Invasion of alien macroalgae in the Venice Lagoon, a pest or a resource?

Alien macroalgae, mostly invasive species, are a constant concern for coastal areas, especially in the northern Adriatic Sea where several taxa have colonized the main transitional environments. A revision of the alien macroalgae in the Venice Lagoon shows that, currently, the number of valid non-indigenous species (NIS) is 29, and this number is growing steadily. On the basis of numerous surveys carried out in the last decade the total alien standing crop (SC) was estimated to be ca. 146,534 tonnes fresh weight (fw), i.e. 32% of the total species SC (ca. 456,000 tonnes fw) measured in May–June 2014 in the whole lagoon. The most abundant species were the invasive Agarophyton vermiculophyllum (approx. 66,383 tonnes fw), Agardhiella subulata and Hypnea cervicornis (approx. 36,714 and 28,305 tonnes fw, respectively). These species grow mainly free-floating and usually colonize the soft substrata of the lagoon. Two other invasive species, Sargassum muticum and Undaria pinnatifida, grow mainly on the docks of Venice historical centre, therefore their distribution is significantly more limited. Recent studies have shown that A. vermiculophyllum in protected, eutrophic areas produced a biomass of approx. 24 kg fw m y. This species can replace Ulvaceae with positive effects on the environment and the presence of macrofaunal taxa. Indeed, historical data analysis shows that, despite the high presence of NIS, biodiversity is increasing. In addition, A. vermiculophyllum, S. muticum, U. pinnatifida, A. subulata and Solieria filiformis may be promising species for phycocolloid and antioxidant or cosmetic productions.


Introduction
Over the last 2-3 decades, the Mediterranean Sea has experienced an increasing introduction of non-indigenous species (NIS) (Zenetos et al. 2010(Zenetos et al. , 2012Petrocelli et al. 2013;Verlaque et al. 2015). In the presence of suitable ecological conditions some NIS can colonise a new environment and often naturalize (Richardson et al. 2000). Heenan et al. (1998) define "naturalized species" those that "form a wild population self-maintained by seed or vegetative reproduction or occur repeatedly in the wild". Clement and Foster (1994) consider naturalized "the species able to be established extensively amongst native vegetation so as to appear native". However, their presence can be limited to a few individuals or extended populations that colonize only one area or most areas. In many cases, when they spread in more than 10 areas in the world they are considered "invasive" (Galil et al. 2014).
Depending on their abundance or trophic level, the NIS affect the autochthonous populations both directly and indirectly. Frequently, they occupy the ecological niche of native species that regress or are completely replaced. In some cases, they prey or feed on eggs and larval stages of other species as in the case of the bluefish Pomatomus saltatrix Linnaeus and the sea nut Mnemiopsis leidyi A. Agassiz. Sometimes their impact on other species or the environment is indirect as when are harvested for economic reasons. Manila clam Ruditapes philippinarum (Adams and Reeve) is a perfect example of indirect impact since it is an edible species of high economical value. The harvest of this species destroys the seabed, resuspends high amounts of sediments, nutrients and pollutants Dalla Valle et al. 2003) and affects the macrofauna (Pranovi and Giovanardi 1994). As a consequence the fishing economy of the lagoons of the northern Adriatic Sea has taken advantage of this important resource, but the environmental costs are incalculable (Orel et al. 2000). However, the effects of the introduction of alien species must not be generalized. Often new introductions remain localized in small areas or integrate with native species without significant effects on biodiversity or the environment. Some species may even be of commercial interest, but it is important to know how to exploit them without damaging the environment.
Environments particularly affected by the invasion of macroalgal NIS include the transitional water systems (TWS) in the well-studied areas of the lagoon of Thau in France (Verlaque 2001;Boudouresque et al. 2011), the lagoon of Venice, the lagoons and ponds of the Po Delta and the Mar Piccolo of Taranto in Italy (Sfriso and Marchini 2014;Marchini et al. 2015;Petrocelli et al. 2013). These basins are affected by extensive anthropogenic impacts associated with the most important pathways of introduction: shellfish farming, shipping, live seafood trade and recreational boating (Occhipinti-Ambrogi et al. 2011;Petrocelli et al. 2013). The number of NIS is continuously changing, because of the constant report of new species. The update and validation of NIS records is therefore crucial to provide reliable check-lists of these species, as required by the European Community (2008).
Among NIS, macrophytes are one of the most relevant taxa, especially in commercial or tourist ports and in TWS characterized by intense aquaculture and the presence of large fish markets.
Thirty-two non-indigenous macroalgae were reported from the Italian coasts until 2009 (Occhipinti-Ambrogi et al. 2011). In 2012 the "Allochthonous Species Group" (GSA) of the Italian Society of Marine Biology (SIBM) produced a new list, containing 35 taxa: 26 Rhodophyta, 5 Ochrophyta, 3 Chlorophyta and 1 seagrass (GSA-SIBM 2012). That list included 7 species considered invasive: Caulerpa taxifolia (Vahl) C. Agardh, Caulerpa cylindracea Sonder, Asparagopsis armata Harvey J.N. Norris et Fredericq, Codium fragile subsp. fragile (Suringar) Hariot, Agarophyton vermiculophyllum (Ohmi) Gurgel, Sargassum muticum (Yendo) Fensholt and Undaria pinnatifida (Harvey) Suringar (Zenetos et al. 2010). Caulerpa taxifolia and C. cylindracea are known to impact the environment severely. The first one is a toxic alga that significantly reduces the biodiversity of the environments it colonizes. The second one creates a dense network of stolons that hinders the presence of other species. Codium fragile subsp. fragile is scarce in the Italian coasts and does not affect the other species. Finally, A. armata (Kraan and Barrington 2005), A. vermiculophyllum (Sousa et al. 2010), S. muticum (Milledge and Nielsen 2016) and U. pinnatifida (Yamanaka and Akiyama 1993) are species that are farmed and can have interesting cosmetic, pharmaceutical, human and animal food uses and different environmental impacts.
Among the invasive species, the last 4 taxa (C. fragile subsp. fragile, A. vermiculophyllum, S. muticum and U. pinnatifida ) are present in the Venice Lagoon, the Italian hotspot of allochthonous species introduction (Sfriso and Marchini 2014;Marchini et al. 2015). The first NIS recorded for Venice was Codium fragile subsp. fragile in 1978 (Sfriso 1987). This macroalga was first recorded in the Etang de Thau, France, by Mars (1966). In Venice it colonizes the artificial breakwaters of the lagoon inlets, the areas near the lagoon mouths and all sites characterized by high water renewal whereas inside the lagoon it is uncommon. Sargassum muticum and Undaria pinnatifida, which were recorded, for the first time, in the Etang de Thau in 1971 (Anonymous 1980) and 1980(Pérez et al. 1984, respectively, were found in Venice in 1992 by Gargiulo et al. (1992) and by Rismondo et al. (1993). Both species grow attached to hard substrata during the cold season and can reach a biomass up to 10-15 kg fw m -2 . Their presence is restricted to a few areas, particularly the historic centers of Venice and Chioggia, and are of local concern because of the size they reach (on average 3-5 m). Agarophyton vermiculophyllum was first recorded in the Mediterranean Sea in the lagoons of the Po Delta and in the Venice Lagoon in 2008 as Gracilaria vermiculophylla . It was found growing attached to stones or mollusc shells, especially oyster shells, in the soft substrata of the lagoons and, also, free-floating in the water column. After few years it had become the most abundant NIS although it had colonized only protected and eutrophic areas . Other NIS which were recorded in the Venice Lagoon and have spread throughout the basin are Agardhiella subulata (C. Agardh) Kraft et Wynne (Curiel et al. 2005) and Hypnea cervicornis J. Agardh, reported as H. flexicalulis Y. Yamagishi and M. Masuda by Wolf et al. (2011). Both species were observed growing prevalently free-floating in the water column such as A. vermiculophyllum but their spread was limited and often hindered by the fast growth of Ulvaceae or Gracilariaceae. Finally, many other species have a limited spread or biomass depending on their presence and size.
This paper aims to report the distribution and total standing crop (SC) of the NIS that have colonized the Venice Lagoon. The main causes of success of the most abundant species and their impact on the environment have been analysed. Particular attention has been given to the invasive species A. vermiculophyllum, which showed the largest expansion and the most interesting results both for the environment, preventing anoxic crises, and its abundance and commercial interest.

Materials and methods
Macrophytes were collected in the framework of some programmes carried out to assess the ecological status of the lagoon by applying the Macrophytes Quality Index (MaQI, , according to the requirements of the European Water Directive 2000/60/EC. The distribution and SC of the most common taxa in the Venice Lagoon ( Figure 1) were determined by analyzing the NIS records during some surveys carried out in the soft substrata of the whole lagoon in summer and autumn 2011 (118 sites) and 2014 (88 sites). Additional data refer to the sampling of 35 sites in the North lagoon between 2015 and 2017 during the implementation of the project Life12 NAT/IT/000331 (2012)  At each site, macrophyte samples were collected by scraping the bottom in a ray of 15-20 m for approx. 1m with a rake of 30 cm. The number of sub-samples depended on the variability of the macroalgae recorded in order to obtain the maximum number of species but it was never less than 6. In soft bottoms, this sampling method allowed to collect almost all the species present in the site. A comparison with the number of species collected by SCUBA divers in the same sites did not show significant differences (Sample number = 30, p = 0.68, F = 0.170, Stastistica 10, StatSoft, Tulsa, USA). The dominant taxa were weighed with an electronic balance (precision ± 1 g) after a short centrifugation with a kitchen centrifuge to remove excess water. All visibly different species were fixed with a 4% formaldehyde solution and identified in laboratory with a stereoscope and a light microscope.
Specimens of dubious identification were silica gel-dried and analysed through the DNA barcoding method using different molecular markers. The plastid large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcL) was chosen for Rhodophyta since it is one of the most commonly used markers in the analysis of these algae  whereas for Chlorophyta, the selected gene was the plastid elongation factor tufA according to Saunders and Kucera (2010). DNA extraction, amplification and sequencing of the chosen marker were accomplished as described in Cecere et al. (2011) and in Wolf et al. (2011Wolf et al. ( , 2012Wolf et al. ( , 2014Wolf et al. ( , 2018. In detail, genomic DNA was extracted by using the Genomic DNA purification kit (Thermo Scientific™) for Rhodophyta samples or following the procedures outlined in Saunders and Kucera (2010) for Chlorophyta. The rbcL gene was amplified using the primer combination F57-R753, F577-R1381 and F577-RrbcSstart, as described by Freshwater and Rueness (1994) and the primers tufGF4 (Saunders and Kucera 2010) and tufAR (Fama` et al. 2002) were employed to amplify the tufA gene. The PCR amplification profile followed Cecere et al. (2011) for the rbcL gene (initial denaturation of 2 min at 94 °C followed by 30 cycles of 40 s at 94 °C, 40 s at 50 °C and 40 s at 72 °C, with a final 5 min extension at 72 °C) and Saunders and Kucera (2010) for the tufA marker (initial denaturation of 4 min at 94 °C followed by 38 cycles of 1 min at 94 °C, 30 s at 45 °C and 1 min at 72 °C with a final 7 min extension at 72 °C). The amplification products were cleaned by enzymatic reaction. Sequencings of PCR products were performed on automated ABI DNA sequencers, with the same primer pairs used in the amplification reactions. The SeqMan II program from the Lasergene software package (DNAStar©, Madison, Wisconsin, USA) was used to assemble the final consensus sequences, which were then compared with other publicly available sequences using BLAST v.2.0 software (Altschul et al. 1990). The barcode sequences obtained during these studies were deposited in the International Nucleotide Sequence Database (INSD) through the European Nucleotide Archive (ENA) platform. GenBank accession numbers are reported in Supplementary material (Table S1).
The mean biomass of each taxon was determined as the mean value of the biomass recorded at the different stations in 2011 and 2014. The estimation of the total SC was obtained by dividing the total lagoon water surface (432 km 2 ) by the number of sites (i.e. 432 km 2 /118 sites and 432/88 sites).
The linear extension of hard substrata (island docks, lagoon inlets, breakwaters and rocky panels) was also sampled. On average their extension is approx. 100 km 2 and the mean width is ca. 3 m. The final estimation of the SC of each species is the sum of its distribution in soft and hard substrata.
On hard substrata sampling was carried out by SCUBA divers who collected all visibly different species or macroalgal assemblages. The use of a frame was in fact not adequate  because macroalgae are distributed differently on the bottom depending on the exposure (waves, air at low tide, depth, etc.), light availability and interferences of local disturbances such as currents and anthropogenic structures.

Results
The revision of the number of all the macroalgal taxa up to 2018 showed that the number of valid alien macroalgae, i.e. only species with unequivocal identity and non-indigenous status, is 29 (Sfriso et al. 2019a). Table S1 reports the morphological descriptions of the analyzed species whereas the biomass estimation is shown in Table 1.
The first record was Codium fragile subsp. fragile in 1978 (Sfriso 1987), but the highest number of introductions occurred in the decades 1988-1998 and 1998-2008 (Figure 2) with 0.9 and 1.0 new introductions per year, respectively. In the decades 1988-1998 and 1998-2008 the recorded NIS were identified mainly on morphological features whereas in the decade 2008-2018 by applying the genetic-molecular analyses, a higher number of new species was discovered.
A confirmation of the progressive NIS spread in the soft bottoms of the whole lagoon can be obtained by the analysis of the species found by our research team during the campaigns carried out in 2011, 2014 (Regione Veneto et al. 2012Veneto et al. , 2015 and 2015,2016,2017 in the North lagoon during the implementation of the project LIFE12/NAT/IT/000331 (2012) SeResto (www.lifeseresto.eu) ( Figure 3). During the surveys in the whole lagoon, 13 NIS were recorded. On average, in 2011 they had colonized 16.5% of the 118 sampling sites and in 2014 that percentage had increased to 18% (Figure 3a).
In 2011 and 2014 the taxa recorded more frequently were Agardhiella subulata, which had colonized 49.7% of the sites, and the microscopic Uronema marinum and Hypnea cervicornis with 47.1% and 45.5% of the total sampling sites, respectively. The colonization of U. marinum in 2014 extended up to ca. 60.2% of the sampling sites, but its biomass was negligible.
Other widespread taxa were Solieria filiformis (32.3%) and Polysiphonia schneideri Stuercke and Freshwater (17.3%) previously misidentified as Polysiphonia denudata (Dillwyn) Greville ex Harvey. The presence of other NIS was < 10% of the total sites ( Figure 3b). Additional information on the colonization ability of NIS in the lagoon was provided by the monitoring of choked areas such as the northern region of the North basin (Ca' Zane and Palude Maggiore, Figure 1). There, 9 NIS were recorded with a spread ranging from 24.4 to 29.9% of the 35 sampling sites (Figure 3c), although the area is clearly separated from the rest of the lagoon by islands and tidal lands. In those areas the most widespread species were P. schneideri (on average 73.3%), A. subulata (65.7%) and H. cervicornis (61.0%). Uronema marinum (23.8%) and S. filiformis (15.2%) were also common, whereas the other species were rare ( Figure 3d).
The list of the 29 NIS that colonize the lagoon is ranked according to their SC. Three taxa: P. schneideri, Melanothamnus japonicus (Bailey) Díaz-Tapia and Maggs and Aglaothamnion halliae (Collins) Aponte, D.L. Ballantine and J.N. Norris are new records for the Mediterranean Sea (Wolf et al. 2018). The first two species: P schneideri and M. japonicus, formerly misidentified as P. denudata and M. harvey (Bailoey) Díaz-Tapia and Maggs, have spread almost over the whole lagoon. Aglaothamnion halliae is rare and was recorded only occasionally in a limited area of the northern historical centre of Venice.
In the whole lagoon (water surface approx. 432 km 2 ), the total alien SC was estimated to be ca. 146,521 tonnes fw (Table 1). Agarophyton vermiculophyllum accounted for 45.3% of the total alien SC (Figure 4). Five other taxa with a SC > 1% (i.e. 1465 tonnes) were: A. subulata (25.1%), H. cervicornis (19.3%), S. muticum (3.29%), S. dotyi (3.26%) and S. filiformis (2.57%). These 6 taxa accounted for approx. 98.8% of the total alien SC (Figure 4). Another 9 taxa displayed a remarkable SC, ranging from 1.25 to 517 tonnes fw, although  However, despite the fact that the NIS standing crop was approx. 32% of that of the macroalgae of the entire lagoon (Table 1), macroalgal biodiversity is increasing (Table 2). Data on the number of macroalgal taxa have been available since the end of the 1930s. Schiffner and Vatova (1938) reported a macroalgal check-list that, after the nomenclature revision, accounted for 141 taxa. In subsequent years the number of taxa decreased due to eutrophication and nuisance macroalgal proliferation (Pignatti 1962;Sfriso 1987;Solazzi et al. 1991). It increased again since the end of the 1990s when the abnormal macroalgal biomass of nuisance macroalgae collapsed (Sfriso and Marcomini 1996;Sfriso and Facca 2007). Currently the number of macroalgal taxa is 323 (Sfriso et al. 2019a) (Table 2). The 29 macrolagal taxa and their corresponding SC are listed below. Images of all species are shown in Figures 5, 6, 7. (Figure 5.1) was one of the last introductions (Sfriso et al. 2010, as Gracilaria vermiculophylla) but it is also the NIS which had the highest SC (ca. 66,383 tonnes fw, Table 1), although it had colonized only the choked areas of the lagoon where water was turbid and eutrophic. The occupied lagoon area is ca. 84.7 km 2 (19.6% of the total lagoon water surface). In those areas it was the dominant species and in some cases it reached 8-10 kg fw m -2 but its mean SC could be estimated in ca. 784 g fw m -2 .

2.
Agardhiella subulata (C. Agardh) Kraft andWynne ( Figure 5.2) was first found by Curiel et al. (2003). This species had spread over a wide lagoon area showing a total SC of 36,714 tonnes fw (Table 1). Differently from A. vermiculophyllum, this macroalga usually grows attached to hard substrata, nevertheless it had colonized almost half of the lagoon surface (i.e. ca. 215 km 2 ). The taxon was found mainly in turbid areas where it was often associated to A. vermiculophyllum. Its estimated SC was ca. 171 g fw m -2 .
3. Hypnea cervicornis J. Agardh (Figure 5.3) was the 3 rd most abundant species. It was recorded for the first time by Wolf et al. (2011) as Hypnea flexicaulis Y.Yamagishi and M.Masuda. It grew prevalently free-floating with an estimated SC of ca. 28,305 tonnes fw. Its populations covered a surface of ca. 197 km 2 with a mean SC of ca. 144 g fw m -2 .
4. Sargassum muticum (Yendo) Fensholt ( Figure 5.4) the 4 th species in terms of SC, was found in the lagoon by Gargiulo et al. (1992). It grew over ca. 70 km of linear hard substrata around the docks of Venice, Chioggia, Pellestrina, Lido, other small islands and the breakwaters that protect the lagoon inlets. In addition, scattered thalli had colonised a surface of ca. 9.5 km 2 of oyster banks at west and south-west of the historical centre of Venice. Overall, the estimated SC was ca. 4,825 tonnes fw, with a mean biomass of ca. 10 kg fw m -2 in hard substrata. It was the largest species present in the lagoon. On average, the thalli were 3-5 m long, but they could reach 8 m in deeper waters .
5. Scytosiphon dotyi M.J. Wynne ( Figure 5.5) was first recorded by Curiel et al. (1996). It is another very abundant Phaeophycea with a total SC of ca. 4,775 tonnes fw. It colonized the hard substrata of islands, breakwaters and docks (ca. 100 km) with a mean SC of ca. 75 g fw m -2 , but it also colonized ca. 216 km 2 , about half of the lagoon water surface. In this case, the biomass was much lower, ca. 21.5 g fw m -2 . This species was present only in winter and early spring.
6. Solieria filiformis (Kützing) P.W. Gabrielson ( Figure 5.6) found by Curiel et al. (2005), covered ca. 140 km 2 with a mean SC of ca. 27 g fw m -2 accounting for ca. 3,768 tonnes fw. It grew free-floating in the soft bottoms and was frequently associated with A. vermiculophyllum and A. subulata. 7. Polysiphonia morrowii Harvey (Figure 6.7) is a very abundant species particularly in late winter and spring. It was recorded for the first time by Curiel et al. (2001). It colonized the hard substrata of islands, breakwaters and the city docks along a linear surface of ca. 100 km 2 with a mean SC of 590 g fw m -2 but it was present in other 7.3 km 2 of the lagoon surface where it had a mean SC of ca. 46.5 g fw m -2 . Overall, we estimated a total SC of ca. 517 tonnes fw.
8. Polysiphonia schneideri Stuercke and Freshwater (Figure 6.8) was recently identified through molecular analyses (Wolf et al. 2018). This species had been in the lagoon for many years but misidentified as Polysiphonia Figure 6. Images of 9 NIS with a biomass ranging from > 1 to hundreds of tonnes: 7) Polysiphonia morrowii and some long periaxial cells; 8) Polysiphonia schneideri and a cross-section highlighting the number of periaxial cells; 9) Ulvaria obscura with a cross-section of the blade; 10) Melanothamnus japonicus and a spermatangial branch formed at the first dichotomy of a modified trichoblast with 1-2 sterile apical cells; 11) Undaria pinnatifida; 12) Grateloupia turuturu with a cystocarp sunken in the medullar part of the blade; 13) Uronema marinum and apical cell with several zooids; 14) Antithamnion nipponicum highlighting globularelliptic gland cells; 15) Codium fragile subsp. fragile with utricles characterized by conical tips. All photos were provided courtesy of Adriano Sfriso. denudata (Dillwyn) Greville ex Harvey in Hooker. It was very common in the lagoon and covered ca. 77.7 km 2 of soft substrata with ca. 4.24 g m -2 and 100 linear km of artificial rock substrata with ca. 230 g m -2 , for a total SC of ca. 398 tonnes fw. 9. Ulvaria obscura (Kützing) Gayral (Figure 6.9) was first recorded by Sfriso et al. (2002) as Monostroma obscurum (Kützing) J. Agardh. That species was very common and showed a highly variable SC in different years. It usually grows attached to mollusc shells, but under favourable conditions this macroalga can form free-floating biomasses, especially between the shoots of the seagrasses Zostera marina Linnaeus and Cymodocea nodosa (Ucria) Ascherson. During the samplings of 2011 and 2014, we observed that it had covered ca. 29.3 km 2 with a mean SC of ca. 11.0 g fw m -2 . Overall, the total estimated SC was ca. 323 tonnes fw. (Figure 6.10) is also a new record for the Mediterranean Sea (Wolf et al. 2018). It had been misidentified as Melanothamnus harveyi (Bailey) Díaz-Tapia and Maggs in Díaz-Tapia et al., a species reported for the Venice Lagoon since 1998 as Polysiphonia harvey Bailey (Bellemo et al. 1999). On the whole, M. japonicus covered 39.7 km 2 of soft substrata with ca. 5.25 g m -2 fw and 100 linear km of artificial rock substrata with ca. 210 g m -2 , for a total SC of ca. 272 tonnes fw. The presence of the cryptic species M. harvey was not confirmed for the Venice Lagoon, as reported in Wolf et al. (2018). Additional surveys are necessary to understand if both species are present before excluding M. harveyi definitively.

Melanothamnus japonicus (Bailey) Díaz-Tapia and Maggs
11. Undaria pinnatifida (Harvey) Suringar (Figure 6.11) that dates back to 1992 (Rismondo et al. 1993) displayed a SC of ca. 143 tonnes fw. It grew strongly attached by thick rhizoids (haptera) to docks and dams along the canals of the historical centre of Venice, Chioggia and Lido where hydrodynamic and nutrient availability were high. Small patches were also present on oyster banks for a total surface of ca. 1 ha around the National Electricity Authority (ENEL) pylons, in front of the Industrial areas of Porto-Marghera. This species was not present in the breakwaters of the lagoon inlets or in areas with low concentrations of nutrients.
12. Grateloupia turuturu Yamada et al. (Figure 6.12) was first recorded in 1989 as Grateloupia doryphora (Tolomio 1993). The surveys carried out in 2011 and 2014 reported that it had colonised ca. 2.6 km 2 of soft bottom with a mean SC of 46.5 g m -2 fw, accounting for ca. 87 tonnes. This species formed abundant populations on the docks of the historical centre of Venice and other islands but its SC was hardly higher than 60 g fw m -2 . Overall, the total estimated SC was ca. 87 tonnes fw.
13. Uronema marinum Womersley (Figure 6.13) is the smallest NIS species. The first record was in 2008 in the Venice Lagoon, the lagoons of the Po Delta and in Pialassa della Baiona in the Emilia-Romagna Region . It was never recorded in other lagoons or marine coasts of the Mediterranean Sea. Uronema marinum was found in ca. 203 km 2 (47.1% of the examined stations), especially in stagnant and eutrophic areas rich in orthophosphates. In those areas it formed dense populations, that covered larger macroalgae almost completely. Scattered filaments were also present near the lagoon inlets, but they were mostly attached to floating thalli of Gracilariaceae and Solieriaceae coming from the inner areas. Due to the microscopic size of the species, an estimation of its SC was very difficult. However, we estimated a mean SC of ca. 0.04 g fw m -2 , accounting for ca. 8.1 tonnes fw.
14. Codium fragile subsp. fragile (Suringar) Hariot (Figure 6.14) was first recorded in 1978 by Sfriso (1987). It is a species considered invasive or ''widespread NIS'' because it had been recorded in the European Seas of 10 or more countries (Galil et al. 2014). However, the spread of this species was not successful in the lagoon of Venice, where it colonized only the breakwaters of some islands for a total extension of ca. 4.0 km and a mean SC of ca. 62.5 g fw m -2 , accounting for ca. 1,250 kg fw.
15. Antithamnion nipponicum Yamada and Inagaki (Figure 6.15) recorded in 1994 by Curiel et al. (1996) was very common along the sea coastline, but it was rare in the lagoon. In fact, it was found only in ca. 6.13 km 2 , ca. 1.42% of the entire lagoon sampled in 2011 and 2014. The species is a small epiphyte that presented a mean SC of ca. 3.07 tonnes fw.
16. Grateloupia yinggehaiensis H.W. Wang and R.X. Luan in D. Zhao et al. (Figure 7.16) is a thin ribbon-like macroalga that was first recorded in 2008 in the Brentella canal, which is affected by the warm water discharges from the cooling system of Fusina hydroelectric power plant (Wolf et al. 2014). This species, which had never been recorded in other Mediterranean lagoons, had colonized only ca. 200 m of the canal banks with a mean SC of. ca. 33 g fw m -2 , accounting for ca. 6.6 kg fw overall.
17. Lomentaria hakodatensis Yendo (Figure 7.17) similarly to G. yinggehaiensisis, is a species that was found only in a very restricted lagoon area, ca. 120 m of linear docks, at Chioggia near the wholesale fish market. Its SC was ca. 47 g fw m -2 and accounted for a total SC of 5.6 kg fw. It was first recorded in 2000 by Curiel et al. (2006).
18. Colaconema codicola (Børgesen) H. Stegenga, J.J. Bolton, and R.J. Anderson (Figure 7.18) is a small species that grew as epiphyte on C. fragile subsp. fragile and therefore was found in the same lagoon areas. Its first record dates back to 1978 (Sfriso and Marchini 2014). It is a common species but its estimated SC was ca. 0.5 kg fw.
In addition to those common species, 9 rare macroalgae were recorded during our surveys: (Figure 7.19) was recorded in 1996 as Sorocarpus sp. by Curiel et al. (1999). Later it was found many times in the North-East area of the historical centre of Venice. The gross morphology of the thalli is very similar to that of many other Ectocarpales and its identification is very difficult.  Photos 16,17,18,19,20,22,23,24,26,27,28, were provided courtesy of Adriano Sfriso. Photo 21, courtesy of Armeli-Minicante (2013a); photo 25, courtesy of Fletcher (1987); photo 29 courtesy of Wolf et al. 2018).

Botrytella parva (Takamatsu) Kim
H.G.Choi Y.S.OH and I.K, Lee, which are very common and abundant in the lagoon. (Figure 7.22) was recorded for the first time in 2003 by Curiel el al. (2003). That species was reported to grow in the benthos around the city of Chioggia, but it was not present anywhere else in the lagoon. Therefore, it must be considered very rare and with a negligible SC.

Aglaothamnion feldmanniae Halos
23. Leathesia marina (Lyngbye) Decaisne (Figure 7.23) was recorded in 1996 by . This species was found rarely and its SC is not quantifiable.
24. Bonnemaisonia hamifera Hariot (Figure 7.24) was first recorded in 1995 by Curiel el al. (1996). Later, it was occasionally reported by the same authors, but in 40 years we have never recorded this species again during our surveys in the lagoon.
25. Halothrix lumbricalis (Kützing) Reinke (Figure 7.25) was reported only in 1992 by Gargiulo et al. (2000). It is a very small species with a negligible SC and after 1992 was never found again.
26. Spermothamnion cymosum (Harvey) De Toni (Figure 7.26) was first recorded in the Mediterranean Sea in 2010 at the Certosa island in the historical centre of Venice (Armeli-Minicante 2013b). In that place it was common, but its SC was negligible (< 0.5 kg fw).
27. Ulva australis Areschoug (Figure 7.27) was first recorded in 2011 by Manghisi et al. (2011) and identified through molecular analyses as Ulva pertusa Kjellman by Wolf et al. (2012). However, its morphology is very similar to other Ulvaceae such as U. rigida C. Agardh and U. laetevirens Areschoug, for this reason its identification is difficult and its distribution in the lagoon is unknown.
28. Ulva californica Wille in F.S. Collins, Holden and Setchell (Figure 7.28) is distinguishable from the widespread U. linza Linnaeus. Ulva californica was first reported for the Mediterranean Sea by Wolf et al. (2012) based on the molecular analyses of some specimens collected in the lagoon of Venice. Later, there were no other reports of this species in the lagoon and its distribution is unknown.
29. Aglaothamnion halliae (Collins) Aponte, D.L. Ballantine and J.N. Norris (Figure 7.29) was a small species found, for the first time in the Mediterranean Sea in 2017 near the trans-lagoon bridge that connects Venice to the mainland (Wolf et al. 2018). Its distribution seemed limited to the area of the first record and its SC was negligible.

Discussion
The Venice Lagoon hosts a higher number of NIS compared to other Italian transitional water systems. That number is very similar to the records found in the Thau Lagoon (France) (Verlaque 2001), and many new species are continuously being recorded (Marchini et al. 2015;Sfriso et al. 2019a). Across the entire lagoon, A. vermiculophyllum is the most abundant species, accounting for ca. 45.3% (i.e. 66,383 tonnes fw) of the total NIS SC. This percentage rises to 89.7% (i.e. 131,402 tonnes fw) when A. subulata and H. cervicornis SC are added (Table 1). All three species grow attached to the hard substrata and detach from the bottom to colonize large areas of the lagoon by free-floating. Another three NIS, two of which grow attached to hard substrata (Sargassum muticum and Scytosiphon dotyi) and one prevalently free-floating (Solieria filiformis), increase the NIS SC to 98.8% of the total. All the other species are negligible even if some of them show a biomass on the order of hundreds of tonnes. Among them Undaria pinnatifida is particularly interesting because it is in association with Sargassum muticum. In late winter and spring, both species colonize the hard substrata of the historical centre of Venice and cause significant problems to navigation. However, their SC is insignificant (< 1%) when it is compared with other algae such as Ulva rigida and Ulva laetevirens that used to colonize the lagoon (Sfriso and Facca 2007). At the end of their life cycle, these macroalgae detach from the hard substrata but they are not a real inconvenience for people because their decomposition is slow. In contrast, Ulvaceae decompose very quickly and trigger extensive anoxic crises leading to the death of the benthic and fish fauna and the emission of stinking or toxic substances such as hydrogen sulphide (Sfriso 1987).
Two more new entries for the Mediterranean Sea, Polysiphonia schneideri and Melanothamnus japonicus (Wolf et al. 2018), formerly misidentified as P. denudata and M. harveyi, respectively, have a very similar morphology and showed a significant SC (398 and 272 tonnes, respectively). The presence of M. harvey and P. denudata has not yet been confirmed, although several samples have been analyzed (Wolf et al. 2018).
The quick spread of NIS was confirmed during the surveys in the soft substrata of the whole lagoon in 2011 and 2014 and in 35 choked lagoon sites of the northern lagoon in the framework of the project Life12 NAT/IT/000331 (2012) SeResto during 2015-2017 (Bonometto et al. 2017;www.lifeseresto.eu). The highest percentage of colonization was in protected areas, between 24.4 and 29.8 % of the sampling sites, but it decreased to only 16.5-18.0% of the sites of the whole lagoon (Figure 3), even if the number of species was higher. It is interesting to note that in the protected areas of the northern lagoon the presence of A. vermiculophyllum was sporadic because waters were clear and oligotrophic (www.lifeseresto.eu).
Agarophyton vermiculophyllum, Sargassum muticum, Undaria pinnatifida and Codium fragile subsp. fragile, are 4 widespread NIS which Galil et al. (2014) defined "invasive" because they had been recorded in 10 or more countries. Among them, only the first three species are widespread in the lagoon and A. vermiculophyllum is confirmed to be the most invasive one (Figure 8). This species which was first recorded in the western part of the industrial area of Porto Marghera in May 2008, in few years had colonized a great part of the choked areas with biomasses up to 8-10 kg fw m -2 . The most probable introduction vector was the importation of oysters and clam seeds from other European countries (Marchini et al. 2015). The Venice Lagoon is in fact characterized by extensive shellfish farming activities largely based on the aquaculture of Ruditapes philippinarum Adams and Reeve and Cassostrea gigas Thunberg. In 2011, A. vermiculophyllum spread also in Valle Millecampi and Valle di Brenta in the southern lagoon, and some samples were also recorded near Venice airport in the northern basin. In 2014 the species established stable populations in almost all the choked areas of the lagoon, except the northernmost sites where water is clear and oligotrophic as recorded in 2011 (Facca et al. 2014) and 2014 (Sfriso et al. 2019a). These results confirm the fast-spreading trend of A. vermiculophyllum, that was reported for other countries in Northern Europe where, in the early 2000s it had been introduced with the importation of oysters from Virginia (USA) (Rueness 2005;Thomsen et al. 2005Thomsen et al. , 2007. In 2014, the spread of A. vermiculophyllum was particularly massive in the northern side of the bridge that connects Venice to the mainland ( Figure 8). There, this species almost completely replaced U. rigida, due to the high concentration of phycocyanin which gives it a black color (black Gracilaria) and allows its growth in turbid environments that are prohibitive for other species. Its rapid spread in 2014 and the following years and the replacing of U. rigida prevented the anoxic crisis that in 2013 affected the area by causing the decomposition of approx. 10,000 tonnes fw of biomass in a few days. The anoxic crisis causing the death of the fish and benthic macrofauna and the release of nauseating smell of sulphides occurred on the night of the Redeemer Feast that Venetians celebrate on boats, compromising the festivities (Bastianini et al. 2013). Contrary to the Ulvaceae, that rapidly collapse when water temperature exceeds 25-26 °C, A. vermiculophyllum survives at temperatures higher than 30 °C. Under extreme temperatures its decomposition it very slow and doesn't trigger anoxic events. Because of its resistance to high temperatures and no danger of anoxic crises in the areas it populates, A. vermiculophyllum is probably the first record of a NIS whose introduction showed a positive effect on the environment. Moreover, the measurements of relative growth rates (RGRs), biomass production, agar, protein and antioxidant yields of A. vermiculophyllum carried out in the choked areas of the lagoon showed that this species could be a very interesting commercial resource for industrial applications. The annual biomass yield ranged from 12.8 kg fw m -2 in turbid waters to 22.0 kg fw m -2 in clear waters Sfriso et al. 2017a). In moderately turbid waters, the production of A. vermiculophyllum was similar to that of Ulva rigida, but in very turbid areas it was the only macroalga able to grow. The potential agar yield was estimated in 0.66-1.14 kg m -2 (Sfriso et al. 2017a) whereas the mean production of proteins was ca. 0.4 kg m -2 (Sfriso et al. 2017b). This abundant species showed also a high concentration of antioxidants such as phycoerythrin, phycocyanin and allophycocyanin. In particular, the concentration of phycoerythrin, one of the most fluorescent compounds used to highlight small cancer nodes at the beginning of their formation is up to > 2 mg g -1 fw with a production of 17-27 g m -2 y -1 ). In addition, all these compounds have effective anti-aging properties and antioxidant effects approx. 2-10 times higher than vitamin C (Sonani et al. 2014).
On the contrary, Sargassum muticum and Undaria pinnatifida have a negative impact on the environment because of the floating biomasses deplaced by currents (S. muticum) and the long thalli (S. muticum and U. pinnatifida) attached to the docks of the historical centre of Venice and islands hinder the navigation of small boats . However, from an economical point of view these two taxa could be a relevant resource for their alginate content. These phycocolloids, have gelling properties similar to agar-agar and are used as thickeners and stabilizers in the food, pharmaceutical and cosmetic industries. In addition, S. muticum is rich in phenolic compounds that have shown excellent qualities as antioxidants when extracted at temperatures below 40 °C (Le Lann et al. 2008;Sabeena Farvin and Jacobsen 2013). Phlorotannins are an important fraction of these polyphenols. Their half-life in the human body reaches 12 hrs while terrestrial polyphenols' 30-180 minutes (Namvar et al. 2013). The antioxidant effects of these compounds act in synergy with α-γ trocopherols and carotenoids (Sabeena Farvin and Jacobsen 2013). Undaria pinnatifida is also widely used in the eastern countries for soup preparations and contains fucosan, a pigment used in cosmetics whose action has shown effects on the suppression of weight gain and the improvement of lipid metabolism (Jeon et al. 2010). These species have also anti-inflammatory, anti-obesity, anti-cancer and UV-preventive properties (Milledge and Nielsen 2016).
High concentrations of phycocolloids are present also in the NIS Agardhiella subulata and Solieria filiformis which could be a precious source of carrageenans (Murano et al. 1997;Toffanin et al. 1997). Indeed, carrageenans are widely used as thickening, gelling and stabilizing agents in pharmaceutical, cosmetic, printing and textile compounds (Campo et al. 2009;Robledo and Freile-Pellegrin 2010).
In China, South Korea, Japan, the Philippines and Malaysia currently, ca. 1 M tonnes of algae are collected each year for the extraction of polysaccharides, with a phycocolloid production reaching 55,000 tonnes y -1 , which means a market value of 585 Mln US$ (213 M US $ from the production of alginates, 132 Mln US$ from agar and 240 Ml US$ from carrageenan (Hugh 2003)) and a mean income of 10,600 US$ tonne -1 y -1 . However, Europe has a significant cultural deficit in the area of algae (with the exception of Ireland and Iceland, which traditionally consume certain types of algae) and only lately Governments have begun to favour the development of this market. The Regulation EU 2015/2283 on "novel foods" and the Legislative Decree 152/2006, which integrates and updates the previous environmental standards, have led to the transformation of the algal biomass from special waste into resource, encouraging its collection, sale and marketing. Therefore, the presence of species of high economical value, notwithstanding their status of alien species, should be reconsidered also because the species that are really dangerous for the Venice Lagoon are Ulvaceae and Cladophoraceae for the serious environmental impacts they can trigger (Sfriso 1987;Ménesguen 2018). The other NIS (Table 1) instead, due to their ecological behaviour or low SC, have a negligible impact on the ecological status of the lagoon and, in general, their presence has enriched the biodiversity of the environment. In fact, the biomass of nuisance macroalgae in 2014 was approx. 10% of that recorded in 1980 before the introduction of the first NIS (Sfriso and Facca 2007;Sfriso et al. 2019b) and the decline of Ulvaceae (Sfriso and Marcomini 1996). Moreover, the number of total macroalgal taxa in the lagoon had shown a decreasing trend in the period from 1938, when the first complete check-list was produced, until 1991 (97 taxa) when Ulvaceae proliferated (Table 2). In the following years, when the biomass decreased, the number of taxa increased remarkably (up to 323 taxa in 2018), although the NIS introduction (Sfriso et al. 2019a). In fact, many dominant NIS are seasonal or naturalized species growing along with native species. The only NIS that showed a strong spatial competition with native species was A. vermiculophyllum for its ability to adapt in choked, turbid and nutrient-enriched environments and replace Ulvaceae. The presence of this species doesn't trigger anoxic crises and is associated to a high benthic biodiversity: small fish such as Aphanius fasciatus Valenciennes a species present in the IUCN Red List of Threatened Species, Version 2019.2, many other commercial fish such as Zosterisessor ophiocephalus Pallas and Atherina boyeri Risso tipycal of TWS of the Adriatic Sea and various species of shrimps. The same high biodiversity, that in the presence of Ulvaceae was hampered by anoxic crises, was also recorded by Nyberg et al. (2009) and Ramus et al. (2017) along the coasts of northern Europe and Virginia. Nyberg and coauthors found 92 taxa associated with A. vermiculophyllum, especially Malacostraca, Gastropoda and Florideophyceae. The diversity of the associated taxa was not affected by A. vermiculophyllum biomass. In Virginia and Sweden animal abundances were positively correlated with the biomass of algae and plants associated with A. vermiculophyllum. Moreover, Ramus et al. (2017) found that in the absence of native habitat formers, such as naked sediment or the areas colonised by Ulvaceae in the Italian TWS, those NIS had an overall positive, density-dependent impact across a diverse set of ecosystem processes (e.g., abundance and richness of nursery areas). Their work supports the idea that where native foundation species are lost, invasive habitat formers may be considered a source of valuable ecosystem functions.

Conclusions
This paper gives an overview of alien species that have invaded the Venice Lagoon since the 1978. The list has been updated with the most recent nomenclature (Guiry and Guiry 2019) and the record of three new species: Polysiphonia schneideri, Malanothamnus japonicus and Aglaothamnion halliae which had never been reported before in the Mediterranean Sea. P. schneideri and M. japonicus were present in the lagoon, but they had been misidentified as other morphologically similar species (P. denudata and M. harvey, respectively). The presence of these cryptic species is now confirmed, not only in Venice Lagoon but also in the Mediterranean Sea. Moreover, an estimate of the SC of the NIS which colonise the lagoon has been carried out for the first time after extensive monitoring surveys in the soft and hard bottoms of this basin. Among the 29 NIS present in the lagoon three: A. vermiculophyllum, A. subulata and H. cervicornis, are the most widespread species, they account for 89.7% of the total NIS SC of ca. 131,402 tonnes fw. Three additional species, S. muticum, S. dotyi and S. filiformis, increases the NIS SC to 98.8% of the total alien species which corresponds to ca. 1/3 of the total macroalgal SC of the lagoon.
Contrary to other NIS that often threaten biodiversity and human wellbeing, such as the recent introduction of the ctenophore Mnemiopsis leidyi A. Agassiz, 1865, which is a pest for eggs and larvae and the marine biodiversity, the introduction of these macroalgae had positive effects and biodiversity is steadily increasing (Sfriso et al. 2019a). Moreover, many species are rich in precious substances useful for the food, cosmetic and pharmaceutical sectors. In the future, those important commercial compounds could be extracted from those NIS instead of being imported from Asian countries. Finally, the colonization and naturalization of A. vermiculophyllum, a species that grows in eutrophic and turbid waters, has confirmed the positive impact on the other taxa and the whole environment as reported by other researchers (Nyberg et al. 2009;Ramus et al. 2017) for Northern Europe. Indeed, A. vermiculophyllum is able to replace Ulvaceae and does not trigger anoxic events and the death of fish and macrofauna.