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Introduction: Light gradients are ubiquitous in marine systems as light reduces exponentially with depth. Seagrasses have a set of mechanisms that help them to cope with light stress gradients. Physiological photoacclimation and clonal integration help to maximize light capture and minimize carbon losses. These mechanisms can shape plants minimum light requirements (MLR), which establish critical thresholds for seagrass survival and help us predict ecosystem responses to the alarming reduction in light availability. Methods: Using the seagrass Cymodocea nodosa as a case study, we compare the MLR under di fferent carbon model scenarios, which include photoacclimation and/or self-facilitation (based on clonal integration) and that where parameterized with values from field experiments. Results: Physiological photoacclimation conferred plants with increased tolerance to reducing light, approximately halving their MLR from 5-6% surface irradiance (SI) to ? 3% SI. In oligotrophic waters, this change in MLR could translate to an increase of several meters in their depth colonization limit. In addition, we show that reduced mortality rates derived from self-facilitation mechanisms (promoted by high biomass) induce bistability of seagrass meadows along the light stress gradient, leading to abrupt shifts and hysteretic behaviors at their deep limit. Discussion: The results from our models point to (i) the critical role of physiological photoacclimation in conferring greater resistance and ability to recover (i.e., resilience), to seagrasses facing light deprivation and (ii) the importance of self-facilitating reinforcing mechanisms in driving the resilience and recovery of seagrass systems exposed to severe light reduction events. [ABSTRACT FROM AUTHOR]

Plant–herbivore interactions are often regulated by a series of direct and indirect buffer mechanisms (compensatory or defensive plant responses, population control, alternative resources) that can determine the relative stability of the system. In plant‐dominated marine environments, these mechanisms are particularly important given how vulnerable these systems are to strong consumer pressure. Here, we evaluate the presence and, where possible, the strength, of five mechanisms that get activated under high herbivory pressure and are capable of dampening their effects in a seagrass meadow ecosystem: 1) seagrass compensatory growth, 2) increase in plant resistance, 3) availability of alternative resources, 4) predatory control and 5) density‐dependent control. We assessed these mechanisms for the interaction between the Mediterranean seagrass Posidonia oceanica and the sea urchin Paracentrotus lividus through a short‐term series of controlled field and laboratory experiments after simulated or natural herbivory events. Of the five mechanisms tested, we found that three mechanisms – availability of alternative resources, increased predation rate and reduction in sea urchin numbers – effectively dampened herbivory and worked as potential buffering mechanisms. In contrast, plant compensatory growth and resistance did not show clear responses. While compensatory growth and plant resistance are direct plant mechanism to tolerate the effects of herbivory, the rest are indirect mechanisms that begin with a modification of a plant trait (i.e. canopy height) that influences other species preference and/or behaviour, which in turn influences plant consumption. These adaptive behaviours may be a crucial and often overlooked factor in the remarkable resilience that Mediterranean seagrass ecosystems show to herbivory. [ABSTRACT FROM AUTHOR]

Aim: The global vertical depth distribution of seagrass species remains poorly understood. Locally, the abundance and distribution of seagrasses is determined by light penetration, but at global levels each seagrass species has very distinct maximum distributional depth ranges, indicating that plant‐associated traits must also influence their specific depth ranges. Seagrass‐specific attributes, such as plant size or architecture, growth or reproductive strategy and their physiological and/or morphological acclimatization potential, have been suggested to be responsible for this variety of vertical distributions. We investigate here whether these species‐specific traits drive differences in the global maximum vertical distribution of seagrasses. Location: Global. Time period: Publications between 1982 and 2020. Major taxa studied: Seagrasses (order Alismatales). Methods: We tested whether the species‐specific maximum vertical distribution of seagrasses can be predicted by (1) their rhizome diameter (a proxy for plant size); (2) their functional resilience (growth/reproductive strategy); or (3) their acclimatization capacity. For the last aspect, we used a systematic review followed by meta‐analytical approaches to select key seagrass traits that could potentially acclimatize to extreme light ranges across different seagrasses. Results: We found that vertical distribution is best explained by the species‐specific acclimatization capacity of various seagrass traits, including saturation irradiance (physiological trait), leaves per shoot (morphological trait) and above‐ground biomass (structural trait). In contrast, our results indicate no predictive power of seagrass size or growth/reproductive strategy on the vertical distribution of seagrasses. Main conclusions: Across the globe, the ability of seagrass species to thrive at a wide range of depths is strongly linked to the species‐specific acclimatization capacity of key traits at different organizational levels. [ABSTRACT FROM AUTHOR]

In the last three decades, quantitative approaches that rely on organism traits instead of taxonomy have advanced different fields of ecological research through establishing the mechanistic links between environmental drivers, functional traits, and ecosystem functions. A research subfield where trait-based approaches have been frequently used but poorly synthesized is the ecology of seagrasses; marine angiosperms that colonized the ocean 100M YA and today make up productive yet threatened coastal ecosystems globally. Here, we compiled a comprehensive trait-based response-effect framework (TBF) which builds on previous concepts and ideas, including the use of traits for the study of community assembly processes, from dispersal and response to abiotic and biotic factors, to ecosystem function and service provision. We then apply this framework to the global seagrass literature, using a systematic review to identify the strengths, gaps, and opportunities of the field. Seagrass trait research has mostly focused on the effect of environmental drivers on traits, i.e., “environmental filtering” (72%), whereas links between traits and functions are less common (26.9%). Despite the richness of trait-based data available, concepts related to TBFs are rare in the seagrass literature (15% of studies), including the relative importance of neutral and niche assembly processes, or the influence of trait dominance or complementarity in ecosystem function provision. These knowledge gaps indicate ample potential for further research, highlighting the need to understand the links between the unique traits of seagrasses and the ecosystem services they provide. [ABSTRACT FROM AUTHOR]

Climate-driven species redistributions are reshuffling the composition of marine ecosystems. How these changes alter ecosystem functions, however, remains poorly understood. Here we examine how impacts of herbivory change across a gradient of tropicalization in the Mediterranean Sea, which includes a steep climatic gradient and marked changes in plant nutritional quality and fish herbivore composition. We quantified individual feeding rates and behaviour of 755 fishes of the native Sarpa salpa, and non-native Siganus rivulatus and Siganus luridus. We measured herbivore and benthic assemblage composition across 20 sites along the gradient, spanning 30° of longitude and 8° of latitude. We coupled patterns in behaviour and composition with temperature measurements and nutrient concentrations to assess changes in herbivory under tropicalization. We found a transition in ecological impacts by fish herbivory across the Mediterranean from a predominance of seagrass herbivory in the west to a dominance of macroalgal herbivory in the east. Underlying this shift were changes in both individual feeding behaviour (i.e. food choice) and fish assemblage composition. The shift in feeding selectivity was consistent among temperate and warm-affiliated herbivores. Our findings suggest herbivory can contribute to the increased vulnerability of seaweed communities and reduced vulnerability of seagrass meadows in tropicalized ecosystems. [ABSTRACT FROM AUTHOR]

Summary: The prevalence of local adaptation and phenotypic plasticity among populations is critical to accurately predicting when and where climate change impacts will occur. Currently, comparisons of thermal performance between populations are untested for most marine species or overlooked by models predicting the thermal sensitivity of species to extirpation.Here we compared the ecological response and recovery of seagrass populations (Posidonia oceanica) to thermal stress throughout a year‐long translocation experiment across a 2800‐km gradient in ocean climate. Transplants in central and warm‐edge locations experienced temperatures > 29°C, representing thermal anomalies > 5°C above long‐term maxima for cool‐edge populations, 1.5°C for central and < 1°C for warm‐edge populations.Cool‐edge, central and warm‐edge populations differed in thermal performance when grown under common conditions, but patterns contrasted with expectations based on thermal geography. Cool‐edge populations did not differ from warm‐edge populations under common conditions and performed significantly better than central populations in growth and survival.Our findings reveal that thermal performance does not necessarily reflect the thermal geography of a species. We demonstrate that warm‐edge populations can be less sensitive to thermal stress than cooler, central populations suggesting that Mediterranean seagrasses have greater resilience to warming than current paradigms suggest. [ABSTRACT FROM AUTHOR]

The relative benefits of group foraging change as animals grow. Metabolic requirements, competitive abilities and predation risk are often allometric and influenced by group size. How individuals optimise costs and benefits as they grow can strongly influence consumption patterns. The shoaling fish Sarpa salpa is the principal herbivore of temperate Posidonia oceanica seagrass meadows. We used in-situ observations to describe how ontogeny influenced S. salpa individual feeding behaviour, shoaling behaviour and group foraging strategies, and its potential consequences to seagrass meadows. Shoaling was strongly influenced by body length: shoals were highly length-assorted and there was a clear positive relationship between body length and shoal size. Foraging strategies changed dramatically with shoal size. Small shoals foraged simultaneously and scattered over large areas. In contrast, larger shoals (made of larger individuals) employed a potentially cooperative strategy where individuals fed rotationally and focused in smaller areas for longer times (spot feeding). Thus, as individuals grew, they increased their potential impact as well, not merely because they consumed more, but because they formed larger shoals capable of considerably concentrating their grazing within the landscape. Our results indicate that ontogenetic shifts in group foraging strategies can have large ecosystem-wide consequences when the species is an important ecosystem modifier. [ABSTRACT FROM AUTHOR]

Abstract There is increasing uncertainty of how marine ecosystems will respond to rising temperatures. While studies have focused on the impacts of warming on individual species, knowledge of how species interactions are likely to respond is scant. The strength of even simple two-species interactions is influenced by several interacting mechanisms, each potentially changing with temperature. We used controlled experiments to assess how plant-herbivore interactions respond to temperature for three structural dominant macrophytes in the Mediterranean and their principal sea urchin herbivore. Increasing temperature differentially influenced plant-specific growth, sea urchin growth and metabolism, consumption rates and herbivore preferences, but not movement behaviour. Evaluating these empirical observations against conceptual models of plant-herbivore performance, it appears likely that while the strength of herbivory may increase for the tested macroalga, for the two dominant seagrasses, the interaction strength may remain relatively unchanged or even weaken as temperatures rise. These results show a clear set of winners and losers in the warming Mediterranean as the complex factors driving species interactions change. Highlights • Multiple mechanisms influence interactions, each likely modified by temperature. • Mediterranean macrophyte-herbivore interactions show complex contrasts. • Herbivory on the two main Mediterranean seagrasses is expected to decrease. • A key canopy-forming macroalgae however is likely to suffer increased herbivory. • Warming is creating winners and losers in temperate waters as interactions change. [ABSTRACT FROM AUTHOR]

Multi-specific seagrass meadow assemblages dominate most tropical intertidal regions but the relative role of environmental stress in determining distribution patterns is still uncertain. Here we combine observational and experimental approaches to examine aerial exposure as a factor driving species occurrence patterns in intertidal meadows of the Andaman archipelago, where up to 6 seagrass species co-occur. In the studied meadow, patterns of exposure did not map onto distance from the coast, instead creating a patchy matrix of exposure, based on fine-scale bathymetric differences. Distributional surveys showed that seagrass species were similarly patchy, often tracking the degree of aerial exposure during low tide. While some species (Halophila ovalis, Halophila minor, and Thalassia hemprichii) frequently occurred in submerged or subtidal areas and were rarely found in completely exposed areas, other species (Cymodocea rotundata , Halophila beccarii, and Halodule uninervis) also occupied areas that were subject to partial or complete aerial exposure during low tide. To confirm this pattern, we used field-based transplant experiments, employing a natural gradient of tidal exposure to subject six seagrass species to different desiccation exposure times. After a month, H. beccarii and H. uninervis transplants survived in areas that sustained more than 3 h of aerial tidal exposure without significant mortality, compared with other species (H. ovalis, H. minor, T. hemprichii, C. rotundata) that showed dramatic shoot mortality at the same exposure regimes. For all species, 4 h represented the upper limit of exposure, in both experimental and distributional studies. However, despite their wider tolerance of exposure to air, H. beccarii and H. uninervis did not dominate the entire meadow. This could be a result either of their poor tolerance to other environmental factors or their lower competitive abilities among other mechanisms. This suggests that in tropical multi-specific meadows, strong environmental filters could override clear intertidal zonation to create patchy matrices based on species tolerances. Key findings of the research showing seagrass species distribution in the intertidal region that can be explained based on individual species tolerance to aerial exposure. Seagrass symbol source: https://vecta.io/symbols/304/flora-seagrass-sav. [Display omitted] • Intertidal seagrass meadows get exposed to air as water recedes during low tide. • Small-scale bathymetric variation creates a complex matrix of aerial exposure. • Seagrass species distributions are similarly patchy, tracking aerial exposure. • Transplant experiments showed strong species-wise differences in exposure tolerance. • Strong environmental filters drive the assembly of multispecies intertidal meadows. Synthesis : Our work shows that in shallow intertidal seagrass meadows, the fine-scale distribution of exposure acts as a strong environmental filter determining distributional patterns and likely mediating species diversity. [ABSTRACT FROM AUTHOR]

To manage trace metal pollution it is critical to determine how much temporal trends can be attributed to local or large-scale sources. We tracked changes in metal content in the seagrass Posidonia oceanica , along the NW Mediterranean from 2003 to 2010. While Cu, Cd and Ni showed a large inter-site variation, likely due to local factors, Fe, Mn and Pb showed little local variation and synchronous interannual variability across sites, most likely due to large-scale sources. Zn showed equal importance of local and large-scale sources of variation. Temporal trends of Ni, Zn, Cd, Cu remained almost stable. In contrast, Fe, Mn and Pb slightly increased in the last decade. These trends suggest that metals like Cu, Cd, Ni can be effectively managed at local scale. Whereas, elements like Fe, Mn and Pb have an important large-scale component that needs to be managed across the frontiers of national jurisdictions. [ABSTRACT FROM AUTHOR]

Spatial and temporal heterogeneity in flowering occur in many plant species with abiotic pollination and may confer fitness advantages through mechanisms such as predator satiation or pollination efficiency. Environmental factors such as light quality or quantity and temperature play an important role in inducing synchronization on wide geographic scales. On a smaller geographic scale, external factors such as resource availability and herbivory are theorized to trigger flowering, while genetic factors may also play an important role., In this study, we assessed the importance of ecological and genetic factors in shaping seascape-level spatial heterogeneity in flowering of the seagrass Posidonia oceanica. By investigating spatially close sites (<20 km) with similar seascape configurations and depth, we assume that major environmental drivers (temperature and light) were equivalent., We assessed four ecological factors (productivity, leaf nitrogen and carbon content and herbivory) and three genetic factors (heterozygosity, relatedness and clonality) to assess three hypotheses for synchronized flowering in P. oceanica: (i) clone synchronization (internal clock hypothesis), (ii) variation in nutrient availability, potentially caused by spatial heterogeneity in herbivory rates or nutrient translocation via clonal integration (resource budget hypothesis) or (iii) kin selection and sibling synchronization., Internal relatedness and heterozygosity had a significant positive effect on the abundance of flowers. Moreover, productivity and genotypic richness (clonality) were negatively associated with flower density, although at a lower level of significance. In addition, we found that clones were almost exclusively shared among mass-flowering patches and patches without mass-flowering, respectively., Synthesis. The results shed new light on seagrass flowering patterns and on the mechanisms of flower synchronization at the patch level within a wider spatial scale. We found support for the kin selection hypothesis and indirect evidence for the resource budget hypothesis. Thus, a combination of mainly genetic but also ecological factors causes the observed heterogeneous flowering patterns in Posidonia oceanica seascapes. In addition, we found a strong positive relationship between the number of flowers and heterozygosity, adding evidence to the controversial association between heterozygosity and fitness when a limited number of loci are used. To our knowledge, this study is the first to link both ecological and genetic factors with flower abundance in a species with a presumed masting strategy. [ABSTRACT FROM AUTHOR]

Herds of dugong, a largely tropical marine megaherbivore, are known to undertake long-distance movements, sequentially overgrazing seagrass meadows in their path. Given their drastic declines in many regions, it is unclear whether at lower densities, their grazing is less intense, reducing their need to travel between meadows. We studied the effect of the feeding behaviour of a small dugong population in the Andaman and Nicobar archipelago, India to understand how small isolated populations graze seagrasses. In the seven years of our observation, all recorded dugongs travelled either solitarily or in pairs, and their use of seagrasses was limited to 8 meadows, some of which were persistently grazed. These meadows were relatively large, contiguous and dominated by short-lived seagrasses species. Dugongs consumed approximately 15% of meadow primary production, but there was a large variation (3–40% of total meadow production) in consumption patterns between meadows. The impact of herbivory was relatively high, with shoot densities c. 50% higher inside herbivore exclosures than in areas exposed to repeated grazing. Our results indicate that dugongs in the study area repeatedly graze the same meadows probably because the proportion of primary production consumed reduces shoot density to levels that are still above values that can trigger meadow abandonment. This ability of seagrasses to cope perhaps explains the long-term site fidelity shown by individual dugongs in these meadows. The fact that seagrass meadows in the archipelago are able to support dugong foraging requirements allows us to clearly identify locations where this remnant population persists, and where urgent management efforts can be directed. [ABSTRACT FROM AUTHOR]

Evaluating the efficacy of management actions to improve environmental quality is often difficult because there may be considerable lags before ecosystem management actions translate into measurable indicator responses. These delays make it difficult to justify often-expensive remedial actions to prevent eutrophication. Therefore, it is critical to identify reliable, rapid and sensitive indicators to detect degradation and environmental quality improvement. We evaluate the efficacy of a set of indicators based on the seagrass Posidonia oceanica to reliably and quickly detect ecosystem improvements using a 7-year (2003–2010) dataset of 10 stations along the Catalan coast (north-western Mediterranean Sea). In the Catalan region, environmental agencies have invested heavily on wastewater treatment, resulting in significant reductions (ca. 75%) in the BOD 5 discharged to coastal waters from 2003 to 2010. These improvements were clearly reflected at the regional level (i.e. for all the stations averaged) in six biochemical seagrass indicators from our dataset. These indicators were directly related to eutrophication (nitrogen, δ 15 N, phosphorus and total non-structural carbohydrates content in rhizomes, δ 34 S and δ 13 C in seagrass rhizomes and N content in epiphytes). In contrast, seagrass structural indicators, related to seagrass abundance or meadow structure (density, cover) did not show any sign of overall recovery during the monitored period. These results confirm that biochemical seagrass indicators are the most sensitive to water quality improvements within management time-scales (7–10 years) for slow-growing species like P. oceanica . Given the budgetary restrictions under which most management actions operate, the availability of decision-support tools that function at appropriate time-scales is crucial to help managers validate the relative success of their remedial efforts. Our results indicate that low inertia, biochemical seagrass indicators fit this task, and can be a robust set of tools to include in monitoring programmes. [ABSTRACT FROM AUTHOR]

Extensive home ranges of marine megafauna present a challenge for systematic conservation planning because they exceed spatial scales of conventional management. For elusive species like dugongs, their management is additionally hampered by a paucity of basic distributional information across much of their range. The Red Sea is home to a wide-spread, globally important but data-poor population of dugongs. We surveyed the north-eastern Red Sea in the waters of NEOM, Kingdom of Saudi Arabia, to locate feeding sites and determine priority areas for dugong conservation. We conducted large-scale in-water surveys of dugong feeding trails across 27 seagrass meadows that span 0.7 degree of latitude and recorded nine seagrass species and 13 dugong feeding sites. Spread over ∼4‚061 km2 of nearshore and offshore waters, many of these sites clustered around five main core feeding areas. Dugong feeding trails were mostly recorded at sites dominated by the fast-growing pioneer seagrasses Halodule uninervis , Halophila ovalis and/or H. stipulacea. Multispecific meadows with pioneer seagrasses tended to be sheltered and shallow, reflecting a similar spatial pattern to the identified dugong feeding sites. Often close to hotels and fishing harbours, these high-use dugong areas are subject to high boat traffic, fishing, and coastal development which places considerable pressures on this vulnerable mammal and its seagrass habitat. The rapidly accelerating coastal development in the northern Red Sea directly threatens the future of its dugong population. Although our sampling focuses on feeding signs in early successional seagrasses, the results are valuable to spatial conservation planning as they will trigger overdue conservation interventions for a globally threatened species in a data-poor area. Urgent dugong conservation management actions in the northern Red Sea should focus on shallow waters sheltered by coastal lagoons, bays and the lee of large islands. • Pioneer seagrasses are important forage for dugongs in north-eastern Red Sea. • Feeding sites excavated by dugongs are found in shallow sheltered waters. • Dugong feeding sites are vulnerable to anthropogenic stressors. • Timely interventions are needed to conserve the dugong population of Red Sea. [ABSTRACT FROM AUTHOR]

The functioning of ecosystems can be strongly driven by landscape attributes. Despite its importance, however, our understanding of how landscape influences ecosystem function derives mostly from species richness and abundance patterns, with few studies assessing how these relate to actual functional rates., We examined the influence of landscape attributes on the rates of herbivory in seagrass meadows, where herbivory has been identified as a key process structuring these relatively simple systems. The study was conducted in three representative Posidonia oceanica meadows. The principal herbivores in these meadows are the fish Sarpa salpa and the sea urchin Paracentrotus lividus, and we hypothesized that differences in their interaction with landscape attributes would significantly influence herbivory rates., We measured herbivore abundance, herbivory rates, primary production and plant quality (C:N) in seagrass patches embedded either in rock or in sand (matrix attribute), in patches either near or far from a rocky reef (distance attribute) and at the edges and interior of meadows., Our results show that matrix and meadow edges significantly affected the actual levels of herbivory. Herbivory rates were higher in seagrass patches embedded in a rocky matrix compared to those on sand, and herbivory at the centre of seagrass meadows was higher than at the edges. In contrast, patch distance to rocky reefs did not affect herbivory. Neither herbivore abundance nor food quality explained the patterns across different landscape attributes. This suggests that variation in herbivory across the landscape may be related much more to behavioural differences between species in their evaluation of risk, movement and food preference in relation to the landscape structure., Our results indicate that richness and abundance patterns may mask critical interactions between landscape attributes and species responses, which result in considerable heterogeneity in the way key functional processes like herbivory are distributed across the ecosystem mosaic. [ABSTRACT FROM AUTHOR]

We used modelling and field approaches to assess the influence of beach nourishment on a relatively distant Posidonia oceanica seagrass meadow in the NW Mediterranean. Both sediment transport models and in situ observations showed that, two years after the nourishment and under wave climates prevalent during the study period, sediment movement was restricted to shallow waters (<7 m), above meadow shallow limit. The only significant impact on seagrass meadows during this period was an increase in fine sediments, associated with vertical rhizome growth rates 1.5–1.7 times higher than normal. However, running the model with data of wave climate over several decades indicates that strong storms able to transfer these sediments much deeper, potentially burying meadows, occur with a return period of about 25 years. Taken together, our results suggest that beach nourishment could result in significant sub-lethal and lethal consequences for seagrasses that may go unnoticed with short-term evaluations. [ABSTRACT FROM AUTHOR]

Prioritizing efforts for conserving rare and threatened species with limited past data and lacking population estimates is predicated on robust assessments of their occupancy rates. This is particularly challenging for elusive, long-lived and wide-ranging marine mammals. In this paper we estimate trends in long-term (over 50years) occupancy, persistence and extinction of a vulnerable and data-poor dugong (Dugong dugon) population across multiple seagrass meadows in the Andaman and Nicobar archipelago (India). For this we use hierarchical Bayesian dynamic occupancy models accounting for false negatives (detection probability<1), persistence and extinction, to two datasets: a) fragmentary long-term occurrence records from multiple sources (1959–2004, n = 40 locations), and b) systematic detection/non-detection data from current surveys (2010–2012, n = 57). Dugong occupancy across the archipelago declined by 60% (from 0.45 to 0.18) over the last 20 years and present distribution was largely restricted to sheltered bays and channels with seagrass meadows dominated by Halophila and Halodule sp. Dugongs were not found in patchy meadows with low seagrass cover. In general, seagrass habitat availability was not limiting for dugong occupancy, suggesting that anthropogenic factors such as entanglement in gillnets and direct hunting may have led to local extinction of dugongs from locations where extensive seagrass meadows still thrive. Effective management of these remnant dugong populations will require a multi-pronged approach, involving 1) protection of areas where dugongs still persist, 2) monitoring of seagrass habitats that dugongs could recolonize, 3) reducing gillnet use in areas used by dugongs, and 4) engaging with indigenous/settler communities to reduce impacts of hunting. [ABSTRACT FROM AUTHOR]

The dominant paradigm of top-down control of ecological communities through direct consumption pathways is giving way to a more nuanced understanding of trophic interactions with the recognition that subtler indirect effects can often play an important role in structuring communities. Direct and indirect trophic and non-trophic processes could well be acting simultaneously within the same food web, and their overall effects may even interact with each other., We studied indirect interactions in a relatively simple trophic system comprising a single producer, two herbivores and a guild of predators. In particular, we assessed whether (i) the principal herbivore fish, Sarpa salpa, is capable of modifying a seagrass trait, canopy height, by grazing and (ii) whether grazing-induced habitat alteration can trigger an environment-mediated modification of the interaction between herbivorous sea urchins Paracentrotus lividus and their predators., We tested these hypotheses with field experiments including fish herbivore-exclusion experiments (to examine the ability of S. salpa to modify seagrass habitats) and predation experiments using tethered sea urchins in a meadow with varying canopy heights (to test whether habitat modification can mediate urchin predation risk)., The effect of S. salpa herbivory was highly significant. Canopy height in herbivore-excluded plots was more than 3·5 times higher than in uncaged control plots. In addition, adult sea urchin predation risk in the most highly grazed plots was 4-5 times higher than plots with higher canopy heights. In contrast, predation risk on juvenile urchins was not influenced by canopy height., Our results show that predation pressure on a key herbivore can be modified both by the environmental context within which it finds itself and by the actions of another herbivore that modifies the plant traits that create this environmental context. These act as two discrete pathways that interact in potentially nonlinear ways, mediating top-down control in these ecosystems. Herbivores, particularly when acting as ecosystem engineers, may have the potential to mediate and increase predation risk, as they substantially modify habitat structure, with consequences for refuge availability, among others. Assessing these indirect interactions is not just important to understand the functioning of the system itself but may have important management and conservation consequences. [ABSTRACT FROM AUTHOR]

Abstract: Quantifying the uncertainty associated with monitoring protocols is essential to prevent the misclassification of ecological status and to improve sampling design. We assessed the Posidonia oceanica multivariate index (POMI) bio-monitoring program for its robustness in classifying the ecological status of coastal waters within the Water Framework Directive. We used a 7-year data set covering 30 sites along 500km of the Catalonian coastline to examine which version of POMI (14 or 9 metrics) maximises precision in classifying the ecological status of meadows. Five factors (zones within a site, sites within a water body, depth, years and surveyors) that potentially generate classification uncertainty were examined in detail. Of these, depth was a major source of uncertainty, while all the remaining spatial and temporal factors displayed low variability. POMI 9 matched POMI 14 in all factors, and could effectively replace it in future monitoring programs. [Copyright &y& Elsevier]

Herbivory is now recognized as an important structuring agent in seagrass meadows but the attack pattern and tissue damage of consumers are highly variable. Nutritional preferences of herbivores and/or easy access to resources may cause differences in biomass loss among tissues that damage the plant in functionally distinctive ways. The two main Mediterranean herbivores, the fish Sarpa salpa (L.) and the sea urchin Paracentrotus lividus (Lmk.), remove higher amounts of intermediate and external shoot leaves, respectively. To test whether this selective feeding can have different consequences on the allocation patterns of nutrient within plants, we simulated the effect of both herbivores by clipping external and intermediate leaves (plus unclipped controls) of Posidonia oceanica (L.) and we measured plant tolerance in terms of shoot growth and leaf nutrient supply to new tissue using isotopic markers. As expected, control treatments displayed high carbon and nutrient supply from external leaves (83% of the total N and 84% of the total C incorporated by the shoot). When subjected to clipping, the remaining leaves enhanced carbon and nitrogen supply compared with the control by 16% of N and 36% of C-in the intermediate clipping-and by over 100% of N and 200% of C-in the external clipping-to compensate for the nutrient lost. However, only in the case of fish herbivory (intermediate clipping), enhanced supply alone was able to fully compensate for the nutrient losses. In contrast, this mechanism is not completely effective when external leaves are clipped (urchin herbivory). Yet, the consequences of this nutrient loss under sea urchin herbivory are not apparent from the nutrient content of the new tissue, suggesting that there are other sources of nitrogen income (uptake or reallocation from rhizomes). Our study does not only confirm the tolerance of P. oceanica to herbivory, but also constitutes the first evidence of leaf-specific, compensatory nutrient supply in seagrasses. [ABSTRACT FROM AUTHOR]

Within-plant variation in susceptibility to herbivory can significantly influence the ecological and evolutionary consequences of plant-herbivore interactions. Seagrasses are marine angiosperms characterised by substantial intra-individual differences in multiple traits, such as nutrients, chemical and structural defences and epibiotic load, all of which can strongly influence herbivore preferences. We quantified the within-plant feeding choices of the two main consumers of the temperate seagrass Posidonia oceanica--the fish Sarpa salpa and the sea urchin Paracentrotus lividus--and determined the plant traits that explained their foraging strategies. We found strong within-plant heterogeneity in both seagrass susceptibility to herbivory and chemical composition, but different consumers exhibited contrasting feeding choices. S. salpa preferred the most nutritious and chemically defended younger leaves, suggesting a full adaptation to consuming this macrophyte and a greater impact of this herbivore on the plant. In contrast, P. lividus consistently preferred the older leaves covered by epibionts, probably attenuating the relative impact of this consumer and helping to explain the weak effects usually recorded for this echinoid in undisturbed meadows. Artificial diet experiments showed that morphology and fine-scale structural defences were the primary determinant of urchin feeding choices, with nutrient content and chemical defences being of secondary importance. Epibiosis did not strongly influence fish feeding, but it did have a strong 'shared-doom' effect on urchin consumption. This effect was driven by a distinct preference towards a mixed diet that included both host tissues and their epibiotic community. [ABSTRACT FROM AUTHOR]

The article presents a study on the ability of Posidonia (P.) oceanica to counteract human-induced or natural changes in sedimentation. The researchers conducted manipulative experiments in distinct P. oceanica meadows, which differed in terms of depth and exposure. They also observed the impact of burial intensity on P. oceanica sublethal responses and shoot population decline and the effect of burial timing on P. oceanica shoot population decline. They found that the different levels of sediment burial significantly affected the branching, rhizome growth and shoot density of P. oceanica.

The article describes an experiment of water column enrichment in an oligotrophic seagrass bed of Posdonia oceanica in northwestern Mediterranean Sea. The authors mention that they monitored monthly seasonal changes in fish herbivore behavior after changes in qualitative and quantitative features, which include nutrient content and biomass availability, of primary producers as a result of nutrient fertilization. They found that nutrient addition caused alterations in both seagrass and epiphytes throughout late spring and summer. They also reveal that the nutritional changes of food organoleptic properties concurred within the seasonal peak of herbivory by the fish Sarpa salpa.

Nitrogen inputs to coastal environments can considerably alter the abundance of primary producers. However, how herbivores modify their trophic signatures and adjust to changes in food resource conditions remains controversial. Here, we assess the effect of nutrient availability on the diet shifts of the two main Mediterranean herbivores, the Sparid fish Sarpa salpa L. and the sea urchin Paracentrotus lividus (Lmk.) that feed mostly on the seagrass Posidonia oceanica L. (Delile), epiphytes and benthic macroalgae. To do this, we (1) investigate the patterns of isotopic composition (δ13C and δ15N signatures) of the two herbivores and their potential food sources in three areas of contrasting nutrient conditions and, (2) we assess the diet shift along this nutrient gradient by estimating the isotopic nutrient enrichment (i.e., the contribution of δ13C and δ15N signatures in consumers’ tissues relative to potential food sources). Food web signatures of δ13C were similar among the three study sites, and no patterns of δ13C shift were observed in their diets. In contrast, there was a consistent increase in N contents and δ15N along the nutrient gradient for all primary producers and their consumers. The rate of δ15N enrichment was also clearly distinctive between the two herbivores: in P. lividus it increased by 61% along the nutrient gradient, while in S. salpa it remained constant. Our results suggest that sea urchins behave as facultative omnivores and feed on vegetable or mixed diets depending on the trophic status of the system. It is unclear, however, if this modification is behavioral or the consequence of mere changes in the availability of food items, as animal epiphytes (e.g., hydrozoans, bryozoans and ascidians) can also became more abundant on seagrass leaves under increased nutrient conditions. In contrast, adult fish appear to feed on vegetal material independent of nutrient availability in the ecosystem. [ABSTRACT FROM AUTHOR]

The article explores the combination of field and laboratory-based manipulative experiment and manipulating controlled conditions of featured seagrass habitat between the sea urchin Paracentrotus lividus and its predators. It states that predation on sea urchins which are important structure agents in seagrass communities modulated by habitat structural complexity to determine predation pressure on the main invertebrate herbivore of the temperate seagrass. The sear urchin Paracentrotus lividus attributes the complexity of seagrass habitat with exposure of the root-rhizome layer on local sedimentary conditions.

Herbivory can induce changes in plant traits that may involve both tolerance mechanisms that compensate for biomass loss and resistance traits that reduce herbivore preference. Seagrasses are marine vascular plants that possess many attributes that may favour tolerance and compensatory growth, and they are also defended with mechanisms of resistance such as toughness and secondary metabolites. We quantified phenotypic changes induced by herbivore damage on the temperate seagrass Posidonia oceanica in order to identify specific compensatory and resistance mechanisms in this plant, and to assess any potential trade-offs between these two strategies of defence. We simulated three natural levels of fish herbivory by repeatedly clipping seagrass leaves during the summer period of maximum herbivory. Compensatory responses were determined by measuring shoot-specific growth, photosynthetic rate, and the concentration of nitrogen and carbon resources in leaves and rhizomes. Induced resistance was determined by measuring the concentration of phenolic secondary metabolites and by assessing the long-term effects of continued clipping on herbivore feeding preferences using bioassays. Plants showed a significant ability to compensate for low and moderate losses of leaf biomass by increasing aboveground growth of damaged shoots, but this was not supported by an increase in photosynthetic capacity. Low levels of herbivory induced compensatory growth without any measurable effects on stored resources. In contrast, nitrogen reserves in the rhizomes played a crucial role in the plant’s ability to compensate and survive herbivore damage under moderate and high levels of herbivory, respectively. We found no evidence of inducibility of long-term resistance traits in response to herbivory. The concentration of phenolics decreased with increasing compensatory growth despite all treatments having similar carbon leaf content, suggesting reallocation of these compounds towards primary functions such as cell-wall construction. [ABSTRACT FROM AUTHOR]

Abstract: We propose here a multivariate index based on structural and functional attributes of the Posidonia oceanica ecosystem (Posidonia oceanica multivariate index: POMI) to assess the ecological status of coastal waters following WFD requirements. POMI is based on the combination, through principal component analysis, of physiological, morphological, structural and community level seagrass descriptors (or metrics), univocally related to environmental quality. Scores on the first axis are normalised to a 0–1 scale (EQR) using reference and worst sites. The index was tested by sampling 22 seagrass beds in the Catalan coast (ca. 500km, NW Mediterranean). The results show a clustering of variables on both sides of the first axis of the PCA, indicating a common relation of all metrics with ecological status. Moreover, the values (EQR) obtained reflect human pressure levels. The proposed index allows a practical and relatively simple assessment of the ecological status of coastal waters, and contains a considerable amount of ecological information, which can be useful for managing purposes. [Copyright &y& Elsevier]

Plant–herbivore interactions have strong ecological and evolutionary consequences, but have been traditionally overlooked in marine higher plants. Despite recent advances in seagrass ecology that highlight the importance of herbivory, the mechanisms that regulate the feeding behaviour of seagrass consumers remain largely unknown. Herbivores have been shown to reduce the sexual reproductive success of seagrasses through direct consumption of inflorescences and seeds, but we know little about intraspecific variation in susceptibility to grazing of different seagrass tissues. We contrasted the relative palatability of reproductive and vegetative tissues of the temperate seagrass Posidonia oceanica in the field, and we assessed the feeding preferences among these tissues of the main consumers of the plant, the fish Sarpa salpa and the urchin Paracentrotus lividus. Moreover, we identified the plant traits that explained the observed feeding behaviour. We provide strong evidence for herbivore selectivity among seagrass tissues. In the field, 70–90% of inflorescences were damaged by herbivores compared to 3–60% of leaves of similar age. In feeding assays, the urchin P. lividus showed over a twofold preference for reproductive tissue at various stages of development. By contrast, we detected no feeding activity on either leaves or inflorescences from the fish S. salpa, which is known to migrate to deeper waters soon after flowering starts and during the period of fruit maturation. Despite being the preferred food of urchins, inflorescences were chemically defended, had higher levels of phenolics and lower nutrient and calorific content than leaves. We experimentally demonstrated that leaf structural defences are the primary factor in determining urchin feeding preferences. Removal of plant structure results in a drastic shift in urchin selectivity towards the most nutritious and less chemically defended leaf tissue, indicating that multiple mechanisms of defence to herbivory may coexist in seagrasses. [ABSTRACT FROM AUTHOR]

Eutrophication is one of the most relevant man-induced changes occurring in coastal waters. The identification and assessment of specific responses to eutrophication in seagrasses can provide a useful tool for the detection of changes in the water quality in coastal zones, given the wide range of distribution of these organisms. In this study, we combine a correlational (across-sites comparison) and a manipulative (fertilization experiment) approach to evaluate the usefulness and potential of alkaline phosphatase activity (APA) in the endemic Mediterranean seagrass Posidonia oceanica as an eutrophication biomarker. Our results showed that APA decreases promptly following nutrient additions, the response being maintained except during the winter period. APA also varies across natural meadows under different levels of nutrient discharges at scales relevant for monitoring purposes. AP activity seems to be an optimal ‘physiological biomarker’ that responds promptly and reliably to a pulse of eutrophication exposure. However, other considerations, such as the seasonality (the response disappears in winter), suggest its use with some caution and, as far as possible, as a complement of other bio-indicators. [ABSTRACT FROM AUTHOR]

We investigated the effects of defoliation by herbivores on leaf elongation and foliar nutrient dynamics in six tropical seagrasses ( Thalassia hemprichii , Thalassodendron ciliatum, Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis and Syringodium isoetifolium ). Defoliation (clipping) treatment resulted in decreased leaf elongation in three of the six seagrasses studied. Mean shoot re-growth of clipped plant plots was 45%, 51%, and 53% lower than in controls in Thalassodendron ciliatum , Syringodium isoetifolium , and Cymodocea serrulata , respectively. Clipping had little effect on the nitrogen content of young, fully expanded leaves, except in one species, Thalassodendron ciliatum (20% reduction in nitrogen). However, there was significantly less nitrogen in senesced leaves of treated plants than in the leaves of control plants of Thalassodendron ciliatum, Halodule uninervis, Cymodocea serrulata and Cymodocea rotundata. Nitrogen resorption efficiencies from leaves averaged 35% higher in defoliated plants than in controls in five of the six species, supporting the hypothesis that resorption in seagrasses may be plastic in response to changes in the magnitude of internal nutrient pools. Seagrasses in this study seem to be variably affected by herbivore pressure. Except for one species ( Thalassia hemprichii ), where growth and nitrogen pools did not respond to the treatment, clipping increased the nitrogen resorption efficiency of seagrasses, partially compensating for the reduction in growth that occurred following defoliation. The magnitude of the fall in leaf elongation rate in response to defoliation was correlated with the species'resorption proficiency, indicating that the levels to which nitrogen can be reduced in senescing leaves is probably not constant in all seagrass species, and this plasticity will determine the success of resorption as a nitrogen conservation mechanism when nitrogen losses occur as a consequence of herbivory. [ABSTRACT FROM AUTHOR]

To assess the effects of shading by epiphytes on the carbon balance of the seagrass Posidonia oceanica, we examined the patterns of within-shoot epiphyte abundance on leaves, and their variability with season and depth. Epiphyte biomass was found to depend on leaf age (larger epiphyte load on older tissues), leaf side (more epiphytes on the inner face than on the outer) and depth (more epiphytes in deep meadows). Depth differences were maximum in spring and disappeared in late summer. Percent light absorbed (absorptance) by epiphytes was measured; light absorptance followed an exponential-saturated model with epiphyte biomass. Combining these data of percent light absorbed, within-shoot epiphyte biomass distribution and an existing carbon balance model, we conclude that reduction in carbon gains caused by epiphyte shading is relatively small, and greater in deep meadows (8.8% on average, with values up to 14.2% in May) than that in shallow water (4.7% on average with maximum values of 7% in August). This is mainly due to the accumulation of epiphytes on old tis- sues, which contribute only marginally to the carbon gain of the plant. Using the same procedure, we modelled the effects of a doubling in epiphyte biomass, conserving the observed within-shoot distribution. The result was a very small additional carbon loss. However, using the same biomass but modifying the distribution (shifting the same abundance towards younger leaf age classes), the reduction in carbon gains was dramatic, particularly in deep meadows (between 21% and 41%, depending on the shift used). Therefore, it is the epiphyte growth timing rate rather than the final biomass reached which seems to be a key control for Posidonia oceanica survival, especially near the deep limit of its distribution. [ABSTRACT FROM AUTHOR]

Thalassodendron ciliatum shoots were collected from natural populations of fished and unfished protected seagrass meadows to assess the herbivory of fish and sea urchins. Fish herbivory was restricted to unfished seagrass meadows, while sea urchin herbivory took place in fished as well as unfished areas where fish appeared not to be effective urchin predators. The results of this study confirm that grazing by sea urchins is important in tropical seagrass ecosystems and indicate that herbivorous fish graze, and probably consume, substantial amounts of seagrass production in fishing-protected habitats. The fact that much of the information on seagrass herbivory comes from heavily-fished meadows indicates that large-scale studies, which include unfished areas, are necessary to provide reliable spatial patterns of seagrass grazing distribution. [ABSTRACT FROM AUTHOR]

Seagrass beds are increasingly impacted by human activities in coastal areas, particularly in tropical regions. The objective of this research program was to study seagrass beds characteristics under various environmental conditions in the French Antilles (FA, Caribbean Sea). A total of 61 parameters, from plant physiology to seagrass ecosystem, were tested along a gradient of anthropogenic conditions, distributed across 11 sites and 3 islands of the FA. A selection of 7 parameters was identified as relevant for the monitoring of seagrass meadows in the framework of public policies. They combined "early warning indicators" (e.g. nutrients and some trace metals) and long-term responding parameters (e.g. shoot density) adapted to management time scales. The ecological status of seagrass meadows was evaluated using a PCA. This work is a first step towards monitoring and management of seagrass meadows in the FA. • A panel of seagrass meadows descriptors were studied along a gradient of anthropogenic conditions in Caribbean region. • Bioindication capacity of two native seagrass species has been confirmed. • The most relevant parameters for monitoring local seagrass beds health and water quality were identified. • An ecological status of the seagrass meadows was provided using a multimetric approach. [ABSTRACT FROM AUTHOR]

Apart from directly influencing individual life histories of species, climate change is altering key biotic interactions as well, causing community processes to unravel. With rising temperatures, disruptions to producer-consumer relationships can have major knock-on effects, particularly when the producer is a habitat-forming species. We studied how sea surface temperature (SST) modifies multiple pathways influencing the interaction between the foundational seagrass species, Posidonia oceanica , and its main consumer, the fish Sarpa salpa in the Mediterranean Sea. We used a combination of a field-based temperature gradient approaches and experimental manipulations to assess the effect of temperature on seagrass performance (growth) and fish early life history (larval development) as well as on the interaction itself (seagrass palatability and fish foraging activity). Within the range of temperatures assessed, S. salpa larvae grew slightly faster at warmer conditions but maintained their settlement size, resulting in a relatively small reduction in pelagic larval duration (PLD) and potentially reducing dispersion. Under warmer conditions (>24 °C), P. oceanica reduced its growth rate considerably and seemed to display fewer deterring mechanisms as indicated by a disproportionate consumption in choice experiments. However, our field-based observations along the temperature gradient showed no change in fish foraging time, or in other aspects of feeding behaviour. As oceans warm, our results indicate that, while S. salpa may show little change in early life history, its preference towards P. oceanica might increase, which, together with reduced seagrass growth , could considerably intensify the strength of herbivory. It is unclear if P. oceanica meadows can sustain such an intensification, but it will clearly add to the raft of pressures this threatened ecosystem already faces from global and local environmental change. • Warming can cause disruptions to plant-herbivore interactions with major knock-on effects. • High temperature had no effect on S. salpa foraging activity but increased larval growth rate. • Warming negatively affected P. oceanica by reducing its growth rate and making it more palatable for fish. • Increased herbivory and decreased plant growth will intensify herbivory pressure in warming seas. [ABSTRACT FROM AUTHOR]

The seagrass Posidonia oceanica is able to reproduce by asexual formation of plantlets directly in the inflorescence. Pseudoviviparous plantlets were observed during May 2004 in 26% of the inflorescences in a P. oceanica meadow off the island of Formentera (Balearic Islands, Western Mediterranean Sea). We do not know how frequently pseudovivipary occurs in Posidonia oceanica , nor the mechanisms triggering it, but this strategy can contribute significantly to short-distance dispersal and meadow maintenance, which is especially relevant in a species whose meadows can persist for thousands of years and whose colonization of new space occurs very slowly. [ABSTRACT FROM AUTHOR]

Seagrass meadows, key ecosystems supporting fisheries, carbon sequestration and coastal protection, are globally threatened. In Europe, loss and recovery of seagrasses are reported, but the changes in extent and density at the continental scale remain unclear. Here we collate assessments of changes from 1869 to 2016 and show that 1/3 of European seagrass area was lost due to disease, deteriorated water quality, and coastal development, with losses peaking in the 1970s and 1980s. Since then, loss rates slowed down for most of the species and fast-growing species recovered in some locations, making the net rate of change in seagrass area experience a reversal in the 2000s, while density metrics improved or remained stable in most sites. Our results demonstrate that decline is not the generalised state among seagrasses nowadays in Europe, in contrast with global assessments, and that deceleration and reversal of declining trends is possible, expectingly bringing back the services they provide. Seagrass meadows are important but one of the most threatened ecosystems globally. Here the authors analyse data about extent and density of seagrasses in Europe from 1869 to 2016, and find evidence of recent trend reversal for declining European seagrass meadows. [ABSTRACT FROM AUTHOR]

We used the disturbance resulting from a once in a 100-yr storm on the northwest Mediterranean coast to examine the extent of the disturbance, the tolerance thresholds to burial, and the medium-term response of the long-lived Posidonia oceanica seagrass. Sediment burial at 12 surveyed areas was particularly strong in shallow meadows, with 23% of their surfaces buried, on average, under more than 10 cm of sediment. In contrast, less than 5% of the meadow was affected at deeper locations. At three sites, we tracked short-term mortality along a gradient of sediment burial. Survival response to burial was clearly nonlinear, with a significant threshold at 4-5 cm, beyond which shoot mortality was 100%. To track medium-term potential recovery, we established permanent plots subject to three sediment burial levels (0-5, 5-10, and > 10 cm burial) in four meadows. Where the initial shoot mortality was 100%, we recorded no shoot recovery over the 4-yr period. In the remaining plots, where some shoots remained alive, we detected either further mortality or shoot recovery of 7% per year on average. Extreme storm events can result in sudden catastrophic losses of seagrass cover in shallow P. oceanica meadows. In the long term and due to the long return time of such storms, the species may still be able to recover despite its low recovery potential. However, added anthropogenic stressors, including climate change, may seriously test the ability of long-lived shallow seagrass ecosystems to resist high-intensity natural disturbances and may be critical for its persistence. [ABSTRACT FROM AUTHOR]

Fragmentation is a major agent for seagrass meadow decline, yet little is known about how it interacts with processes like herbivory, an important functional driver of seagrass meadows. The interaction with external stressors like fragmentation could exacerbate the effects of internal ecosystem drivers like herbivory, with distinct implications for ecosystem management., We used manipulative field experiments to assess these interactive effects in two Posidonia oceanica seagrass meadows. We monitored replicated plots in small and large patches in two meadows suffering fragmentation with and without herbivores (using exclusion cages) to test whether fragment size and herbivory could act together to alter ecosystem functioning. We measured changes in defoliation rates, primary production, canopy height and nutrient content in all plots after 4 months of herbivore exclusion., Our results show that herbivores increased defoliation rates resulting in reduced primary production, nutrient content and canopy structure (canopy height). Patch size (fragment) on its own also reduced primary production, nutrient content and canopy structure. We also observed significant additive interactions between herbivores and fragmentation on canopy structure and production responses. In addition, small patches showed nutrient limitation but were able to accumulate more carbohydrate reserves, probably due to a higher light availability. This may explain why small patches can persist under significant herbivore pressure., Synthesis. While fragmentation has already been identified as an important external agent of seagrass decline, the combination of fragmentation and herbivory can seriously exacerbate structural losses and affect primary production, profoundly compromising the role of seagrasses as habitat-forming ecosystems. These interactions between external stressors and internal drivers may result in large unexpected consequences that may flow on to the rest of the ecosystem. [ABSTRACT FROM AUTHOR]

Highlights: [•] We assessed the impact of the invasive Lophocladia lallemandii in seagrasses. [•] Seagrass mortality was 2.5–5 times higher in invaded than in turf-free sites. [•] Seagrass mortality increased when the algal turf biomass exceeded 200–300gDWm−2. [•] L. lallemandii could represent an important treat to P. oceanica resilience. [ABSTRACT FROM AUTHOR]

Highlights: [•] Empirical measures of man–animal conflict often do not match community perceptions. [•] Lakshadweep fishers linked catch declines to grazing of seagrass by green turtles. [•] At high densities, turtle grazing reduced seagrass canopy heights and lowered fish abundances. [•] Indirect costs of conflict far outweigh direct costs (like damage to gear) by turtles. [•] Managing conflict requires acknowledgment of these hidden indirect losses. [Copyright &y& Elsevier]

Abstract: Secondary metabolites play an important role in protecting plants from herbivores or pathogens. Despite this, few studies have investigated the presence and the concentration of those compounds over broad spatial scales in seagrasses. Here we analysed the presence and abundance of specific phenolic compounds of Zostera noltii from four different meadows located across the Atlantic and the Mediterranean coast. Three phenolic acids have been found in the samples of Z. noltii: rosmarinic, caffeic and zosteric acids. The proportion of the three compounds varied substantially with the major proportion comprising rosmarinic acid (RA, mean value 94%) followed by zosteric acid (ZA, mean value 4%) and caffeic acid (CAF, mean value 2%). Z. noltii metabolic compounds varied notably between sites. The highest concentrations in RA were found in the Cadiz Bay with values of 11,254 (55 SD) μg g−1 dry wt, while the lowest were observed in the Alfacs Bay (Ebro) with values of 823 (7 SD) μg g−1 dry wt. The highest concentrations in ZA were also observed in the Cadiz Bay with values of 727 (6 SD) μg g−1 dry wt, while the lowest were observed in the Sa Nitja Bay (Menorca Island) with values of 20 (4 SD) μg g−1 dry wt. Finally, the concentrations in CAF remain very low at each site (17–69 μg g−1 dry wt), with the lowest observed in Arcachon. Their proportions remained relatively constant, indicating that most phenolic acids responded together. Documenting the presence of those compounds in living tissues of Z. noltii and how they vary in abundance between seagrass meadows across large geographical scales is a crucial first step to understand the large-scale level response of the plant to potential pathogens, herbivore outbreaks or other ecological processes. [Copyright &y& Elsevier]

Abstract: In this study we examine the influence of non-monsoon sediment arrival on the high-diversity SE Indian seagrass meadows of the Palk Bay and the Gulf of Mannar. We used a gradient-based approach to examine the influence of increasing sediment loads on species composition and shoot density. In addition, for the ubiquitous seagrass (Cymodocea serrulata), we tested the influence of sediment on its biomass and productivity. We identified three sites in Palk Bay and four sites in Gulf of Mannar (SE India) along a gradient of sediment input. At each of the seven locations, sediment traps were deployed to measure sedimentation rates. Nine seagrass cores were taken systematically along 50m transects at a constant sub-tidal depth to measure shoot density and biomass. A few shoots of C. serrulata were marked to estimate the above ground seagrass growth rate. Our results indicate that sedimentation rates that ranged from 8.6 to 62.4mgDWcm−2 d−1 could not explain species composition of the meadow or shoot density of the observed species. C. serrulata was, by far, the most abundant species and present in all sediment conditions. Sedimentation rates did not alter shoot elongation rates in C. serrulata, ranging from 1.54±0.29 SD to 0.25±0.02SDcmd−1, but in contrast, increased vertical rhizome elongation rate. This increase was reflected in an increase in below ground biomass along the sediment gradient (R 2 =0.582, p =0.01). C. serrulata appears to be able to adapt to the sediment dynamics in this area by allocating resources to rhizomes and roots to counteract burial and stabilizing sediments. Given that siltation is one of the most important threats to seagrass meadows, understanding the species-specific adaptive mechanisms of seagrass species in these high-sediment, high diversity South Asian meadows is an important first step in ensuring their long-term survival and functioning. [Copyright &y& Elsevier]

Abstract: The demand for sensitive biological tools to assess the environmental quality of coastal waters at broad spatial scales is increasing. Many of the tools used are based on the taxonomic composition of biotic assemblages. They usually require a valuable taxonomic expertise while are unique reflecting the overall ecosystem integrity. Here, we evaluate the potential indicator value of several features of the epiphytic community (overall assemblage composition, species richness, and proportion of the main taxonomic groups) growing on the seagrass Posidonia oceanica leaves. We do so by empirically examining their changes along a disturbance gradient where multiple human activities have interactive and cumulative impacts, sampling at different spatial scales and at two different depths (5 and 15 m). Our results show that the specific composition of the epiphytic assemblages (i.e. species composition) closely reflects, in the deep meadows, the combined effects of different anthropogenic stressors along the gradient, showing an integrative and non-specific response. Similarly, an increase in the proportion of hydrozoans, and a decrease in the proportion of rhodophytes and chlorophytes are observed in deep meadows along the gradient. In shallow meadows, grazing and biotic features of the seagrass seem the main forcing factors determining species composition, and therefore masking the response of epiphytes to the deterioration gradient. After address the effect of natural sources of variability (water depth, within- and between-meadow heterogeneity), changes in epiphyte assemblages and in the proportion of hydrozoans, rhodophytes and chlorophytes in relatively deep meadows seem promising monitoring tools for detecting coastal environmental deterioration. [Copyright &y& Elsevier]

Abstract: Processes acting on the early-life histories of marine organisms can have important consequences for the structuring of benthic communities. In particular, the degree of coupling between larval supply and adult abundances can wield considerable influence on the strength of trophic interactions in the ecosystem. These processes have been relatively well described in rocky systems and soft-sediment communities, and it is clear that they are governed by very different bottlenecks. Seagrass meadows make interesting study systems because they bear structural affinities to both soft sediments as well as rocky substrates. We examined the early-life history of Paracentrotus lividus, one of the dominant herbivores in Mediterranean seagrass meadows, to identify the drivers of population dynamics in this species. We measured spatial and temporal variability in sea urchin post-settlement in 10 Posidonia oceanica meadows in the North-Western Mediterranean over a period of two years, and compared the numbers with the one-year old cohort a year later (i.e. the new population recruitment) as well as between successive size–age groups. Urchin post-settlers differed substantially between meadows but were present in both years in all meadows surveyed, suggesting that larval supply was not limiting for any of the studied sites. However, in six of the studied meadows, the one-year cohort of urchins was absent in both years, indicating that post-settlement processes strongly affected urchins in these meadows. In contrast, in four of the studied meadows, there was a strong coupling between post-settlers and one-year cohort individuals. These meadows were structurally different from the others in that they were characterised by an exposed matrix of rhizomes forming a dense seagrass mat. This mat apparently strongly mediates post-settlement mortality, and its presence or absence dictates the successful establishment of urchin populations in seagrass meadows. As the population aged, the relationship between size–age groups decreased evidencing the action of other processes. Yet, these results indicate that differences in physical structure are a vital bottleneck for sea urchin populations in seagrass meadows. Exploring the interaction between ecosystem structure and early-life history may provide a broader and more unified framework to understand the dynamics of a range of benthic habitats, including rocky substrates, soft sediments and seagrass meadows. [Copyright &y& Elsevier]

Abstract Patchy landscapes behave differently from continuous ones. Patch size can influence species behaviour, movement, feeding and predation rates, with flow-on consequences for the diversity of species that inhabit these patches. To understand the importance of patchiness on regional species pools, we measured decapod richness and abundance in several seagrass patches with contrasting sizes. Additionally, we evaluated potential drivers of patch-specific species distribution including resource abundance, predator habitat use and the structural complexity of patches. Our results showed a non-random distribution of decapod species: small patches were clear hotspots of diversity and abundance, particularly of larger-bodied epifaunal decapods. Interestingly, these hotspots were characterized by lower nutrient resources, lower canopy height, but also lower predator use. Small fish invertivores such as Coris julis and several species of Symphodus were mostly restricted to large patches. These resident predators may be critical in clumping predation in large patches with consequences for how biodiversity of their prey is distributed across the seascape. Our results highlight the idea that a habitat mosaic with both large and small seagrass patches would potentially bolster biodiversity because preys and predators may seek refuge in patches of different sizes. Highlights • Decapod species are unevenly distributed in marine landscapes. • Small patches can be hotspots of diversity and abundance. • Predation by fish the main factor determining decapod richness and abundances. • Seascapes with a mixture of large and small patches can bolster biodiversity. [ABSTRACT FROM AUTHOR]

Changes in light and sediment conditions can sometimes trigger abrupt regime shifts in seagrass meadows resulting in dramatic and unexpected die-offs of seagrass. Light attenuates rapidly with depth, and in seagrass systems with non-linear behaviours, can serve as a sharp boundary beyond which the meadow transitions to bare sand. Determining system behaviour is therefore essential to ensuring resilience is maintained and to prevent stubborn critical ecosystem transitions caused by declines in water quality. Here we combined field and modelling studies to explore the transition from meadow to bare sand in the seagrass Cymodocea nodosa at the limit of its depth distribution in a shallow, light-limited bay. We first describe the relationship between light availability and seagrass density along a depth gradient in an extensive unfragmented meadow (Alfacs bay, NE Spain). We then develop a simple mechanistic model to characterise system behaviour. In the field, we identified sharp decline in shoot density beyond a threshold of ∼1.9 m depth, shifting from a vegetated state to bare sand. The dynamic population model we developed assumes light-dependent growth and an inverse density-dependent mortality due to facilitation between shoots (mortality rate decreases as shoot density increases). The model closely tracked our empirical observations, and both the model and the field data showed signs of bistability. This strongly suggests that the depth limit of C. nodosa is a critical transition driven by photosynthetic light requirements. While the mechanisms still need to be confirmed with experimental evidence, recognizing the non-linear behaviour of C. nodosa meadows is vital not only in improving our understanding of light effects on seagrass dynamics, but also in managing shallow-water meadows. Given the shallow threshold (<2m), light-limited systems may experience significant and recalcitrant meadow retractions with even small changes in sediment and light conditions. Understanding the processes underlying meadow resilience can inform the maintenance and restoration of meadows worldwide. • Cymodocea nodosa meadow shows an abrupt decline in shoot density beyond a threshold of available light at the depth limit. • Bi-stability trends, at the depth limit, suggest alternative stable state dynamics in the seagrass meadow. • We propose a general mechanistic model based on light-dependent growth and density-dependent mortality in the meadow. • The advantages of mixing modeling and observational studies are evident in the following results. [ABSTRACT FROM AUTHOR]

Managing coastal development requires a set of tools to adequately detect ecosystem and water column degradation, but it also demands tools to detect any post-disturbance improvement. Structural seagrass indicators (such as shoot density or cover) are often used to detect or assess disturbances, but while they may be very sensitive to the impact itself, it is unclear if those indicators on their own can effectively reflect recovery at time scales relevant to managers. We used the construction of a harbour affecting a nearby Posidonia oceanica seagrass community to test the ability of a set of indicators (structural and others) to detect alterations and to evaluate their sensitivity to recovery of environmental quality after harbour construction was complete and the disturbance ceased. We used a Beyond Before After Control Impact (BBACI) design to evaluate effects on one impacted and three control meadows where we used structural, morphological, community and physiological indicators (26 in total) to asses disturbance impacts. Additionally, we measured some of the potential environmental factors that could be altered during and after the construction of the harbour and are critical to the survival of the seagrass meadow (light, sediment organic matter, sediment accrual). Harbour construction caused a clear increase in sediment organic matter and in sediment deposition rates, especially fine sand. Light availability was also reduced due to suspended sediments. Sediment and light conditions returned to normal levels 5 and 15 months after the construction began. As expected, seagrass structural indicators responded unequivocally to these environmental changes, with clear reductions in shoot density. Additionally, reduced light conditions quickly resulted in a decline in carbohydrate content in affected meadows. Unexpectedly, we also recorded a significant increase in metal content in plant tissues. No response was detected in the physiological indicators related to eutrophication (e.g. N and P content in tissues) and in morphological (shoot biomass) and community (epiphyte biomass) indicators. More than three years after the completion of the harbour, structural indicators did not show any sign of recovery. In contrast, physiological indicators, mainly heavy metal and carbohydrates content, were much better in detecting the improvement of the environmental conditions over the fairly short period of this study. These results indicate that while structural indicators are critical to evaluate the immediate effect of disturbances and the recovery on impacted systems, specific physiological indicators may be much better suited to determining the timing of environmental quality recovery. The design of impact and monitoring protocols in the wake of coastal developmental projects need to consider the differential effectiveness and time-response of measured indicators carefully. [ABSTRACT FROM AUTHOR]