Current extent and future opportunities for living shorelines in Australia

Living shorelines aim to enhance the resilience of coastlines to hazards while simultaneously delivering co-benefits such as carbon sequestration. Despite the potential ecological and socio-economic benefits of living shorelines over conventional engineered coastal protection structures, application is limited globally. Australia has a long and diverse coastline that provides prime opportunities for living shorelines using beaches and dunes, vegetation, and biogenic reefs, which may be either natural ('soft' approach) or with an engineered structural component ('hybrid' approach). Published scientific studies, however, have indicated limited use of living shorelines for coastal protection in Australia. In response, we combined a national survey and interviews of coastal practitioners and a grey and peer-reviewed literature search to (1) identify barriers to living shoreline implementation; and (2) create a database of living shoreline projects in Australia based on sources other than scientific literature. Projects included were those that had either a primary or secondary goal of protection of coastal assets from erosion and/or flooding. We identified 138 living shoreline projects in Australia through the means sampled starting in 1970; with the number of projects increasing through time particularly since 2000. Over half of the total projects (59 %) were considered to be successful according to their initial stated objective (i.e., reducing hazard risk) and 18 % of projects could not be assessed for their success based on the information available. Seventy percent of projects received formal or informal monitoring. Even in the absence of peer-reviewed support for living shoreline construction in Australia, we discovered local and regional increases in their use. This suggests that coastal practitioners are learning on-the-ground, however more generally it was stated that few examples of living shorelines are being made available, suggesting a barrier in information sharing among agencies at a broader scale. A database of living shoreline projects can increase knowledge among practitioners globally to develop best practice that informs technical guidelines for different approaches and helps focus attention on areas for further research.

The cycle of seagrass life: From flowers to new meadows

Understanding sexual reproduction and recruitment in seagrasses is crucial to their conservation and restoration. Flowering, seed production, seed recruitment, and seedling establishment data for the seagrass Posidonia australis was collected annually between 2013 and 2018 in meadows at six locations around Rottnest Island, Western Australia. Variable annual rates of flowering and seed production were observed among meadows between northern and southern sides of the island and among years. Meadows on the northern shore consistently flowered more intensely and produced more seeds across the years of the survey. Inter-site variation in clonal diversity and size of clones, seed production, wind and surface currents during pollen and seed release, and the large, but variable, impact of seed predation are likely the principal drivers of successful recruitment into established meadows and in colonizing unvegetated sands. The prolific but variable annual reproductive investment increases the probability of low levels of continuous recruitment from seed in this seagrass, despite high rates of abiotic and biotic disturbance at seedling, shoot, and patch scales. This strategy also imparts a level of ecological resilience to this long-lived and persistent species.

Ocean connectivity and habitat characteristics predict population genetic structure of seagrass in an extreme tropical setting

Understanding patterns of gene flow and processes driving genetic differentiation is important for a broad range of conservation practices. In marine organisms, genetic differentiation among populations is influenced by a range of spatial, oceanographic, and environmental factors that are attributed to the seascape. The relative influences of these factors may vary in different locations and can be measured using seascape genetic approaches. Here, we applied a seascape genetic approach to populations of the seagrass, Thalassia hemprichii, at a fine spatial scale (~80 km) in the Kimberley coast, western Australia, a complex seascape with strong, multidirectional currents greatly influenced by extreme tidal ranges (up to 11 m, the world's largest tropical tides). We incorporated genetic data from a panel of 16 microsatellite markers, overwater distance, oceanographic data derived from predicted passive dispersal on a 2 km-resolution hydrodynamic model, and habitat characteristics from each meadow sampled. We detected significant spatial genetic structure and asymmetric gene flow, in which meadows 12-14 km apart were less connected than ones 30-50 km apart. This pattern was explained by oceanographic connectivity and differences in habitat characteristics, suggesting a combined scenario of dispersal limitation and facilitation by ocean current with local adaptation. Our findings add to the growing evidence for the key role of seascape attributes in driving spatial patterns of gene flow. Despite the potential for long-distance dispersal, there was significant genetic structuring over small spatial scales implicating dispersal and recruitment bottlenecks and highlighting the importance of implementing local-scale conservation and management measures.

Marine heatwave and reduced light scenarios cause species-specific metabolomic changes in seagrasses under ocean warming

Climate change and extreme climatic events, such as marine heatwaves (MHWs), are threatening seagrass ecosystems. Metabolomics can be used to gain insight into early stress responses in seagrasses and help to develop targeted management and conservation measures. We used metabolomics to understand the temporal and mechanistic response of leaf metabolism in seagrasses to climate change. Two species, temperate Posidonia australis and tropical Halodule uninervis, were exposed to a combination of future warming, simulated MHW with subsequent recovery period, and light deprivation in a mesocosm experiment. The leaf metabolome of P. australis was altered under MHW exposure at ambient light while H. uninervis was unaffected. Light deprivation impacted both seagrasses, with combined effects of heat and low light causing greater alterations in leaf metabolism. There was no MHW recovery in P. australis. Conversely, the heat-resistant leaf metabolome of H. uninervis showed recovery of sugars and intermediates of the tricarboxylic acid cycle under combined heat and low light exposure, suggesting adaptive strategies to long-term light deprivation. Overall, this research highlights how metabolomics can be used to study the metabolic pathways of seagrasses, identifies early indicators of environmental stress and analyses the effects of environmental factors on plant metabolism and health.

Sulfide intrusion in a habitat forming seagrass can be predicted from relative abundance of sulfur cycling genes in sediments

Sulfide intrusion from sediments is an increasingly recognized contributor to seagrass declines globally, yet the relationship between sediment microorganisms and sulfide intrusion has received little attention. Here, we use metagenomic sequencing and stable isotope (³⁴S) analysis to examine this relationship in Cockburn Sound, Australia, a seagrass-dominated embayment with a gradient of sulfide stress and seagrass declines. There was a significant positive relationship between sulfide intrusion into seagrasses and sulfate reduction genes in sediment microbial communities, which was greatest at sites with long term seagrass declines. This is the first demonstration of a significant link between sulfur cycling genes present in seagrass sediments and sulfide intrusion in a habitat-forming seagrass that is experiencing long-term shoot density decline. Given that microorganisms respond rapidly to environmental change, the quantitative links established in this study can be used as a potential management tool to enable the prediction of sulfide stress on large habitat forming seagrasses; a global issue expected to worsen with climate change.

Historic and contemporary biogeographic perspectives on range-wide spatial genetic structure in a widespread seagrass

Historical and contemporary processes drive spatial patterns of genetic diversity. These include climate-driven range shifts and gene flow mediated by biogeographical influences on dispersal. Assessments that integrate these drivers are uncommon, but critical for testing biogeographic hypotheses. Here, we characterize intraspecific genetic diversity and spatial structure across the entire distribution of a temperate seagrass to test marine biogeographic concepts for southern Australia. Predictive modeling was used to contrast the current Posidonia australis distribution to its historical distribution during the Last Glacial Maximum (LGM). Spatial genetic structure was estimated for 44 sampled meadows from across the geographical range of the species using nine microsatellite loci. Historical and contemporary distributions were similar, with the exception of the Bass Strait. Genetic clustering was consistent with the three currently recognized biogeographic provinces and largely consistent with the finer-scale IMCRA bioregions. Discrepancies were found within the Flindersian province and southwest IMCRA bioregion, while two regions of admixture coincided with transitional IMCRA bioregions. Clonal diversity was highly variable but positively associated with latitude. Genetic differentiation among meadows was significantly associated with oceanographic distance. Our approach suggests how shared seascape drivers have influenced the capacity of P. australis to effectively track sea level changes associated with natural climate cycles over millennia, and in particular, the recolonization of meadows across the Continental Shelf following the LGM. Genetic structure associated with IMCRA bioregions reflects the presence of stable biogeographic barriers, such as oceanic upwellings. This study highlights the importance of biogeography to infer the role of historical drivers in shaping extant diversity and structure.

Tissue-specific transcriptome profiles identify functional differences key to understanding whole plant response to life in variable salinity

Plants endure environmental stressors via adaptation and phenotypic plasticity. Studying these mechanisms in seagrasses is extremely relevant as they are important primary producers and functionally significant carbon sinks. These mechanisms are not well understood at the tissue level in seagrasses. Using RNA-seq, we generated transcriptome sequences from tissue of leaf, basal leaf meristem and root organs of Posidonia australis, establishing baseline in situ transcriptomic profiles for tissues across a salinity gradient. Samples were collected from four P. australis meadows growing in Shark Bay, Western Australia. Analysis of gene expression showed significant differences between tissue types, with more variation among leaves than meristem or roots. Gene ontology enrichment analysis showed the differences were largely due to the role of photosynthesis, plant growth and nutrient absorption in leaf, meristem and root organs, respectively. Differential gene expression of leaf and meristem showed upregulation of salinity regulation processes in higher salinity meadows. Our study highlights the importance of considering leaf meristem tissue when evaluating whole-plant responses to environmental change.

This article has an associated First Person interview with the first author of the paper.

Summary: Differences in seagrass leaf, meristem and root transcriptomes across variable salinities are due to tissue-specific processes. Leaf meristem contained the broadest process range, indicating preferential use for inferring plant-wide activity.

Extensive polyploid clonality was a successful strategy for seagrass to expand into a newly submerged environment

Polyploidy has the potential to allow organisms to outcompete their diploid progenitor(s) and occupy new environments. Shark Bay, Western Australia, is a World Heritage Area dominated by temperate seagrass meadows including Poseidon's ribbon weed, Posidonia australis. This seagrass is at the northern extent of its natural geographic range and experiences extremes in temperature and salinity. Our genomic and cytogenetic assessments of 10 meadows identified geographically restricted, diploid clones (2n = 20) in a single location, and a single widespread, high-heterozygosity, polyploid clone (2n = 40) in all other locations. The polyploid clone spanned at least 180 km, making it the largest known example of a clone in any environment on earth. Whole-genome duplication through polyploidy, combined with clonality, may have provided the mechanism for P. australis to expand into new habitats and adapt to new environments that became increasingly stressful for its diploid progenitor(s). The new polyploid clone probably formed in shallow waters after the inundation of Shark Bay less than 8500 years ago and subsequently expanded via vegetative growth into newly submerged habitats.

Cable bacteria at oxygen-releasing roots of aquatic plants: a widespread and diverse plant-microbe association

Cable bacteria are sulfide-oxidising, filamentous bacteria that reduce toxic sulfide levels, suppress methane emissions and drive nutrient and carbon cycling in sediments. Recently, cable bacteria have been found associated with roots of aquatic plants and rice (Oryza sativa). However, the extent to which cable bacteria are associated with aquatic plants in nature remains unexplored. Using newly generated and public 16S rRNA gene sequence datasets combined with fluorescence in situ hybridisation, we investigated the distribution of cable bacteria around the roots of aquatic plants, encompassing seagrass (including seagrass seedlings), rice, freshwater and saltmarsh plants. Diverse cable bacteria were found associated with roots of 16 out of 28 plant species and at 36 out of 55 investigated sites, across four continents. Plant-associated cable bacteria were confirmed across a variety of ecosystems, including marine coastal environments, estuaries, freshwater streams, isolated pristine lakes and intensive agricultural systems. This pattern indicates that this plant-microbe relationship is globally widespread and neither obligate nor species specific. The occurrence of cable bacteria in plant rhizospheres may be of general importance to vegetation vitality, primary productivity, coastal restoration practices and greenhouse gas balance of rice fields and wetlands.

Reproductive Output, Synchrony across Depth and Influence of Source Depth in the Development of Early Life stages of Kelp

Ecklonia radiata is the main foundation species in Australian temperate reefs, yet little has been published on its reproduction and how this may change across its depth range (1-50+ m). In this study, we examined differences in sporophyte morphology and zoospore production during a reproductive season and across four depths (7, 15, 25, and 40 m). Additionally, we examined differences in germination rate, survival, and morphological traits of gametophytes obtained from these four depths, cultured under the same light and temperature conditions. Multivariate morphology of sporophytes differed significantly between deep (~40 m) and shallow sites (7 and 15 m), but individual morphological traits were not significantly different across depths. Total spore production was similar across depths but the peak of zoospore release was observed in February at 15 m of depth (6,154 zoospores · mm-2 of tissue) and the minimum observed in January at 7, 25, and 40 m (1,141, 987, and 214 zoospores · mm-2 of tissue, respectively). The source depth of zoospores did not have an influence in the germination rate or the survival of gametophytes, and only gametophytes sourced from 40 m sites presented significantly less surface area and number of branches. Overall, these results indicate that E. radiata's reproductive performance does not change across its depth range and that kelp beds reproducing in deeper areas may contribute to the replenishment of their shallow counterparts. We propose that deep kelps may constitute a mechanism of resilience against climate change and anthropogenic disturbances.

Seagrass losses since mid-20th century fuelled CO2 emissions from soil carbon stocks

Seagrass meadows store globally significant organic carbon (C_org ) stocks which, if disturbed, can lead to CO₂ emissions, contributing to climate change. Eutrophication and thermal stress continue to be a major cause of seagrass decline worldwide, but the associated CO₂ emissions remain poorly understood. This study presents comprehensive estimates of seagrass soil C_org erosion following eutrophication-driven seagrass loss in Cockburn Sound (23 km² between 1960s and 1990s) and identifies the main drivers. We estimate that shallow seagrass meadows (<5 m depth) had significantly higher C_org stocks in 50 cm thick soils (4.5 ± 0.7 kg C_org /m² ) than previously vegetated counterparts (0.5 ± 0.1 kg C_org /m² ). In deeper areas (>5 m), however, soil C_org stocks in seagrass and bare but previously vegetated areas were not significantly different (2.6 ± 0.3 and 3.0 ± 0.6 kg C_org /m² , respectively). The soil C_org sequestration capacity prevailed in shallow and deep vegetated areas (55 ± 11 and 21 ± 7 g C_org  m^-2  year^-1 , respectively), but was lost in bare areas. We identified that seagrass canopy loss alone does not necessarily drive changes in soil C_org but, when combined with high hydrodynamic energy, significant erosion occurred. Our estimates point at ~0.20 m/s as the critical shear velocity threshold causing soil C_org erosion. We estimate, from field studies and satellite imagery, that soil C_org erosion (within the top 50 cm) following seagrass loss likely resulted in cumulative emissions of 0.06-0.14 Tg CO_2-eq over the last 40 years in Cockburn Sound. We estimated that indirect impacts (i.e. eutrophication, thermal stress and light stress) causing the loss of ~161,150 ha of seagrasses in Australia, likely resulted in the release of 11-21 Tg CO_{2 -eq} since the 1950s, increasing cumulative CO₂ emissions from land-use change in Australia by 1.1%-2.3% per annum. The patterns described serve as a baseline to estimate potential CO₂ emissions following disturbance of seagrass meadows.

Root microbiomes as indicators of seagrass health

The development of early warning indicators that identify ecosystem stress is a priority for improving ecosystem management. As microbial communities respond rapidly to environmental disturbance, monitoring their composition could prove one such early indicator of environmental stress. We combined 16S rRNA gene sequencing of the seagrass root microbiome of Halophila ovalis with seagrass health metrics (biomass, productivity and Fsulphide) to develop microbial indicators for seagrass condition across the Swan-Canning Estuary and the Leschenault Estuary (south-west Western Australia); the former had experienced an unseasonal rainfall event leading to declines in seagrass health. Microbial indicators detected sites of potential stress that other seagrass health metrics failed to detect. Genera that were more abundant in 'healthy' seagrasses included putative methylotrophic bacteria (e.g. Methylotenera and Methylophaga), iron cycling bacteria (e.g. Deferrisoma and Geothermobacter) and N2 fixing bacteria (e.g. Rhizobium). Conversely, genera that were more abundant in 'stressed' seagrasses were dominated by putative sulphur-cycling bacteria, both sulphide-oxidising (e.g. Candidatus Thiodiazotropha and Candidatus Electrothrix) and sulphate-reducing (e.g. SEEP-SRB1, Desulfomonile and Desulfonema). The sensitivity of the microbial indicators developed here highlights their potential to be further developed for use in adaptive seagrass management, and emphasises their capacity to be effective early warning indicators of stress.

Variation in reproductive effort, genetic diversity and mating systems across Posidonia australis seagrass meadows in Western Australia

Populations at the edges of their geographical range tend to have lower genetic diversity, smaller effective population sizes and limited connectivity relative to centre of range populations. Range edge populations are also likely to be better adapted to more extreme conditions for future survival and resilience in warming environments. However, they may also be most at risk of extinction from changing climate. We compare reproductive and genetic data of the temperate seagrass, Posidonia australis on the west coast of Australia. Measures of reproductive effort (flowering and fruit production and seed to ovule ratios) and estimates of genetic diversity and mating patterns (nuclear microsatellite DNA loci) were used to assess sexual reproduction in northern range edge (low latitude, elevated salinities, Shark Bay World Heritage Site) and centre of range (mid-latitude, oceanic salinity, Perth metropolitan waters) meadows in Western Australia. Flower and fruit production were highly variable among meadows and there was no significant relationship between seed to ovule ratio and clonal diversity. However, Shark Bay meadows were two orders of magnitude less fecund than those in Perth metropolitan waters. Shark Bay meadows were characterized by significantly lower levels of genetic diversity and a mixed mating system relative to meadows in Perth metropolitan waters, which had high genetic diversity and a completely outcrossed mating system. The combination of reproductive and genetic data showed overall lower sexual productivity in Shark Bay meadows relative to Perth metropolitan waters. The mixed mating system is likely driven by a combination of local environmental conditions and pollen limitation. These results indicate that seagrass restoration in Shark Bay may benefit from sourcing plant material from multiple reproductive meadows to increase outcrossed pollen availability and seed production for natural recruitment.

Depth moderates loss of marine foundation species after an extreme marine heatwave: could deep temperate reefs act as a refuge?

Marine heatwaves (MHWs) have been documented around the world, causing widespread mortality of numerous benthic species on shallow reefs (less than 15 m depth). Deeper habitats are hypothesized to be a potential refuge from environmental extremes, though we have little understanding of the response of deeper benthic communities to MHWs. Here, we show how increasing depth moderates the response of seaweed- and coral-dominated benthic communities to an extreme MHW across a subtropical–temperate biogeographical transition zone. Benthic community composition and key habitat-building species were characterized across three depths (15, 25 and 40 m) before and several times after the 2011 Western Australian MHW to assess resistance during and recovery after the heatwave. We found high natural variability in benthic community composition along the biogeographic transition zone and across depths with a clear shift in the composition after the MHW in shallow (15 m) sites but a lot less in deeper communities (40 m). Most importantly, key habitat-building seaweeds such as Ecklonia radiata and Syctothalia dorycarpa which had catastrophic losses on shallow reefs, remained and were less affected in deeper communities. Evidently, deep reefs have the potential to act as a refuge during MHWs for the foundation species of shallow reefs in this region.

Too hot to handle: Unprecedented seagrass death driven by marine heatwave in a World Heritage Area

The increased occurrence of extreme climate events, such as marine heatwaves (MHWs), has resulted in substantial ecological impacts worldwide. To date, metrics of thermal stress within marine systems have focussed on coral communities, and less is known about measuring stress relevant to other primary producers, such as seagrasses. An extreme MHW occurred across the Western Australian coastline in the austral summer of 2010-2011, exposing marine communities to summer seawater temperatures 2-5°C warmer than average. Using a combination of satellite imagery and in situ assessments, we provide detailed maps of seagrass coverage across the entire Shark Bay World Heritage Area (ca. 13,000 km² ) before (2002 and 2010) and after the MHW (2014 and 2016). Our temporal analysis of these maps documents the single largest loss in dense seagrass extent globally (1,310 km² ) following an acute disturbance. Total change in seagrass extent was spatially heterogeneous, with the most extensive declines occurring in the Western Gulf, Wooramel Bank and Faure Sill. Spatial variation in seagrass loss was best explained by a model that included an interaction between two heat stress metrics, the most substantial loss occurring when degree heating weeks (DHWm) was ≥10 and the number of days exposed to extreme sea surface temperature during the MHW (DaysOver) was ≥94. Ground truthing at 622 points indicated that change in seagrass cover was predominantly due to loss of Amphibolis antarctica rather than Posidonia australis, the other prominent seagrass at Shark Bay. As seawater temperatures continue to rise and the incidence of MHWs increase globally, this work will provide a basis for identifying areas of meadow degradation, or stability and recovery, and potential areas of resilience.

Automatic Hierarchical Classification of Kelps Using Deep Residual Features

Across the globe, remote image data is rapidly being collected for the assessment of benthic communities from shallow to extremely deep waters on continental slopes to the abyssal seas. Exploiting this data is presently limited by the time it takes for experts to identify organisms found in these images. With this limitation in mind, a large effort has been made globally to introduce automation and machine learning algorithms to accelerate both classification and assessment of marine benthic biota. One major issue lies with organisms that move with swell and currents, such as kelps. This paper presents an automatic hierarchical classification method local binary classification as opposed to the conventional flat classification to classify kelps in images collected by autonomous underwater vehicles. The proposed kelp classification approach exploits learned feature representations extracted from deep residual networks. We show that these generic features outperform the traditional off-the-shelf CNN features and the conventional hand-crafted features. Experiments also demonstrate that the hierarchical classification method outperforms the traditional parallel multi-class classifications by a significant margin (90.0% vs. 57.6% and 77.2% vs. 59.0%) on Benthoz15 and Rottnest datasets respectively. Furthermore, we compare different hierarchical classification approaches and experimentally show that the sibling hierarchical training approach outperforms the inclusive hierarchical approach by a significant margin. We also report an application of our proposed method to study the change in kelp cover over time for annually repeated AUV surveys.

Australian vegetated coastal ecosystems as global hotspots for climate change mitigation

Policies aiming to preserve vegetated coastal ecosystems (VCE; tidal marshes, mangroves and seagrasses) to mitigate greenhouse gas emissions require national assessments of blue carbon resources. Here, we present organic carbon (C) storage in VCE across Australian climate regions and estimate potential annual CO₂ emission benefits of VCE conservation and restoration. Australia contributes 5–11% of the C stored in VCE globally (70–185 Tg C in aboveground biomass, and 1,055–1,540 Tg C in the upper 1 m of soils). Potential CO₂ emissions from current VCE losses are estimated at 2.1–3.1 Tg CO₂-e yr^-1, increasing annual CO₂ emissions from land use change in Australia by 12–21%. This assessment, the most comprehensive for any nation to-date, demonstrates the potential of conservation and restoration of VCE to underpin national policy development for reducing greenhouse gas emissions.

Policies aiming to preserve vegetated coastal ecosystems (VCE) to mitigate greenhouse gas emissions require national assessments of blue carbon resources. Here the authors assessed organic carbon storage in VCE across Australian and the potential annual CO₂ emission benefits of VCE conservation and find that Australia contributes substantially the carbon stored in VCE globally.

A review of protocols for the experimental release of kelp (Laminariales) zoospores

ABSTRACT

Kelps (order Laminariales) are foundation species in temperate and arctic seas globally, but they are in decline in many places. Laminarian kelp have an alternation of generations and this poses challenges for experimental studies due to the difficulties in achieving zoospore release and gametophyte growth. Here, we review and synthesize the protocols that have been used to induce zoospore release in kelps to identify commonalities and provide guidance on best practices. We found 171 papers, where zoospore release was induced in four kelp families from 35 different ecoregions. The most commonly treated family was Laminariaceae, followed by Lessoniaceae and the most studied ecoregion was Central Chile, followed by the Southern California Bight. Zoospore release generally involved three steps: a pretreatment which included cleaning of the reproductive tissue to eliminate epiphytic organisms, followed by desiccation of the tissue, and finally a postdesiccation immersion of the reproductive material in a seawater medium for zoospore release. Despite these commonalities, there was a high degree of variation in the detail within each of these steps, even among studies within genera and from the same ecoregions. This suggests either that zoospore release may be relatively insensitive across the Laminariales or that little methods optimization has been undertaken. We suggest that greater attention to standardization of protocols and reporting of methodology and optimization would improve comparisons of kelp zoospore release across species and locations and facilitate a broader understanding of this key, but understudied life history stage.

OPEN RESEARCH BADGES

This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.0kh1f8j.

Oxygen loss from seagrass roots coincides with colonisation of sulphide-oxidising cable bacteria and reduces sulphide stress

Seagrasses thrive in anoxic sediments where sulphide can accumulate to phytotoxic levels. So how do seagrasses persist in this environment? Here, we propose that radial oxygen loss (ROL) from actively growing root tips protects seagrasses from sulphide intrusion not only by abiotically oxidising sulphides in the rhizosphere of young roots, but also by influencing the abundance and spatial distribution of sulphate-reducing and sulphide-oxidising bacteria. We used a novel multifaceted approach combining imaging techniques (confocal fluorescence in situ hybridisation, oxygen planar optodes, and sulphide diffusive gradients in thin films) with microbial community profiling to build a complete picture of the microenvironment of growing roots of the seagrasses Halophila ovalis and Zostera muelleri. ROL was restricted to young root tips, indicating that seagrasses will have limited ability to influence sulphide oxidation in bulk sediments. On the microscale, however, ROL corresponded with decreased abundance of potential sulphate-reducing bacteria and decreased sulphide concentrations in the rhizosphere surrounding young roots. Furthermore, roots leaking oxygen had a higher abundance of sulphide-oxidising cable bacteria; which is the first direct observation of these bacteria on seagrass roots. Thus, ROL may enhance both abiotic and bacterial sulphide oxidation and restrict bacterial sulphide production around vulnerable roots, thereby helping seagrasses to colonise sulphide-rich anoxic sediments.

Effects of desalination brine and seawater with the same elevated salinity on growth, physiology and seedling development of the seagrass Posidonia australis

Desalination has the potential to provide an important source of potable water to growing coastal populations but it also produces highly saline brines with chemical additives, posing a possible threat to benthic marine communities. The effects of brine (0%, 50%, 100%) were compared to seawater treatments with the same salinity (37, 46, 54 psu) for seagrass (Posidonia australis) in mesocosms over 2 weeks. There were significant differences between brine and salinity treatments for photosynthesis, water relations and growth. Germinating seedlings of P. australis were also tested in brine treatments (0%, 25%, 50%, 100%) over 7 weeks followed by 2.5 weeks recovery in seawater. Growth was severely inhibited only in 100% brine. These experiments demonstrated that brine increased the speed and symptoms of stress in adult plants compared to treatments with the same salinity, whereas seedlings tolerated far longer brine exposure, and so could potentially contribute to seagrass recovery through recruitment.

Seagrass ecosystem trajectory depends on the relative timescales of resistance, recovery and disturbance

Seagrass ecosystems are inherently dynamic, responding to environmental change across a range of scales. Habitat requirements of seagrass are well defined, but less is known about their ability to resist disturbance. Specific means of recovery after loss are particularly difficult to quantify. Here we assess the resistance and recovery capacity of 12 seagrass genera. We document four classic trajectories of degradation and recovery for seagrass ecosystems, illustrated with examples from around the world. Recovery can be rapid once conditions improve, but seagrass absence at landscape scales may persist for many decades, perpetuated by feedbacks and/or lack of seed or plant propagules to initiate recovery. It can be difficult to distinguish between slow recovery, recalcitrant degradation, and the need for a window of opportunity to trigger recovery. We propose a framework synthesizing how the spatial and temporal scales of both disturbance and seagrass response affect ecosystem trajectory and hence resilience.

Metagenomic Evidence of Microbial Community Responsiveness to Phosphorus and Salinity Gradients in Seagrass Sediments

Sediment microorganisms can have profound influence on productivity and functioning of marine ecosystems through their critical roles in regulating biogeochemical processes. However, the identity of sediment microorganisms that mediate organic matter turnover and nutrient cycling in seagrass sediments is only poorly understood. Here, we used metagenomic sequencing to investigate shifts in the structure and functioning of the microbial community of seagrass sediments across a salinity and phosphorus (P) availability gradient in Shark Bay, WA, Australia. This iconic ecosystem is oligotrophic and hypersaline with abundant seagrass meadows that directly contribute Shark Bay's status as a World Heritage Site. We show that sediment phosphonate metabolism genes as well as enzyme activities increase in hypersaline conditions with lower soluble reactive phosphate in the water column. Given that sediment organic P content is also highest where P concentrations in the water column are low, we suggest that microbial processing of organic P can contribute to the P requirements of seagrasses at particularly oligotrophic sites. Seagrass meadows are often highly productive in oligotrophic waters, and our findings suggest that an increase in the functional capacity of microbial communities in seagrass sediments to break down organic P may contribute to the high productivity of seagrass meadows under oligotrophic conditions. When compared to soil and sediment metagenomes from other aquatic and terrestrial ecosystems, we also show microbial communities in seagrass sediments have a disproportionately high abundance of putative phosphorus and sulfur metabolism genes, which may have played an important evolutionary role in allowing these angiosperms to recolonize the marine environment and prosper under oligotrophic conditions.

Genomic comparison of two independent seagrass lineages reveals habitat-driven convergent evolution

Seagrasses are marine angiosperms that live fully submerged in the sea. They evolved from land plant ancestors, with multiple species representing at least three independent return-to-the-sea events. This raises the question of whether these marine angiosperms followed the same adaptation pathway to allow them to live and reproduce under the hostile marine conditions. To compare the basis of marine adaptation between seagrass lineages, we generated genomic data for Halophila ovalis and compared this with recently published genomes for two members of Zosteraceae, as well as genomes of five non-marine plant species (Arabidopsis, Oryza sativa, Phoenix dactylifera, Musa acuminata, and Spirodela polyrhiza). Halophila and Zosteraceae represent two independent seagrass lineages separated by around 30 million years. Genes that were lost or conserved in both lineages were identified. All three species lost genes associated with ethylene and terpenoid biosynthesis, and retained genes related to salinity adaptation, such as those for osmoregulation. In contrast, the loss of the NADH dehydrogenase-like complex is unique to H. ovalis. Through comparison of two independent return-to-the-sea events, this study further describes marine adaptation characteristics common to seagrass families, identifies species-specific gene loss, and provides molecular evidence for convergent evolution in seagrass lineages.

Determining light stress responses for a tropical multi-species seagrass assemblage

Existing mitigations to address deterioration in water clarity associated with human activities are based on responses from single seagrass species but may not be appropriate for diverse seagrass assemblages common to tropical waters. We present findings from a light experiment designed to determine the effects of magnitude and duration of low light on a mixed tropical seagrass assemblage. Mixed assemblages of three commonly co-occurring Indo-West Pacific seagrasses, Cymodocea serrulata, Halodule uninervis and Halophila ovalis were grown in climate-controlled tanks, where replicate pots were subjected to a gradient in light availability (0.9-21.6 mols PAR m^-2 day^-1) for 12 weeks. Increased shading resulted in declines in growth and changes in cellular and photosynthesis responses for all species, although time-scale and magnitude of response were species-specific. Applying management criteria (e.g. thresholds) relevant to one species may under- or over-estimate potential for impact on other species and the meadow as a whole.

Short-term Responses of Posidonia australis to Changes in Light Quality

Seagrass meadows are highly productive ecosystems that provide ecosystem services to the coastal zone but are declining globally, particularly due to anthropogenic activities that reduce the quantity of light reaching seagrasses, such as dredging, river discharge and eutrophication. Light quality (the spectral composition of the light) is also altered by these anthropogenic stressors as the differential attenuation of wavelengths of light is caused by materials within the water column. This study addressed the effect of altered light quality on different life-history stages of the seagrass Posidonia australis, a persistent, habitat-forming species in Australia. Aquarium-based experiments were conducted to determine how adult shoots and seedlings respond to blue (peak λ = 451 nm); green (peak λ = 522 nm); yellow (peak λ = 596 nm) and red (peak λ = 673 nm) wavelengths with a control of full-spectrum light (λ = 400 - 700 nm, at 200 μmol photons m^-2 s^-1). Posidonia australis adults did not respond to changes in light quality relative to full-spectrum light, demonstrating a capacity to obtain enough photons from a range of wavelengths across the visible spectrum to maintain short-term growth at high irradiances. Posidonia australis seedlings (<4 months old) grown in blue light showed a significant increase in xanthophyll concentrations when compared to plants grown in full-spectrum, demonstrating a pigment acclimation response to blue light. These results differed significantly from negative responses to changes in light quality recently described for Halophila ovalis, a colonizing seagrass species. Persistent seagrasses such as P. australis, appear to be better at tolerating short-term changes in light quality compared to colonizing species when sufficient PPFD is present.

Low Light Availability Alters Root Exudation and Reduces Putative Beneficial Microorganisms in Seagrass Roots

Seagrass roots host a diverse microbiome that is critical for plant growth and health. Composition of microbial communities can be regulated in part by root exudates, but the specifics of these interactions in seagrass rhizospheres are still largely unknown. As light availability controls primary productivity, reduced light may impact root exudation and consequently the composition of the root microbiome. Hence, we analyzed the influence of light availability on root exudation and community structure of the root microbiome of three co-occurring seagrass species, Halophila ovalis, Halodule uninervis and Cymodocea serrulata. Plants were grown under four light treatments in mesocosms for 2 weeks; control (100% surface irradiance (SI), medium (40% SI), low (20% SI) and fluctuating light (10 days 20% and 4 days 100%). 16S rDNA amplicon sequencing revealed that microbial diversity, composition and predicted function were strongly influenced by the presence of seagrass roots, such that root microbiomes were unique to each seagrass species. Reduced light availability altered seagrass root exudation, as characterized using fluorescence spectroscopy, and altered the composition of seagrass root microbiomes with a reduction in abundance of potentially beneficial microorganisms. Overall, this study highlights the potential for above-ground light reduction to invoke a cascade of changes from alterations in root exudation to a reduction in putative beneficial microorganisms and, ultimately, confirms the importance of the seagrass root environment - a critical, but often overlooked space.

Disturbance Is an Important Driver of Clonal Richness in Tropical Seagrasses

Clonality is common in many aquatic plant species, including seagrasses, where populations are maintained through a combination of asexual and sexual reproduction. One common measure used to describe the clonal structure of populations is clonal richness. Clonal richness is strongly dependent on the biological characteristics of the species, and how these interact with the environment but can also reflect evolutionary scale processes especially at the edge of species ranges. However, little is known about the spatial patterns and drivers of clonal richness in tropical seagrasses. This study assessed the spatial patterns of clonal richness in meadows of three tropical seagrass species, Thalassia hemprichii, Halodule uninervis, and Halophila ovalis, spanning a range of life-history strategies and spatial scales (2.5-4,711 km) in Indonesia and NW Australia. We further investigated the drivers of clonal richness using general additive mixed models for two of the species, H. uninervis and H. ovalis, over 8° latitude. No significant patterns were observed in clonal richness with latitude, yet disturbance combined with sea surface temperature strongly predicted spatial patterns of clonal richness. Sites with a high probability of cyclone disturbance had low clonal richness, whereas an intermediate probability of cyclone disturbance and the presence of dugong grazing combined with higher sea surface temperatures resulted in higher levels of clonal richness. We propose potential mechanisms for these patterns related to the recruitment and mortality rates of individuals as well as reproductive effort. Under a changing climate, increased severity of tropical cyclones and the decline in populations of mega-grazers have the potential to reduce clonal richness leading to less genetically diverse populations.

Sediment tolerance mechanisms identified in sponges using advanced imaging techniques

Terrestrial runoff, resuspension events and dredging can affect filter-feeding sponges by elevating the concentration of suspended sediments, reducing light intensity, and smothering sponges with sediments. To investigate how sponges respond to pressures associated with increased sediment loads, the abundant and widely distributed Indo-Pacific species Ianthella basta was exposed to elevated suspended sediment concentrations, sediment deposition, and light attenuation for 48 h (acute exposure) and 4 weeks (chronic exposure). In order to visualise the response mechanisms, sponge tissue was examined by 3D X-ray microscopy and scanning electron microscopy (SEM). Acute exposures resulted in sediment rapidly accumulating in the aquiferous system of I. basta, although this sediment was fully removed within three days. Sediment removal took longer (>2 weeks) following chronic exposures, and I. basta also exhibited tissue regression and a smaller aquiferous system. The application of advanced imaging approaches revealed that I. basta employs a multilevel system for sediment rejection and elimination, containing both active and passive components. Sponges responded to sediment stress through (i) mucus production, (ii) exclusion of particles by incurrent pores, (iii) closure of oscula and pumping cessation, (iv) expulsion of particles from the aquiferous system, and (v) tissue regression to reduce the volume of the aquiferous system, thereby entering a dormant state. These mechanisms would result in tolerance and resilience to exposure to variable and high sediment loads associated with both anthropogenic impacts like dredging programs and natural pressures like flood events.

Timing anthropogenic stressors to mitigate their impact on marine ecosystem resilience

Better mitigation of anthropogenic stressors on marine ecosystems is urgently needed to address increasing biodiversity losses worldwide. We explore opportunities for stressor mitigation using whole-of-systems modelling of ecological resilience, accounting for complex interactions between stressors, their timing and duration, background environmental conditions and biological processes. We then search for ecological windows, times when stressors minimally impact ecological resilience, defined here as risk, recovery and resistance. We show for 28 globally distributed seagrass meadows that stressor scheduling that exploits ecological windows for dredging campaigns can achieve up to a fourfold reduction in recovery time and 35% reduction in extinction risk. Although the timing and length of windows vary among sites to some degree, global trends indicate favourable windows in autumn and winter. Our results demonstrate that resilience is dynamic with respect to space, time and stressors, varying most strongly with: (i) the life history of the seagrass genus and (ii) the duration and timing of the impacting stress.

Identifying critical recruitment bottlenecks limiting seedling establishment in a degraded seagrass ecosystem

Identifying early life-stage transitions limiting seagrass recruitment could improve our ability to target demographic processes most responsive to management. Here we determine the magnitude of life-stage transitions along gradients in physical disturbance limiting seedling establishment for the marine angiosperm, Posidonia australis. Transition matrix models and sensitivity analyses were used to identify which transitions were critical for successful seedling establishment during the first year of seed recruitment and projection models were used to predict the most appropriate environments and seeding densities. Total survival probability of seedlings was low (0.001), however, transition probabilities between life-stages differed across the environmental gradients; seedling recruitment was affected by grazing and bioturbation prevailing during the first life-stage transition (1 month), and 4-6 months later during the third life-stage transition when establishing seedlings are physically removed by winter storms. Models projecting population growth from different starting seed densities showed that seeds could replace other more labour intensive and costly methods, such as transplanting adult shoots, if disturbances are moderated sufficiently and if large numbers of seed can be collected in sufficient quantity and delivered to restoration sites efficiently. These outcomes suggest that by improving management of early demographic processes, we could increase recruitment in restoration programs.

Belowground stressors and long-term seagrass declines in a historically degraded seagrass ecosystem after improved water quality

Continued seagrass declines in ecosystems with improved water quality may be driven by sediment stressors. One of the most cited examples of a seagrass ecosystem with declines is Cockburn Sound, Western Australia, where 75% of seagrasses (2169 ha) were lost in the 1960s-1980s due to poor water quality. Water quality has subsequently improved in Cockburn Sound, yet shoot density declines continue in some areas. Here, we investigated if sediment stressors (sulfide intrusion and heavy metals) contributed to declining Posidonia sinuosa shoot densities in Cockburn Sound. Seagrass δ³⁴S were depleted at sites with a history of seagrass declines, indicating seagrasses at these sites were under sulfide stress. Heavy metals (Fe, Zn, Mn, Cr, Cu and Cd) in sediments and seagrasses did not show clear patterns with shoot density or biomass, and largely decreased from similar measurements in the late 1970s. However, seagrass cadmium concentrations were negatively correlated to seagrass biomass and shoot density. High cadmium concentrations interfere with sulfur metabolism in terrestrial plants, but impacts on seagrasses remain to be explored. Given that sulfide intrusion can prevent recolonization and drive seagrass declines, management plans in degraded seagrass ecosystems should include management of sediment stressors and water quality to provide comprehensive management.

Restricted gene flow and local adaptation highlight the vulnerability of high-latitude reefs to rapid environmental change

Global climate change poses a serious threat to the future health of coral reef ecosystems. This calls for management strategies that are focused on maximizing the evolutionary potential of coral reefs. Fundamental to this is an accurate understanding of the spatial genetic structure in dominant reef-building coral species. In this study, we apply a genotyping-by-sequencing approach to investigate genome-wide patterns of genetic diversity, gene flow, and local adaptation in a reef-building coral, Pocillopora damicornis, across 10 degrees of latitude and a transition from temperate to tropical waters. We identified strong patterns of differentiation and reduced genetic diversity in high-latitude populations. In addition, genome-wide scans for selection identified a number of outlier loci putatively under directional selection with homology to proteins previously known to be involved in heat tolerance in corals and associated with processes such as photoprotection, protein degradation, and immunity. This study provides genomic evidence for both restricted gene flow and local adaptation in a widely distributed coral species, and highlights the potential vulnerability of leading-edge populations to rapid environmental change as they are locally adapted, reproductively isolated, and have reduced levels of genetic diversity.

Effects of high salinity from desalination brine on growth, photosynthesis, water relations and osmolyte concentrations of seagrass Posidonia australis

Highly saline brines from desalination plants expose seagrass communities to salt stress. We examined effects of raised salinity (46 and 54psu) compared with seawater controls (37psu) over 6weeks on the seagrass, Posidonia australis, growing in tanks with the aim of separating effects of salinity from other potentially deleterious components of brine and determining appropriate bioindicators. Plants survived exposures of 2-4weeks at 54psu, the maximum salinity of brine released from a nearby desalination plant. Salinity significantly reduced maximum quantum yield of PSII (chlorophyll a fluorescence emissions). Leaf water potential (Ψ_w) and osmotic potential (Ψ_π) were more negative at increased salinity, while turgor pressure (Ψ_p) was unaffected. Leaf concentrations of K⁺ and Ca²⁺ decreased, whereas concentrations of sugars (mainly sucrose) and amino acids increased. We recommend leaf osmolarity, ion, sugar and amino acid concentrations as bioindicators for salinity effects, associated with brine released in desalination plant outfalls.

Accelerating Tropicalization and the Transformation of Temperate Seagrass Meadows

Climate-driven changes are altering production and functioning of biotic assemblages in terrestrial and aquatic environments. In temperate coastal waters, rising sea temperatures, warm water anomalies and poleward shifts in the distribution of tropical herbivores have had a detrimental effect on algal forests. We develop generalized scenarios of this form of tropicalization and its potential effects on the structure and functioning of globally significant and threatened seagrass ecosystems, through poleward shifts in tropical seagrasses and herbivores. Initially, we expect tropical herbivorous fishes to establish in temperate seagrass meadows, followed later by megafauna. Tropical seagrasses are likely to establish later, delayed by more limited dispersal abilities. Ultimately, food webs are likely to shift from primarily seagrass-detritus to more direct-consumption-based systems, thereby affecting a range of important ecosystem services that seagrasses provide, including their nursery habitat role for fishery species, carbon sequestration, and the provision of organic matter to other ecosystems in temperate regions.

The Genome of a Southern Hemisphere Seagrass Species (Zostera muelleri)

Seagrasses are marine angiosperms that evolved from land plants but returned to the sea around 140 million years ago during the early evolution of monocotyledonous plants. They successfully adapted to abiotic stresses associated with growth in the marine environment, and today, seagrasses are distributed in coastal waters worldwide. Seagrass meadows are an important oceanic carbon sink and provide food and breeding grounds for diverse marine species. Here, we report the assembly and characterization of the Zostera muelleri genome, a southern hemisphere temperate species. Multiple genes were lost or modified in Z. muelleri compared with terrestrial or floating aquatic plants that are associated with their adaptation to life in the ocean. These include genes for hormone biosynthesis and signaling and cell wall catabolism. There is evidence of whole-genome duplication in Z. muelleri; however, an ancient pan-commelinid duplication event is absent, highlighting the early divergence of this species from the main monocot lineages.

Climate-driven regime shift of a temperate marine ecosystem

Ecosystem reconfigurations arising from climate-driven changes in species distributions are expected to have profound ecological, social, and economic implications. Here we reveal a rapid climate-driven regime shift of Australian temperate reef communities, which lost their defining kelp forests and became dominated by persistent seaweed turfs. After decades of ocean warming, extreme marine heat waves forced a 100-kilometer range contraction of extensive kelp forests and saw temperate species replaced by seaweeds, invertebrates, corals, and fishes characteristic of subtropical and tropical waters. This community-wide tropicalization fundamentally altered key ecological processes, suppressing the recovery of kelp forests.

Photosynthetic response to globally increasing CO2 of co-occurring temperate seagrass species

Photosynthesis of most seagrass species seems to be limited by present concentrations of dissolved inorganic carbon (DIC). Therefore, the ongoing increase in atmospheric CO2 could enhance seagrass photosynthesis and internal O2 supply, and potentially change species competition through differential responses to increasing CO2 availability among species. We used short-term photosynthetic responses of nine seagrass species from the south-west of Australia to test species-specific responses to enhanced CO2 and changes in HCO3 (-) . Net photosynthesis of all species except Zostera polychlamys were limited at pre-industrial compared to saturating CO2 levels at light saturation, suggesting that enhanced CO2 availability will enhance seagrass performance. Seven out of the nine species were efficient HCO3 (-) users through acidification of diffusive boundary layers, production of extracellular carbonic anhydrase, or uptake and internal conversion of HCO3 (-) . Species responded differently to near saturating CO2 implying that increasing atmospheric CO2 may change competition among seagrass species if co-occurring in mixed beds. Increasing CO2 availability also enhanced internal aeration in the one species assessed. We expect that future increases in atmospheric CO2 will have the strongest impact on seagrass recruits and sparsely vegetated beds, because densely vegetated seagrass beds are most often limited by light and not by inorganic carbon.

Heat stress of two tropical seagrass species during low tides - impact on underwater net photosynthesis, dark respiration and diel in situ internal aeration

Seagrasses grow submerged in aerated seawater but often in low O2 sediments. Elevated temperatures and low O2 are stress factors. Internal aeration was measured in two tropical seagrasses, Thalassia hemprichii and Enhalus acoroides, growing with extreme tides and diel temperature amplitudes. Temperature effects on net photosynthesis (PN ) and dark respiration (RD ) of leaves were evaluated. Daytime low tide was characterized by high pO2 (54 kPa), pH (8.8) and temperature (38°C) in shallow pools. As PN was maximum at 33°C (9.1 and 7.2 μmol O2  m(-2) s(-1) in T. hemprichii and E. acoroides, respectively), the high temperatures and reduced CO2 would have diminished PN , whereas RD increased (Q10 of 2.0-2.7) above that at 33°C (0.45 and 0.33 μmol O2  m(-2)  s(-1) , respectively). During night-time low tides, O2 declined resulting in shoot base anoxia in both species, but incoming water containing c. 20 kPa O2 relieved the anoxia. Shoots exposed to 40°C for 4 h showed recovery of PN and RD , whereas 45°C resulted in leaf damage. These seagrasses are 'living near the edge', tolerant of current diel O2 and temperature extremes, but if temperatures rise both species may be threatened in this habitat.

Reproduction at the extremes: pseudovivipary, hybridization and genetic mosaicism in Posidonia australis (Posidoniaceae)

BACKGROUND AND AIMS

Organisms occupying the edges of natural geographical ranges usually survive at the extreme limits of their innate physiological tolerances. Extreme and prolonged fluctuations in environmental conditions, often associated with climate change and exacerbated at species' geographical range edges, are known to trigger alternative responses in reproduction. This study reports the first observations of adventitious inflorescence-derived plantlet formation in the marine angiosperm Posidonia australis, growing at the northern range edge (upper thermal and salinity tolerance) in Shark Bay, Western Australia. These novel plantlets are described and a combination of microsatellite DNA markers and flow cytometry is used to determine their origin.

METHODS

Polymorphic microsatellite DNA markers were used to generate multilocus genotypes to determine the origin of the adventitious inflorescence-derived plantlets. Ploidy and genome size were estimated using flow cytometry.

KEY RESULTS

All adventitious plantlets were genetically identical to the maternal plant and were therefore the product of a novel pseudoviviparous reproductive event. It was found that 87 % of the multilocus genotypes contained three alleles in at least one locus. Ploidy was identical in all sampled plants. The genome size (2 C value) for samples from Shark Bay and from a separate site much further south was not significantly different, implying they are the same ploidy level and ruling out a complete genome duplication (polyploidy).

CONCLUSIONS

Survival at range edges often sees the development of novel responses in the struggle for survival and reproduction. This study documents a physiological response at the trailing edge, whereby reproductive strategy can adapt to fluctuating conditions and suggests that the lower-than-usual water temperature triggered unfertilized inflorescences to 'switch' to growing plantlets that were adventitious clones of their maternal parent. This may have important long-term implications as both genetic and ecological constraints may limit the ability to adapt or range-shift; this seagrass meadow in Shark Bay already has low genetic diversity, no sexual reproduction and no seedling recruitment.

Reconstruction of centennial-scale fluxes of chemical elements in the Australian coastal environment using seagrass archives

The study of a Posidonia australis sedimentary archive has provided a record of changes in element concentrations (Al, Fe, Mn, Pb, Zn, Cr, Cd, Co, As, Cu, Ni and S) over the last 3000 years in the Australian marine environment. Human-derived contamination in Oyster Harbor (SW Australia) started ~100 years ago (AD ~1900) and exponentially increased until present. This appears to be related to European colonization of Australia and the subsequent impact of human activities, namely mining, coal and metal production, and extensive agriculture. Two contamination periods of different magnitude have been identified: Expansion period (EXP, AD ~1900-1970) and Establishment period (EST, AD ~1970 to present). Enrichments of chemical elements with respect to baseline concentrations (in samples older than ~115 cal years BP) were found for all elements studied in both periods, except for Ni, As and S. The highest enrichment factors were obtained for the EST period (ranging from 1.3-fold increase in Cu to 7.2-fold in Zn concentrations) compared to the EXP period (1.1-fold increase for Cu and Cr to 2.4-fold increase for Pb). Zinc, Pb, Mn and Co concentrations during both periods were 2- to 7-fold higher than baseline levels. This study demonstrates the value of Posidonia mats as long-term archives of element concentrations and trends in coastal ecosystems. We also provide preliminary evidence on the potential for Posidonia meadows to act as significant long-term biogeochemical sinks of chemical elements.