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Monitoring non-indigenous macroalgae in a Mediterranean MPA: Lessons from a short-temporal variability of pristine habitats invasion
Abstract
Since there is no local management for Marine Protected Areas (MPAs) to prevent the establishment of macroalgae NIS, successful efforts to contrast their spread probably should concentrate in the conservation of resistant habitats. The study aimed to evaluate the role of depth and habitat in the spread of macroalgal non-indigenous species (NIS). A multifactorial sampling design was employed to compare the abundance of NIS macroalgae over a one-year period across different substrates and depths within a Mediterranean MPArea. Moreover, the ecological quality of macroalgal assemblages in relation to invasion was assessed through the ALien Biotic IndEX (ALEX). The NIS macroalgae found in the MPA, Caulerpa cylindracea, Acrothamnion preissii, Womersleyella setacea and Falkenbergia sp., were overall quite low in abundance and, accordingly, the ALEX index had generally high values. The substrate, depth and time significantly affected NIS abundance; however, the lack of significant interannual variability (December 2018–December 2019), both in pooled and separated NIS abundance, suggested that the occurrence of these four NISs will no further expand in the MPA at the scale of habitat. The highlighted patterns in NIS abundance suggest the importance of detailed sampling designs of monitoring taking into consideration different habitats and depths and using suitable sampling methods to assess NIS establishment and spread, such as those required by international directives.
... On the contrary, the cavities created by the accretion of encrusting Rhodophyta increase the complexity of coralligenous concretions, favoring the spreading and anchoring of the Caulerpa stolons [36]. Our results contrast with recent observations carried out within a Marine Protected Area (Capo Carbonara MPA) in the southern Tyrrhenian Sea, where coralligenous reefs were among the habitats most exposed to C. cylindracea and further non-indigenous species settlement [37]. ...
... A strong seasonal variation, with a maximum development in summer and regression in winter, was highlighted in all the investigated habitats: this is consistent with what has been previously described for shallow habitats [8,38]. Conversely, our findings did not confirm the maintenance of biomass during winter in deeper habitats [31]: seasonal variation in C. cylindracea percentage cover differed with results provided by Piazzi et al. as well [37]. Both photoperiod and temperature are the main factors responsible for the observed pattern, considering the southern origin of the IAS. ...
The role of habitat degradation on the spread of the alien green alga Caulerpa cylindracea
is reported here by comparing observations achieved through a multi-year assessment on three Mediterraneans habitats, namely Posidonia oceanica meadows, Phyllophora crispa turf, and coralligenous reefs. Due to the peculiarity of the study site, both natural-reference and impacted conditions were investigated. C. cylindracea occurred in all the studied habitats under impacted conditions. High susceptibility to the invasion characterized impacted P. oceanica, where Caulerpa cover reached 70.0% in summer months. C. cylindracea cover did not differ significantly among conditions in P. crispa
turf, where values never exceeded 5.0%. Conversely, the invasive green algae was low in abundance and patchily distributed in coralligenous reefs. Our results confirmed that habitat loss enhances the spread of C. cylindracea, although with different magnitudes among habitats. Dead matte areas of P. oceanica represented the most vulnerable habitat among those analyzed, whereas coralligenous reefs were less susceptible to the invasion under both the studied conditions.
... In fact, the taxa identified in the two assemblages were nearly the same, although the recruited assemblages were dominated by quick-growing species such as Sphacelaria spp., while the mature assemblages on untouched thalli were mostly characterised by slow-growing forms such as Peyssonnelia spp., Halimeda tuna and Flabellia petiolata, supporting the theory that algal succession occurs CCA epibiota (Piazzi et al., 2011). Regarding the presence of introduced species, the Rhodophyta Womersleyella setacea was more abundant on recruited than in mature assemblages, suggesting that mature assemblages may control the spread of this species probably through competitive interactions (Piazzi et al., 2021a) and, on the contrary, the lack of a complex assemblage may enhance the spread of opportunistic fast-growing introduced species. In addition, invasive species such as W. setacea were found more abundant in the warmer site (i.e. ...
Biogenic structures edified by crustose coralline algae (CCA) are known to enhance biodiversity, but the mechanisms involved are only partially defined. This study aims at contributing to the knowledge of the interaction between habitat-builder CCA and epibiont assemblages through a manipulative approach. To achieve this objective, transplantations of the CCA Lithophyllum stictiforme were performed to test the following hypotheses: 1) the structure of L. stictiforme epibiont assemblage changes between depth and biogeographic area; 2) L. stictiforme epibiont assemblage is different and more diversified than the assemblage recruited on bare rock; 3) the early L. stictiforme epibiont assemblage differ from the mature L. stictiforme epibiont assemblage. A total of 63 epibiontic taxa were found on the transplanted L. stictiforme thalli, such as 7 Chlorophyta, 10 Ochrophyta, 40 Rhodophyta, 1 Sarcomastigophora, 1 Porifera, 4 Bryozoa. The sessile assemblage recruited on transplanted L. stictiforme thalli depended on sites and depths, and it had a higher taxa richness compared to that colonizing the bare rock. The early recruited assemblage on L. stictiforme transplanted thalli compared to the mature assemblage on untouched thalli was different mostly in the relative abundance of taxa, rather than in species composition. These manipulative experiments highlighted for the first time the role of a common CCA on the development and biodiversity of coralligenous assemblages, contributing to emphasize the importance of CCA conservation efforts.
... In general, healthy and diverse communities, as for example in MPAs, should hypothetically minimize the presence of NIS, but recent studies have also highlighted a positive effect on NIS settlement. It seems indeed that MPAs can both protect against NIS and enhance invasion success, but this aspect is still poorly investigated and depend to the target species considered (Burfeind et al., 2013;Gallardo et al., 2017;Giakoumi and Pey, 2017;Piazzi et al., 2021). The protection from NIS could occur due to biodiversity and biomass growth of native species that leave no niche available for other colonizers (Giakoumi and Pey, 2017;Blanco et al., 2020); while the facilitation could take place through other mechanisms, like the fishing restrictions inside the MPAs, which also include the unintended protection of NIS, and the higher diversity of predators and parasites of native species in MPAs (Burfeind et al., 2013). ...
Biotic resistance is considered an important driver in the establishment of non-indigenous species (NIS), but experiments in the marine environment have led to contradictory results. In this context, a transplant experiment of fouling communities was carried out over five months. Settlement panels were moved from low impact (species-rich native communities) to high impact sites by NIS in two Italian areas to test the biotic resistance hypothesis. Fouling communities displayed significant differences among treatments before and after the transplant, thus indicating the maintenance of a peculiar fouling community in transplanted panels. On the other hand, newly recruited species were similar between treatments and neither a facilitation nor a mitigation role from native fouling communities on NIS was observed. Our results highlight the importance to better investigate the factors affecting the high variability obtained in experiments testing this hypothesis, with the aim to identify potential solutions for NIS management in ports.
... A modified version of ALEX has been recently proposed to evaluate biological invasions in macroalgal assemblages (Piazzi et al., 2015a). ALEX was applied in a Marine Protected Area where a recreational-fishing port is present, supporting the suitability of the index to detect spatial and habitat differences within a MPA where some non-indigenous macroalgae are at early stages of spread 2021a). Moreover, ALEX was also employed in impact evaluation studies . ...
Macroalgae have been utilized as biological indicators of ecosystem health in many monitoring programs worldwide. These programs have utilized various methods to quantify macroalgal community structures. The aim of this study was to provide an overview of current progress by reviewing techniques and methods in both monitoring programs and impact evaluation studies that use macroalgal assemblage data. A total of 215 papers were selected and divided into four categories: macroalgal assemblage monitoring, macroalgal mapping, developing and employing ecological indices based on macroalgae, and developing and employing generic ecological indices including macroalgae. The number and goals of macroalgal monitoring programs are very different among geographical areas. In Europe, the recent European Union Directives led to the development of indices as tool to monitor the ecological quality of coastal systems. In other geographic regions, most studies focused on mapping the distribution of kelps or Fucales. This demonstrates the necessity to harmonize marine macroalgal monitoring, identifying common metrics and approaches in sampling design, field measurements, taxonomic resolution and data management, in order to develop standardized procedures which may allow data obtained to be compared.
... This metric discriminated stations according to different levels of degradation, therefore confirming variability across a gradient of human-mediated impacts. The ALEX index was also used in different situations [86][87][88][89]. ...
Biopollution by alien species is considered one of the main threats to environmental health. The marine environment, traditionally less studied than inland domains, has been the object of recent work that is reviewed here. Increasing scientific evidence has been accumulated worldwide on ecosystem deterioration induced by the development of massive non-indigenous population outbreaks in many coastal sites. Biopollution assessment procedures have been proposed, adopting criteria already used for xenochemical compounds, adjusting them to deal with alien species invasions. On the other hand, prevention and mitigation measures to reduce biopollution impact cannot always mimic the emission countermeasures that have been successfully applied for chemical pollutants. Nevertheless, in order to design comprehensive water-quality criteria, risk assessment and management strategies, based on scientific knowledge, have been developed in a similar way as for chemical pollution. The Mediterranean Sea is a well-known case of alien species invasion, mainly linked to the opening of the Suez Canal. Non-indigenous species have caused well-documented changes in many coastal ecosystems, favoured by concomitant changes induced by global warming and by the heavy load of nutrients and pollutants by various anthropogenic activities. Naval commercial traffic and leisure boats are among the most active vectors of spread for alien species inside the Mediterranean, and also towards other ocean regions. The scientific evidence gathered and summarized in this review suggests that effective management actions, under a precautionary approach, should be put in place in order to control introductions of species in new areas. These management measures are already established in international treaties and national legislations, but should be enforced to prevent the disruption of the dynamic ecological equilibria in the receiving environment and to control the direct adverse effects of alien species.
... This effect may be more severe if invasive turf species are involved [40], such as the introduced Rhodophyta Womersleyella setacea (Hollenberg) R.E. Norris which has been described as a relevant component of coralligenous algal turfs in the Capo Carbonara MPA [76]. ...
In this work, the consequences of a local gorgonian coral mortality on the whole coralligenous assemblage were studied. A Before/After-Control/Impact sampling design was used: the structure of the coralligenous assemblage was compared before and after the gorgonian mortality event at the mortality site and two control sites. At the mortality site, a relevant decrease in alpha and beta diversity occurred, with a shift from a stratified assemblage characterized by gorgonians and other invertebrates to an assemblage dominated by algal turfs; conversely, neither significant variations of the structure nor decrease in biodiversity were observed at the control sites. The assemblage shift involved the main taxa in different times: in autumn 2018, a large proportion of the plexaurid coral Paramuricea clavata died, but no significant changes were observed in the structure of the remaining assemblage. Then, in autumn 2019, algal turfs increased significantly and, one year later, the abundance of the gorgonian Eunicella cavolini and bryozoans collapsed. Although the mechanisms of the assemblage shift following gorgonian loss will remain uncertain and a cause-effect relationship cannot be derived, results suggest the need for detecting signs of gorgonian forests stress in monitoring programs, which should be considered early indicators of their condition. in the coralligenous monitoring programs for detecting any sign of gorgonian forests stress which should be considered an early indicator of the assemblage condition.
Coralligenous reefs are calcareous structures edified mostly by coralline algae that characterize the circalittoral zone of the Mediterranean Sea. However, in some cases coralline algae can constitute peculiar infralittoral biogenic reefs which have been studied less than the circalittoral ones. This study aims to contribute to the knowledge of infralittoral coralligenous reefs by describing their macroalgal assemblages on a large rocky platform off the Tuscany coasts, northwestern Mediterranean Sea. To this end, a multifactorial sampling design was used to describe the structure of the assemblages and to evaluate the variability of the system at multiple spatial scales. A total of 71 macroalgal taxa were found on the coralline algae. Macroalgal assemblages were well structured, with high biodiversity values. The dominant taxa included both photophilous and sciaphilous species, guaranteeing peculiar characteristics in these assemblages, above all if compared with the typical infralittoral and circalittoral macroalgal communities of the same geographic area. The assemblages showed greater variability at a small and intermediate spatial scale than at a large scale. Although infralittoral coralligenous outcrops constitute a peculiar system, they are still poorly understood and should not only be the object of specific studies but also be included in monitoring programmes.
In the last decades, climate change and human pressures have increasingly and dramatically impacted the ocean worldwide, calling for urgent actions to safeguard coastal marine ecosystems. The European Commission, in particular, has set ambitious targets for member states with two major directives, the Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD), both designed to protect the marine environment in EU waters. Diverse biotic indices have accordingly been developed to assess water and habitat quality. The WFD adopts four Biological Quality Elements (BQEs), whereas the MSFD recommends a set of eleven qualitative descriptors. The borderline between water quality and habitat quality is hard to trace and so far most assessments have involved the use of a few indices and were mainly related to a single BQE or qualitative descriptor. In this study, thanks to the availability of a large dataset encompassing a wide array of descriptors, we compared the performance of 11 biotic indices relative to three habitats/biotic components (reefs, seagrass, and fish) of the Marine Protected Area (MPA) of Capo Carbonara (SE Sardinia, Italy). The aim was to assess whether the indices were consistent in defining the environmental status in the MPA investigated. We used the graphical approach RESQUE (REsilience and QUality of Ecosystem), which enabled us to obtain a single and comprehensive measure of the status of the environment by integrating several metrics. This approach was applied here to different habitats for the first time. All indices were consistent with each other in confirming the good status of Capo Carbonara MPA. The use of RESQUE provided insights to interpret the differences between water quality, defined according to the WFD, and habitat quality, defined according to the MSFD. Differences between the two EU directives, in terms of either requirements or goals, have long been discussed but the present study highlights for the first time that they are congruent in their assessment of the environmental status of marine ecosystems.
The green alga Caulerpa cylindracea Sonder is one of the most infamous and threatening invasive species in the Mediterranean Sea. Since 1985, it started rapidly spreading to all Mediterranean regions causing many ecological changes on natural communities. In the present study, we present an example of this proliferation with the first record in the Marine Protected Area of Tremiti Island (MPATI) in the South Adriatic Sea. Fifteen sites along the coast and 5 different depths have been investigated. Our results provide eveidence of a wide invasion of this pest in three islands, San Domino, San Nicola and Capraia. This study fills a particular data gap in the ongoing biomonitoring of invasive seaweeds in the Mediterranean Sea representing a base line of this invasive species for the MPATI. Keywords: Environmental science, Ecology, Aquatic ecology, Ecosystem change, Flora, Biological pollutants, Invasive macrophytes, Mediterranean sea, Caulerpa cylindracea, Bioinvasion, Tropicalisation
Stochastic perturbations can trigger major ecosystem shifts. Marine systems have been severely affected in recent years by mass mortality events related to positive thermal anomalies. Although the immediate effects in the species demography affected by mortality events are well known, information on the mid- to long-term effects at the community level is much less documented. Here, we show how an extreme warming event replaces a structurally complex habitat, dominated by long-lived species, by a simplified habitat (lower species diversity and richness) dominated by turf-forming species. On the basis of a study involving the experimental manipulation of the presence of the gorgonian Paramuricea clavata, we observed that its presence mitigated the effects of warming by maintaining the original assemblage dominated by macroinvertebrates and delaying the proliferation and spread of the invasive alga Caulerpa cylindracea. However, due to the increase of sediment and turf-forming species after the mortality event we hypothesize a further degradation of the whole assemblage as both factors decrease the recruitment of P.clavata, decrease the survival of encrusting coralligenous-dwelling macroinvertebrates and facilitate the spreading of C. cylindracea.
The biotic resistance hypothesis predicts that more diverse communities should have greater resistance to invasions than species-poor communities. However for facultative and obligate epiphytic invaders a high native species richness, abundance and community complexity might provide more resources for the invader to thrive to. We conducted surveys across space and time to test for the influence of native algal species abundance and richness on the abundance of the invasive facultative epiphytic filamentous alga Lophocladia lallemandii in a Mediterranean Cystoseira balearica seaweed forest. By removing different functional groups of algae, we also tested whether these relationships were dependent on the complexity and abundance of the native algal community. When invasion was first detected, Lophocladia abundance was positively related to species richness, but the correlation became negative after two years of invasion. Similarly, a negative relationship was also observed across sites. The removal experiment revealed that more complex native communities were more heavily invaded, where also a positive relationship was found between native algal richness and Lophocladia, independently of the native algal abundance. Our observational and experimental data show that, at early stages of invasion, species-rich seaweed forests are not more resistant to invasion than species-poor communities. Higher richness of native algal species may increase resource availability (i.e. substrate) for invader establishment, thus facilitating invasion. After the initial invasion stage, native species richness decreases with time since invasion, suggesting negative impacts of invasive species on native biodiversity. © 2018 Springer International Publishing AG, part of Springer Nature
The aim of the study was to evaluate the effectiveness of the application of multiple environmental descriptors through an asymmetrical sampling design to detect possible impacts related to the Costa Concordia event on the coastal marine environment. The Costa Concordia shipwreck occurred on a submerged rocky reef in the north-western Mediterranean Sea and the wreck was removed 2 years later. To achieve the proposed objective two main coastal ecosystems, the seagrass Posidonia oceanica and coralligenous assemblages were studied using two ecological indices, PREI and ESCA, respectively. Both indices show a lower ecological quality in the disturbed sites compared with the control ones. Differences between the disturbed and control sites observed in both studied ecosystems would seem to indicate an increase of turbidity around the shipwreck as the most plausible cause of impact. The concurrent use of different ecological indices and asymmetrical sampling designs allowed detection of differences in ecological quality of the disturbed sites compared with the controls. This approach may represent an interesting tool to be employed in impact evaluation studies.
The Asian kelp Undaria pinnatifida (‘Wakame’) is one of the most widespread invasive non-native species in coastal marine habitats and is fast approaching cosmopolitan status, yet its interactions with native species are poorly understood. Within the Plymouth Sound (UK) Special Area of Conservation (SAC), Undaria has become a conspicuous and important component of assemblages in shallow rocky reef habitats, where it co-exists with native canopy-forming brown macroalgae. We examined the hypothesis that rocky reef habitats supporting dense macroalgal canopies will have more biotic resistance to the invasion of Undaria compared with reef habitats supporting disturbed or sparse native canopies. Field surveys were completed at two spatial scales and sampling resolutions, and a short-term field-based canopy removal experiment was conducted to examine the influence of native macroalgal assemblages on the abundance, cover, biomass and morphology of Undaria. Field surveys indicated that Undaria was negatively related to the cover of macroalgal ‘competitors’, particularly Laminaria spp. However, multiple, large Undaria sporophytes were observed within dense native canopies, suggesting that disturbance to, or the absence of, canopies is not a prerequisite for Undaria colonisation. The short-term canopy removal experiment indicated that Undaria functions primarily as a pioneer species in this system. Where native canopies were left intact, Undaria sporophytes were far less abundant and were generally smaller with lower biomass compared with those in disturbed patches. The spread of Undaria into natural habitats is inhibited by the presence of native competitors, particularly large perennial species such as Laminaria spp., although the persistence of intact dense canopies does not completely prevent assimilation of Undaria into native assemblages.
There is increasing evidence that the severity of the ecological impact of non-native species does not necessarily scale linearly with their abundance in the introduced range. Nonetheless, the potential of low abundance invaders to alter the resilience of native communities to disturbance has been poorly explored. On Mediterranean rocky reefs, we tested the hypothesis that (1) a pulse disturbance opening gaps within canopy stands formed by the fucoid seaweed Cystoseira brachycarpa facilitates the establishment of the non-native seaweed Caulerpa cylindracea and that (2), once established, the seaweed can reduce the recovery of macroalgal canopies. In July 2011, C. cylindracea was experimentally transplanted in small and large plots that were either cleared of the canopy or left untouched. After 45 months, the cover of C. cylindracea was greatest in small canopy-removal plots, without, however, achieving values exceeding ~10%. Nonetheless, such a low abundance of C. cylindracea caused a threefold reduction in canopy recovery. The establishment of C. cylindracea in canopy-removal plots did not alter the structure of the understory assemblages or the cover of turf-forming, erect and encrusting algae and sessile invertebrates. Our results suggest that some non-native species may be stronger competitors than natives, despite their low abundance. This property has important implications for control programs since not achieving the total eradication of the targeted invader would make little progress towards the mitigation of its impacts. Finally, our results show that non-native species acting as passengers of change can ultimately promote the persistence of alternative degraded states.
The marine Site of Community Importance (SCI ITB040020) of the network Natura 2000 is included almost entirely within the perimeter of the Marine Protected Area Capo Carbonara (South East Sardinia, Italy) which is characterized by a high biodiversity, with 720 species and 30 marine habitats reported. Hence, it is recognised as a Specially Protected Area of Mediterranean Importance (SPAMI). During the summer the area is affected by anthropogenic impacts due to recreational activities (e.g. fishing, pleasure boating, swimming, diving) that represents potential threats to some species and protected habitats. This study is part of the project LIFE+ RES MARIS (LIFE13 NAT / IT / 000 433), addressed to the restoration and conservation of marine and terrestrial ecosystems included in the SCI under Habitats Directive 92/43/EEC. The aims is to evaluate the conservation status of the priority habitat 1120 “Posidonia beds” through mapping and characterization of the Posidonia oceanica meadow in areas that previous studies have identified as the most affected by anchoring, for a total surface of about 400 ha. Acoustic methods (Side Scan Sonar), direct visual surveys in SCUBA diving and underwater towed camera are applied. Assessments of coverage of P. oceanica, types of substrate, continuity of the meadow, presence of dead matte, Cymodocea nodosa and benthic alien species (e.g. Caulerpa cylindracea) are reported. Results are crucial for enhancing the knowledge of the current distribution and status of the priority habitat 1120 and is basic for planning further concrete conservation and monitoring measures.
The spread of the green macroalga Caulerpa cylindracea is one of the most threatening invasions in the
Mediterranean Sea. Many correlative and experimental studies have focused on different aspects of C. cylindracea invasion. This paper aims to evaluate the main factors influencing the spread of this alga through an overview of the results from 47 published papers on this topic; a conceptual model
synthesizing the main biotic and abiotic factors that influence C. cylindracea spread was proposed. Mechanical destruction of habitats, enhanced sedimentation rate and nutrients loading directly promote the spread of C. cylindracea. Indirect effects due to factors that foster the spread of turf-forming algae at the expense of canopy-forming species and decrease substrate complexity emerged as an important determinant
of the success of C. cylindracea. Ultimately, the spread of C. cylindracea appears to be regulated by a complex net of interactions between abiotic and biotic factors. Our conceptual model represents a general tool for the rapid assessment of the factor underpinning the spread of non-native species, a critical step for the control of non-native species that have successfully established viable populations.
Biological invasions threaten biodiversity worldwide. Nonetheless, a unified theory linking disturbance and resistance to invasion through a mechanistic understanding of the changes caused to biodiversity is elusive. Building on different forms of the disturbance-biodiversity relationship and on the Biotic ResistanceHypothesis (BRH), we constructed conceptual models showing that, according to the main biodiversity mechanism generating invasion resistance (complementary vs. identity effects), disturbance can either promote or hinder invasion. Following the Intermediate Disturbance Hypothesis (IDH), moderate levels of disturbance (either frequency or intensity) are expected to enhance species richness. This will promote invasion resistance when complementarity is more important than species identity. Negative effects of severe disturbance on invasion resistance, due to reductions in species richness, can be either overcompensated or exacerbated by species identity effects, depending on the life-traits becoming dominant within the native species pool. Different invasion resistance scenarios are generated when the diversity-disturbance relationship is negative or positive monotonic. Predictions from these models were experimentally tested on rocky reefs. Macroalgal canopies differing in species richness (1 vs. 2
vs. 3) and identity, were exposed to either a moderate or a severe pulse disturbance. The effects of different canopy-forming species on the seaweed, Caulerpa cylindracea, varied from positive (Cystoseira crinita) to neutral (Cystoseira barbata) to negative (Cystoseira compressa). After 2 years, severely disturbed plots were monopolized by C. compressa and supported less C. cylindracea. Our study shows that the effects of disturbance on invasion depend upon its intensity, the main mechanism through which biodiversity generates invasion resistance and the life-traits selected within the native species pool. Disturbance can sustain invasion resistance when promoting the dominance of competitively subordinate species possessing traits that allow outperforming invaders.
The study compared the structure of mobile epifaunal assemblages associated with Mediterranean Cystoseira beds between areas invaded and not invaded by Lophocladia lallemandii. A total of 150 taxa were identified: 42 Polychaeta, 78 Arthropoda, 26 Mollusca and 4 Echinodermata. Epifaunal assemblages differed between areas invaded and not invaded by Lophocladia lallemandii when the invasive species reached maximum values of cover and biomass, while differences between conditions were not significant in other periods of the year. The main differences were the greater abundance of amphipods, isopods and polychaetes in invaded areas and the greater abundance of molluscs and decapods in non-invaded areas, while richness and total abundance of organisms were not significantly different between conditions. The effects of Lophocladia lallemandii invasion on Cystoseira-associated assemblages seem to be limited to the period of vegetative growth of the alga and reversible during the period of its vegetative rest.
The species introduction process involves a donor region, a vector, a corridor, a recipient region, a candidate species and a variety of filters that hinder the process. All these parameters are of importance for understanding the success or the failure of a non-native species. Some characteristics of the recipient area, such as low species diversity, disturbances and climate warming, which are often said to facilitate the introduction process in the terrestrial realm, may be of far lesser importance in the marine environment. In contrast, the role of the vector and of the corridor might have been underestimated.
Kappaphycus alvarezii is a commercially important red alga being intentionally introduced in marine waters worldwide for the production of kappa carrageenan. Its introduction into the Gulf of Mannar Biosphere Reserve during the 1990s and its subsequent escape from cultivation sites have paved the way for its invasion into the coral reef ecosystem of Kurusadai Island. Since the report of its invasion in 2008, removal of K. alvarezii from the reefs has been started by means of manual removal (hand plucking). This article details the unsuccessful attempt and negative impact of the eradication programme. Regrowth of K. alvarezii from removal points and drifting broken fragments resulting during removal have led to further establishment in the reef environment. Variation in the morphology of K. alvarezii populations after their removal has been observed. A significant reduction in the cover of coral and native algae due to the increase in abundance of K. alvarezii was evident from the study. The need for immediate scientific control measures to eradicate the invasive alga is discussed.
The ESCA (Ecological Status of Coralligenous Assemblages) index was recently developed to evaluate the ecological quality of coralligenous habitat. The study aims to improve the first index proposal through testing response to different sources of anthropogenic pressures and optimizing the sampling effort. ESCA was calculated on 14 sites and tested against a gradient of human pressures. Moreover, the main scales of spatial variability of assemblages were evaluated and values of index obtained with different sampling designs were compared. Results showed that studied sites resulted in high, good or moderate ecological status, according to an increasing gradient of the anthropization level. Values of ESCA index obtained with different methods have been compared and photographic method provides EQR values lower than destructive method. Spatial variability of assemblages was higher at large and small scales than at intermediate ones. Two locations for each study site and 20 replicated samples for each location may be consider the best sampling combination providing reliable values of ESCA index for the evaluation of ecological quality of coralligenous assemblages.
Human disturbances, such as anchoring and dredging, can cause physical removal of seagrass rhizomes and shoots, leading to the fragmentation of meadows. The introduced green alga, Caulerpa race-mosa, is widely spread in the North-West Mediterra-nean and, although it can establish in both degraded and pristine environments, its ability to become a dominant component of macroalgal assemblages seems greater in the former. The aim of this study was to estimate whether the spread of C. racemosa depends on the intensity of disturbance to the canopy structure of Posidonia oceanica. A field experiment was started in July 2010 when habitat complexity of a P. oceanica meadow was manipulated to simulate mechanical disturbances of different intensity: rhi-zome damage (High disturbance intensity = H), leaf removal (Low disturbance intensity = L), and undis-turbed (Control = C). Disturbance was applied within plots of different size (40 9 40 cm and 80 9 80 cm), both inside and at the edge of the P. oceanica meadow, according to an orthogonal multifactorial design. In November 2011 (16 months after the start of the experiment), no C. racemosa was found inside the seagrass meadow, while, at the edge, the cover of the seaweed was dependent on disturbance intensity, being greater where the rhizomes had been damaged (H) than in leaf removal (L) or undisturbed (C) plots. The results of this study indicate that physical disturbance at the margin of seagrass meadows can promote the spread of C. racemosa.
Biotic indices are mainly aimed at assessing levels of deterioration caused by chemical or organic pollution. However, no biotic index to date has been developed to detect impacts of alien species on benthic communities. In this paper, a new biotic index, namely ALEX, is proposed to address the objectives of the Water Framework Directive and was tested in Mersin Bay (Levantine Sea, Turkey). Species were divided into four biogeographic groups, namely native species, transient species, established species and invasive species, and the metric considers the relative importance of these groups in samples. The index classified the ecological status of some stations which are shallow, and close to harbor and river mouths as bad or poor in February and October. The ALEX values were positively and significantly correlated with total nitrogen, silicate and silt percentage in sediment, and negatively correlated with depth and the distance from the harbor.
The effects of nutrients dynamics on
biological invasions in marine habitats have not been
fully investigated. The present study aimed to evaluate
the role of nutrient pollution in determining the
successful invasion of the introduced seaweed Caulerpa
racemosa var. cylindracea in a Mediterranean
subtidal rocky habitat. To this end, a manipulative
field experiment was carried out by supplying the
water column with nutrients for a one-year period.
Afterwards C. racemosa was transplanted in both
nutrient-enriched and control macroalgal assemblages.
Results showed that the penetration and
biomass of C. racemosa in the assemblages previously
treated with nutrients were higher than in control ones,
indicating that communities stressed by nutrient
pollution are more vulnerable to invasion. The nutrient
enrichment changed the structure of natural macroalgal
assemblages, mainly by encouraging the growth of
turf species. No significant effects were detected for
the erect and prostrate layers and for the number of
species, suggesting that the susceptibility of the
community to invasion may depend more on the
morpho-functional identity of macroalgae rather than
on the diversity of assemblages. Changes in the
availability of seawater nutrients may foster the spread
of C. racemosa, both by enhancing its growth and
eroding the natural resistance of macroalgal assemblages.
Thus, the maintenance of good water quality
may play an important role in containing both C.
racemosa and other introduced seaweeds.
Habitat‐forming invasive species have complex impacts on native communities. Positive above ground and negative below ground impacts are reported, suggesting that habitat‐forming invasive species may affect community components differently. Furthermore, such effects may vary depending on the density of the invader. We determined the responses of community components to different densities of the invasive green alga Caulerpa taxifolia in southeastern Australia. Initially we investigated differences in soft‐sediment faunal communities (above and below ground) across a biomass gradient at two invaded sites. Caulerpa taxifolia biomass was positively associated with the composition and abundance of the epifaunal community, but negatively correlated with the abundance of infauna. To examine the response of common community members in more detail, we caged two species of mollusk (the infaunal bivalve, Anadara trapezia and the epifaunal gastropod, Batillaria australis) across the same biomass gradient to determine lethal and sublethal effects of C. taxifolia biomass on individuals. Survivorship of A. trapezia was low when C. taxifolia was above 300 g m−2. Negative sublethal effects were also density‐dependent with A. trapezia tissue weight being lowest above this same C. taxifolia biomass. The proportion of B. australis surviving was unaffected by C. taxifolia biomass. However, the total number of live B. australis recovered in cages increased as C. taxifolia biomass increased, providing further evidence of positive density dependent effects (in line with the survey data) of C. taxifolia on epifauna. Finally, we removed C. taxifolia from plots of differing C. taxifolia biomass and followed community change for 5 months. Community change following C. taxifolia removal was also density dependent as recovery 5 months post‐removal depended on the initial biomass of C. taxifolia, suggesting a lag in the recovery of communities due to residual environmental effects post‐removal (i.e. hysteresis). We have shown that the effects of a habitat‐forming invasive species are biomass dependent and also affect community components differently, suggesting that, globally, the impact of these types of invaders may be context dependent.
Kappaphycus alvarezii (Doty) Doty (Rhodophyta: Solieriaceae) is a Philippine-derived macroalga introduced into the Gulf of Mannar Marine Biosphere Reserve, South India for mariculture in 2000. Here we report its bioinvasion on branching corals (Acropora sp.) in the Kurusadai Island. Qualitative data collected using underwater photography clearly indicated its invasion and establishment on live and dead corals as well as coral rubbles and pavements. It specifically invaded Acropora sp. as monospecific beds with extraordinary phenotypic plasticity in the form of thallus, thickness of its major axis and lateral branching. It shows remarkable shadowing and smothering effects over the coral colonies. The primary and secondary branches are much reduced in the invaded algal colonies. Quantitative data on its live cover on corals and biomass production are also reported. These observations are discussed with available limited information on bioinvasion of K. alvarezii on coral reefs. Our findings disprove all arguments and misapprehensions reported earlier about this species as coral-friendly and as a safe candidate for mariculture for the production of carrageenan under wild conditions in the Gulf of Mannar. Our observations underscore the need for urgent reconsideration of its cultivation in a biologically diverse ecosystem, the Gulf of Mannar.
Understanding the mechanisms that facilitate or inhibit invasion of exotic seaweeds is crucial in assessing the threat posed by their incursion and to define control options. In this paper, we consider how life history characteristics of the invading species and properties of the recipient environment influence the likelihood of inva- sion, giving particular emphasis to how disturbance influ- ences the establishment, spread and persistence of introduced seaweed populations. Very few commonali- ties in key life history traits emerge since each species possesses a unique set of traits that confers a high capacity for invasiveness. Consequently, for seaweeds at least, predictions of invasibility based on life history char- acters alone are unlikely to be useful. In contrast, it is clear that disturbance is an important process in the establishment of these invasive species. With the pos- sible exception of Caulerpa taxifolia, disturbance appears to be a critical factor that is either a key requirement (e.g., Codium fragile ssp. tomentosoides, Sargassum muticum and Undaria pinnatifida), or which accelerates (e.g., Fucus serratus) establishment and spread. The role of disturbance in the persistence of the invaders is more complex and depends on the species concerned. In sev- eral cases there is substantial evidence for positive feed- back mechanisms that enable introduced species to persist in the absence of the disturbance factor that facil- itated establishment in the first place. These circum- stances define examples of ecological hystereses that pose particular challenges for management and control. The evidence suggests that, in several cases, preventing anthropogenically mediated disturbance to canopies of native seaweeds should be considered as a potential control option to minimise the risk of establishment of exotic species at high densities. However, for these kinds of introduced species, once they are established, control options that primarily target the disturbance are unlikely to represent viable management options.
The occurrence of filamentous turf-forming macroalgae assemblages has been widely reported in tropical and temperate oceans. In the Mediterranean Sea the development of dense and persistent turfs has recently started and has been linked to the spread of the two introduced filamentous Rhodophyta Acrothamnion preissii and Womersleyella setacea. In this work algal assemblages on rocky bottoms and rhizomes of Posidonia oceanica were studied during one year in three localities of the northwestern Mediterranean Sea (Leghorn, Vada Shoals and Gorgona Island) characterised by different sedimentation rate and composition of particulate matter. Dead matte of P. oceanica was also studied on Vada Shoals. Results showed that turfs are able to develop on all habitats investigated, but different patterns are found. Turfs were abundant on rhizomes of P. oceanica in all localities studied while on rocks they reached high cover at Leghorn and Vada but not on Gorgona Island. The dead matte of P. oceanica showed patterns similar to those found on rocky bottoms. Turfs seem positively influenced by sedimentation while neither the nature of particulate matter nor other polluting substances had any important result on their spread. The development of turfs negatively affects species number and diversity of invaded communities, while cover did not show significant variations. Womersleyella setacea replaced Acrothamnion preissii on rocky bottoms and on dead matte, but not on Posidonia oceanica rhizomes where Acrothamnion preissii was dominant everywhere. In spite of the occurrence of competition between the two algae, it did not seem to limit the development of turfs as they reached similar cover values independently of the dominant species.
A study of the meadows of the invasive Caulerpa racemosa var. cylindracea (Sonder) Verlaque, Huisman et Boudouresque was carried out over one year at Marseilles (Provence, France) where the alga is thriving, probably since 1994, in the cold waters of the north western Mediterranean Sea. At an early phase of colonisation, the C. racemosa meadow is characterized by a patchy distribution pattern. Several years are necessary to obtain a dense and continuous meadow. In one area colonized for more than 4 years, C. racemosa has developed a continuous meadow with wide seasonal variations. Maximum development was reached in autumn (biomass: 82±3g DWm−2; length of stolons: 1162±86mm−2; number of apices: 8360±405m−2; number of erect axes: 20955±1499m−2) and the minimum from winter to early spring (respectively, 0.3±0.1g DWm−2; 3±1mm−2; 220±55 apicesm−2; 35±15 erect axesm−2). Seasonal variations in the growth rate were highly significant. The season of high growth lasted from June to October. The apical growth rate of a stolon reached a maximum of 7.5±0.3mmday−1 in early October, then began to decrease significantly from the end of October to December, before becoming nearly nil from January to early May. Annual net production rate expressed in terms of stolon length and biomass was estimated as 5801mm−2a−1 and 612g DWm−2a−1, respectively. During the growth period, the turnover rate of the C. racemosa stolons was estimated at from 25 to 46 days. The growth rate was closely correlated to the seawater temperature (R2=0.83), whereas no significant correlation was found between growth and irradiance. During the growth period, a decrease in temperature rapidly affects the growth rate, which soon recovers its earlier level when the temperature rises again. In winter, the growth rate decreased rapidly with the seasonal drop in the seawater temperature. Grazing by fish (Sarpa salpa and Boops boops) can also affect the growth rate from September to December by consumption of the erect axes and stolon apices, enhancing the ramification of stolons. Seasonal changes at Marseilles are much sharper than those reported for warmer Mediterranean localities (French Riviera, Italy, Croatia): in winter and early spring C. racemosa meadows decreased and locally disappeared, leaving a barren substrate. C. racemosa survives the lower winter seawater temperatures of the north-western Mediterranean Sea probably in the form of zygotes and/or small fragments (rhizoids, stolons, propagules).
Although eutrophication and biological invasions are recognized as important elements of global change, few studies have assessed how their synergism can affect structural and functional traits of marine ecosystems. The present study aimed to evaluate the effect of seawater nutrient enrichment on the spread of the introduced Chlorophyta Caulerpa racemosa var. cylindracea and the interactive effects of nutritional enrichment and C. racemosa invasion on the native macroalgal assemblages of a subtidal rocky bottom. To achieve these objectives, a manipulative field experiment was carried out at 24 m depth in the northwestern Mediterranean Sea by adding C. racemosa and supplying the water column with nutrients. The following treatments were applied: non-manipulated assemblages, nutrient enrichment, addition of C. racemosa, and both nutrient enrichment and addition of C. racemosa. Results showed that C. racemosa spread was enhanced by nutrient enrichment. Moreover, a significant interaction between nutritional increase and C. racemosa invasion was detected: the effects of C. racemosa invasion on native macroalgal assemblages were amplified by nutrient enrichment. Results suggested that synergism between effects of different anthropogenic impacts can have severe consequences on the integrity of marine ecosystems.
A venerable generalization about community resistance to invasions is that more diverse communities are more resistant to invasion. However, results of experimental and observational studies often conflict, leading to vigorous debate about the mechanistic importance of diversity in determining invasion success in the field, a well as other ecosystem properties, such as productivity and stability. In this study, we employed both field experiments and observational approaches to assess the effects of diversity on the invasion of a subtidal marine invertebrate community by three species of nonindigenous ascidians (sea squirts). In experimentally assembled communities, decreasing native diversity increased the survival and final percent cover of invaders, whereas the abundance of individual species had no effect on these measures of invasion success. Increasing native diversity also decreased the availability of open space, We limiting resource in this system, by buffering against fluctuations in the cover of individual species. This occurred because temporal patterns of abundance differed among species, so space was most consistently and completely occupied when more species were present. When we held diversity constant, but manipulated resource availability, we found that the settlement and recruitment of new invaders dramatically increased with increasing availability of open space. This suggests that the effect of diversity on invasion success is largely due to its effects on resource (space) availability. Apart from invasion resistance, the increased temporal stability found in more diverse communities may itself be considered an enhancement of ecosystem function. In field surveys, we found a strong negative correlation between native-species richness and the number and frequency of normative invaders at the scale of both a single quadrat (25 X 25 cm), and an entire site (50 X 50 in). Such a pattern suggests that the means by which diversity affects invasion resistance in our experiments is important in determining the distribution of invasive species in the field. Further synthesis of mechanistic and observational approaches should be encouraged, as this will increase our understanding of the conditions under which diversity does (and does not) play an important role in determining the distribution of invaders in the field.
In the present work, coralligenous phytobenthic com-munities of the north-western Mediterranean Sea were studied in three deep sublittoral habitats: islands, conti-nental coasts and offshore banks. Species and morpho-logical groups were utilised as descriptors. A combination of univariate and multivariate analysis was used to describe variability in coralligenous macroalgal assemblages of vertical rocky cliffs between habitats and at different spatial scales within each habitat. In addition, temporal dynamics in the structure of macroalgal assem-blages on horizontal and vertical surfaces were studied for one location. A total of 99 macroalgal species was identified, among them 12 Fucophyceae, 10 Chlorophy-ta, and 77 Rhodophyta. Results showed no major differ-ences between assemblages of the rocky habitats studied. Spatial variability within each habitat appeared higher at the smallest scale investigated for all variables considered. In the coralligenous assemblages of the bank, patterns related to the inclination of substratum were not significant, except for filamentous species and turf layer. In contrast, the assemblages showed signifi-cant seasonal differences mostly related to the disap-pearance of many turf-species and the decrease in cover of most erect algae, principally foliose and corticated-terete forms, in the cold season.
Macroalgal assemblages of Posidonia oceanica rhizomes of the western Mediterranean have been studied, in order to describe the structure of this community and to evaluate changes due to recent macroalgal invasions. To achieve these objectives, P. oceanica rhizomes were sampled from 21 beds distributed throughout the western Mediterranean Sea and macroalgal assemblages were studied using both species and functional groups as descriptors. Moreover, modifications linked to temporal fluctuations and depth were evaluated in an insular and in a continental bed. A total of 74 macroalgal species were found, among them 7 Chlorophyta, 8 Fucophyceae and 59 Rhodophyta. The species number varied from 5±0 at Vada to 18±0.58 at Panarea Island. The total percentage cover ranged from 19.5±1.8 at Cabo de Creus to 77.2±13.3 at La Maddalena Island. Filamentous species were dominant in the localities colonized by Acrothamnion preissii or Womersleyella setacea, while a high functional diversity occurred where the two introduced species were not present or they did not constitute dense turfs; the importance of crustose and foliose species was remarkable, while low cover of articulate and corticated-terete species was found at all localities. Localities where turfs were not developed were grouped together, even if they appeared more dispersed than those dominated by filamentous algae. The two beds studied over a 1 year period showed higher values of macroalgal cover in March and December, although significant differences were not detected with ANOVA in relation to either depth or season. The development of turfs, which occurs widely in western Mediterranean seagrass beds, seems to modify strongly the structure of macroalgal assemblages of rhizomes, mostly via the decrease in species and functional diversity.
The study aimed at contributing to the development of methods to assessing the effects of human disturbance on coralligenous reefs. The effects of the presence of a fish farm on coralligenous reefs were evaluated using the STAR (STAndaRdized coralligenous evaluation procedure) sampling procedure. An asymmetrical sampling design was used to compare the aquaculture site with two reference sites in areas unaffected by human pressure. The response of different ecological indices (ESCA, Ecological Status of Coralligenous Assemblages; ISLA, Integrated Sensitivity Level of coralligenous Assemblages; COARSE, COralligenous Assessment by Reef Scape Estimate) and descriptors (α-diversity, β-diversity and Sensitivity Level) of this kind of disturbance was compared. Results indicate that coralligenous reefs are vulnerable to aquaculture fish cages, and differences in the structure of coralligenous assemblages between the disturbed and the reference sites were mostly due to the decrease in β-diversity. On the contrary, no significant differences in the number of taxa/groups were highlighted. Encrusting Corallinales, erect Rhodophyta, Dictyotales, Fucales and Halimeda tuna were more abundant in reference sites than in disturbed site, while Peyssonnelia spp. and algal turfs had the opposite trend. Conversely, no significant differences between conditions were found in the abundance of sessile invertebrates. The study supports the suitability of the STAR approach to be employed in impact evaluation assessments, such as in monitoring programs. The present study is a first attempt to combine three different ecological indices (ESCA, ISLA and COARSE) within a unified approach, in order to assess the status of coralligenous reefs subjected to a moderate human-induced disturbance. The inconsistent response of the different indices highlights the advantage of applying different indices and descriptors to evaluate the variable human pressures on natural systems.
The establishment and spread of Non-Indigenous Species (NIS) can be facilitated through perturbations that lower the resilience of native communities to invasion. After a disturbance to a rocky subtidal macroalgal assemblage in the Mediterranean Sea (Italy) by a shipwreck (Costa Concordia), we compared native and non-native seaweed community structure at the disturbed site and two reference sites using traditional abundance measures, in addition to employment of the ALien Biotic IndEX (ALEX). ALEX has been recently proposed as a potential multi-metric indicator to evaluate biological invasions in Mediterranean macroalgal assemblages, taking into account both the relative abundance of macroalgal NIS to native seaweed abundances and the invasive traits of those NIS. We found that macroalgal NIS were initially more abundant at the disturbed site, but four months later were decreased and there were no difference with the reference sites. Native macroalgal cover at the disturbed site was lower than at reference sites in the initial survey, but unexpectedly decreased significantly four months later. Despite the decline in the cover of macroalgal NIS, ALEX was consistently lower through time at the disturbed site, due to concurrent declines in native seaweed abundances. This suggests that evaluation of macroalgal NIS should not be based on cover of macroalgal NIS alone and that the relative cover of native species should also be considered. ALEX may represent a versatile tool to be employed in both monitoring programs and impact evaluation studies.
A modified version of the ALien Biotic IndEX (ALEX) has been recently proposed to evaluate biological invasions
in macroalgal assemblages. ALEX was applied in a Marine Protected Area where a recreational-fishing port is
present testing the following hypotheses: ALEX increases with the distance from the port, it changes between the
two directions off the port and it changes among three different habitats: Cystoseira beds, algal turf and dead
matte of the seagrass Posidonia oceanica. A total of 78 native macroalgal taxa and 4 introduced species were
found, the Chlorophyta Caulerpa cylindracea and the Rhodophyta Apoglossum gregarium, Acrothamnion preissii and
Womersleyella setacea. All study sites were in high quality status highlighting that the assemblages investigated
were at an early stage of NIS invasion. However, ALEX detected different values among conditions and habitats
within the MPA, suggesting a local dynamics of NIS spread and different resistance to invasion of the investigated
habitats.
This study investigates whether eutrophication can affect the capacity of the canopy alga Cystoseira brachycarpa to impede or limit the spread of the introduced species Caulerpa cylindracea. By means of a manipulative field study (16 months long), the effects of nutrient enrichment and C. cylindracea removal were tested on the canopy-alga and the associated macroalgal community. Results highlighted deep changes through time due to nutrient enrichment, as C. brachycarpa decreased and Halopteris scoparia increased in cover. Furthermore, C. brachycarpa was also affected by the presence of the introduced species Caulerpa cylindracea which, in turn, was found significantly advantaged by nutrient enrichment. Overall, our findings suggest that eutrophication can drive the substitution of Cystoseira with H. scoparia, leading to the shift from canopy to opportunistic species, which are unable to avoid the spread of C. cylindracea.
This paper reports the results of a study about the geographical distribution of A. preissii in the Tuscan Archipelago. Moreover, vertical distribution, seasonal dynamics and reproductive phenology of A. preissii, on hard bottom and Posidonia oceanica beds, were investigated around Gorgona Island. Some considerations regarding the effects of A. preissii turfs on benthic communities are reported. -from Authors
Coralligenous concretions, the unique calcareous formations of biogenic origin in Mediterranean benthic environments, are produced by the accumulation of encrusting algae growing in dim light conditions. This review provides an overview of the results obtained by the main studies dealing with these formations, including the environmental factors which influence the development of coralligenous communities, their distribution, types, assemblages, builders and eroders, the biotic relationships and processes that create and destroy coralligenous assemblages, their dynamics and seasonality, and the functioning of several outstanding and key species. Special attention is devoted to the biodiversity of coralligenous communities and a first estimation of the number of species reported for this habitat is provided. Major disturbances affecting coralligenous communities are discussed, ranging from large-scale events that are probably related to global environmental changes to degradation by waste water or invasive species. Degradation by fishing activities and by divers is also considered. Finally, the main gaps in current scientific knowledge of coralligenous communities are listed and some recommendations are made regarding their protection. © R. N. Gibson, R. J. A. Atkinson, and J. D. M. Gordon, Editors Taylor & Francis.
The two alien green algae Caulerpa taxifolia and Caulerpa cylindracea (formerly Caulerpa racemosa var. cylindracea) are among the most notorious and threatening invasive species in the Mediterranean Sea. From the beginning of their introduction, C. taxifolia aroused a great concern due to its initial exponential spread and impacts on native ecosystems, whilst C. cylindracea received lower attention. In this paper, the distribution of the two aliens over the last 30 years in the Italian side of the Ligurian Sea (NW Mediterranean Sea) is reviewed and the kinetics of their spreading is compared. We combined available data on habitat occupancy with data on substratum cover obtained during underwater surveys to compute the average annual spreading rates for both aliens. C. taxifolia had an impressive expansion phase from 1984 to 2000 but then, its dispersal rate showed lower than that predicted and the species did not persist in areas formerly colonized. Today, abundance of this species is strongly declined as it disappeared from most of the attained areas. On the contrary, C. cylindracea exhibited an impressive and constant expansion from the beginning of its first appearance in the Ligurian Sea and, today, it is still increasing its range and habitat occupancy. Its abundance results 1.5-times greater than that reached by C. taxifolia during the period of its maximum expansion. The divergent kinetics of spreading of the two aliens prevents any possibility of generalization of the future behaviour of invasive species in the Mediterranean Sea: continuous and periodic surveys are thus mandatory to understand the kinetics of expansion and to define species-specific models.
A three-year experimental study was performed to evaluate the interactive effects of topographic complexity and sedimentation in determining the structure of rocky macroalgal assemblages. The following hypotheses were tested: i) the structure of macroalgal assemblages varies according to the complexity of the substratum; ii) high sediment deposition reduces variations in the structure of assemblages among substrata characterized by different complexity. At natural levels of sediment deposition, greater substratum complexity enhanced species richness and favored the development of assemblages dominated by architecturally complex species, such as large corticated Rhodophyta. Under high sediment deposition, turfs became the main component of macroalgal assemblages, although different filamentous forms responded differently to substratum complexity. In addition, high sediment deposition increased the abundance of the invasive Chlorophyta, Caulerpa cylindracea, on low complexity substrata, but decreased it on high complexity substrata. These results show that an increase in sediment deposition can dampen variations between assemblages associated to substrata characterized by different complexity, with consequent reduction of both alpha (i.e., species loss) and beta diversity (i.e., decreased small-scale variation in community structure).
Predicting community susceptibility to invasion has become a priority for preserving biodiversity. We tested the hypothesis that the occurrence and abundance of the seaweed Caulerpa racemosa in the north-western (NW) Mediterranean would increase with increasing levels of human disturbance. Data from a survey encompassing areas subjected to different human influences (i.e. from urbanized to protected areas) were fitted by means of generalized linear mixed models, including descriptors of habitats and communities. The incidence of occurrence of C. racemosa was greater on urban than extra-urban or protected reefs, along the coast of Tuscany and NW Sardinia, respectively. Within the Marine Protected Area of Capraia Island (Tuscan Archipelago), the probability of detecting C. racemosa did not vary according to the degree of protection (partial versus total). Human influence was, however, a poor predictor of the seaweed cover. At the seascape level, C. racemosa was more widely spread within degraded (i.e. Posidonia oceanica dead matte or algal turfs) than in better preserved habitats (i.e. canopy-forming macroalgae or P. oceanica seagrass meadows). At a smaller spatial scale, the presence of the seaweed was positively correlated to the diversity of macroalgae and negatively to that of sessile invertebrates. These results suggest that C. racemosa can take advantage of habitat degradation. Thus, predicting invasion scenarios requires a thorough knowledge of ecosystem structure, at a hierarchy of levels of biological organization (from the landscape to the assemblage) and detailed information on the nature and intensity of sources of disturbance and spatial scales at which they operate.
The European Marine Strategy Framework Directive (MSFD) requires EU Member States (MS) to achieve Good Environmental Status (GEnS) of their seas by 2020. We address the question of what GEnS entails especially with regard to the level at which targets are set (descriptors, criteria, indicators), to scales for assessments (regional, sub-divisions, site-specific), and to difficulties in putting into practice the GEnS concept. We propose a refined and operational definition of GEnS, indicating the data and information needed to all parts of that definition. We indicate the options for determining when GEnS has been met, acknowledge the data and information needs for each option, and recommend a combination of existing quantitative targets and expert judgement. We think that the MSFD implementation needs to be less complex than shown for other similar directives, can be based largely on existing data and can be centred on the activities of the Regional Seas Conventions.
In this study, the macroalgal community of Posidonia oceanica rhizomes was described in relation to different levels of development of the two introduced Rhodophyceae Acrothamnion preissii and Womersleyella setacea. Fifteen P. oceanica meadows were chosen in the western Mediterranean. In each meadow, two samples of five shoots were made at 4 and 15 metres deep. Sixty-five macroalgal species were found: seven Chlorophyta, eight Fucophyceae and 50 Rhodophyta. A. preissii was distributed from the Balearic Islands to Tuscan Archipelago and along the Ustica Island. W. setacea was present in all samples except for Tunisia. The development of algal turfs changed the structure of epiphytic communities. In the colonised areas, A. preissii and W. setacea formed paucispecific populations. Diversity diminished with the increasing of turf developments. Turfs trap sediments, forming a stratum that prevents the development of other algal species. Where the two species were both present, A. preissi reached higher values of dominance. The development of algal turfs seems not limited by the competition between A. preissii and W. setacea. Turf cover was the same independently of the dominant species.
Introduced algae have become a prominent component of the marine flora in many regions worldwide. In the NE Pacific, the introduced Japanese alga Sargassum muticum (Yendo) Fensholt is common and abundant in shallow, subtidal, rocky habitats, but its effects on subtidal, benthic communities in this region have not previously been studied. I measured the response of native species to experimental manipulation of S. muticum in field experiments in the San Juan Islands of Washington State. Native canopy (brown) and understory (red) algae were more abundant in plots from which S, muticum had been removed, and the native kelp Laminaria bongardiana (the most abundant species of brown alga in the absence of S. muticum) grew more than twice as fast in plots where S. multicum was absent. The negative effects of S. muticum on native algae appear to be a result of shading, rather than changes in water flow, sedimentation, or nutrient availability. S. muticum also had a strongly negative indirect effect on the native sea urchin Stronglyocentrotus droebachiensis by reducing abundances of the native kelp species on which it prefers to feed. My results indicate that S. muticum has a substantial impact on native communities in this region, including effects at multiple trophic levels. Because of their worldwide distribution and capacity to alter native communities, non-indigenous algae are potentially important agents of global ecological change.
Coexistence of species is generally attributed to the interacting roles of competition, predation, and disturbance. Overgrowth is considered to be an important mechanism of competition for space. and species are often ranked in hierarchies based on their abilities to overgrow. In some marine habitats, however, encrusting algae dominate primary substrata despite a dense permanent cover of epiphytes, suggesting that factors other than competition could be important in influencing their distributions. The spatial relationships and competitive interactions between encrusting algae and overgrowing overgrowing filamentous, turf-forming algae were investigated on a subtidal rocky reef (Mediterranean Sea. Italy). Quantitative observations and field experiments were done from 1992 to 1998: (1) to investigate the relative patterns of distribution and abundance of crusts and turf and how they differed across space and time, (2) to test whether spatial relationships between crusts and turf were influenced by various characteristics of the substratum and by disturbance from wave action. (3) to investigate spatial and temporal patterns of recruitment and the mechanisms by which crusts colonize space, and (4) to rest whether trusts and turf compete for space. Crusts were always among the first colonizers of available bare rock and were subsequently overgrown by turf. Despite variations in spatial and temporal recruitment of crusts, this pattern was never reversed. Covers of crusts and turf were not significantly affected by disturbance. There was some evidence for competition, but this did not result in the local exclusion of either crusts or turf. Despite extensive and persistent cover of turf, encrusting algae were abundant and were able to live, grow, and reproduce beneath the: turf over lung periods with little adverse effect. Similarly, crusts did not limit the distribution of turf, although they slowed its rate of growth. The capabilities of turf and crusts to overgrow and tolerate overgrowth, respectively, were identified as the major determinants of the structure of this two-layered assemblage and the probable basis of the success of these species in coexisting as the dominant algal forms. Overgrowth was not equivalent to competitive subordination and displacement, emphasizing the need fur caution when interpreting competitive abilities from observed patterns of overgrowth. Overall, results suggested that, in some habitats, tolerance may be more important than competition in maintaining coexistence of species and influencing community structure.
Seaweeds have invaded ecosystems along the coasts of many countries where they can displace native algae and seagrasses, reduce biodiversity and impair habitat of fish and invertebrates. The most widespread and notorious cases have been introductions of Caulerpa taxifolia, which now infests over 20,000 ha of Mediterranean subtidal zones. Few attempts to control seaweed invasions have been successful, due to often harsh and highly variable physical conditions in marine environments, and the lack of efficacious methods. Use of heat, copper, chlorine, salt, freshwater and various mechanical (removal) approaches have been successful in reducing or eradicating some infestations. Biological control by herbivorous mollusks and sea urchins has been investigated, but has yet to result in any operational programs. Nutrient inputs from near-shore sources have exacerbated the spread of some species (e.g., off the Florida coast). To counter the increase in seaweed introductions and the spread of these species, it will be useful to adapt, where feasible, methods that have proven successful in controlling freshwater weeds. New methods will need to be developed as well. This will require better communication among researchers and managers working to reduce introductions and negative impacts of these seaweeds. © 2007 by Walter de Gruyter GmbH & Co. KG, 10785 Berlin All rights reserved.
Caulerpa taxifolia (Vahl) C. Agardh is a green tropical seaweed recently introduced in the Mediterranean. Since 1984, when for the first time it was recorded along the French Riviera, it has rapidly spread into a wide range of sublittoral habitats and has contributed to the decay and regression of the seagrasses Cymodocea nodosa (Ucria) Ascherson and Posidonia oceanica (L.) Delile. Descriptive field studies have indicated a strong habitat effect on C. taxifolia performance and suggested a positive influence of seagrasses, particularly of P. oceanica, on both blade density and size of the alga.
Thalli of the invasive alga Caulerpa racemosa (Cauler- pales, Chlorophyta) were measured monthly at 22 m depth off Villefranche-sur-Mer (southeastern France, Mediterranean Sea). C. racemosa was perennial at this site. Stolons (attaining up to 1 km/m 2 ) comprised most of the thallus, with stolon biomass up to 12 times higher than the frond biomass. Biomass, stolon and frond lengths, and the number of fronds varied over the year, being lowest in winter. Compared to other species of Caulerpa in the Mediterranean Sea, C. racemosa showed lower total biomass and lower allocation to fronds relative to stolons.
Invasive species are considered an increasing global threat to the marine ecosystems. Understanding which factors can accelerate or reduce invasion success is one of the main goals of invasion ecology. The present study investigated the interactive effects of propagule pressure and native functional diversity on the invasion success of the invasive alga Sargassum muticum (Yendo) Fensholt over a 1-year invasion process (i.e. settlement, recruitment and colonisation). Invasibility of macroalgal assemblages with different functional diversity was assessed under high and low propagule pressure. Synthetic assemblages resembling natural marine macroalgal assemblages were built and used as model systems. Recruitment discs were used to record early settlement at the assemblages. Recruitment and colonisation success were monitored in the field 2 and 10-months after exposure to propagule rain at the laboratory. In addition, availability of resources (space and photosynthetic active radiation, PAR) was also monitored for each assemblage and used as predictive variables in the analyses. Results showed that the effects of propagule pressure and functional diversity varied over the different invasion stages. Contrasting to previous studies on S. muticum, space availability did not play a significant role explaining recruitment success of the invader. PAR, however, was a key resource in the invasion success of S. muticum. Our findings showed that the invasion success of S. muticum over the different invasion stages was influenced differently by recipient assemblage traits. Hence, invasion success may better be explained by the interaction of environmental factors that covary with species diversity and identity at different invasion stages.
In this study, a combination of univariate and multivariate analysis was used to compare macroalgal assemblages of Mediterranean coralligenous system in areas invaded by the alien species Caulerpa racemosa var. cylindracea and Womersleyella setacea with reference assemblages. Results showed that assemblages strongly differed between invaded and reference areas in relation to both assemblage structure and species richness. Moreover, significant differences were detected between assemblages colonized by different invaders: C. racemosa var. cylindracea seemed to affect mostly encrusting organisms, while turf algae decreased in areas invaded by W. setacea. The present study showed that invasions of alien macroalgae could represent a serious threat for Mediterranean coralligenous assemblages.