Seagrass Posidonia Oceanica Research Articles

In situ experiments on the effect of low pH on the ultrastructure of the seagrasses Cymodocea nodosa and Posidonia oceanica

The present study investigates the impacts of low pH on the cell structure of the seagrasses Posidonia oceanica (L.) Delile and Cymodocea nodosa (Ucria) Ascherson. The study was applied with in situ experiments at the Castello Aragonese of Ischia (Naples, Italy), where shallow submarine vents, lowering the pH, can be used as natural laboratories. Shoots of the seagrasses were transferred from the control area (pH 8.1) to the two venting areas (pH 7.8 and 6.8) for different times. Epidermal cells of young leaves were examined using transmission electron microscopy (TEM) and tubulin immunofluorescence. After one week at pH 7.8, the cell structure of Posidonia oceanica was normal, while in Cymodocea nodosa microtubule (MT) network and cell structure were affected. In addition, in C. nodosa, ultrastructural analysis revealed a gradual degradation of the nuclei, a disorganization of the chloroplasts, and an increase in the number of mitochondria and dictyosomes. The exposure of both plants for 3 weeks at pH 6.8 resulted in the aggregation and finally in the dilation of the endoplasmic reticulum (ER) membranes. Tubulin immunofluorescence revealed that after three weeks, the MT cytoskeleton of both plants was severely affected. All these alterations can be considered as indications of an apoptotic like programmed cell death (AL-PCD) which may be executed in order to regulate stress response.

The Natural Capital Value of the Seagrass Posidonia oceanica in the North-Western Mediterranean

Posidonia oceanica is an endemic Mediterranean seagrass used as a ‘biological quality element’ in monitoring programmes of the EU Water Framework Directive, providing information about coastal ecosystems status. The regression of P. oceanica meadows caused a growing interest among policy makers to assess the value of seagrasses and to increase their protection. An evaluation of P. oceanica meadows located in the Ligurian-Provençal basin (NW Mediterranean) through a biophysical approach is here developed. Six meadows located in Liguria (Italy) and Corsica (France) were investigated by applying the emergy analysis to assess the natural capital (NC) stocked by leaves and rhizomes components. Results highlighted the importance of carrying out an analysis of the variations in the NC value in both components: rhizomes defined the growth stage and the capacity to store NC over time; leaves provided information on the variability due to disturbances in the water column. Emergy analysis allows defining the NC, in terms of resources needed to maintain the meadows and to provide services to coastal communities. This research is inserted into the effort of incorporating the NC evaluation into marine planning and decision making to achieve nature conservation goals, while ensuring the sustainable exploitation of marine resources.

Dimethylsulfoniopropionate and dimethylsulfoxide in Posidonia oceanica

The present work aims at determining the natural variability of dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) contents in the seagrass Posidonia oceanica, which is the largest producer of these molecules reported to date among coastal autotrophs. Seagrass leaf samples were collected during a period of 3.5 years in the pristine Revellata Bay (Calvi, northwestern Corsica, France). The DMSP content ranged from 25 to 265 µmol.gfw−1; DMSO from 1.0 to 13.9 µmol.gfw−1. The dynamics of the two molecules were closely linked, the DMSO content being equivalent to 3.5% of the DMSP content, all leaf samples considered (n = 423 samples and 414 DMSP(O) data pairs). The annual growth cycle of the seagrass diluted the initial stocks of the two molecules. Temperature indirectly affected molecule content dynamics through their direct effect on the seagrass productivity and biomass. Inter-annual variations in DMSP(O) content in relation to shallow water temperature might further indicate that DMSP(O) could have been involved in the physiological response of P. oceanica to heat stress. Finally, middle-aged leaf tissues with an organosulfur molecule content similar to the average value calculated for the seagrass leaf bundle appeared to be the best choice of sample material to study DMSP and DMSO in that species. More research is needed to elucidate the biosynthetic pathways of these molecules in seagrasses, the evolutionary reasons for such a high production in P. oceanica and the physiological functions they play.

Trait gradients inform predictions of seagrass meadows changes to future warming

Comparing populations across temperature gradients can inform how global warming will impact the structure and function of ecosystems. Shoot density, morphometry and productivity of the seagrass Posidonia oceanica to temperature variation was quantified at eight locations in Sardinia (western Mediterranean Sea) along a natural sea surface temperature (SST) gradient. The locations are spanned for a narrow range of latitude (1.5°), allowing the minimization of the effect of eventual photoperiod variability. Mean SST predicted P. oceanica meadow structure, with increased temperature correlated with higher shoot density, but lower leaf and rhizome width, and rhizome biomass. Chlorophyll a (Chl-a) strongly impacted seagrass traits independent of SST. Disentangling the effects of SST and Chl-a on seagrass meadow shoot density revealed that they work independently, but in the same direction with potential synergism. Space-for-time substitution predicts that global warming will trigger denser seagrass meadows with slender shoots, fewer leaves, and strongly impact seagrass ecosystem. Future investigations should evaluate if global warming will erode the ecosystem services provided by seagrass meadows.

The Cost of Brine Dilution in the Desalination Plants of Alicante

To reduce the environmental impact of desalination plants, a good dilution of the brine is needed. Brine dilution may be carried out using diffusers, by mixing the concentrate with other effluents, or with seawater bypassing. Seawater bypassing increases the energy consumption of the plant but, thus far, this energy consumption has not been estimated. The environmental impact statement (EIS) of desalination plants in Alicante establishes a system of seawater bypassing for diluting brine and protecting the Posidonia oceanica seagrass meadows. The aim of this paper is to quantify the energy consumption of brine dilution, which was necessary for meeting the environmental requirements from 2012 to 2018. During the research period, the plants’ operation was variable, as it depended on the supply needs. The results indicate that the energy consumption of the dilution systems fluctuated between 2,135,315 kWh in 2012 and 685,988 kWh in 2013, with an average consumption of 1,205,952 kWh for the selected period. The energy cost in 2012 was EUR 179,556, while that for 2013 was EUR 60,787, with an average annual cost of EUR 91,690. This interannual variability is due to the difference in the production values of the plants and in the dilution ratio, which oscillated between 2.5 and 7.5 seawater:brine. In addition, the dilution showed an additional cost of the energy consumed by the desalination plants of around 1.7% on average. However, it also allowed the fulfillment of the established requirements in the EIS and the protection of the Posidonia oceanica seagrass from the discharge of the desalination plants.

Seascape Context as a Driver of the Fish Community Structure of Posidonia oceanica Meadows in the Adriatic Sea

Abstract Marine underwater habitats dominated by seagrass Posidonia oceanica play an essential role in fish community assembly, affecting taxonomic and functional diversity, abundance and fish behavior. The value of seagrasses as habitat depends on the spatial arrangement of the seascape elements and the availability of alternative habitats. Little is known about the effect of the seascape context of P. oceanica meadows on fish assemblages in the Mediterranean Sea. To identify P. oceanica meadows’ relative importance as a habitat for fishes, fish communities in the Croatian Adriatic Sea were investigated, using SCUBA lure-assisted visual census. The results show a significant effect of different arrangements of P. oceanica meadows’ seascape elements and surrounding habitats on fish community structure. Fragmented mosaic meadows with P. oceanica growing directly on and between rocky-algal reefs/boulders had significantly higher fish abundances compared to both types of continuous meadows (bordering rock and bordering sand). Continuous meadows bordering sand harbored the highest number of unique species. Evidence that alternative structured habitats within proximity to seagrass beds may affect the community structure of associated fish assemblages is provided, highlighting the need to consider P. oceanica meadows’ seascape context in conservation management and experimental design for fish community structure.

An Overview of New Insights into the Benefits of the Seagrass Posidonia oceanica for Human Health.

Posidonia oceanica (L.) Delile is a Mediterranean-endemic angiosperm often described for its great ecological importance. Despite evidence of a millennia-old relationship between P. oceanica and humans, as well as traditional medicine applications, the potential benefits of P. oceanica for human health have been documented only recently. This review aims to compile newly acquired knowledge on P. oceanica bioactive properties that allow the scientific community to look at this plant as a promising source of natural therapeutical products for human health. Experimental investigations conducted in both in vitro cellular-based and in vivo animal models pave the way for new research projects aiming at the development of alternative and complementary therapeutic strategies based on P. oceanica against a wide range of pathological conditions.

Managing biotic interactions during early seagrass life stages to improve seed‐based restoration

Abstract Seagrasses are declining globally, and effective restoration actions to promote the recovery of degraded meadows are urgently needed. Harnessing positive plant interactions during early life stages is considered a valuable strategy to improve terrestrial and coastal habitat restoration. Yet, its application to seagrass restoration is still in infancy, and very little is known on the role played by biotic interactions in shaping newly established populations. We assessed the feasibility of manipulating intraspecific and interspecific plant interactions to enhance seed‐based restoration success using the seagrass Posidonia oceanica as a model. Specifically, we investigated in mesocosm whether seed germination increased with increasing seed density and whether increasing seedling density and planting a pioneer seagrass promoted facilitation among seedlings. To do this, seedlings were grown either as a single individual or in clumps of medium and high density, with and without Cymodocea nodosa, for 2 years encompassing their most critical growth stage. Germination of P. oceanica seeds was not affected by seed density. Posidonia oceanica seedlings planted at medium and high density showed higher survival than those planted individually but only in the presence of C. nodosa. Seedlings planted at medium and at high density with C. nodosa performed better than those grown at low density or without C. nodosa due to a positive joint effect of intraspecific and interspecific interaction. Cymodocea nodosa plants grown with P. oceanica seedlings at high density were larger than those grown alone, indicating a mutualistic relationship. Synthesis and applications. Our results show that in seagrasses positive interactions during early life stages can be promoted by planting seedlings in dense clumps in association with an early successional species. The incorporation of this novel nature‐based approach in seed‐based restoration could accelerate the recolonization of degraded seagrass habitats. In planning future seed‐based restoration interventions, managers should assess both intraspecific and interspecific interactions established by seedlings of target species for identifying proper planting density/spatial configuration and potential benefactor species promoting facilitative mechanisms to maximize seedling planting success.

Sizing the carbon sink associated with Posidonia oceanica seagrass meadows using very high-resolution seismic reflection imaging

Among blue carbon ecosystems, seagrass meadows have been highlighted for their contribution to the ocean carbon cycle and climate change mitigation derived from their capacity to store large amounts of carbon over long periods of time in their sediments. Most of the available estimates of carbon stocks beneath seagrass meadows are based on the analysis of short sediment cores in very limited numbers. In this study, high-resolution seismic reflection techniques were applied to obtain an accurate estimate of the potential size of the organic deposit underlying the meadows of the Mediterranean seagrass Posidonia oceanica (known as ‘matte’). Seismic profiles were collected over 1380 km of the eastern continental shelf of Corsica (France, Mediterranean Sea) to perform a large-scale inventory of the carbon stock stored in sediments. The seismic data were ground-truthed by sampling sediment cores and using calibrated seismo-acoustic surveys. The data interpolation map highlighted a strong spatial heterogeneity of the matte thickness. The height of the matte at the site was estimated at 251.9 cm, being maximum in shallow waters (10–20 m depth), near river mouths and lagoon outlets, where the thickness reached up to 867 cm. Radiocarbon dates revealed the presence of seagrass meadows since the mid-Holocene (7000–9000 cal yr BP). Through the top meter of soil, the matte age was estimated at 1656 ± 528 cal yr BP. The accretion rate showed a high variability resulting from the interplay of multiple factors. Based on the surface area occupied by the meadows, the average matte thickness underneath them and the carbon content, the matte volume and total Corg stock were estimated at 403.5 ± 49.4 million m3 and 15.6 ± 2.2 million t Corg, respectively. These results confirm the need for the application of large-scale methods to estimate the size of the carbon sink associated with seagrass meadows worldwide.

Poor success of seagrass Posidonia oceanica transplanting in a meadow disturbed by power line burial

Local disturbances drive the decrease of the area covered by Posidonia oceanica in the Mediterranean. Mechanical impacts during the development of coastal infrastructures alter sea floor and the recolonization of benthic community will depend on the recovery of pre-disturbance environmental conditions and on the intrinsic characteristics of the local community that was disturbed. We transplanted 468 rhizome fragments and 450 seedlings of P. oceanica in a meadow disturbed by the trenching and deployment of a power line to evaluate the suitability of the disturbed sea floor for rehabilitating P. oceanica meadows. We quantify and compare the survivorship and vegetative development of the transplanted/planted (i.e. fragments/seedlings) material in the two types of the unconsolidated substrata left after infrastructure deployment works finished: sand and burlap bags filled with coarse gravel. The latter was used as a corrective measure for topographic restoration. Three experimental plots with sixteen transplanted fragments or twenty-five seedlings were placed at each substratum type at three different depths (i.e. 15, 20 and 25 m). Our results show that the transplanting of P. oceanica rhizome fragments in the disturbed substrata had low survival rates (0–31%) after 40–48 months. The survivorship of seedlings was lower than that of fragments. Our results highlight the importance of substratum for P. oceanica recovery after mechanical impact; disturbed, non-consolidated substrata will preclude P. oceanica rehabilitation through planting. Preservation of meadow substratum (i.e. dead matte) is a critical element that coastal infrastructure projects should consider to enable future recovery of P. oceanica meadows.

Corollospora mediterranea: A Novel Species Complex in the Mediterranean Sea

The genus Corollospora, typified by the arenicolous fungus Corollospora maritima, consists of twenty-five cosmopolitan species that live and reproduce exclusively in marine environments. Species of this genus are known to produce bioactive compounds and can be potentially exploited as bioremediators of oil spill contaminated beaches; hence their biotechnological importance. In this paper, nine fungal strains isolated in the Mediterranean Sea, from the seagrass Posidonia oceanica (L.) Delile, from driftwood and seawater contaminated by an oil spill, were investigated. The strains, previously identified as Corollospora sp., were examined by deep multi-loci phylogenetic and morphological analyses. Maximum-likelihood and Bayesian phylogeny based on seven genetic markers led to the introduction of a new species complex within the genus Corollospora: Corollospora mediterranea species complex (CMSC). The Mediterranean Sea, once again, proves an extraordinary reservoir of novel fungal species with a still undiscovered biotechnological potential.

Identification of Ecological Hotspots for the Seagrass Posidonia oceanica via Metapopulation Modeling

The seagrass Posidonia oceanica is a benthic foundation species endemic to the Mediterranean Sea. It is a key component of coastal seascapes across the Mediterranean large marine ecosystem, where it plays fundamental ecological, physical, and economic roles. Despite the importance of this iconic seagrass species, a quantitative assessment of the interplay between local dynamics and basin-wide dispersal patterns is still lacking. Here we propose a Mediterranean-scale metapopulation model for P. oceanica, accounting for both demographic processes (inter-annual survival, vegetative growth, fruit production, seed establishment) and the spatial connectivity provided by current-driven dispersal of seagrass fruits. Model simulations are used to identify hotspots of seagrass population abundance, realized connectivity, and long-distance dispersal. Our results indicate that P. oceanica multi-functional hotspots, defined as species-suitable areas that rank high in all of the considered functional roles, are unevenly distributed in the four main sub-basins of the Mediterranean Sea, and along both the European and the African coastline. Our analysis also allows us to outline a remarkable geographical gap in protection: in fact, while many of the hotspots located along European coasts occur close to protected sites, the great majority of the hotspots lying on African coasts lack any form of protection. The identification of hotspots of P. oceanica metapopulation dynamics can thus help select regions that may serve as priority candidates for focusing conservation 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.

Net community metabolism of a Posidonia oceanica meadow

AbstractWe report a 12‐yr data set (August 2006–October 2018) of nearly continuous estimates (n = 3275) of gross primary production (GPP), community respiration (CR), and net community production (NCP) in a Posidonia oceanica seagrass meadow, computed from O2 measurements on a mooring at 10 m bottom depth in the Bay of Revellata (Corsica). Both NCP and CR were correlated to GPP and followed the leaf biomass seasonal cycle. The meadow was net autotrophic (NCP of 23 ± 8 mol O2 m−2 yr−1, GPP [83 ± 16 mol O2 m−2 yr−1] > −CR [−60 ± 9 mol O2 m−2 yr−1]), in agreement with oxygen oversaturation (104% at annual scale, 101% in winter, and 109% in summer). Calcification (CAL) and CaCO3 dissolution (DIS) rates were evaluated from dissolved inorganic carbon measurements in benthic chamber incubations (August 2006–2009). The meadow was found to be a net sink of CaCO3 (DIS > CAL) at an annual rate of 7 mol CaCO3 m−2 yr−1 that matched estimates of CaCO3 deposition on the meadow by sedimentation from the water column. CAL from epiphyte coralline algae was correlated to GPP, but CAL : GPP ratio (0.1) was lower than reported for coralline algae in cultures (0.6) due to the additional contribution of Posidonia to GPP. Both NCP and net DIS contributed to an annual CO2 sink of −30 mol CO2 m−2 yr−1 distinctly stronger than the estimated net air‐sea CO2 flux (−1 mol CO2 m−2 yr−1). This suggests that CO2 input by vertical mixing and/or transport by horizontal advection also strongly contribute to the net atmospheric CO2 exchange.

Exploring the convergence of natural flows for the generation of natural capital stocks in marine ecosystems

Marine ecosystems are open, complex, adaptive, and hierarchical systems highly integrated through the exchange of matter and energy flows. This flows exchange allows marine ecosystems to operate at different scales acting as dissipative structures, building natural capital stocks capable of generating several ecosystem services vital for human well-being. Humans derive a wide range of goods and services from marine ecosystems while, at the same time, generate several impacts causing biodiversity loss and seriously affecting their capacity to provide benefits to humans. Effective management strategies are crucial to conserve healthy and diverse marine and coastal ecosystems, maintain the valuable functions and services they provide, and allow for sustainable human activities. In recent years, Marine Protected Areas (MPAs) have been increasingly acknowledged worldwide as important tools to conserve biodiversity and achieve human well-being and sustainable development goals. Assessing the value of natural capital and ecosystem services is crucial to raise awareness on their importance, support conservation strategies, and ensure the sustainable management of marine ecosystems. This study aimed at calculating biomass and emergy-based indicators to assess the value of natural capital stocks in a Mediterranean MPA. The assessment was performed through a biophysical and trophodynamic environmental accounting model fed with field biomass data collected through ad hoc sampling campaigns performed in the MPA. Four main macro-habitats were investigated: sciaphilic hard bottom (coralligenous bioconstructions), photophilic hard bottom, soft bottom, and Posidonia oceanica seagrass beds. The biomass density of the main autotrophic and heterotrophic taxonomic groups identified in the four macro-habitats of the MPA was evaluated. Based on this biomass matrix, the emergy value of natural capital stocks was assessed. The Posidonia oceanica seagrass beds habitat showed the highest biophysical value (2.32·1019 sej) at MPA scale, while coralligenous bioconstructions resulted the habitat with the highest biophysical value per unit area (2.72·1012 sej m−2). In addition, to complement the biophysical assessment with an economic perspective, the emergy-based indicators were converted into monetary units. The total value of natural capital of the whole MPA resulted in about 46 M€. The results of this study can support local managers and policy makers in the development of management strategies to ensure nature conservation and sustainable human activities. They can be also used as a benchmark for the assessment of natural capital value at larger scales in support of a proper consideration and inclusion of nature value into processes of policy making.

Environmental Engineering Techniques to Restore Degraded Posidonia oceanica Meadows

Seagrass planting techniques have shown to be an effective tool for restoring degraded meadows and ecosystem function. In the Mediterranean Sea, most restoration efforts have been addressed to the endemic seagrass Posidonia oceanica, but cost-benefit analyses have shown unpromising results. This study aimed at evaluating the effectiveness of environmental engineering techniques generally employed in terrestrial systems to restore the P. oceanica meadows: two different restoration efforts were considered, either exploring non-degradable mats or, for the first time, degradable mats. Both of them provided encouraging results, as the loss of transplanting plots was null or very low and the survival of cuttings stabilized to about 50%. Data collected are to be considered positive as the survived cuttings are enough to allow the future spread of the patches. The utilized techniques provided a cost-effective restoration tool likely affordable for large-scale projects, as the methods allowed to set up a wide bottom surface to restore in a relatively short time without any particular expensive device. Moreover, the mats, comparing with other anchoring methods, enhanced the colonization of other organisms such as macroalgae and sessile invertebrates, contributing to generate a natural habitat.

Mechanical properties of lightweight gypsum composites comprised of seagrass Posidonia oceanica and pine (Pinus sylvestris) wood fibers

• Seagrass ( Posidonia oceanica ) and pine wood ( Pinus sylvestris ) fibers used as reinforcing components in plaster. • Incorporation of wood fibers improves mechanical properties of plaster. • Addition of seagrass and wood fibers enhances plaster toughness. • The number and length of the added fibers influences the fracture mechanism of the plaster. Considering the current environmental awareness and the increasing interest in advanced and sustainable materials, the use of natural fibers has become a common practice owing to their appealing characteristics. This study assesses the mechanical properties with respect to bending and compression, impact bending resistance as well as the hardness of gypsum plaster composed with the Mediterranean seagrass ( Posidonia oceanica) and pine wood ( Pinus sylvestris ) fibers. The addition of fibers from 1 to 6 wt% led to a reduction of density from 5% to 30%, respectively. In terms of mechanical properties, composites containing up to 2% wood fiber develop enhanced flexural and compression strength by 28% and 4% respectively; however, a further addition worsened these properties. Composites comprised of seagrass yielded a decrease in strength; yet, the fracture energy absorbed by the material before it completely lost its load-bearing capacity increased. Correspondingly, the impact resistance of the seagrass composites was 57% higher than that of unreinforced plaster. Surface hardness tests indicated that the smaller wood fibers are more effective in transferring the load on a small scale and therefore can improve localized strength compared to larger seagrass fibers. The addition of seagrass and wood fibers presents a sustainable and ecological way to improve the major properties of gypsum products.

Discovering symbiosis in the supralittoral: bacterial metabarcoding analysis from the hepatopancreas of Orchestia and Tylos (Crustacea)

Herein, we investigated for the first time the microbiota from the hepatopancreas of the talitrid amphipod Orchestia montagui and the oniscidean isopod Tylos ponticus, two crustacean species associated with beached leaves of the seagrass Posidonia oceanica (Posidonia banquette). We carried out a metabarcoding analysis based on several hypervariable regions of the bacterial 16S rRNA gene. At the same time, three representative habitats of the supralittoral zone, namely leaf surface of Posidonia banquette, sandy shore and surface seawater were examined by means of bacterial metabarcoding analysis. Our data analyses indicated that the bacterial microbiota from the hepatopancreas and that from the marine habitats form distinct groups in terms of the taxonomic composition. Comparative analyses revealed that some bacteria were associated with the hepatopancreas of the syntopic O. montagui and T. ponticus but were not detected in the supralittoral habitats. This suggests that distinct components of the microbial community may be uniquely adapted within the hepatopancreas. The microbiota from the hepatopancreas of O. montagui was more complex than that of T. ponticus. However, the putative presence of polysaccharide-degrading bacteria in both species indicated symbiotic microbiomes that might be involved in the functionality of the hepatopancreas. A deeper understanding of halophilic hepatopancreatic symbionts in the supralittoral zone also has broad relevance to applied research, particularly to the biotechnological sector related to marine biomass conversion.

Epiphytic hydroids on Posidonia oceanica seagrass meadows are winner organisms under future ocean acidification conditions: evidence from a CO2 vent system (Ischia Island, Italy)

Effects of ocean acidification (OA) on the plant phenology and colonization/settlement pattern of the hydrozoan epibiont community of the leaves of the seagrass Posidonia oceanica have been studied at volcanic CO2 vents off Ischia (Italy). The study was conducted in shallow Posidonia stands (2.5–3.5 m depth), in three stations on the north and three on the south sides of the vent’s area (Castello Aragonese vents), distributed along a pH gradient. At each station, 10–15 P. oceanica shoots were collected every three months for one-year cycle (Sept 2009–2010). The shoot density of Posidonia beds in the most acidified stations along the gradient (pH < 7.4) was significantly higher than that in the control area (pH = 8.10). On the other hand, we recorded lower leaf lengths and widths in the acidified stations in the whole year of observations, compared to those in the control stations. However, the overall leaf surface (Leaf Area Index) available for epiphytes under ocean acidification conditions was higher on the south side and on both the most acidified stations because of the higher shoot density under OA conditions. The hydrozoan epibiont community on the leaf canopy accounted for seven species, three of which were relatively abundant and occurring all year around (Sertularia perpusilla, Plumularia obliqua, Clytia hemisphaerica). All hydroids species showed a clear tolerance to low pH levels, including chitinous and non-calcifying forms, likely favoured also by the absence of competition for substratum with the calcareous forms of epiphytes selected against OA.

Does Warming Enhance the Effects of Eutrophication in the Seagrass Posidonia oceanica?

Seagrass meadows are disappearing at rates comparable to those reported for mangroves, coral reefs, and tropical rainforests. One of the main causes of their decline is the so-called cultural eutrophication, i.e., the input of abnormal amounts of nutrients derived from human activities. Besides the impact of eutrophication at a local scale, the occurrence of additional stress factors such as global sea warming may create synergisms in detriment of seagrass meadows’ health. In the present study, we aimed to evaluate if plants undergoing chronic cultural eutrophication and plants growing in relatively pristine waters are more (or less) sensitive to heat stress, nutrient load and the combination of both stressors. To address this question, a mesocosm experiment was conducted using Posidonia oceanica collected from two environments with different nutrients load history. Plants were exposed in controlled conditions to high nutrient concentrations, increased temperature and their combination for 5 weeks, to assess the effect of the single stressors and their interaction. Our results revealed that plants experiencing chronic cultural eutrophication (EU) are more sensitive to further exposure to multiple stressors than plants growing in oligotrophic habitats (OL). OL and EU plants showed different morphological traits and physiological performances, which corroborates the role of local pressures in activating different strategies in response to global environmental changes. EU-plants appeared to be weaker during the treatments, showing the greatest percentage of mortality, particularly under increased temperature. Temperature and nutrient treatments showed opposite effects when tested individually and an offset response when combined. The activation of physiological strategies with high energetic expenses to cope with excess of nutrients and other stressors, could affect plants present and future persistence, particularly under eutrophic conditions. Our results represent a step forward in understanding the complex interactions that occur in natural environments. Moreover, unraveling intraspecific strategies and the role of local acclimation/adaptation in response to multiple stressors could be crucial for seagrass conservation strategies under a climate change scenario.