stellar opacities 1 , atomic physics in general 2,3 and topics associated to inertial confinement fusion 4 . Experiments have been conducted on the LULI2000 laser facility (LULI, CNRS, Ecole Polytechnique) [START_REF] Luli | LULI[END_REF] with an Ultra-High-Intensity (UHI) laser delivering 10 17 to 10 19 W.cm -2 on target to create plasmas with temperatures of several 100 eV and densities higher than 1 g.cm -3 , although only within a limited volume. These campaigns thus required a high-resolution monochromatic X-ray imager to characterize the plasma emission. FUHRI (Fresnel Ultra High-Resolution Imager) is an x-ray imager with a demonstrated resolution lower than 5 µm for an energy bandwidth limited to 100 eV centered around the Al Heβ line at 1860 eV. It consists in a transmission Fresnel phase zone plate [START_REF] Kirz | [END_REF] (FZP) associated to a multilayer (ML) mirror for spectral selection. The present article describes an advanced version, FUHRIx2, featuring two channels each based on the FUHRI principle. The first one, "Al", identical to the previous configuration, images the Al Heβ line while the second, "Ti", images the Heα line of titanium at 4750 eV with 100 eV bandwidth. The FUHRIx2 design is detailed in section II, followed by measurements of its FZP resolutions and ML mirror reflectance on a synchrotron facility (section III) and on a laser facility (section IV). Section V presents first results obtained on an UHI experiment.

II. FUHRIx2 presentation

The FUHRIx2 diagnostic is based on the high-resolution quasi-monochromatic x-ray imager FUHRI design presented in a previous paper 7 . The FUHRIx2 chip has two FZP engraved on the same silicon substrate for simultaneous imaging at two photon energies (Figure 1). Adding another channel to the diagnostic was a necessity for the UHI experiment as the experimental target were composed of a mixed material of Al and Ti. These two line self-emission image are characteristics of two different plasma temperatures and can be analyzed along with spectroscopic data.

FZPs are focusing optics that can operate in the X-ray domain [START_REF] Jenkins | Fundamentals of Optics[END_REF] . They consist of a circular transmission grating: alternating N concentric transparent and opaque zones [START_REF] Kamiya | [END_REF]10 . The FZPs for FUHRIx2 are manufactured using an electroplating method by Applied Nanotools INC 11 . Their central area features a beam block (see Figure 1) to prevent direct emission 7 (i.e. the 0 th order) to reach the detector.

One FZP was designed for the Al Heβ line (1860 eV) or Al Lyβ line (2050 eV, Table 1) and the other for the Heα line emission of Ti (~4700 eV, Table 1). Both the central beam block surface and the surface outside the FZPs are covered by a 3 µm electroplated gold layer to stop most of the hard x-rays. Both images are detected by a Princeton PI-MTE CCD camera.

The focal distance (fλ) of a FZP is directly linked to the wavelength of the X-rays: (1) where 𝑟 ( is the first zone radius and 𝜆 the considered wavelength. Metrology studies were already performed to measure the Al FZP efficiency on a synchrotron radiation facility. To complete those data, additional resolution measurements have been conducted for the FZP both on a synchrotron radiation facility and on a laser facility (EQUINOX).

𝑓 " = " $ % &

FZP type

III. Metrology on a synchrotron radiation facility

The metrology of the components was performed at the Four-Crystal Monochromator (FCM) beamline in the Physikalisch-Technische Bundesanstalt (PTB) laboratory at the synchrotron radiation facility Bessy II 1 . The accessible photon energy range of this beamline extends from 1.75 keV to 11 keV using either Si or InSb crystals as monochromator. The zone plates were placed in an ultra-high-vacuum reflectometer 2 , which provides 0.001° angular resolution for the sample and the detector.

FZP spatial resolution

Resolution measurement method

The resolution of the FZP was measured with the knife edge method (cf. Figure 2). The monochromatic beam was collimated to have a size of 0.5 mm x 0.5 mm. First, the piezo-blade covers all the synchrotron beam signal. Then it is removed gradually (Figure 2), with a step size of 1 µm or 0.2 µm, to reveal the focal spot until it totally gets out of the beam. The signal was recorded using an 8 mm diameter photodiode. The resolution is obtained as the full width at half maximum (FWHM) of the derivative of the measured signal as a function of blade position. An example of such a result for the Al FZP of FUHRIx2 is presented on Figure 3. As both FZP were placed on the same support the resolution of the Ti FZP was also measured at 253 mm distance from the blade. Resolution of 5 µm was obtained for the Ti FZP and remain stable within at least ±25 eV energy range around the energy of 4700 eV.

Resolution measurement of FUHRIx2 FZPs

ML Mirror reflectivity measurements

The components of the multilayer mirrors were deposited by Axo-Dresden 12 . The thickness of each layer can be calculated from the X-ray reflective peaks according the Bragg formula.

The "Al" ML mirror consists of 50 pairs of Pd/B4C layers (2.6 nm/2.5 nm). The measured and calculated reflectivity curves are compared on Figure 5-a using a Debye Waller roughness of 0.65 nm.

The "Ti" ML mirror is made of 60 stacks of Mo/Si periodical layers. The measured reflectance versus photon energy at 1.975° grazing angle is shown on Figure 5-b. The theoretical reflectance (dashed curve) was calculated by the XRV software [START_REF] Moreno | Software X-ray Vision applied by Caminotec[END_REF][START_REF] Parratt | [END_REF] , considering a 0.6 nm mean interface substrate layer. From the fitting of the experimental results, we obtain the interface roughness of 0.4 nm.

(a) (b)

IV. Metrology on a laser facility

The EQUINOX laser facility 13,14 features a 800 nm-wavelength Ti:sapphire laser producing 0.3 J pulses, focused by an off-axis parabola. The laser pulse duration can be adjusted from 80 fs to 30 ps. As the focal spot diameter is about 50 µm, the irradiance on a flat solid target varies between 10 14 and 10 17 W/cm 2 , and mostly produces X-rays in the 100 eV -2 keV range. The laser can operate at 10 Hz for signal accumulation.

The experimental set-up used was the same as that described in ref. [7] and is shown on Figure 6. Our X-ray source is created by irradiating a large Ti or Al foil target with the laser beam. We placed a gold grid (10 µm/40 µm periods) at a distance d from the source, the FZP at a distance p from the grid and a CCD camera at a distance q from the FZP. Using a ML mirror in combination with the FZP selects a specific photon energy band and the corresponding focal distance (Eq. 1). However, in this experiment the geometrical constraints did not allow us to use a ML mirror, this absence caused the whole spectrum to be integrated, thus degrading the resolution significantly. The following resolution measurements must thus be considered as an upper limit. For an optimal result in this situation, the best focusing distance is that of the average energy across the whole spectrum. In the Al case, this average is at 1630 eV, leading to a 222 mm focal distance. For the Ti spectrum, with our experimental conditions, the average is close to the Heα lines (4726.7 eV and 4749.2 eV) thus the focal distance is 250 mm (see Table 1).

For each configuration we calculated the p distance with the thin lens formula: The signal is collected with a Princeton CCD camera, with a pixel size of 13.5 µm X 13.5 µm by accumulating 100 shots (Figure 7). 2.

( * = ( + + ( - (2)

Resolution measurements

Note that, for the Al configuration (Figure 7-a), the area inside the white circle is strongly affected by the FZP beam stop. The grid is located relatively far from the plasma to allow a large field of view, but this reduce the photon flux and add diffractions effect on the beam stop, creating the "converging" effect in the white circle of Figure 7-a. The average value of the resolution measured using the sharp edge method is (5.4±1) µm with a mean 24% contrast defined as: (highest signal -lowest signal)/(highest signal + lowest signal). To estimate the effect of an error made on the FZP position, to reproduce this situation we did two resolutions measurement with the grid out of focus (-4 mm and +3 mm). In both case, we measured an average of (7.3±1) µm resolution with a 26% contrast.

For the Ti configuration, the resolution measured here is (7.5±1) µm with a 20% contrast. This suggests that the quality of the Ti FZP is slightly lower than the Al, which confirms the measurement made in Part. III.1. However, the value differences Part III. and Part IV. are likely influenced by the non-monochromaticity of the x-ray source. As in the latter measurement there are no spectral selection, all the energies are being detected on the CCD camera adding more background and blurring to our signal.

V. FUHRIx2 results on a UHI experiment

We implemented FUHRIx2 during a laser-plasma interaction experiment on the LULI2000 facility. When an UHI laser pulse is focused on a thin target (<10 µm), the suprathermal electrons generate a return current which heats a small volume isochorically 15 . One objective of the experiment was to measure the spatial distribution of x-ray emission caused by this hot plasma. These emission lines are temperature dependent, thus providing some insight on the spatial temperature distribution.

The laser delivered pulses from 10 to 15 J in 1.3 ps at a wavelength of 526 nm focused on CH/Ti-Al/CH layered targets (Al and Ti are mixed) with an intensity of 1.6 10 19 W.cm -2 . FUHRIx2 was used to investigate the spatial temperature distribution of the aluminum and titanium layer's heated region. The thermal X-ray emission was focused by the FZPs, and the ML mirror, placed in front of the CCD camera, selected the He-like emission from Al and Ti. FUHRIx2 was placed at 22.5° from the target rear normal. The target-FZPs distance was 𝑝 = 266.2 mm and the detector-FZPs distance 𝑞 = 4100 mm corresponding to a magnification around 15.2 and a collection solid angle of 9 10 -6 sr. The "Al" ML mirror angle was 𝜃 = 4° and the "Ti" ML mirror angle was 𝜃 = 2°. Two configurations were used for FUHRIx2. The first was optimized for 2 channels at 1850 eV (Al Heβ) and 4700 eV (Ti Heα), while the second was optimized for 2050 eV only (Al Hβ). The parameters are summarized in Table 3, and the transmission of FUHRIx2 for two Al configurations are presented on Figure 8 An example of images obtained with FUHRIx2 is presented on Figure 9-a. We can see the "Ti" image on the left side and the "Al" image on the right side as well as a small part of the direct beam. To compare different lines, we analyzed shots on CH/Al-Ti/CH with the same laser energy (~16 J) and pulse duration. Sample images for each energy are presented on Figure 9-abc. Note that the two images on figure 9bc are from the same shot. The FWHM was estimated using a 2D-Gaussian fit of the images and the results are sown on Table 4. Here we are looking at the same plasma at different energy. For a set of plasma conditions, each emission lines of Al and Ti will have a specific emission zone size and brightness. Thus, we will be able to compare the size and the absolute emission measured with FUHRIx2 with theoretical calculations to verify our emission models. Detailed analysis of all the experimental data will be conducted in another paper. What we can observe here is that the emission area for each X-ray line is smaller for higher X-ray energies. This match corresponds to the heat propagation inside the target: lower temperatures (i.e. lower X-ray energies) are found far from the heat source.

Lines Al Heβ Al Lyβ Ti Heα

VI. Conclusions

Resolution element

Resolution element

X position (microns)

Resolution element

We improved our previous high-resolution X-ray imager by adding an additional measurement channel allowing us to capture simultaneously the size of the emission zone of two different x-ray emission lines with resolutions of (3.4 ±0.3) µm for the Al lines and of (5.5 ±0.3) µm for the Ti line. We proved the versatility of our system by changing the observed energy of one of our channels.

The investigation of chromatism effect on diagnostic performances will be pursued in a future article using x-ray tracing and wave propagation methods.

Figure 1 :

 1 Figure 1 : FUHRIx2 setup. The two channels correspond to the Al and Ti emission focused on the CCD camera after spectral selection by ML mirrors. The top panel show a photograph of the substrate with the two FZPs.

Figure 2 :Figure 3 :

 23 Figure 2: Knife edge method

Figure 4 -Figure 4 :

 44 Figure 4-a shows the results for the Al FZP at a photon energy of 1852 eV. At a FZP-blade distance of 253 mm, a minimum FWHM of (3.4 ±0.3) µm was achieved. In Figure-b the resolution measurement at different energies for the Al FZP are shown for a FZP-blade distance of 253 mm. A ±5 eV variation of the energy induces a spatial resolution variation of a factor x2.

Figure 5 :

 5 Figure 5: (a) "Al" FZP ML mirror reflectance. (b) "Ti" FZP ML mirror reflectance.

Figure 6 :

 6 Figure 6: Generic experimental set-up

Figure 7 :

 7 Figure 7: FUHRIx2 grid radiography image realized at the EQUINOX facility for the (a) Al and (b) Ti given in Table2.

Figure 8 :

 8 Figure 8: FUHRIx2 Al total transmission for two different mirror angles, superimposed with a typical X-ray spectrum from an UHI experiment. Unfortunately, we were not able to measure the resolution of each FUHRIx2 channel, we can only give an estimation of the resolution based on our previous paper 7 using FZPs resolution measurement on synchrotron (Part. III) and on a laser facility (Part. IV). Based on the measured resolution of the CCD camera (𝜎 ::; = [2.3 ± 0.15] µm) and the resolution of the FZP (𝜎 BCDCEFG = [1.5 ± 0.3] µ𝑚 and 𝜎 BCDCEIJ = [5.0 ± 0.3] µ𝑚), we obtain a total resolution for each channel of: 𝜎 IKLEFG = [2.7 ± 0.3] µ𝑚 and 𝜎 IKLEIJ = [5.5 ± 0.3] µ𝑚.

Figure 9 :

 9 Figure 9: Sample image of FUHRIx2 for a 1.6 10 19 W.cm -2 intensity shot on a mixed target of CH/Al-Ti/CH for (a) the Al FZP at 1850 eV (Heβ), (b) the Ti FZP at 4700 eV (Heα), and (c) the Al FZP at 2050 eV (Lyβ).

Table 1 :

 1 FZPs characteristics

	Al (1860 eV)	Ti (4700 eV)

Table 2 :

 2 Resolution measurements experiment parameters for the Al and Ti configurations.

	Target	Number	Filters	d (mm) p (mm) q (mm) q/p	q/(d+p)	Resolution	Contrast
		of shots
	Al (a)	100	MylarAl (2 µm/0.15 µm) Be (10 µm)	185	243	2782	11.4	6.5	(5.4±1) µm	24%
	Ti (b)	100	Ti (8 µm) Be (10 µm)	153.3	274.7	2782	10.1	6.5	(7.5±1) µm	20%

  with integrated spectra.

	Target	Filters	p (mm)	q (mm)	q/p	ϴ
	CH/Ti-Al/CH	PP (10 µm)	266.2	4100	15.2	4°(Al) / 2°(Ti)
		Be (10 µm)				Or 3.65°(Al) / 2°(Ti)

Table 3 :

 3 Experiment setup parameters.

Table 4 :

 4 Emission zone size for each energy.

VII. ACKNOWLEDGMENTS

The authors acknowledge the LULI2000 team, the LULI target lab, as well as Levent Cibik and Swenja Schreiber (both PTB) for their help during the experiments as well as Laboratoire de Photonique et de Nanostructures (LPN-CNRS, Marcoussis, France) lab team for their help in imaging FZP with a scanning electron microscope.

VIII. References