Coralligenous Assemblages Research Articles

Threats to macroalgal coralligenous assemblages in the Mediterranean Sea

Coralligenous habitat is one of the most important coastal systems of the Mediterranean Sea. This paper evaluates the main threats to macroalgal coralligenous habitats through a review of the relevant literature. Sedimentation, nutrient enrichment and biological invasions can cause severe alterations in the structure of coralligenous assemblages due to the regression of perennial structuring species and increases in populations of ephemeral algae. Under pristine conditions, mechanical disturbance seems to be easily mitigated by the recovery capability of coralligenous assemblages; however, such disturbances seriously affect coralligenous structure if they occur in concert with other stressors. Important synergetic effects among all the studied anthropogenic disturbances are also highlighted. The main consequences of the considered stressors are the loss of the complexity of macroalgal assemblages and the deterioration of both alpha and beta diversity.

Methane-Derived Authigenic Carbonates (MDAC) in northern-central Adriatic Sea: Relationships between reservoir and methane seepages

In the Adriatic basin, gas venting characterized by pockmarks, gas bubbling and other fluid escape related features, is diffused and has been linked or to shallow gases trapped in the Late Pleistocene and below the Holocene maximum flooding surface or to leakage from more deeply buried Pliocene to Pleistocene foredeep succession. Some seepages are associated with carbonate crusts, but a possible correlation between methane leakage and carbonate precipitation in this basin never reached a clear definition. Carbonate concretions recovered by different areas of the Adriatic Sea (northern, central and southern) at different depths (from ca 30 to ca 300m) have been analysed. Different fabrics, petrography and mineralogy can be evidenced for the different sets of samples; all of them show a carbonate content mainly represented by high-Mg calcite: up to 66% in weight of total carbonates for the samples from central Adriatic (close to Bonaccia gas field, offshore Ancona), and 79 to 96% of the total carbonate weight for the other samples that are mainly biogenic. These differences are also reflected by the δ13C values, ranging between −19‰ and −29‰ VPDB, for the samples collected in the Bonaccia area and moderately positive values for all the other samples.Bonaccia samples contain carbonate cements that record the contribution of methane-derived carbon and have been interpreted as formed below the sea floor, in correspondence of the Sulphate-Methane Transition Zone. Carbonates recovered from the southern and northern Adriatic basin, do not show clear evidence of methane-derived carbon and are mainly made up by organisms typically recovered in the Adriatic coralligenous assemblage. Nonetheless micro-morphological evidences suggest influence of bacterial sulphate reduction and methanogenesis in precipitation of some of these carbonates.

Exploring the effects of invasive algae on the persistence of gorgonian populations

The effects of invasive species on native fauna are understudied in marine ecosystems, even though the impact of introduced species must be taken into consideration for proper conservation and management. The main aim of this study was to investigate the effects of invasive algae on one of the highest diversity assemblages in the Mediterranean, the coralligenous. Two manipulative experiments were designed to study the effects of overgrowth of invasive species on juvenile colonies of the Mediterranean gorgonian Paramuricea clavata, an important structural species in coralligenous assemblages. The experiments were conducted in two French marine protected areas, Port-Cros National Park and Scandola Regional Park, which are separated by hundreds of km and are invaded by Caulerpa racemosa and Womersleyella setacea, respectively. Both invasive species had a strong and consistently negative effect on different components of fitness. This finding was demonstrated by lower survivorship, higher necrosis rates and lower biomass in P. clavata colonies exposed to invasive algal overgrowth compared to the controls. Because invasive species affected the viability of juvenile colonies of P. clavata, we conclude that the persistence of P. clavata populations affected by invasion can be jeopardized, especially in the case of populations already impacted by mass mortalities associated with climate warming.

A rapid photographic method detects depth gradient in coralligenous assemblages

Coralligenous assemblages represent the unique calcareous formations of biogenic origin in the Mediterranean Sea. Despite their importance in terms of biodiversity and biomass production, community analyses remain scarce. Actual sampling of these assemblages is complicated to carry out because their depth distribution (down to −120m) necessitates complex diving logistics. We highlight a rapid, cost-effective, objective and accurate method for the sampling of coralligenous assemblages and tested its efficiency in delineating a depth gradient. We compared seven photographic methods for estimating the percentage cover of sessile organisms: visual estimates (VS) with the aid of a 25, 64 or 100 square-grid and random-point-quadrats (RQ) with 25, 64 or 100 random points or 64 stratified random points. Comparisons were made using two simulated quadrats for which percent cover values were known. RQ with 64 random points was the method that accumulated the highest number of advantages. Using this method, two field sites were sampled by divers at three depths (−50, −60 and −70m) with increasing replication (10, 20, 30 and 40 photographic quadrats). The communities deduced from the 30 and 40 photos were similar. Community analyses showed an effect of depth nested in site on the assemblages observed. With increasing depth, encrusting algae get replaced by Porifera. Dissimilarity between −50m and −60m/−70m was mainly due to Crambe tailliezi abundance. This methodology will be a useful tool for managers and administrators; it guarantees fast abundance estimation, non-destructive repeated sampling, the possibility of comparison among researchers and the permanent record of deep-sea communities.

A preliminary study toward an index based on coralligenous assemblages for the ecological status assessment of Mediterranean French coastal waters

Despite the great contribution of coralligenous communities to Mediterranean biodiversity (second key-ecosystem after Posidonia oceanica meadows), they were never considered in the establishment of multimetric indices for ecological status assessment of marine environment. In this paper, we describe a method to evaluate the ecological status of coralligenous assemblages along Mediterranean French coasts. Several metrics were selected from literature for coralligenous assemblage description and include functional and structural information: percent cover of visible non-vagile species (using photographic quadrats along a transect) and gorgonian demography. Thirty eight field stations were sampled for these metrics in PACA (Provence-Alpes-Côte-d’Azur) region in June 2010 and considered for their morphology (bank, rim), geographical orientation and principal current direction (North, East, West, South) and depth (from −30 to −84m). Metrics found to be linked to human pressures using ANCOVA and multiple correlation matrix were selected to be included in the index. The index (Coralligenous Assemblage Index, CAI) that we proposed was based on three selected metrics (Bryozoa percent cover, sludge percent cover, builder species percent cover) and considers depth; it was positively and significantly linked to anthropization (related to water quality). The 38 stations studied with theoretically good to bad environmental conditions were classified in levels of status in accordance with our field work knowledge. CAI variation was validated with three stations sampled with 30 other photos. This index could be an effective tool for the assessment of the ecological quality of coralligenous communities. It could be applied in the context of the Marine Strategy Framework Directive as well as in conservation and sustainable management of the marine environment.

Combining literature review, acoustic mapping and in situ observations: an overview of coralligenous assemblages in Liguria (NW Mediterranean Sea)

A review and update of the existing knowledge on the coralligenous assemblages of Liguria (NW Italy) was conducted as an essential step towards management measures for their conservation according to the EU Marine Strategy Framework Directive. By combining a literature review, acoustic mapping and in situ observations on a geographic information systems platform, we were able to assess the distribution and heterogeneity of coralligenous assemblages and the main pressures affecting them. The reliability of the literature was previously estimated using a dependability index. The coralligenous assemblages cover an area of 130.9 ha and range from 10 to 113 m depth. Twelve different biological facies (five of them not included in the EUNIS list) were identified and four main geomorphotypes (plungingcliffs, paleocliffs, rockfalls and shoals) were recognized. Incident light values influenced the distribution of four facies in Portofino promontory. Pressures were found on 33% of the coralligenous assemblages investigated, mainly due to fishing activities, mass mortality events, invasive species and occasional mucilaginous events. Our results showed a high spatial, geomorphological and biological heterogeneity of coralligenous assemblages in Liguria.

Rapid Biodiversity Assessment and Monitoring Method for Highly Diverse Benthic Communities: A Case Study of Mediterranean Coralligenous Outcrops

Increasing anthropogenic pressures urge enhanced knowledge and understanding of the current state of marine biodiversity. This baseline information is pivotal to explore present trends, detect future modifications and propose adequate management actions for marine ecosystems. Coralligenous outcrops are a highly diverse and structurally complex deep-water habitat faced with major threats in the Mediterranean Sea. Despite its ecological, aesthetic and economic value, coralligenous biodiversity patterns are still poorly understood. There is currently no single sampling method that has been demonstrated to be sufficiently representative to ensure adequate community assessment and monitoring in this habitat. Therefore, we propose a rapid non-destructive protocol for biodiversity assessment and monitoring of coralligenous outcrops providing good estimates of its structure and species composition, based on photographic sampling and the determination of presence/absence of macrobenthic species. We used an extensive photographic survey, covering several spatial scales (100s of m to 100s of km) within the NW Mediterranean and including 2 different coralligenous assemblages: Paramuricea clavata (PCA) and Corallium rubrum assemblage (CRA). This approach allowed us to determine the minimal sampling area for each assemblage (5000 cm2 for PCA and 2500 cm2 for CRA). In addition, we conclude that 3 replicates provide an optimal sampling effort in order to maximize the species number and to assess the main biodiversity patterns of studied assemblages in variability studies requiring replicates. We contend that the proposed sampling approach provides a valuable tool for management and conservation planning, monitoring and research programs focused on coralligenous outcrops, potentially also applicable in other benthic ecosystems.

From global to local genetic structuring in the red gorgonian Paramuricea clavata: the interplay between oceanographic conditions and limited larval dispersal

Defining the scale of connectivity among marine populations and identifying the barriers to gene flow are tasks of fundamental importance for understanding the genetic structure of populations and for the design of marine reserves. Here, we investigated the population genetic structure at three spatial scales of the red gorgonian Paramuricea clavata (Cnidaria, Octocorallia), a key species dwelling in the coralligenous assemblages of the Mediterranean Sea. Colonies of P.clavata were collected from 39 locations across the Mediterranean Sea from Morocco to Turkey and analysed using microsatellite loci. Within three regions (Medes, Marseille and North Corsica), sampling was obtained from multiple locations and at different depths. Three different approaches (measures of genetic differentiation, Bayesian clustering and spatially explicit maximum-difference algorithm) were used to determine the pattern of genetic structure. We identified genetic breaks in the spatial distribution of genetic diversity, which were concordant with oceanographic conditions in the Mediterranean Sea. We revealed a high level of genetic differentiation among populations and a pattern of isolation by distance across the studied area and within the three regions, underlining short effective larval dispersal in this species. We observed genetic differentiation among populations in the same locality dwelling at different depths, which may be explained by local oceanographic conditions and which may allow a process of local adaptation of the populations to their environment. We discuss the implications of our results for the conservation of the species, which is exposed to various threats.

Effects of nutrient enrichment on macroalgal coralligenous assemblages

Effects of eutrophication on marine ecosystems have been widely studied, even if both the effects on deep subtidal rocky assemblages and response of different successional stages to nutrients impact are still not clear. In this context, the study aimed to evaluate the effects of nutrient enrichment on Mediterranean macroalgal assemblages associated with coralligenous habitat. A manipulative field experiment was carried out by supplying both mature and early successional stages of assemblages with nutrients. A total of 62 macroalgal species were identified. Multivariate and univariate analyses showed that the structure of both mature and early successional macroalgal assemblages of coralligenous significantly varied between areas treated with nutrients and not treated areas. Moreover, differences were stronger when macroalgal assemblages were in the early successional stage than in the mature one. Results highlighted the role played by nutrients in determining the structure of macroalgal coralligenous assemblages, furthermore suggesting possible synergetic effects with other kinds of disturbances.

Coralligenous habitat: patterns of vertical distribution of macroalgal assemblages

The present study investigates patterns of distribution of macroalgal coralligenous assemblages in relation to depth and evaluates the role of different environmental conditions on these patterns. Two depths (30 and 40 m) were investigated off small islands and off continental coasts in order to select two different environmental conditions. Results showed differences between depths in the structure of assemblages around islands, while along the continental coasts these patterns were not evident. Moreover, differences between assemblages related to different environmental conditions were more evident in the shallower zone of distribution of the coralligenous habitat. This correlative study did not allow us to identify any cause-effect relationship, but patterns we detected agree with those of other studies, suggesting that alterations in the environmental conditions may be the cause of the decrease in differences among assemblages developing at different depths and may lead to a higher spatial homogenization and an impoverishment of the whole subtidal system.

Species composition and patterns of diversity of macroalgal coralligenous assemblages in the north-western Mediterranean Sea

This paper contributes to the knowledge of macroalgal coralligenous systems. The species composition of coralligenous assemblages of Tuscany's coasts and islands (north-western Mediterranean Sea) was examined and different aspects of alpha and beta diversity were evaluated. The encrusting layer of coralligenous habitat in the study areas mostly comprised the Corallinales Mesophyllum alternans. A total of 187 epiphytic macroalgal species was observed, among them 29 Ochrophyta, 14 Chlorophyta and 144 Rhodophyta. In the studied area, coralligenous assemblages show a similar structure across large scales, as 41species were common to all the studied locations. Multivariate analysis showed that locations along continental coasts were distinct from islands. Vertical and horizontal substrata were distinct on islands, but not in coastal locations. The studied assemblages showed a high diversity, especially in relation to the smaller spatial scales examined.

Coralligenous habitat in the northern Adriatic Sea: an overview

AbstractThe rocky outcrops irregularly scattered in the sandy‐muddy sea bed of the Northern Adriatic, formed by a base rock and a superimposed concretion of vegetal and animal organisms, are considered an infralittoral coralligenous habitat, according to the most recent definition. In the last four decades, research has mainly concentrated on their geology because of the question of the origin of the base rock – very similar to beachrock – which has only recently been attributed to sedimentary cementation induced by methane seeps. Studies on their macrobenthic assemblages have also been published recently, but true comparisons among them are difficult because they mostly refer only to a few selected phyla or to short‐term observations. Very few papers deal with the whole of the animal and plant communities, including all taxonomic groups and the most important environmental variables. On the basis of the literature data concerning several outcrops in the Gulf of Venice, we suggest that the Adriatic reefs differ slightly from the classic deeper coralligenous assemblages around the Mediterranean coast because they have smaller concretions of algae builders. We also suggest that the Northern Adriatic coralligenous reefs are younger than the other reefs occurring in the Mediterranean Sea.

Variability of Mediterranean coralligenous assemblages subject to local variation in sediment deposition

This study tested whether the development of coralligenous assemblages on horizontal and vertical surfaces differed between localities of high and low sediment deposition. The development and structure of these assemblages varied in predictable ways according to the level of sediment deposition. These differences were largely independent of the orientation of substratum. Turfs were more extensive in areas of high sediment deposition while erect and encrusting algae were most extensive in areas of low sediment deposition. Encrusting invertebrates characterised vertical surfaces and were most extensive in areas of high sediment deposition. These results are consistent with studies from other temperate regions of the globe, suggesting that effects of sedimentation on temperate coasts are substantial and widespread.

Spatial and temporal variability of Mediterranean macroalgal coralligenous assemblages in relation to habitat and substratum inclination

In the present work, coralligenous phytobenthic communities of the north-western Mediterranean Sea were studied in three deep sublittoral habitats: islands, continental coasts and offshore banks. Species and morphological 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 assemblages on horizontal and vertical surfaces were studied for one location. A total of 99 macroalgal species was identified, among them 12 Fucophyceae, 10 Chlorophyta, and 77 Rhodophyta. Results showed no major differences 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 significant seasonal differences mostly related to the disappearance of many turf-species and the decrease in cover of most erect algae, principally foliose and corticatedterete forms, in the cold season.

Variability at different spatial scales of a coralligenous assemblage at Giannutri Island (Tuscan Archipelago, northwest Mediterranean)

This study was carried out on the rocky cliffs of Giannutri Island (Tyrrhenian Sea, Italy) to test the hypothesis that coralligenous assemblages are consistent within the bathymetric range considered (25–35 m depth) over three different spatial scales (1000 m, 100 m and 10 m). A multi-factorial sampling design was used to assess patterns of vertical distribution in the studied area. Data on the percent cover of algae and invertebrates were collected at three depths (25, 30 and 35 m) using a photographic method, and percentage cover was obtained using a visual method. Analysis of the results using ANOVA indicated that the distribution and abundance of algae and some invertebrates of the sublittoral assemblages are clearly heterogeneous. Algae, sponges and bryozoans showed significant variability in distribution and abundance at different depths, but this variability was not consistent amongst transects. Ascidians did not show any variability, while the abundance of anthozoans differed significantly amongst transects. We concluded that heterogeneity in the distribution and abundance of the taxa analysed is related to the smallest spatial scale investigated (10’s of m). Some of the possible causes of the observed variability are discussed.