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Exotic species are a growing global ecological threat; however, their overall effects are insufficiently understood. While some exotic species are implicated in many species extinctions, others can provide benefits to the recipient communities. Here, we performed a meta-analysis to quantify and synthesize the ecological effects of 76 exotic marine species (about 6% of the listed exotics) on ten variables in marine communities. These species caused an overall significant, but modest in magnitude (as indicated by a mean effect size of g  < 0.2), decrease in ecological variables. Marine primary producers and predators were the most disruptive trophic groups of the exotic species. Approximately 10% (that is, 2 out of 19) of the exotic species assessed in at least three independent studies had significant impacts on native species. Separating the innocuous from the disruptive exotic species provides a basis for triage efforts to control the marine exotic species that have the most impact, thereby helping to meet Aichi Biodiversity Target 9 of the Convention on Biological Diversity. A meta-analysis reveals that the presence of exotic species has a modest but significantly negative impact on the ecological properties of native marine communities and identifies the exotic species that exert the most harmful effects.

Eelgrass Zostera marina L. meadows are major structural and trophic components of coastal ecosystems. The role of eelgrass in ecosystem functioning depends on biomass and production of the meadows, which can fluctuate greatly during an annual cycle and be major temporal drivers of changes in the coastal zone. We analysed magnitude and seasonality of eelgrass aboveground biomass, shoot density and production across temperature and latitude gradients over the majority of the species’ distributional range, and investigated to what extent temperature and/or light drive differences in these values. Eelgrass phenology (timing of peak biomass, start and end of the growing season) showed strong effects of temperature and latitude, indicating that seasonality was considerably advanced in warm areas at low latitudes compared to cold areas at high latitudes. Magnitude of peak aboveground biomass, length of the growing season, mean annual shoot density and aboveground production did not change significantly with either temperature or latitude, indicating that these parameters were controlled mainly by other factors. Annual variation in aboveground biomass and shoot density was significantly smaller in areas with low summer temperature, indicating that while warm-water populations may show substantial temporal variation in biomass, cold-water meadows are less dynamic. These findings were supported by cold-water populations having a larger mean annual biomass and a greater investment in belowground parts. In all significant regressions, temperature was a better predictor of population dynamics than latitude. This indicates that eelgrass phenology might advance considerably in response to global warming, and suggests that the distributional range of this species might be moving northwards. Given the key role of eelgrass in coastal ecosystems, these climate-induced changes might entail substantial impacts on waterbirds, fish, invertebrates and other organisms exploiting these meadows.

Accurately forecasting the response of global biota to warming is a fundamental challenge for ecology in the Anthropocene. Within-species variation in thermal sensitivity, caused by phenotypic plasticity and local adaptation of thermal limits, is often overlooked in assessments of species responses to warming. Despite this, implicit assumptions of thermal niche conservatism or adaptation and plasticity at the species level permeate the literature with potentially important implications for predictions of warming impacts at the population level. Here we review how these attributes interact with the spatial and temporal context of ocean warming to influence the vulnerability of marine organisms. We identify a broad spectrum of thermal sensitivities among marine organisms, particularly in central and cool-edge populations of species distributions. These are characterized by generally low sensitivity in organisms with conserved thermal niches, to high sensitivity for organisms with locally adapted thermal niches. Important differences in thermal sensitivity among marine taxa suggest that warming could adversely affect benthic primary producers sooner than less vulnerable higher trophic groups. Embracing the spatial, temporal and biological context of within-species variation in thermal physiology helps explain observed impacts of ocean warming and can improve forecasts of climate change vulnerability in marine systems. This article is part of the theme issue ‘Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen’.

Marine vegetated habitats occupy a small fraction of the ocean surface, but contribute about 50% of the carbon that is buried in marine sediments. In this Review the potential benefits of conservation, restoration and use of these habitats for coastal protection and climate change mitigation are assessed. Marine vegetated habitats (seagrasses, salt-marshes, macroalgae and mangroves) occupy 0.2% of the ocean surface, but contribute 50% of carbon burial in marine sediments. Their canopies dissipate wave energy and high burial rates raise the seafloor, buffering the impacts of rising sea level and wave action that are associated with climate change. The loss of a third of the global cover of these ecosystems involves a loss of CO 2 sinks and the emission of 1 Pg CO 2 annually. The conservation, restoration and use of vegetated coastal habitats in eco-engineering solutions for coastal protection provide a promising strategy, delivering significant capacity for climate change mitigation and adaption.

Seagrass meadows are highly productive habitats found along many of the world's coastline, providing important services that support the overall functioning of the coastal zone. The organic carbon that accumulates in seagrass meadows is derived not only from seagrass production but from the trapping of other particles, as the seagrass canopies facilitate sedimentation and reduce resuspension. Here we provide a comprehensive synthesis of the available data to obtain a better understanding of the relative contribution of seagrass and other possible sources of organic matter that accumulate in the sediments of seagrass meadows. The data set includes 219 paired analyses of the carbon isotopic composition of seagrass leaves and sediments from 207 seagrass sites at 88 locations worldwide. Using a three source mixing model and literature values for putative sources, we calculate that the average proportional contribution of seagrass to the surface sediment organic carbon pool is ∼50%. When using the best available estimates of carbon burial rates in seagrass meadows, our data indicate that between 41 and 66 gC m−2 yr−1 originates from seagrass production. Using our global average for allochthonous carbon trapped in seagrass sediments together with a recent estimate of global average net community production, we estimate that carbon burial in seagrass meadows is between 48 and 112 Tg yr−1, showing that seagrass meadows are natural hot spots for carbon sequestration.

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.

Seagrass ecosystems rank amongst the most efficient natural carbon sinks on earth, sequestering CO 2 through photosynthesis and storing organic carbon (C org ) underneath their soils for millennia and thereby, mitigating climate change. However, estimates of C org stocks and accumulation rates in seagrass meadows (blue carbon) are restricted to few regions, and further information on spatial variability is required to derive robust global estimates. Here we studied soil C org stocks and accumulation rates in seagrass meadows across the Colombian Caribbean. We estimated that Thalassia testudinum meadows store 241 ± 118 Mg C org ha −1 (mean ± SD) in the top 1 m-thick soils, accumulated at rates of 122 ± 62 and 15 ± 7 g C org m −2  year −1 over the last ~ 70 years and up to 2000 years, respectively. The tropical climate of the Caribbean Sea and associated sediment run-off, together with the relatively high primary production of T. testudinum , influencing biotic and abiotic drivers of C org storage linked to seagrass and soil respiration rates, explains their relatively high C org stocks and accumulation rates when compared to other meadows globally. Differences in soil C org storage among Colombian Caribbean regions are largely linked to differences in the relative contribution of C org sources to the soil C org pool (seagrass, algae Halimeda tuna , mangrove and seston) and the content of soil particles < 0.016 mm binding C org and enhancing its preservation. Despite the moderate areal extent of T. testudinum in the Colombian Caribbean (661 km 2 ), it sequesters around 0.3 Tg CO 2 year −1 , which is equivalent to ~ 0.4% of CO 2 emissions from fossil fuels in Colombia. This study adds data from a new region to a growing dataset on seagrass blue carbon and further explores differences in meadow C org storage based on biotic and abiotic environmental factors, while providing the basis for the implementation of seagrass blue carbon strategies in Colombia.

The metabolic rates of seagrass communities were synthesized on the basis of a data set on seagrass community metabolism containing 403 individual estimates derived from a total of 155 different sites. Gross primary production (GPP) rates (mean ± SE = 224.9 ± 11.1 mmol O2 m−2 d−1) tended to be significantly higher than the corresponding respiration (R) rates (mean ± SE = 187.6 ± 10.1 mmol O2 m−2 d−1), indicating that seagrass meadows tend to be autotrophic ecosystems, reflected in a positive mean net community production (NCP 27.2 ± 5.8 mmol O2 m−2 d−1) and a mean P/R ratio above 1 (1.55 ± 0.13). Tropical seagrass meadows tended to support higher metabolic rates and somewhat lower NCP than temperate ones. The P/R ratio tended to increase with increasing GPP, exceeding, on average, the value of 1 indicative of metabolic balance for communities supporting a GPP greater than 186 mmol O2 m−2 d−1, on average. The global NCP of seagrass meadows ranged (95% confidence limits of mean values) from 20.73 to 50.69 Tg C yr−1 considering a low global seagrass area of 300,000 km2 and 41.47 to 101.39 Tg C yr−1 when a high estimate of global seagrass area of 600,000 km2 was considered. The global loss of 29% of the seagrass area represents, therefore, a major loss of intense natural carbon sinks in the biosphere.

Vertical migration to reach cooler waters is a suitable strategy for some marine organisms to adapt to ocean warming. Here, we calculate that realized vertical isotherm migration rates averaged −6.6  +  18.8 m dec −1 across the global ocean between 1980 and 2015. Throughout this century (2006–2100), surface isotherms are projected to deepen at an increasing rate across the globe, averaging −32.3 m dec −1 under the representative concentration pathway (RCP)8.5 ‘business as usual’ emissions scenario, and −18.7 m dec −1 under the more moderate RCP4.5 scenario. The vertical redistribution required by organisms to follow surface isotherms over this century is three to four orders of magnitude less than the equivalent horizontal redistribution distance. However, the seafloor depth and the depth of the photic layer pose ultimate limits to the vertical migration possible by species. Both limits will be reached by the end of this century across much of the ocean, leading to a rapid global compression of the three-dimensional (3D) habitat of many marine organisms. Phytoplankton diversity may be maintained but displaced toward the base of the photic layer, whereas highly productive benthic habitats, especially corals, will have their suitable 3D habitat rapidly reduced. A projection of ocean surface isotherm deepening under emissions scenarios RCP8.5 and RCP4.5 reveals that the potential habitat of many marine organisms will rapidly become tightly compressed between depth levels imposed by isotherm deepening, the thickness of the photic layer and the seafloor.

Following widespread deterioration of coastal ecosystems since the 1960s, current environmental policies demand ecosystem recovery and restoration. However, vague definitions of recovery and untested recovery paradigms complicate efficient stewardship of coastal ecosystems. We critically examine definitions of recovery and identify and test the implicit paradigms against well-documented cases studies based on a literature review. The study highlights a need for more careful specification of recovery targets and metrics for assessing recovery in individual ecosystems. Six recovery paradigms were identified and examination of them established that partial (as opposed to full) recovery prevails, that degradation and recovery typically follow different pathways as buffers act to maintain the degraded state, and that recovery trajectories depend on the nature of the pressure as well as the connectivity of ecosystems and can differ between ecosystem components and among ecosystems. A conceptual model illustrates the findings and also indicates how restoration efforts may accelerate the recovery process.