Seagrass collapse due to synergistic stressors is not anticipated by phenological changes

Resistance of Posidonia oceanica seedlings to warming: Investigating the importance of the lag-phase duration between two heat events to thermo-priming

The increase of marine heat waves (MHWs) occurrence is exacerbated in Mediterranean Sea and temperature resilience-enhancing strategies on key species, such as the seagrass Posidonia oceanica, need to be investigated. "Priming" describes a stimulus that prepares an organism for an improved response to upcoming environmental changes by triggering a memory that remains during a lag-phase. The aim of this study, conducted in Sardinia (Italy), was to investigate whether the development of thermo-primed P. oceanica seedlings is affected by a field simulated MHW depending on the duration of the lag-phase. After the thermo-priming stimulus, seedlings had a 0, 7 or 14 days lag-phase and after that, for each lag-phase group, half of the seedlings experienced a simulated MHW (the other half served as controls). Some other seedlings did not experience either the priming stimulus or the lag-phase. Results did not show any evidence of a memory triggered by the priming stimulus, but they highlighted the importance of an acclimation phase before the highest temperature: seedlings that experienced a gradual increase of temperature had a higher number of leaves and shorter leaf necrosis length compared to seedlings that had a lag-phase between two heat events. Regardless the priming stimulus, MHWs slowed down the development of the leaf and root length. Considering the increase of temperature fluctuations, testing different intensities of priming and different length of lag-phase is necessary to provide information about the adaptive success of the species.

The resilience of transplanted seagrass traits encourages detection of restoration success

Restoration of coastal ecosystems, particularly those dominated by seagrasses, has become a priority to recover the important ecosystem services they provide. However, assessing restoration outcomes as a success or failure remains still difficult, probably due to the unique features of seagrass species and the wide portfolio of practices used on transplanting actions. Here, several traits (maximum leaf length, number of leaves, leaf growth rate per shoot, and leaf elemental carbon and nitrogen contents) of transplanted seagrass Posidonia oceanica were compared to reference meadows in five sites of Western Mediterranean Sea in which restoration were completed in different times. Results have evidenced the resilience of transplanted P. oceanica shoots within a few years since restoration, as traits between treatments changed depending on the elapsed time since settlement. The highlighted stability of the restoration time effect suggests that the recovery of the plants is expected in four years after transplanting.

Effects of acidification on the biogeochemistry of unvegetated and seagrass marine sediments

Many studies addressed ocean acidification (OA) effects on marine life, whereas its effects on sedimentary organic matter (OM) have received less attention. We investigated differences in OM features in sediments from unvegetated and seagrass (Posidonia oceanica) beds in a shallow hydrothermal area (Aeolian Archipelago, Mediterranean Sea), under natural (8.1-8.0) and acidified (7.8-7.9) conditions. We show that a pH difference of -0.3 units have minor effects on OM features in unvegetated sediments, but relevant consequences within acidified seagrass meadows, where OM quantity and nutritional quality are lower than those under natural pH conditions. Effects of acidified conditions on OM biogeochemistry vary between unvegetated and seagrass sediments, with lower C degradation rates and longer C turnover time in the former than in the latter. We conclude that OA, although with effects not consistent between unvegetated and vegetated sediments, can affect OM quantity, composition, and degradation, thus having possible far-reaching consequences for benthic trophic webs.

Climate change and the presence of invasive species will threaten the persistence of the Mediterranean seagrass community

The Mediterranean Sea has been experiencing rapid increases in temperature and salinity triggering its tropicalization. Additionally, its connection with the Red Sea has been favouring the establishment of non-native species. In this study, we investigated the effects of predicted climate change and the introduction of invasive seagrass species (Halophila stipulacea) on the native Mediterranean seagrass community (Posidonia oceanica and Cymodocea nodosa) by applying a novel ecological and spatial model with different configurations and parameter settings based on a Cellular Automata (CA). The proposed models use a discrete (stepwise) representation of space and time by executing deterministic and probabilistic rules that develop complex dynamic processes. Model applications were run under two climate scenarios (RCP 2.6 and RCP 8.5) projected from 2020 to 2100 in four different regions within the Mediterranean. Results indicate that the slow-growing P. oceanica will be highly vulnerable to climate change, suffering vast declines in its abundance. However, the results also show that western and colder areas of the Mediterranean Sea might represent refuge areas for this species. Cymodocea nodosa has been reported to exhibit resilience to predicted climate scenarios; however, it has shown habitat regression in the warmest predicted regions in the easternmost part of the basin. Our models indicate that H. stipulacea will thrive under projected climate scenarios, facilitating its spread across the basin. Also, H. stipulacea grew at the expense of C. nodosa, limiting the distribution of the latter, and eventually displacing this native species. Additionally, simulations demonstrated that areas from which P. oceanica meadows disappear would be partially covered by C. nodosa and H. stipulacea. These outcomes project that the Mediterranean seagrass community will experience a transition from long-lived, large and slow-growing species to small and fast-growing species as climate change progresses.

Undisturbed Posidonia oceanica meadows maintain the epiphytic bacterial community in different environments

Seagrasses harbour different and rich epiphytic bacterial communities. These microbes may establish intimate and symbiotic relationships with the seagrass plants and change according to host species, environmental conditions, and/or ecophysiological status of their seagrass host. Although Posidonia oceanica is one of the most studied seagrasses in the world, and bacteria associated with seagrasses have been studied for over a decade, P. oceanica's microbiome remains hitherto little explored. Here, we applied 16S rRNA amplicon sequencing to explore the microbiome associated with the leaves of P. oceanica growing in two geomorphologically different meadows (e.g. depth, substrate, and turbidity) within the Limassol Bay (Cyprus). The morphometric (leaf area, meadow density) and biochemical (pigments, total phenols) descriptors highlighted the healthy conditions of both meadows. The leaf-associated bacterial communities showed similar structure and composition in the two sites; core microbiota members were dominated by bacteria belonging to the Thalassospiraceae, Microtrichaceae, Enterobacteriaceae, Saprospiraceae, and Hyphomonadaceae families. This analogy, even under different geomorphological conditions, suggest that in the absence of disturbances, P. oceanica maintains characteristic-associated bacterial communities. This study provides a baseline for the knowledge of the P. oceanica microbiome and further supports its use as a putative seagrass descriptor.

Sources of variability in seagrass fatty acid profiles and the need of identifying reliable warming descriptors

Global warming is expected to have inexorable and profound effects on marine ecosystems, particularly in foundation species such as seagrasses. Identifying responses to warming and comparing populations across natural temperature gradients can inform how future warming will impact the structure and function of ecosystems. Here, we investigated how thermal environment, intra-shoot and spatial variability modulate biochemical responses of the Mediterranean seagrass Posidonia oceanica. Through a space-for-time substitution study, Fatty acid (FA) profiles on the second and fifth leaf of the shoots were quantified at eight sites in Sardinia along a natural sea surface temperature (SST) summer gradient (about 4 °C). Higher mean SST were related to a decrease in the leaf total fatty acid content (LTFA), a reduction in polyunsaturated fatty acids (PUFA), omega-3/omega-6 PUFA and PUFA/saturated fatty acids (SFA) ratios and an increase in SFA, monounsaturated fatty acids and carbon elongation index (CEI, C18:2 n-6/C16:2 n-6) ratio. Results also revealed that FA profiles were strongly influenced by leaf age, independently of SST and spatial variability within sites. Overall, this study evidenced that the sensitive response of P. oceanica FA profiles to intra-shoot and spatial variability must not be overlooked when considering their response to temperature changes.

Gene co-expression network analysis for the selection of candidate early warning indicators of heat and nutrient stress in Posidonia oceanica

The continuous worldwide seagrasses decline calls for immediate actions in order to preserve this precious marine ecosystem. The main stressors that have been linked with decline in seagrasses are 1) the increasing ocean temperature due to climate change and 2) the continuous inputs of nutrients (eutrophication) associated with coastal human activities. To avoid the loss of seagrass populations, an "early warning" system is needed. We used Weighed Gene Co-expression Network Analysis (WGCNA), a systems biology approach, to identify potential candidate genes that can provide an early warning signal of stress in the Mediterranean iconic seagrass Posidonia oceanica, anticipating plant mortality. Plants were collected from both eutrophic (EU) and oligotrophic (OL) environments and were exposed to thermal and nutrient stress in a dedicated mesocosm. By correlating the whole-genome gene expression after 2-weeks exposure with the shoot survival percentage after 5-weeks exposure to stressors, we were able to identify several transcripts that indicated an early activation of several biological processes (BP) including: protein metabolic process, RNA metabolic process, organonitrogen compound biosynthetic process, catabolic process and response to stimulus, which were shared among OL and EU plants and among leaf and shoot apical meristem (SAM), in response to excessive heat and nutrients. Our results suggest a more dynamic and specific response of the SAM compared to the leaf, especially the SAM from plants coming from a stressful environment appeared more dynamic than the SAM from a pristine environment. A vast list of potential molecular markers is also provided that can be used as targets to assess field samples.

A Tight Interaction between the Native Seagrass Cymodocea nodosa and the Exotic Halophila stipulacea in the Aegean Sea Highlights Seagrass Holobiont Variations

Seagrasses harbour bacterial communities with which they constitute a functional unit called holobiont that responds as a whole to environmental changes. Epiphytic bacterial communities rapidly respond to both biotic and abiotic factors, potentially contributing to the host fitness. The Lessepsian migrant Halophila stipulacea has a high phenotypical plasticity and harbours a highly diverse epiphytic bacterial community, which could support its invasiveness in the Mediterranean Sea. The current study aimed to evaluate the Halophila/Cymodocea competition in the Aegean Sea by analysing each of the two seagrasses in a meadow zone where these intermingled, as well as in their monospecific zones, at two depths. Differences in holobionts were evaluated using seagrass descriptors (morphometric, biochemical, elemental, and isotopic composition) to assess host changes, and 16S rRNA gene to identify bacterial community structure and composition. An Indicator Species Index was used to identify bacteria significantly associated with each host. In mixed meadows, native C. nodosa was shown to be affected by the presence of exotic H. stipulacea, in terms of both plant descriptors and bacterial communities, while H. stipulacea responded only to environmental factors rather than C. nodosa proximity. This study provided evidence of the competitive advantage of H. stipulacea on C. nodosa in the Aegean Sea and suggests the possible use of associated bacterial communities as an ecological seagrass descriptor.

Collating evidence on the restoration efforts of the seagrass Posidonia oceanica: current knowledge and gaps

Seagrass meadows are important shallow coastal ecosystems due to their contribution to enhancing biodiversity, nutrient cycling, carbon burial, and sediment stabilisation, but the maintenance of their integrity has been threatened by several anthropogenic disturbances. Active restoration is considered a reliable strategy to enhance recovery of seagrass ecosystems, and decision making for correct seagrass restoration management requires relying on valuable information regarding the effectiveness of past restoration actions and experimental efforts. Previous experimental efforts and human-mediated active restoration actions of the slow growing seagrass Posidonia oceanica have been collated here by combining a literature systematic review and questionnaires consulting seagrass ecology experts. Overall, the poor consistency of the available information on P. oceanica restoration may be due to the wide portfolio of practices and methodologies used in different conditions, that supports the need of further field manipulative experiments in various environmental contexts to fill the identified knowledge gaps. The current situation requires an international, collaborative effort from scientists and stakeholders to jointly design the future strategy forward in identifying the best practices that lead to efficient restorations of P. oceanica habitat and functioning.

Plastic pollution of four understudied marine ecosystems: a review of mangroves, seagrass meadows, the Arctic Ocean and the deep seafloor

Plastic pollution is now a worldwide phenomenon affecting all marine ecosystems, but some ecosystems and regions remain understudied. Here, we review the presence and impacts of macroplastics and microplastics for four such ecosystems: mangroves, seagrass meadows, the Arctic Ocean and the deep seafloor. Plastic production has grown steadily, and thus the impact on species and ecosystems has increased, too. The accumulated evidence also indicates that plastic pollution is an additional and increasing stressor to these already ecosystems and many of the species living in them. However, laboratory or field studies, which provide strong correlational or experimental evidence of ecological harm due to plastic pollution remain scarce or absent for these ecosystems. Based on these findings, we give some research recommendations for the future.

Local environment modulates whole-transcriptome expression in the seagrass Posidonia oceanica under warming and nutrients excess

The intensification of anomalous events of seawater warming and the co-occurrence with local anthropogenic stressors are threatening coastal marine habitats, including seagrasses, which form extensive underwater meadows. Eutrophication highly affects coastal environments, potentially summing up to the widespread effects of global climate changes. In the present study, we investigated for the first time in seagrasses, the transcriptional response of different plant organs (i.e., leaf and shoot apical meristem, SAM) of the Mediterranean seagrass Posidonia oceanica growing in environments with a different history of nutrient enrichment. To this end, a mesocosm experiment exposing plants to single (nutrient enrichment or temperature increase) and multiple stressors (nutrient enrichment plus temperature increase), was performed. Results revealed a differential transcriptome regulation of plants under single and multiple stressors, showing an organ-specific sensitivity depending on plants' origin. While leaf tissues were more responsive to nutrient stress, SAM revealed a higher sensitivity to temperature treatments, especially in plants already impacted in their native environment. The exposure to stress conditions induced the modulation of different biological processes. Plants living in an oligotrophic environment were more responsive to nutrients compared to plants from a eutrophic environment. Evidences that epigenetic mechanisms were involved in the regulation of transcriptional reprogramming were also observed in both plants' organs. These results represent a further step in the comprehension of seagrass response to abiotic stressors pointing out the importance of local pressures in a global warming scenario.

Photo-acclimatory thresholds anticipate sudden shifts in seagrass ecosystem state under reduced light conditions

Seagrass ecosystems usually respond in a nonlinear fashion to increasing pressures and environmental changes. Feedback mechanisms operating at the ecosystem level and involving multiple interactions among the seagrass meadow, its associated community and the physical environment are known to play a major role in such nonlinear responses. Phenotypic plasticity may also be important for buffering these ecological thresholds (i.e., regime shifts) as many physiological processes show nonlinear responses to gradual environmental changes, conferring the appearance of resistance before the effects at the organism and population levels are visible. However, the potential involvement of plant plasticity in driving catastrophic shifts in seagrass ecosystems has not yet been assessed. In this study, we conducted a manipulative 6-month light-gradient experiment in the field to capture nonlinearities of the physiological and population responses of the seagrass Cymodocea nodosa to gradual light reduction. The aim was to explore if and how the photo-acclimatory responses of shaded plants are translated to the population level and, hence, to the ecosystem level. Results showed that the seagrass population was rather stable under increasing shading levels through the activation of multilevel photo-acclimative responses, which are initiated with light reduction and modulated in proportion to shading intensity. The activation of photo-physiological and metabolic compensatory responses allowed shaded plants to sustain nearly constant plant productivity (metabolic carbon balance) along a range of shading levels before losing linearity and starting to decline. The species then activated plant- and meadow-scale photo-acclimative responses and drew on its energy reserves (rhizome carbohydrates) to confer additional population resilience. However, when the integration of all these buffering mechanisms failed to counterbalance the effects of extreme light limitation, the population collapsed, giving place to a phase shift from vegetated to bare sediments with catastrophic ecosystem outcomes. Our findings evidence that ecological thresholds in seagrass ecosystems under light limitation can be explained by the role of species' compensatory responses in modulating population-level responses. The thresholds of these plastic responses anticipate the sudden loss of seagrass meadows with the potential to be used as early warning indicators signalling the imminent collapse of the ecosystem, which is of great value for the real-world management of seagrass ecosystems.

Using Unoccupied Aerial Vehicles (UAVs) to Map Seagrass Cover from Sentinel-2 Imagery

Seagrass habitats are ecologically valuable and play an important role in sequestering and storing carbon. There is, thus, a need to estimate seagrass percentage cover in diverse environments in support of climate change mitigation, marine spatial planning and coastal zone management. In situ approaches are accurate but time-consuming, expensive and may not represent the larger spatial units collected by satellite imaging. Hence, there is a need for a consistent methodology that uses accurate point-based field surveys to deliver high-quality mapping of percentage seagrass cover at large spatial scales. Here, we develop a three-step approach that combines in situ (quadrats), aerial (unoccupied aerial vehicle—UAV) and satellite data to map percentage seagrass cover at Turneffe Atoll, Belize, the largest atoll in the northern hemisphere. First, the optical bands of four UAV images were used to calculate seagrass cover, in combination with in situ data. The seagrass cover calculated from the UAV was then used to develop training and validation datasets to estimate seagrass cover in Sentinel-2 pixels. Next, non-seagrass areas were identified in the Sentinel-2 data and removed by object-based classification, followed by a pixel-based regression to calculate seagrass percentage cover. Using this approach, percentage seagrass cover was mapped using UAVs (R2 = 0.91 between observed and mapped distributions) and using Sentinel-2 data (R2 = 0.73). This work provides the first openly available and explorable map of seagrass percentage cover across Turneffe Atoll, where we estimate approximately 242 km2 of seagrass above 10% cover is located. We estimate that this approach offers 30 times more data for training satellite data than traditional methods, therefore presenting a substantial reduction in cost-per-point for data. Furthermore, the increase in data helps deliver a high-quality seagrass cover map, suitable for resolving trends of deteriorating, stable or recovering seagrass environments at 10 m2 resolution to underpin evidence-based management and conservation of seagrass.

Localized effects of offshore aquaculture on water quality in a tropical sea

Aquaculture production has increased steadily in many tropical countries over the past few decades, although impact assessments have been frequently neglected. We investigated the impacts of an offshore barramundi fish farm on water quality in the southern-central Red Sea, a traditionally understudied tropical, oligotrophic, and semi-enclosed basin. Inorganic nutrients, particulate matter, chlorophyll-a, and heterotrophic bacteria were measured periodically over 8 months around the farm. Water down-current from the farm had, on average, more heterotrophic bacteria and chlorophyll-a than up-current (11% and 34% higher, respectively). Ratios of dissolved inorganic nitrogen:phosphorus down-current from the farm were lower than ratios up-current (mean 9.8 vs 16.0, respectively). Phosphate, inorganic nitrogen, and particulate matter showed patterns of enrichment associated with the farm after a fish feeding event. Strategies such as feed optimization and considering hydrodynamics in site selection may improve water quality for future fish farms in Saudi Arabia and other tropical countries.

Seagrasses in an era of ocean warming: a review

Seagrasses are valuable sources of food and habitat for marine life and are one of Earth's most efficient carbon sinks. However, they are facing a global decline due to ocean warming and eutrophication. In the last decade, with the advent of new technology and molecular advances, there has been a dramatic increase in the number of studies focusing on the effects of ocean warming on seagrasses. Here, we provide a comprehensive review of the future of seagrasses in an era of ocean warming. We have gathered information from published studies to identify potential commonalities in the effects of warming and the responses of seagrasses across four distinct levels: molecular, biochemical/physiological, morphological/population, and ecosystem/planetary. To date, we know that although warming strongly affects seagrasses at all four levels, seagrass responses diverge amongst species, populations, and over depths. Furthermore, warming alters seagrass distribution causing massive die-offs in some seagrass populations, whilst also causing tropicalization and migration of temperate species. In this review, we evaluate the combined effects of ocean warming with other environmental stressors and emphasize the need for multiple-stressor studies to provide a deeper understanding of seagrass resilience. We conclude by discussing the most significant knowledge gaps and future directions for seagrass research.

Phenotypic plasticity under rapid global changes: The intrinsic force for future seagrasses survival

Coastal oceans are particularly affected by rapid and extreme environmental changes with dramatic consequences for the entire ecosystem. Seagrasses are key ecosystem engineering or foundation species supporting diverse and productive ecosystems along the coastline that are particularly susceptible to fast environmental changes. In this context, the analysis of phenotypic plasticity could reveal important insights into seagrasses persistence, as it represents an individual property that allows species' phenotypes to accommodate and react to fast environmental changes and stress. Many studies have provided different definitions of plasticity and related processes (acclimation and adaptation) resulting in a variety of associated terminology. Here, we review different ways to define phenotypic plasticity with particular reference to seagrass responses to single and multiple stressors. We relate plasticity to the shape of reaction norms, resulting from genotype by environment interactions, and examine its role in the presence of environmental shifts. The potential role of genetic and epigenetic changes in underlying seagrasses plasticity in face of environmental changes is also discussed. Different approaches aimed to assess local acclimation and adaptation in seagrasses are explored, explaining strengths and weaknesses based on the main results obtained from the most recent literature. We conclude that the implemented experimental approaches, whether performed with controlled or field experiments, provide new insights to explore the basis of plasticity in seagrasses. However, an improvement of molecular analysis and the application of multi-factorial experiments are required to better explore genetic and epigenetic adjustments to rapid environmental shifts. These considerations revealed the potential for selecting the best phenotypes to promote assisted evolution with fundamental implications on restoration and preservation efforts.

New Phenolic Compounds in Posidonia oceanica Seagrass: A Comprehensive Array Using High Resolution Mass Spectrometry

The studies on the Posidonia oceanica Delile (P. oceanica) phenolic composition have been focused on the foliar tissues and have often neglected the phenolic compounds in rhizomes or roots alike. With the current improvements in high resolution mass spectrometry (HRMS) analyzers, such as the Orbitrap MS, there is a new opportunity to more deeply study P. oceanica. One of the benefits is the possibility of conducting an exhaustive phenolic monitoring, which is crucial in the search for new stressor-specific biomarkers of coastal deterioration. For this purpose, the different tissues (leaf, rhizome, and root) of P. oceanica seagrass from several marine sampling areas were analyzed through target, suspected, and non-target screenings. This paper brings a fast and tissues-specific extraction, as well as a detection method of phenolic compounds applying for the first time the potential of HRMS (Exactive Orbitrap) in P. oceanica samples. As a result, 42 phenolic compounds were satisfactorily detected, of which, to our knowledge, 24 were not previously reported in P. oceanica, such as naringenin, naringenin chalcone and pinocembrin, among others. Information here reported could be used for the evaluation of new stressor-specific biomarkers of coastal deterioration in the Mediterranean waters. Furthermore, the followed extraction and analytical method could be considered as a reference protocol in other studies on marine seagrasses due to the exhaustive search and satisfactory results.

Extreme events and conservation of subtidal habitats: Effects of a rainfall flood on coralligenous reefs

The increase of the intensity and frequency of rainfall-dominated flood is considered a main effect of climate change. The present study evaluated the effect of a rainfall flood event on coralligenous reefs. The flooded site was compared to three control sites using a Before/After-Control/Impact (BACI) design. Sites were sampled using the STAR (STAndaRdized coralligenous evaluation procedure) approach and three ecological indices (ESCA, COARSE and ISLA) were calculated. At the disturbed site the number of species per sample, beta diversity, sensitivity levels of assemblages and the values of the three indices were lower after the flooding event, while the same variables did not decrease at the control sites. Algal turf and Dictyotales increased at the disturbed sites after the flood event, while Udoteaceae, erect sponges, bryozoans and Corallium rubrum decreased. This study provides evidence for identifying floods as a further cause of degradation for the coralligenous reef assemblages.

Semiautomated Mapping of Benthic Habitats and Seagrass Species Using a Convolutional Neural Network Framework in Shallow Water Environments

Benthic habitats are structurally complex and ecologically diverse ecosystems that are severely vulnerable to human stressors. Consequently, marine habitats must be mapped and monitored to provide the information necessary to understand ecological processes and lead management actions. In this study, we propose a semiautomated framework for the detection and mapping of benthic habitats and seagrass species using convolutional neural networks (CNNs). Benthic habitat field data from a geo-located towed camera and high-resolution satellite images were integrated to evaluate the proposed framework. Features extracted from pre-trained CNNs and a “bagging of features” (BOF) algorithm was used for benthic habitat and seagrass species detection. Furthermore, the resultant correctly detected images were used as ground truth samples for training and validating CNNs with simple architectures. These CNNs were evaluated for their accuracy in benthic habitat and seagrass species mapping using high-resolution satellite images. Two study areas, Shiraho and Fukido (located on Ishigaki Island, Japan), were used to evaluate the proposed model because seven benthic habitats were classified in the Shiraho area and four seagrass species were mapped in Fukido cove. Analysis showed that the overall accuracy of benthic habitat detection in Shiraho and seagrass species detection in Fukido was 91.5% (7 classes) and 90.4% (4 species), respectively, while the overall accuracy of benthic habitat and seagrass mapping in Shiraho and Fukido was 89.9% and 91.2%, respectively.

Impairment of microbial and meiofaunal ecosystem functions linked to algal forest loss

Habitat loss is jeopardizing marine biodiversity. In the Mediterranean Sea, the algal forests of Cystoseira spp. form one of the most complex, productive and vulnerable shallow-water habitats. These forests are rapidly regressing with negative impact on the associated biodiversity, and potential consequences in terms of ecosystem functioning. Here, by comparing healthy Cystoseira forests and barren grounds (i.e., habitats where the macroalgal forests disappeared), we assessed the effects of habitat loss on meiofaunal and nematode biodiversity, and on some ecosystem functions (here measured in terms of prokaryotic and meiofaunal biomass). Overall, our results suggest that the loss of Cystoseira forests and the consequent barren formation is associated with the loss of meiofaunal higher taxa and a decrease of nematode biodiversity, leading to the collapse of the microbial and meiofaunal variables of ecosystem functions. We conclude that, given the very limited resilience of these ecosystems, active restoration of these vulnerable habitats is needed, in order to recover their biodiversity, ecosystem functions and associated services.

Heat wave intensity can vary the cumulative effects of multiple environmental stressors on Posidonia oceanica seedlings

Climate change is introducing new stressors into already stressed ecosystems. Among these, extreme events such as heat waves play a crucial role in determining the structure of ecosystems. We tested single and combined effects of overgrazing, burial and heat waves on the seedlings of the habitat-forming species Posidonia oceanica. At current heat wave temperatures, overgrazing in isolation had more deleterious effects than seed burial, and effects were synergistic and additive when both factors co-occurred. The combined effect of overgrazing and seed burial with current heat waves could hamper P. oceanica seedling development, with similar or even higher levels than the sole effect of heat waves in the near future (29 °C). The effects of overgrazing and seed burial are expected to be overridden if heat waves temperatures exceed 29 °C. These results suggest that co-occurring environmental stressors, in combination with current heat waves, could compromise the sexual recruitment of this seagrass.

Assessment of the ecological structure of Posidonia oceanica (L.) Delile on the northern coast of Lazio, Italy (central Tyrrhenian, Mediterranean)

The ecological structure of Posidonia oceanica (L.) Delile meadows was evaluated on the northern coast of Lazio, Italy (central Tyrrhenian, Mediterranean sea). This is an infra-littoral zone with a wide range of anthropogenic activities and high geo-morphological variability, which reflects heterogeneity in shoot density, leaf morphology and biomass in fragmented patches. Genetic variability in populations corresponds to the formation of 3 sub-clusters, in the diverse impacted zones (north, centre and south), being correlated to the geographical distance between sites. AMOVA estimated a high genetic variation showing 43.05% individual differences within populations with a marked differentiation among the populations (56.9%) indicated by Fst value (0.57). These results revealed the role of the genetic structure of seagrasses for determining selectivity of fragmented habitat, in response to natural drivers. They showed that site-specific self-recruitment is related to biodiversity capacity and to the geo-morphological characteristic of the coast.

Green tides select for fast expanding Ulva strains

Green tides, the phenomenon whereby large volume of marine environment is taken over by the sea lettuce Ulva spp, are a seasonal occurrence thought to be caused mainly by anthropogenic eutrophication. The aggravation of green tide occurrence since the 1970s could however be due to the amplification of fast-growing strains within these areas. In this study, we compared the growth and metabolite content of 28 green tide Ulva strains against 100 non-green tide strains, under conditions close to those encountered in green tides areas. The aim was to determine whether the presence of specific characteristics intrinsic to green tide strains could in itself be a major factor for their reoccurrence. We confirmed that green tide strains have specific characteristics, with faster tissue expansion, higher protein and pigments, and lower starch content compared to non-green tide ones, thus highlighting a genetic component specific to green tide strains. Dry biomass accumulation, however, was not different between the two types of Ulva strains. Hence, we hypothesise that the selective pressure in green tide areas leads to the amplification of Ulva genotypes best adapted for this environment. Such selection of fast-growing strains would indicate that green tides are likely to become more prevalent and of higher magnitude over the coming years.

Halophila stipulacea descriptors in the native area (Red Sea): A baseline for future comparisons with native and non-native populations

Halophila stipulacea is a small tropical seagrass species native to the Red Sea. Due to its invasive character, there is growing interest in understanding its ability to thrive in a broad range of ecological niches. We studied temporal (February 2014 and July 2014), depth (5, 9, 18 m) and spatial (NB and SB) related dynamics of H. stipulacea meadows in the northern Gulf of Aqaba. We evaluated changes in density, morphometry, biomass, and biochemical parameters alongside the reproductive effort. In both sites, maximal growth and vegetative performance occurred in the summer with a marked increase of 35% in shoot density and 18% in biomass; PAR reduction with season and depth induced a significant increase of 28% in leaf area. Sexual reproduction efforts were only observed in July, and the density of plants carrying male or female flowers decreased significantly with depth. The favorable growth responses of H. stipulacea plants observed in the N-enriched NB site suggests their capacity to acclimate to human-disturbed nearshore environments.

Ecological Function of Phenolic Compounds from Mediterranean Fucoid Algae and Seagrasses: An Overview on the Genus Cystoseira sensu lato and Posidonia oceanica (L.) Delile

Biodiversity is undergoing rapid and worrying changes, partially driven by anthropogenic activities. Human impacts and climate change (e.g., increasing temperature and ocean acidification), which act at different spatial scales, represent the most serious threats to biodiversity and ecosystem structure and function. In the Mediterranean Sea, complex systems such as fucoid algae and seagrasses, characterized by a high associated biodiversity, are regularly exposed to natural and anthropogenic pressures. These systems, particularly sensitive to a variety of stressors, evolved several physiological and biochemical traits as a response to the different pressures which they are subjected to. For instance, they produce a huge quantity of secondary metabolites such as phenolic compounds, to adapt to different environmental stressors and to defend themselves from biological pressures. These natural products are receiving increasing attention due to their possible applications in a wide range of industrial sectors. In this paper we provide an overview on the ecological role of phenolic compounds from the genus Cystoseira sensu lato and Posidonia oceanica (L.) Delile, also highlighting their potential use as ecological biomarkers.

The negative effects of short-term extreme thermal events on the seagrass Posidonia oceanica are exacerbated by ammonium additions

Global warming is increasingly affecting our biosphere. However, in addition to global warming, a panoply of local stressors caused by human activities is having a profound impact on our environment. The risk that these local stressors could modify the response of organisms to global warming has attracted interest and fostered research on their combined effect, especially with a view to identifying potential synergies. In coastal areas, where human activities are heavily concentrated, this scenario is particularly worrying, especially for foundation species such as seagrasses. In this study we explore these potential interactions in the seagrass Posidonia oceanica. This species is endemic to the Mediterranean Sea. It is well known that the Mediterranean is already experiencing the effects of global warming, especially in the form of heat waves, whose frequency and intensity are expected to increase in the coming decades. Moreover, this species is especially sensitive to stress and plays a key role as a foundation species. The aim of this work is thus to evaluate plant responses (in terms of photosynthetic efficiency and growth) to the combined effects of short-term temperature increases and ammonium additions.To achieve this, we conducted a mesocosm experiment in which plants were exposed to three thermal treatments (20°C, 30°C and 35°C) and three ammonium concentrations (ambient, 30 μM and 120 μM) in a full factorial experiment. We assessed plant performance by measuring chlorophyll fluorescence variables (maximum quantum yield (Fv/Fm), effective quantum yield of photosystem II (ΔF/Fm'), maximum electron transport rate (ETRmax) and non-photochemical quenching (NPQ)), shoot growth rate and leaf necrosis incidence. At ambient ammonium concentrations, P. oceanica tolerates short-term temperature increases up to 30°C. However, at 35°C, the plant loses functionality as indicated by a decrease in photosynthetic performance, an inhibition of plant growth and an increase of the necrosis incidence in leaves. On the other hand, ammonium additions at control temperatures showed only a minor effect on seagrass performance. However, the combined effects of warming and ammonium were much worse than those of each stressor in isolation, given that photosynthetic parameters and, above all, leaf growth were affected. This serves as a warning that the impact of global warming could be even worse than expected (based on temperature-only approaches) in environments that are already subject to eutrophication, especially in persistent seagrass species living in oligotrophic environments.

Interactive effects of global warming and eutrophication on a fast-growing Mediterranean seagrass

Coastal ecosystems, such as seagrasses, are subjected to local (e.g. eutrophication) and global (e.g. warming) stressors. While the separate effects of warming and eutrophication on seagrasses are relatively well known, their joint effects remain largely unstudied. In order to fill this gap, and using Cymodocea nodosa as a model species, we assessed the joint effects of warming (three temperatures, 20 °C, 30 °C and 35 °C) with two potential outcomes of eutrophication: (i) increase in nutrients concentration in the water column (30 and 300 μM), and (ii) organic enrichment in the sediment). Our results confirm that temperature in isolation clearly affects plant performance; while plants exposed to 30 °C performed better than control plants, plants exposed to 35 °C showed clear symptoms of deterioration (e.g. decline of photosynthetic capacity, increase of incidence of necrotic tissue). Plants were unaffected by high ammonium concentrations; however, organic enrichment of sediment had deleterious effects on plant function (photosynthesis, growth, demographic balance). Interestingly, these negative effects were exacerbated by increased temperature. Our findings indicate that in addition to the possibility of the persistence of C. nodosa being directly jeopardized by temperature increase, the joint effects of warming and eutrophication may further curtail its survival. This should be taken into consideration in both predictions of climate change consequences and in local planning.

Investigating cellular stress response to heat stress in the seagrass Posidonia oceanica in a global change scenario

Posidonia oceanica meadows are facing global threats mainly due to episodic heat waves. In a mesocosm experiment, we aimed at disentangling the molecular response of P. oceanica under increasing temperature (20 °C-32 °C). The experiment was carried out in spring, when heat waves can potentially occur and plants are putatively more sensitive to heat stress, since they are deprived in carbohydrates reserves after the cold winter months. We aimed to identify the activation of different phases of the cellular stress response (CSR) reaction and the responsive genes activated or repressed in heated plants. A molecular traffic light was proposed as a response model including green (protein folding and membrane protection), yellow (ubiquitination and proteolysis) and red (DNA repair and apoptosis) categories. Additionally, we estimated phenological trait variations to complement the information obtained from the molecular proxies of stress. Despite reduced leaf growth rate, heated plants did not exhibit signs of irreversible damage, probably underlying species pre-adaptation to warm and fluctuating regimes. Gene expression analyses revealed that molecular chaperoning, DNA repair and apoptosis inhibition processes related genes were the ones that mostly responded to high thermal stress and will be target of further investigation and in situ proofing for assessing their use as indicators of P. oceanica performance under sub-lethal heat stress.

Seagrass recovery after fish farm relocation in the eastern Mediterranean

Finfish aquaculture has damaged seagrass meadows worldwide as wastes from the farms can kill these habitat-forming plants. In Cyprus, the Mediterranean endemic Posidonia oceanica is at its upper thermal limits yet forms extensive meadows all around the island. Understanding this under-studied isolated population may be important for the long-term survival of the species given that the region is warming rapidly. When fish farming began around Cyprus in the mid-nineties, cages were moored above seagrass beds, but as production expanded they were moved into deeper water further away from the meadows. Here, we monitored the deepest edge of meadows near fish farms that had been moved into deeper waters as well as at a decommissioned farm site. Four P. oceanica monitoring systems were set up using methods developed by the Posidonia Monitoring Network. Seagrass % coverage, shoot density, % of plagiotropic rhizomes, shoot exposure, leaf morphometry, and sediment organic matter content and grain size were monitored at 11 fixed plots within each system, in 2012-2014 and in 2017. Expansion at the lower depth limit of seagrass meadows was recorded at all monitoring sites. Most other P. oceanica descriptors either did not change significantly or declined. Declines were most pronounced at a site that was far from mariculture activities but close to other anthropogenic pressures. The most important predictor affecting P. oceanica was depth. Monitoring using fixed plots allowed direct comparisons of descriptors over time, removes patchiness and intra-meadow variability increasing our understanding of seagrass dynamics and ecosystem integrity. It seems that moving fish farms away from P. oceanica has helped ensure meadow recovery at the deepest margins of their distribution, an important success story given that these meadows are at the upper thermal limits of the species.

A tale of two algal blooms: Negative and predictable effects of two common bloom-forming macroalgae on seagrass and epiphytes

Recent evidence suggests macroalgal blooms may play a role in the worldwide decline in seagrass, but the shape of the functional relationship between seagrass health and dominant bloom-forming macroalgae is poorly characterized. We tested whether the impact of varying abundances of two cosmopolitan bloom-forming macroalgal genera caused linear/quasi-linear or sudden threshold changes in measures of eelgrass, Zostera marina, meadow health. We conducted two caging experiments in a shallow Z. marina bed (∼1 m depth) in Bodega Harbor, California, USA where we maintained six densities within the range of natural abundances of macroalgae, Ulva (0-4.0 kg m^-2) and Gracilariopsis (0-2.0 kg m^-2), as well as uncaged controls over a 10-week period. Shoot density, blade growth, and epiphyte load were measured every two weeks and algal treatments reset. We did not find support for threshold transitions between algal abundance and measures of seagrass bed health using sigmoidal and broken-stick regression analyses for each data set; these models are commonly used to identify threshold patterns in ecological shifts. Instead, final measurements of shoot density and epiphyte load were best modelled as linear or slightly non-linear declines with increasing Ulva abundance. A negative linear relationship also existed between shoot density and Gracilariopsis abundance and a trend towards linear negative effects on epiphyte load. The similar shape of these functional relationships across different types of algae suggests the relationship may be generalizable. At algal abundances that are commonly observed, we found smooth and predictable negative impacts to Z. marina by decline in shoot density and potential impacts to food webs by loss of epiphytes rather than sudden threshold shifts or "ecological surprises". Our work contrasts with the growing body of literature suggesting highly non-linear shifts in response to human impact; thus, it is important to broaden understanding of shifts to more than just pattern but to the processes that drive different patterns of shifts.

Molecular level responses to chronic versus pulse nutrient loading in the seagrass Posidonia oceanica undergoing herbivore pressure

Seagrasses are key marine foundation species, currently declining due to the compounded action of global and regional anthropogenic stressors. Eutrophication has been associated with seagrass decline, while grazing has been traditionally considered to be a natural disturbance with a relatively low impact on seagrasses. In the recent years, this assumption has been revisited. Here, by means of a 16-month field-experiment, we investigated the molecular mechanisms driving the long-term response of Posidonia oceanica to the combination of nutrient enrichment, either as a chronic (press) or pulse disturbance, and herbivory. Changes in expression levels of 19 target genes involved in key steps of photosynthesis, nutrient assimilation, chlorophyll metabolism, oxidative-stress response and plant defense were evaluated through reverse transcription-quantitative polymerase chain reaction (RT-qPCR). High herbivore pressure affected the molecular response of P. oceanica more dramatically than did enhanced nutrient levels, altering the expression of genes involved in plant tolerance and resistance traits, such as photosynthesis and defense mechanisms. Genes involved in carbon fixation and N assimilation modulated the response of plants to high nutrient levels. Availability of resources seems to modify P. oceanica response to herbivory, where the upregulation of a nitrate transporter gene was accompanied by the decline in the expression of nitrate reductase in the leaves, suggesting a change in plant-nutrient allocation strategy. Finally, press and pulse fertilizations altered nitrate uptake and reduction-related genes in opposite ways, suggesting that taking into account the temporal regime of nutrient loading is important to assess the physiological response of seagrasses to eutrophication.