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Global analysis of seagrass restoration: the importance of large‐scale planting
In coastal and estuarine systems, foundation species like seagrasses, mangroves, saltmarshes or corals provide important ecosystem services. Seagrasses are globally declining and their reintroduction has been shown to restore ecosystem functions. However, seagrass restoration is often challenging, given the dynamic and stressful environment that seagrasses often grow in. From our world‐wide meta‐analysis of seagrass restoration trials (1786 trials), we describe general features and best practice for seagrass restoration. We confirm that removal of threats is important prior to replanting. Reduced water quality (mainly eutrophication), and construction activities led to poorer restoration success than, for instance, dredging, local direct impact and natural causes. Proximity to and recovery of donor beds were positively correlated with trial performance. Planting techniques can influence restoration success. The meta‐analysis shows that both trial survival and seagrass population growth rate in trials that survived are positively affected by the number of plants or seeds initially transplanted. This relationship between restoration scale and restoration success was not related to trial characteristics of the initial restoration. The majority of the seagrass restoration trials have been very small, which may explain the low overall trial survival rate (i.e. estimated 37%). Successful regrowth of the foundation seagrass species appears to require crossing a minimum threshold of reintroduced individuals. Our study provides the first global field evidence for the requirement of a critical mass for recovery, which may also hold for other foundation species showing strong positive feedback to a dynamic environment. Synthesis and applications. For effective restoration of seagrass foundation species in its typically dynamic, stressful environment, introduction of large numbers is seen to be beneficial and probably serves two purposes. First, a large‐scale planting increases trial survival – large numbers ensure the spread of risks, which is needed to overcome high natural variability. Secondly, a large‐scale trial increases population growth rate by enhancing self‐sustaining feedback, which is generally found in foundation species in stressful environments such as seagrass beds. Thus, by careful site selection and applying appropriate techniques, spreading of risks and enhancing self‐sustaining feedback in concert increase success of seagrass restoration.
Journal Article
Seagrass restoration in the greater Southeast Asia region: techniques, species, survival and comparisons among investigations
The greater Southeast Asian region contains the largest global extent of tropical seagrass; however, anthropogenic degradation is estimated to be greater than 7% per year. Although the areal extent of seagrass is presently 36,765 km 2 , Fortes group estimates that 50% of the original seagrass has been degraded from a variety of impacts. One set of solutions to degradation is to restore tropical seagrass successfully, for which information from past results is needed to avoid failures. Van Katwijk, Thorhaug and others provided a global seagrass restoration review of 1,786 trials, but did not include the full Southeast Asian regional information. Thus, we review findings from 228 trials in the greater Southeast Asian region, involving 305,807 restored units with an extent of 372,649 m 2 . Seagrasses planted with varying successes include 13 tropical species and five subtropical or near-subtemperate species. We compare methodologies as well as key factors of light level, energetics, and depth. This review demonstrates the highest survival in seagrass restoration employing sprigs or plugs at medium depths (2–4 m) with adequate light levels in medium to low energetics planting one to several dominant species. Substrate anchors improved successful establishment. Information gaps occur in quantified monitoring of seagrass services reassembled with tropical-seagrass restoration; thus, fisheries’ nursery potentials are not provided. Future actions need national seagrass restoration policies and plans to restore degraded seagrasses. At present, such policies and plans are non-existent in most greater Southeast Asian regional nations, with the exceptions of Australia and the Philippines, although some nations have national plans for restoring corals or mangroves.
Journal Article
The amount of light reaching the leaves in seagrass
Seagrass meadows, and other submerged vegetated habitats, support a wide range of essential ecological services, but the true extents of these services are in many ways still not quantified. One important tool needed to assess and model many of these services is accurate estimations of the systems' primary productivity. Such productivity estimations require an understanding of the underwater light field, especially regarding the amount of light that actually reaches the plants' photosynthetic tissue. In this study, we tested a simple practical approach to estimate leaf light exposure, relative to incoming light at the canopy, by attaching light sensitive film at different positions on leaves of Zostera marina, eelgrass, in four seagrass meadows composed of different shoot density and at two different depths. We found that the light reaching the leaves decreased linearly down through the canopy. While the upper parts of the leaves received approximately the same level of light (photosynthetic photon flux density, PPFD) as recorded with a PAR meter at the canopy top, the average light that the seagrass leaves were exposed to varied between 40 and 60% of the light on top of the canopy, with an overall average of 48%. We recommend that actual light interception is measured when assessing or modelling light depending processes in submerged vegetation, but if this is not achievable a rough estimation for vegetation similar to Z. marina would be to use a correction factor of 0.5 to compensate for the reduced light due to leaf orientation and internal shading.
Journal Article
Restoration of Seagrass Meadows in the Mediterranean Sea: A Critical Review of Effectiveness and Ethical Issues
Some species of seagrasses (e.g., Zostera marina and Posidonia oceanica) have declined in the Mediterranean, at least locally. Others are progressing, helped by sea warming, such as Cymodocea nodosa and the non-native Halophila stipulacea. The decline of one seagrass can favor another seagrass. All in all, the decline of seagrasses could be less extensive and less general than claimed by some authors. Natural recolonization (cuttings and seedlings) has been more rapid and more widespread than was thought in the 20th century; however, it is sometimes insufficient, which justifies transplanting operations. Many techniques have been proposed to restore Mediterranean seagrass meadows. However, setting aside the short-term failure or half-success of experimental operations, long-term monitoring has usually been lacking, suggesting that possible failures were considered not worthy of a scientific paper. Many transplanting operations (e.g., P. oceanica) have been carried out at sites where the species had never previously been present. Replacing the natural ecosystem (e.g., sandy bottoms, sublittoral reefs) with P. oceanica is obviously inappropriate in most cases. This presupposes ignorance of the fact that the diversity of ecosystems is one of the bases of the biodiversity concept. In order to prevent the possibility of seagrass transplanting from being misused as a pretext for further destruction, a guide for the proper conduct of transplanting is proposed.
Journal Article
Positive Ecological Interactions and the Success of Seagrass Restoration
Seagrasses are a valuable environmental resource. They provide a multitude of ecosystem services including nursery habitat, improved water quality, coastal protection, and carbon sequestration. However, seagrasses are in crisis as global coverage is declining at an accelerating rate. Recently, restoration has increased in popularity as a primary conservation action that may help re-establish degraded seagrass beds. With the elevation of restoration efforts as a conservation strategy, new methods that enhance restoration yields need to be explored. Terrestrially, incorporating positive species interactions and feedbacks into planting designs have proven beneficial to enhance restoration success at little additional cost. Decades of work in coastal plant ecosystems, including seagrasses, has shown that positive species relationships and feedbacks are critical for ecosystem stability, expansion, and recovery from disturbance. We reviewed the restoration literature on seagrasses and found, despite the critical role positive interactions play in marine plant systems, less than 8% of studies have tested for the beneficial effects of including positive interactions in seagrass restoration designs. Here we review the full suite of positive species interactions that have been documented in seagrass ecosystems, where they occur, and how they might be integrated into seagrass restoration. The few studies in marine plant communities that have explicitly incorporated positive species interactions and feedbacks have found an increase in plant growth with little additional resource investment. As oceans continue to change and stressors become more prevalent, pioneering restoration methods, such as harnessing positive feedbacks between species, will be key in rehabilitating populations of seagrasses.
Journal Article
Diverse methylotrophic methanogenic archaea cause high methane emissions from seagrass meadows
Marine coastlines colonized by seagrasses are a net source of methane to the atmosphere. However, methane emissions from these environments are still poorly constrained, and the underlying processes and responsible microorganisms remain largely unknown. Here, we investigated methane turnover in seagrass meadows of Posidonia oceanica in the Mediterranean Sea. The underlying sediments exhibited median net fluxes of methane into the water column of ca. 106 μmol CH₄ · m−2 · d−2. Our data show that this methane production was sustained by methylated compounds produced by the plant, rather than by fermentation of buried organic carbon. Interestingly, methane production was maintained long after the living plant died off, likely due to the persistence of methylated compounds, such as choline, betaines, and dimethylsulfoniopropionate, in detached plant leaves and rhizomes. We recovered multiple mcrA gene sequences, encoding for methyl-coenzyme M reductase (Mcr), the key methanogenic enzyme, from the seagrass sediments. Most retrieved mcrA gene sequences were affiliated with a clade of divergent Mcr and belonged to the uncultured Candidatus Helarchaeota of the Asgard superphylum, suggesting a possible involvement of these divergent Mcr in methane metabolism. Taken together, our findings identify the mechanisms controlling methane emissions from these important blue carbon ecosystems.
Journal Article
A review of seagrass ecosystem services: providing nature-based solutions for a changing world
Seagrasses are marine flowering plants, which form extensive meadows in intertidal and shallow water marine environments. They provide a wide range of ecosystem services, which directly or indirectly benefit humans and can be grouped into four broad categories: provisioning (e.g. food production); regulating (e.g. carbon sequestration); supporting (e.g. primary production); and cultural (e.g. recreational, and eco-tourism). This study provides a review of publications focusing on seagrass ecosystem services provision to identify knowledge gaps and improve our understanding of the use of these habitats as nature-based solutions to societal challenges, such as climate change. Results showed that some ecosystem services, namely food provision, carbon sequestration, and maintenance of biodiversity/nursery habitats receive a higher level of focus and attention than others, such as regulation of diseases and social relations, which are rarely, if ever, included in studies. It is clear that in order to fully comprehend the nature-based solution potential held by seagrass ecosystems, studies need to consider ecosystem services as a whole, and also combine and share results across global regions, to better understand the potential impacts of degradation and loss of these ecosystems worldwide. Suggestions include applying novel technologies such as remote sensing and ecological niche modelling to address some of the main gaps in seagrass research, like meadow extent and connectivity within landscapes, to better incorporate preservation of seagrass ecosystems in marine management plans.
Journal Article
Spatiotemporal Trends Observed in 20 Years of Posidonia oceanica Monitoring along the Alicante Coast, Spain
Posidonia oceanica meadows, known to be valuable marine ecosystems, have been reported to be in decline as a result of human activities in recent decades. However, it is still controversial if this decline is a global phenomenon or it is caused by specific disturbances related to human development at a local scale. In order to evaluate changes in P. oceanica meadows, in this study, monitoring data obtained at 14 stations along the Mediterranean coast near Alicante, Spain, over a 20-year period were analyzed. Field data were obtained through the citizen science project POSIMED, which had the aim of carrying out annual monitoring of both shallow and deep P. oceanica meadows along the coast near Alicante and determining whether their ecological status was changing over time. The percentage cover of living P. oceanica and dead matte and shoot density data were used to assess the ecosystem status and to determine whether there had been an overall regional decline in seagrass over the 20-year period. Both cover and density data showed a significant positive trend at most locations. However, the amount of dead matte was noted to slightly increase with time while six shallow and one deep station showed a negative P. oceanica cover trend, indicating that in certain locations meadow regression might be taking place. Shoot density decreased with depth and increased with the amount of rock cover; its correlation with the dead matte percentage was unclear, which probably means that a range of different factors can result in the presence of dead plants. These results support the idea that local disturbances are the cause of seagrass decline in the Mediterranean, thus demonstrating the need for management plans that focus on local stressors of P. oceanica meadows at specific locations. Long-term, large-scale monitoring allows the ecosystem status in the western Mediterranean to be assessed; however, local disturbances can also affect specific locations.
Journal Article
Dynamics and fate of blue carbon in a mangrove–seagrass seascape: influence of landscape configuration and land-use change
ContextSeagrass meadows act as efficient natural carbon sinks by sequestering atmospheric CO2 and through trapping of allochthonous organic material, thereby preserving organic carbon (Corg) in their sediments. Less understood is the influence of landscape configuration and transformation (land-use change) on carbon sequestration dynamics in coastal seascapes across the land–sea interface.ObjectivesWe explored the influence of landscape configuration and degradation of adjacent mangroves on the dynamics and fate of Corg in seagrass habitats.MethodsThrough predictive modelling, we assessed sedimentary Corg content, stocks and source composition in multiple seascapes (km-wide buffer zones) dominated by different seagrass communities in northwest Madagascar. The study area encompassed seagrass meadows adjacent to intact and deforested mangroves.ResultsThe sedimentary Corg content was influenced by a combination of landscape metrics and inherent habitat plant- and sediment-properties. We found a strong land-to-sea gradient, likely driven by hydrodynamic forces, generating distinct patterns in sedimentary Corg levels in seagrass seascapes. There was higher Corg content and a mangrove signal in seagrass surface sediments closer to the deforested mangrove area, possibly due to an escalated export of Corg from deforested mangrove soils. Seascapes comprising large continuous seagrass meadows had higher sedimentary Corg levels in comparison to more diverse and patchy seascapes.ConclusionOur results emphasize the benefit to consider the influence of seascape configuration and connectivity to accurately assess Corg content in coastal habitats. Understanding spatial patterns of variability and what is driving the observed patterns is useful for identifying carbon sink hotspots and develop management prioritizations.
Journal Article