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Abstract: The Maldives was severely hit by massive coral bleaching and subsequent mortality in 1998. The results of reef monitoring in the following years have supported contrasting views about their recovery potential, partly because of the scarcity of information on the situation before 1998. Quantitative data on coral assemblages collected in 1993 in the Rasfari region (North Malé Atoll) may provide a base-line for the evaluation of the present status of the Maldivian reefs. Five years before the 1998 mortality, most coral communities appeared to be similar, in terms of both coral cover and growth-form composition, to those described in 1958 and 1964, notwithstanding increased human pressure and local events such as minor bleaching episodes in 1987 and crown-of-thorns starfish (COTS) attacks in 1989. Three lessons can be learnt from these results to help to understand the present situation, some ten years after the 1998 mass mortality. First, Maldivian reefs proved in the past to be capable of maintaining flourishing coral life despite various disturbances. Second, four years had been sufficient for complete reef recovery after a (minor) bleaching event. Third, recovery after both COTS attack and bleaching follows a predictable path suggesting that the presence of a three-dimensional community structure, which should reduce post settlement mortality of coral recruits, is essential for rapid coral recovery. As coral recruitment remains high and large tabular Acropora colonies are now reappearing, it is expected that Maldivian reefs should return to their original condition within the next few years. [Copyright &y& Elsevier]

Abstract: The performances of two commonly used non-destructive sampling procedures for rocky benthic assemblages (i.e. photography and visually assessed quadrats) were compared. A damaging human activity, date mussel (Lithophaga lithophaga) harvesting (DMH), was chosen. Directly impacted sites were compared with reference conditions (controls). Both visual quadrats and photography were equally able to detect differences between impacted situations and controls. However, visual quadrats showed high variability among replicates and estimated higher species richness for controls, while photography did so for impacts. Pooling photos in a ‘sum photo quadrat’ showed that differences between the two methods are due to sampling unit size rather than sampling procedure itself. As a small sampling unit is unavoidable with photography, visual quadrats should be preferred in investigating shallow rocky reefs for their larger size; however, longer working time underwater with quadrats does not allow for numerous replicates. Pooling many photos to reach sampling sizes comparable with those of quadrats may be a valid alternative to reconcile image resolution and areal coverage. [Copyright &y& Elsevier]

Abstract Shallow hydrothermal vents emit warm water, carbon dioxide, toxic chemicals, nutrients and reduced compounds that altogether mimic climate and human impacts, and are therefore considered as 'natural laboratories' at which can be investigated the effects of these stressors on marine ecosystems. One of the effects more thoroughly investigated is the impact of reduced pH on marine biodiversity. Calcifying organisms, such as corals, are expected to be more affected, but their response to reduced pH values in seawater has been tackled mostly by laboratory studies. Here, we assessed coral recruitment and juvenile survival, two fundamental processes for coral reef maintenance and resilience, in shallow reefs of North Sulawesi (Indonesia) close to hydrothermal vents. Differences in abundance of coral recruits (<5 cm in diameter) and juveniles (5–15 cm in diameter) were evaluated at vent sites and at control sites, on both reef flats and upper slopes. Recruits of Acropora and other broadcasting corals resulted more abundant near vents, while no difference in juvenile survival was observed between vent sites and controls. On the contrary, Pocillopora , which includes many brooders, showed a low density of recruits and low survival rates at vent sites. Vents caused a typical closed water circulation that retained coral larvae on site, and this effect, rather than water acidification or the emission of chemical compounds, was likely to be responsible for increased recruitment of broadcasters. Graphical abstract Image 1 Highlights • Coral recruitment and survival are not affected by water acidification. • Recruits' abundance is enhanced in vent sites compare to control sites. • Hydrothermal vent cause a closed water circulation. • Vent activity promote coral recruitment by retaining coral larvae. • Broadcast-spawning corals, Acropora and most of the Others, seem to be favoured. [ABSTRACT FROM AUTHOR]

Seawater warming is affecting corals worldwide. Mass mortality events are increasingly frequent, and the bioconstruction potential of coral reefs is jeopardized. Revisiting sites surveyed in the past may represent a valuable tool to better understand the health state and ongoing trend of coral reefs. In the Mediterranean Sea the only reef-building coral is the endemic species Cladocora caespitosa. Several mortality events of this coral, due to thermal anomalies, have been reported in the last decades. An impressive change occurred in a population of C. caespitosa revisited 25 years after: smaller colonies became the prevalent size class and larger colonies disappeared, highlighting a loss in the bioconstruction potential. Data collected on C. caespitosa colonies in the Eastern Ligurian Sea (NW Mediterranean) revealed that mortality rate was correlated with colony size, while reproductive potential apparently increased, suggesting hormesis (i.e., enhanced reproductive output under pre-lethal conditions). • Cladocora caespitosa is the only reef-building coral in the Mediterranean Sea. • C. caespitosa has been affected by mass mortality events due to the water warming. • Evaluation of the present coral health status requires long-term data series. • A loss in the bioconstruction potential of C. caespitosa was observed. • Mortality of C. caespitosa concerns predominantly largest colonies. [ABSTRACT FROM AUTHOR]

Abstract: Thermal anomalies up to 4 °C above the climatological mean caused dramatic mass mortalities in benthic ecosystems of the NW Mediterranean during the summer heat waves of 1999 and 2003. Information on the sessile communities of the submarine cave of Bergeggi (Ligurian Sea) was taken in 1986 and 2004, i.e. before and after the two thermal events, using wire-frame photography in four sectors of the cave ecosystem with different morphologies and environmental features. Percent cover data of growth forms (as descriptors of the structural aspects of cave ecosystem) and feeding guilds (functional aspects) were analysed by multivariate and univariate techniques. Differences in trophic organisation and total biotic cover in the four sectors were significant and consistent in the two years. Thus, food web constraints (such as water confinement and trophic depletion), that are mostly dependent on topography, underwent no major change following the thermal events. On the contrary, structural aspects changed across time, 3-dimensional growth forms being replaced by 2-dimensional ones, leading to a general homogenisation of the cave communities. Positive thermal anomalies are thought to have selectively killed erect and massive organisms, their replacement by encrusting organisms possibly representing a phase in cave recolonisation. Submarine caves are poorly resilient ecosystems, and understanding their capacity of recovery after major disturbances is mandatory for their management and conservation. [Copyright &y& Elsevier]

Macrobenthic communities along the Magra Estuary (Ligurian Sea, NW Mediterranean) were investigated between 1988 and 1990 using different taxonomic levels to estimate which level balances the loss of information with the time gained in sorting and identification at coarser levels than species. Samples were collected by dredge in four stations (FO, SC, IG and CF) positioned along the estuarine gradient and in one station (MA) located in the marine area in front of the estuary. The data set consists of 17,200 individuals belonging to six phyla, 10 classes, 32 orders, 75 families, 98 genera and 114 species. In the ordination plot from correspondence analysis at the species level four different benthic communities could be recognised: marine (MA), mouth (FO), lower (SC) and higher estuary (IG+CF). Their relative positions reflected the topographic distribution along the marine–estuarine gradient. The four communities remained stable enough over the 3-year period. The interannual differences among station–year groups were much more significant for the community of the mouth (FO), which showed major temporal drifts. At the genus level, similar results were obtained. At the family level, there was no substantial loss of information. At the order level, the pattern changed: the communities were not clearly distinct and did not scatter in the ordination diagram according to the marine–estuarine gradient. At class level, the plot is not readable. At phylum level, correspondence analysis did not give significant results and no ecological interpretation was possible. [Copyright &y& Elsevier]

Marine caves are unique and vulnerable habitats threatened by multiple global and local pressures. Changes in the sessile epibenthic community of the Grotta Azzurra of Capo Palinuro (Salerno, Italy) during the last 26 years have been evaluated using growth forms and trophic guilds as non-taxonomic descriptors, informative of the structure and the functioning of the cave ecosystem, respectively. Photographic samples were collected in 1992 and 2018 to investigate the stability of the ecosystem response to the three main environmental gradients occurring inside the cave: i) confinement, ii) depth, and iii) light. Benthic communities were well spatially distributed along the three gradients inside the cave according to both the morphology and the feeding strategy of organisms, although the zonation was less marked considering trophic guilds. Gradients persisted and zones remained distinct through time, notwithstanding significant changes in community composition. The trophic organization was preserved throughout the cave, but significant changes in the spatial organization of benthic assemblages were widely observed, mainly due to 3-dimensional growth forms replaced by 2-dimensional (i.e., encrusting) growth forms and by increased sedimentation, thus causing the structural homogenization of the cave community. The outermost and the deepest sectors of the cave experienced higher changes compared to the most confined, blind-ended, sector and to the shallowest depths, which resulted comparatively more stable. From a functional point of view, filter feeders always dominated the communities (even considering their shift from erect to encrusting morphologies) and the few observed changes were due to the reduction of photophilic autotrophs and active ciliate without lophophore and the increase of mixotrophic sponges. As local human pressures remained low through the period considered, the changes observed are likely to be mainly due to global warming, which caused increased sea surface water temperatures and repeated summer heat waves (reported in 1994, 2003, 2008, 2012, and 2015). Regular monitoring programs on marine caves should be promoted to create historical series of data precious to better understand magnitude and pattern of change in the long-term evolution, and to estimate the recovery time of such priority habitats. • We investigated changes in the last 26 years in a submerged marine cave. • Three main environmental gradients were analysed: confinement, depth, and light. • Gradients persisted and zones remained distinct through time. • We observed significant changes in the spatial organization of benthic assemblages. • Changes were likely due to global warming and repeated summer heat waves. [ABSTRACT FROM AUTHOR]

Abstract Marine coastal ecosystems are facing structural and functional changes due to the increasing human footprint worldwide, and the assessment of their long-term changes becomes particularly challenging. Measures of change can be done by comparing the observed ecosystem status to a purposely defined reference condition. In this paper, a geospatial modelling approach based on 2D mapping and morphodynamic data was used to predict the natural position of the upper limit (i.e., the landward continuous front) of Posidonia oceanica seagrass meadows settled on soft bottom. This predictive model, formerly developed at the regional spatial scale, was here applied for the first time at the Mediterranean spatial scale in eight coastal areas of Spain, France, Italy, and Greece showing different coastal morphologies and hydrodynamic characteristics, and affected by a number of natural and/or human local disturbances. The model was effective in measuring the regression (i.e., seaward withdrawal) of the meadow upper limit. In all the meadows investigated the upper limit was regressed, laying deeper than the reference condition, with the proportion of regression ranging from 17.7% to 98.9%. The highest values of regression were found in Spain and in France, and were consistent with the highest levels of fragmentation detected with map analysis and of coastal pressures. This geospatial modelling approach represents an effective tool to define the reference conditions when proper pristine areas or historical data are not available, thus allowing the assessment of long-time changes experienced by seagrass ecosystems due to human impacts. Highlights • Reference conditions are needed to measure long-term change in marine ecosystems. • We modelled the upper (landward) limit of seagrass meadows at Mediterranean scale. • All meadow upper limits appeared regressed and fragmented due to local disturbances. • Modelling represents an effective tool for evaluating seagrass meadow conditions. [ABSTRACT FROM AUTHOR]

Mapping marine coastal habitats provides an important aid to the long-term sustainability of urban development and the conservation of marine biodiversity. Historical maps, when available, may represent effective tools to evaluate quali-quantitative changes over time and to fix reference conditions. Scientific mapping of marine habitats is relatively recent, but the existence of fishery maps could compensate, after a critical evaluation, for the lack of more rigorous historical data. Coralligenous reefs are among the most important marine coastal habitats in the Mediterranean Sea, whose conservation requires, as a first step suggested by the recent European Directives, updated and detailed cartographies. This paper compares the recent cartographies (i.e. 2006 and 2009) on coralligenous reefs distribution in Liguria (NW Mediterranean Sea) with two historical fishery maps (i.e. dating back to the second half of the last century), on a GIS platform. Application of the Reliability Index (RI) showed that both historical sources can be trusted, with some caution. Fishery maps report few quantitative data (e.g. extent in hectares of coralligenous reefs) that are in little agreement with recent cartographies, mainly due to errors in positioning. In contrast, qualitative data (i.e. presence/absence in a specific site) represent the main information found in these maps and agree much more with the recent ones. It is more than likely that many of the rocky bottoms reported only in the fishery maps may host coralligenous reefs, which are apparently unknown in the recent cartographies. Latent information provided by this kind of historical maps may thus be useful for planning new surveys to sea-truthing on sites where no recent information is available and to promote the ecological studies required for their conservation. [ABSTRACT FROM AUTHOR]

A multidisciplinary approach was applied to investigate the role of abiotic constraints in the settlement of Posidonia oceanica on shallow rocks in two coastal areas of the Ligurian Sea (Italy, NW Mediterranean). Meadows developed very shallow upper limits, at 1.5 m depth in both areas, and with a distinctive morphology of stripes growing on rocky outcrops orthogonal to the coastline. Application of a predictive model to indicate the reference condition zone for the meadow upper limit, already validated on meadows developing on soft-bottoms, was not adequate for these rocky substrata as the meadow upper limits were found shallower than model predictions (>5 m depth). Geological and geomorphological characteristics of the rocky shores were analysed through geomechanic and petrographic analyses (i.e. thin sections, SEM analyses, rock hardness tests) whilst the shape and the features of the meadows (i.e. shoot density and maximum leaf length) were assessed through scuba diving surveys. Among the different lithotypes occurring at the sites in the alternating and interbedded outcrops, P. oceanica was passively selected (i.e. due to the seedlings survival and settlement there) on the strongest (i.e. less erodible) lithotypes, whilst the comparatively weaker and more erodible rocks remained unvegetated and covered by a layer of soft-sediments. P. oceanica , settling on specific rocky substrata with favourable lithological and geomechanical characteristics, is able to establish outside the theoretical reference zone predicted by the model for soft sediments due to greater attachment strength and possible resistance to hydrodynamic forces. Combining biological, ecological, mineralogical, geological and geomorphological approaches was effective for explaining the primary role of substratum nature in the spatial variability of seagrass meadows, with geomechanical and lithological characteristics of the rocks being equally important abiotic factors than sedimentological features. [ABSTRACT FROM AUTHOR]

Coral reefs are threatened by multiple global and local disturbances. The Maldives, already heavily hit by the 1998 mass bleaching event, are currently affected also by growing tourism and coastal development that may add to global impacts. Most of the studies investigating effects of local disturbances on coral reefs assessed the response of communities along a horizontal distance from the impact source. This study investigated the status of a Maldivian coral reef around an island where an international touristic airport has been recently (2009–2011) built, at different depths along the reef profile (5–20 m depth) and considering the change in the percentage of cover of five different non-taxonomic descriptors assessed through underwater visual surveys: hard corals, soft corals, other invertebrates, macroalgae and abiotic attributes. Eight reefs in areas not affected by any coastal development were used as controls and showed a reduction of hard coral cover and an increase of abiotic attributes (i.e. sand, rock, coral rubble) at the impacted reef. However, hard coral cover, the most widely used descriptor of coral reef health, was not sufficient on its own to detect subtle indirect effects that occurred down the reef profile. Selecting an array of descriptors and considering different depths, where corals may find a refuge from climate impacts, could guide the efforts of minimising local human pressures on coral reefs. [ABSTRACT FROM AUTHOR]

Sustained functioning of ecosystems is predicted to depend upon the maintenance of their biodiversity, structure and integrity. The large consensus achieved in this regard, however, faces to the objective difficulty of finding appropriate metrics to measure ecosystem functioning. Here, we aim at evaluating functional consequence of the phase shift occurring in meadows of the Mediterranean seagrass Posidonia oceanica , a priority habitat that is undergoing regression in many coastal areas due to multiple human pressures. Structural degradation of the P. oceanica ecosystem, consequent to increasing coastal exploitation and climate change, may result in the progressive replacement of this seagrass by opportunistic macrophytes, either native or alien. Reviewing published information and our personal records, we measured changes in biological habitat provisioning, species richness and biomass associated to each of the alternative states characterizing the phase shift. Then, ecosystem functioning was assessed by computing the exergy associated to each state, exergy being a state variable that measures the ecosystem capacity to produce work. Phase shift was consistently shown to imply loss in habitat provision, species richness, and biomass; structural and compositional loss was parallelled by a reduction of exergy content, thus providing for the first time an objective and integrative measure of the loss of ecosystem functioning following the degradation of healthy seagrass meadows. [ABSTRACT FROM AUTHOR]

Abstract: Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, forms large meadows from the sea surface down to 40 m. The depth of the lower limit of the meadows marks the boundary between the infralittoral and the circalittoral zone, and is said to be normally set by light attenuation underwater, while the role of water movement has been little explored. In this paper, the position (i.e. distance from the shoreline and depth) of P. oceanica meadow lower limits along the whole Ligurian coastline (about 350 km) was related to the annual storm wave base. This depth represents the limit of interaction between waves and seafloor and corresponds to L 0/2, where L 0 is the annual offshore wave length. In all meadows, the lower limit has never been found deeper than the annual storm wave base, and its depth (Zc ) showed related to L0 according to the equation Zc  = 0.32∙L 0 + 5.62. In the coastal tracts affected by the least intense waves, the reduction of water movement with depth represents the most important constraint to the seaward development of the meadow, whereas light availability plays a major role in meadows affected by the most intense waves. The present study represents the first attempt at understanding the role of hydrodynamic factors in setting the depth limit of seagrass meadows. If corroborated by future research at other sites, this will have important implications for both basic and applied science, as it would imply rethinking about the relative importance of water movement and light in seagrass depth distribution, and could allow for a better estimate of the extent of meadow regression in anthropized areas. [Copyright &y& Elsevier]

Abstract: While coastal management activities have long been known to exert a strong influence on the health of marine ecosystems, neither scientists nor administrators have realized that small interventions may lead to disproportionately larger impacts. This study investigated the broad and long-lasting environmental consequences of the construction of an ill-planned, although small (only 12 m long) jetty for pleasure crafts on the hydrodynamic conditions and on the meadow of the seagrass Posidonia oceanica of an embayed cove in the Ligurian Sea (NW Mediterranean). There, P. oceanica used to develop on a high (>1.5 m) matte (a lignified terrace causing seafloor elevation) in which the leaves reach the surface and form a compact natural barrier to waves in front of the beach. Such a so-called ‘fringing reef’ of P. oceanica is today recognized of high ecological value and specific conservation efforts are required. The construction of the jetty implied the cutting of the matte, which directly destroyed part of the fringing reef. In addition, meadow mapping and sedimentological analyses coupled with morphodynamic modelling showed that the ecosystem of the whole cove had been greatly altered by the jetty. We used the geometric planform approach, a proper tool in the study of headland-controlled embayment, both to characterise the present situation of Prelo cove and to simulate the original one, before the jetty was built. In the long term, such a small jetty completely altered the configuration and the hydrodynamic conditions of the whole cove, splitting the original pocket beach into two smaller ones and creating strong rip-currents flowing seaward along the jetty. These rip-currents enhanced erosion of residual shallow portions of the meadow and further modified the sedimentary fluxes in shallow waters. A century after the construction of the jetty, an irreversible environmental damage has occurred, as the slow growing rate of P. oceanica implies that the high matte terrace and the fringing reef will hardly form again, even after the removal of the jetty. The lesson learnt from this study is that even such small, and therefore reputed intrinsically ‘innocent’, interventions on the coastal zone require accurate planning based on interdisciplinary studies to understand and respect the delicate interplay among morphological, hydrodynamic and ecological components. [Copyright &y& Elsevier]

Abstract: Habitat fragmentation in meadows of Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, was investigated at a region-wide spatial scale using a synthetic ecological index, the Patchiness Index (PI). We tested the hypothesis that human impacts are the major factor responsible for habitat fragmentation in P. oceanica meadows contrasting fragmentation of meadows located in “anthropized” areas with that of meadows located in areas with low anthropization and considered as virtually “natural”. We also related fragmentation of meadow with the morphodynamic state of the submerged beach (i.e. distinctive types of beach produced by the topography, the wave climate and the sediment composition) in order to investigate the influence of one natural component on the seagrass meadow seascape. Results demonstrated that fragmentation in the P. oceanica meadows is strongly influenced by the human component, being lower in natural meadows than in anthropized ones, and that it is little influenced by the morphodynamic state of the coast. The use of landscape approaches to discriminate natural disturbance from human impacts that affect seagrass meadows is thus recommended for the proper management of coastal zones. [Copyright &y& Elsevier]