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Poor water quality caused by intensive sugarcane farming has been among the main causes of coastal degradation around the Great Barrier Reef (GBR). Despite regulations and incentives, water quality in the GBR has yet to reach government targets, and there is an urgent need to investigate alternative management approaches. In this study, we have employed a social-ecological approach to understand farmers' attitudes to existing nutrient and sediment management approaches, assess farmers' perceptions of managing coastal wetlands to ameliorate agricultural run-off, and propose how these findings could inform integrative policy development for the GBR. We conducted semi-structured interviews with sugarcane farmers in the Wet Tropics to understand perceptions of ecosystem services, coastal wetlands and industry challenges. We linked our analysis of key issues of concern to farmers with broader environmental and land use issues and have outlined future policy considerations in the face of socio-economic and climatic change. Our conclusions emphasise the need to rethink future land tenure in low-lying areas of North Queensland, the importance of considering ecosystem services provided by coastal wetlands in policy models and the need for a payments for ecosystem services (PES) model to safeguard the future of the GBR.

Geomorphological landforms and processes underpin the valuable ecological functions and ecosystem services performed by river-dominated, tide-dominated and interior mangrove forest systems. At the local scale, mangrove system morphodynamics are characterised by complex interactions between tidal channel flows and shallow-flooded vegetated surfaces, with wave energy dissipation across mangrove margins. Surface elevation change in mangroves reflects the balance between surface sedimentation processes and sub-surface processes and, exceptionally, responses to hurricane and tsunami impacts. Geomorphology has a role to play in assessments of mangrove status under anthropogenic pressures and in providing insights into trajectories for effective mangrove rehabilitation and restoration.

Blue carbon provides opportunities to mitigate climate change while increasing ecosystem services for coastal communities, including climate change adaptation; however, blue carbon ecosystems are vulnerable to climate change, leading to uncertainties in the future efficacy of these ecosystems. In this review, we assess the potential impacts of climate change on blue carbon. Despite uncertainties, carbon sequestration in coastal ecosystems is enhanced by landward migration of blue carbon habitats, maintenance of sediment supply, restoration, and improved water quality. As an example, landward migration of mangroves could result in carbon sequestration of 1.5 Pg by 2100. Mudflats, seaweed beds, and coastal swamp forests could also contribute to climate change mitigation, although there are large data gaps. Achieving the full potential of blue carbon requires protection and restoration of ecosystems and facilitation of changes in ecosystem distributions with climate change, actions that will also deliver adaptation benefits. Conversely, in the worst-case coastal squeeze scenario, losses of 3.4 Pg of sequestered carbon by 2100 could occur.

Macroalgae are important components on coral reefs that underpin food webs but are also indicators of declines in coral reef condition. The sensitivity of macroalgal communities to environmental factors, including climate and pollutants from the land, is important for the future state of coral reefs. We assessed regional variation in the photosynthetic physiology of macroalgae on the Great Barrier Reef and Moreton Bay, Queensland, Australia, using delta C-13, tissue chemistry, and metabolic indicators over broad environmental gradients from six sites spanning 13 degrees of latitude and varying distance from shore. Our data set included 568 samples from 54 genera and 19 macroalgal orders. The delta C-13 of tissues had a strong taxonomic basis (explaining 66% of the variation in delta C-13), varying significantly among algal orders. Particularly low values of delta C-13, indicating direct use of CO2(aq) in photosynthesis, were mainly associated with lineages within the Rhodophyta. We observed declines in delta C-13 with increasing depth (inshore only) and latitude, from offshore to inshore sites and with increasing tissue N and N : P ratio. Variation in delta C-13 of tissues among macroalgal lineages suggests evolution of a range of CO2 acquisition pathways. Patterns in delta C-13 availability are most consistent with higher availability of CO2(aq) in cooler water and inshore sites. delta C-13 in macroalgae could provide an important tool for monitoring changes in CO2(aq) with increasing atmospheric CO2, with changes in ocean circulation and with changes to runoff from the land to the coastal oceans.

Organismal elemental stoichiometry is important at all scales of ecological interactions, particularly in symbiosis. Symbiotic partnerships are found extensively in corals, where coral hosts and their photosynthetic dinoflagellate partners trade essential nutrients. Using an ecological stoichiometry framework, we assessed variations in carbon (C), nitrogen (N), and phosphorus (P) concentrations in coral hosts and symbiotic zooxanthellae. Our aim was to assess whether corals and zooxanthellae differ in the stoichiometry of C, N and P, which may indicate that corals regulate their symbionts by restricting their access to nutrients. We also investigated the influence of biological and environmental factors on the elemental stoichiometry of coral–zooxanthellae symbiosis. Analysing field data from four locations on the Great Barrier Reef using boosted regression tree models, we observed that C:P and N:P ratios are significantly higher in zooxanthellae than in their coral hosts, suggesting P limitation for zooxanthellae, although the degree of P or N limitation in zooxanthellae varied among coral taxa. Coral taxonomy was also a significant driver of coral–zooxanthellae stoichiometry variations at the species level. Geographical location had a small influence on the variations in stoichiometry. Our analyses suggest that coral species may differentially control the access of zooxanthellae to nutrients which may lead to variation in reef coral responses to nutrient enrichment.

This study aims to understand the spatial distribution of coral reefs in the central region of Viet Nam. We classified live coral cover in Son Tra Peninsula (ST) and Cu Lao Cham Island (CLC) in the South-Central Coast Region of Viet Nam using the Maximum Likelihood Classifier on 3 m Planetscope imagery. Confusion matrices and the accuracy of the classifier were assessed using field data (1,543 and 1,560 photographs in ST and CLC, respectively). The results showed that the reef’s width ranged from 30 to 300 m across the study site, and we were able to detect live coral cover across a depth gradient of 2 to 6 m below the sea surface. The overall accuracies of the classifier (the Kappa coefficient) were 76.78% (0.76) and 78.08% (0.78) for ST and CLC, respectively. We found that 60.25% of coral reefs in ST were unhealthy and the live coral cover was less than 50%, while 25.75% and 11.46% of those in CLC were in good and excellent conditions, respectively. This study demonstrates the feasibility of utilizing Planetscope imagery to monitor shallow coral reefs of small islands at a high spatial resolution of 3 m. The results of this study provide valuable information for coral reef protection and conservation.

The Australian Forum for Operational Oceanography (FOO), started in 2015, established a working group to focus on wind-waves. One of the aims of this working group was to identify the key priorities of wind-waves research. This undertaking has been driven by Australian needs, but Australia is just one part of the larger international waves community; results of this process are also relevant to other marine-focused nations. The process to identify research priorities engaged both researchers and stakeholders in a democratic, collaborative, and iterative process. The main steps were 1) soliciting possible research questions via an online survey, 2) reviewing and editing the questions at a face-to-face workshop, and 3) ranking the research questions. A total of 360 survey invitations were emailed to possible participants, with 69 respondents. Half of these were from research organizations, and the remainder from private industry, service providers, or government. The survey gathered a list of ideas that were consolidated to 155 suggestions, which were further reviewed by participants at a wind-waves research symposium and then ranked via a voting process. A second round of online voting was then undertaken that specifically targeted the industry and stakeholder community. The top five priorities were identified, and are referred to here as “tier 1” priorities. A further 10 priorities were identified, and are referred to here as “tier 2,” providing a total of 15 top-ranked priorities.

Wetlands of Melaleuca spp. in Australia form large forests that are highly threatened by deforestation and degradation. In America, Melaleuca has invaded large areas of native wetlands causing extensive damage. Despite their status as an endangered native ecosystem and as a highly invasive one, little is known about their C and N dynamics. In this study, we sampled five Melaleuca wetlands and measured their C and N ecosystem stocks (aboveground biomass and soil), tree accumulation rates, sedimentation rates, and soil stability. Melaleuca wetlands were highly heterogeneous, but most have large ecosystem C [mean ± SE (range); 360 ± 100 (80–670) Mg C ha−1] and N [8100 ± 1900 (1600–13,000) kg N ha−1] stocks. Tree accumulation rates were 5.0 ± 2.1 Mg C y−1 and 26 ± 14 kg N y−1, and surface soil accumulation rates were 0.6 ± 0.2 Mg C ha−1 y−1 and 39 ± 1 kg N y−1. We found evidence of long-term C and N accumulation in the soil, but also of some level of organic decomposition. Overall, we found that Melaleuca wetlands store and accumulate large amounts of C, especially in their trees, and large amounts of N in their soils, suggesting an important role in coastal biogeochemical cycles.

As global mean sea level continues to rise, thresholds corresponding to coastal inundation impacts are exceeded more frequently. This paper aims to relate sea level rise (SLR) observations and projections to their physical on‐the‐ground impacts. Using a large coastal city as an example, we show that in Sydney, Australia, frequencies of minor coastal inundation have increased from 1.6 to 7.8 days per year between 1914 and present day. We attribute over 80% of the observed coastal inundation events between 1970 and 2015 to the predominantly anthropogenic increases in global mean sea level. Further, we find that impact‐producing coastal inundation will occur weekly by 2050 under high‐ and medium‐emission/SLR scenarios and daily by 2100 under high emissions. The proportion of tide‐only coastal inundation events (i.e., where no storm surge is required to exceed flood thresholds) will increase with SLR, such that most coastal inundation events, including those considered historically severe, will become a predictable consequence of SLR and astronomical tides. These findings are important for coastal managers as frequency, severity, and predictability of inundation impacts can all now be related to the amount of SLR (e.g., a planning allowance or SLR projection). By incorporating known historical inundation events, this allows contextualization, visualization, and localization of global SLR and the changing nature of future coastal inundation risk.

Tropical floodplain wetlands are found in low-lying areas that are periodically inundated. During wet periods, these wetlands can receive large amounts of suspended and dissolved material from the catchment, including many potential pollutants. In this study, we use traditional isotope tracers (&delta, 15N and &delta, 13C) along with soil eDNA to investigate the sources of transported materials and potential contaminants in seven forested floodplain wetlands in tropical Australia. We hypothesised that eDNA and isotope tracers in the soil would reflect the land use of the catchment. Our goal was to test whether eDNA could be used as a potential tool to identify and monitor pollutants in floodplain wetlands. The sampling sites were located within catchments that have a mosaic of land types, from well-conserved rainforests to intensive agricultural land uses, such as grazing, sugar cane, wood production, and horticulture. The soil eDNA was comprised of a mix of plant species consistent with the land use of the catchments. Most of the eDNA pool was derived from native trees, accounting for 46.2 &plusmn, 6.5% of the total, while cultivated species associated with agricultural activities contributed to 1&ndash, 24% of the total. From the cultivated species, highest contributions (&gt, 5%) were from Sorghum sp. used for grazing, banana (Musa ornata), melons (Cucumis melo), and Pinus radiata and Juniperus sp. grown for wood production. Interestingly, tropical wetlands on sites 15 km offshore had soil eDNA from agricultural activities of the mainland, highlighting the connectivity of these wetlands, probably during extensive floods. Overall, soil eDNA, more than isotopic tracers, showed promising results for tracing and monitoring potential pollutants in tropical floodplain wetlands that are highly connected and susceptible to environmental degradation.

Predicted sea level fluctuations and sea level rise with climate change will lead to inundation of coastal and estuarine soils. Coastal wetlands usually contain large amounts of organic matter, which can be potential sources of greenhouse gas emissions (GHGs; CO2, CH4, N2O) during decomposition, but there are limited studies on the effects of sea level variation on GHGs in coastal wetlands. We measured the effect of brackish water inundation and wetting and drying cycles on GHG emissions from coastal wetland soil cores that supported four different vegetation types: Apium gravedens (AG), Leptospermum lanigerum (LL), Phragmites australis (PA) and Paspalum distichum (PD) from the estuarine floodplain of the Aire River in south-western Victoria, Australia. Intact soil cores were incubated under either dry, flooded, or a 14 day wet-dry cycle treatments for a total of 56 days at a constant temperature of 23 °C. CO2, CH4, and N2O fluxes were investigated in closed chambers and measured with gas chromatography. In the dry treatment, a positive correlation was found between soil organic carbon (SOC) and CO2 flux, and between SOC and CH4 flux. Higher SOC is indicative of higher amounts of soil organic matter (SOM) which acts as a source of substrate for microbes to produce CO2 or CH4 emissions under aerobic or anaerobic conditions. The NO2− and NO3− concentrations were positively correlated with N2O emissions in the wet-dry cycle treatment. NO2− and NO3− provide a supply of substrate for denitrification. The flooded treatment decreased cumulative CO2 emissions by 34%, 25% and 14% at the LL, PA, PD sites, respectively, and decreased cumulative N2O emissions by 42%, 39% and 43% at the AG, LL and PA sites, compared to the dry treatment. The wet-dry cycle treatment and dry treatment decreased cumulative CH4 emissions for all vegetation types compared to the flooded treatment. The redox potential (Eh) was negatively correlated with CH4 flux and positively correlated N2O flux at all sites. This study highlights the significance of sea level fluctuations when estimating GHG flux from coastal and estuarine floodplains which are highly vulnerable to inundation, and the role of SOC and mineral N as important drivers affecting GHG flux.

The Australian marine research, industry, and stakeholder community has recently undertaken an extensive collaborative process to identify the highest national priorities for wind-waves research. This was undertaken under the auspices of the Forum for Operational Oceanography Surface Waves Working Group. The main steps in the process were first, soliciting possible research questions from the community via an online survey; second, reviewing the questions at a face-to-face workshop; and third, online ranking of the research questions by individuals. This process resulted in 15 identified priorities, covering research activities and the development of infrastructure. The top five priorities are 1) enhanced and updated nearshore and coastal bathymetry; 2) improved understanding of extreme sea states; 3) maintain and enhance the in situ buoy network; 4) improved data access and sharing; and 5) ensemble and probabilistic wave modeling and forecasting. In this paper, each of the 15 priorities is discussed in detail, providing insight into why each priority is important, and the current state of the art, both nationally and internationally, where relevant. While this process has been driven by Australian needs, it is likely that the results will be relevant to other marine-focused nations.

Coastal wetlands play a pivotal role in regulating both carbon (CO2) and methane (CH4) concentrations across the globe. The amount of CO2 and CH4 stored and released by these ecosystems is becoming more understood, in particular, within each aspect of the ecosystem. However, how the dynamics of the ecosystem affect CO2 and CH4 fluxes on a microclimate level is poorly understood, as well as the overall flux of these Greenhouse Gases (GHGs) within temperate, coastal wetlands. Current research primarily focuses on inland wetlands and coastal wetlands in sub-tropical and tropical regions. Thus, this research aims to investigate CO2 and CH4 fluxes within coastal, temperate wetlands, and improve the understanding of how environmental dynamics impact the flux of these critically important Greenhouse Gases (GHGs). To satisfy this aim, the use of the Eddy-Covariance (EC) method was employed. An EC station was installed on the South-West tip of French Island, Victoria, Australia in late February 2018. The collected data demonstrates the challenges with collecting micro-climate data in an ecosystem with ever-changing environmental conditions. The preliminary results indicate how sensitive flux dynamics are within coastal, temperate wetlands, in particular, to factors such as: tidal and seasonal inundation, seasonal vegetation dynamics, and shifting ecological gradients. The data obtained by the EC station provides a preliminary indication of the complexities of accounting for, and understanding, carbon and methane movement through coastal wetlands in general. The full dataset will aid in improving this understanding, specifically for rare, temperate wetland environments, increasing the knowledge base on how flux dynamics of carbon and methane are affected when collected via open-source methods in dynamic environments.

We thank Törnqvist et al. for engaging with our modelling study on the future response of global coastal wetlands to sea-level rise (SLR) and their careful and critical discussion of the presented methods and results. However, we disagree with their suggestion that our modelling approach is inadequate, a claim which relies on two arguments: (1) they argue that our results are inconsistent with the “A/S (accommodation versus sediment supply) theory”; (2) they refer to coastal Louisiana as a case example where our modelling results would deviate from historic observations and future projections of coastal wetland change. However, below we will demonstrate that Törnqvist et al.’s application of the A/S theory is not valid to predict changes in coastal wetland area, and that our global predictions are in line with regional observations and projections for coastal Louisiana and the wider region of the Gulf of Mexico. Taking coastal Louisiana as an example, Törnqvist et al. highlight that ca. 6000 km2 of land are expected to be lost over the coming 50 years due to RSLR and the erosion/drowning of coastal wetlands. However, this figure cannot directly be compared to our results, because it does not account for upland areas being converted to wetlands as sea level rises; it only accounts for seaward losses due to erosion and/or drowning with associated shoreline retreat and land loss3. Equivalent scenario runs of our model (i.e. only considering wetland accretion, but no inland migration) result in a comparable projected wetland loss in Louisiana of ca. 6,900 km2 until 2100, under the medium SLR scenario (RCP4.5). This loss is triggered by insufficient sediment availability for the marshes to keep pace with SLR in situ. Hence, Törnqvist et al.’s claim that our model underestimates future wetland loss on the US Gulf coast is incorrect. Rather, we demonstrate that our global-scale model predictions of wetland losses are comparable to regional estimates.

Conceptual models have described mangrove forest structure in response to tidal inundation, porewater salinity and mangrove water use, however, the importance of feedbacks have not been assessed. We assessed the processes underlying the feedback between water availability and plant water use in mangrove ecosystems by applying the process- and individual-based mangrove model MANGA, evaluating individual tree growth behavior, emerging forest structure, and mangrove species composition. We compared simulation data with field data collected at a field site with a monospecific mangrove forest that showed a strong gradient in tree height and biomass across the intertidal zone. While tidal inundation and the resulting porewater salinity gradient caused variation in mangrove forest structure within the MANGA model, the inclusion of tree water use in the model led to high similarity between model results and field observations. We further show the importance of the feedback between tree water use and porewater salinity for emergence of species zonation patterns. In accordance with particular site observations, we simulated the response to a broad spectrum of modelled porewater salinity inputs of an artificial forest. The modelled forest consists of two distinct species differing in their water use efficiency and salt tolerance. The modelled feedback mechanism produced a strong correlation between mangrove species zonation along the intertidal gradient and porewater salinity inputs at the system boundaries. The results of this study describe and demonstrate the absolute and relative importance of the implemented interacting processes on mangrove vegetation patterns. A better understanding of the role of tidal inundation and the feedback loops between plant water use and porewater salinity in determining mangrove forest structure will help to predict and manage the fate of mangrove ecosystems in a changing climate.

Mangroves sequester large amounts of carbon (C) and they are increasingly recognized for their potential role in climate change mitigation programs. However, there is uncertainty in the C content of many mangrove forests because the amount of C stored in the roots is usually estimated from allometric equations and not from direct field measurements. There are only a handful of allometric equations in mangroves that are used worldwide to estimate root biomass, however, root biomass can vary from the allometric relationship if the environmental conditions are different from those where the equation was developed. In this study, we compiled recent information on how mangrove roots are affected by environmental conditions. Then, we explored the effect of sampling methodology on root biomass estimations. Finally, we compared published values of root biomass from field measurements against our estimations from allometric equations. The goal was to calculate the uncertainty associated with the estimation of root biomass and thus, the belowground C content of mangroves. The results showed that sampling methodology has a significant effect on root biomass estimations. The highest biomass estimations are reported where both live and dead roots are measured and when the roots are sampled by digging trenches. When comparing measured values against estimations from allometric equations, on average the general allometric equation provided root biomass values that were 40 ± 12% larger than those obtained from field measurements with cores. The result suggests that either: (a) sampling with cores largely underestimates root biomass, or (b) allometric equations overestimate root biomass when used outside the region where they were developed. The uncertainty in root biomass estimates from allometric equations corresponds to 4–15% of the ecosystem C stock (trees + soil), with higher uncertainties in forests with low tree density and low interstitial salinity. We provide a statistical model that includes salinity, forest density and root biomass to correct for this systematic bias. The estimated uncertainty is important to consider when quantifying C budgets at large spatial scales and to validate methodological approaches to C stock estimations.

Aim: Global warming and eutrophication are major threats to coastal environments worldwide. As a result of differences between temperate and tropical ecosystems in nutrient availability, nitrogen (N): phosphorus (P) coupling and carbon retention, primary productivity and biotic interactions in the tropics are predicted to have stronger responses to increased nutrients than in temperate ecosystems. Habitats that occur across broad climatic ranges, such as mangrove forests, provide an opportunity to test this hypothesis by investigating the responses of herbivores to nutrient enrichment in temperate versus tropical latitudes on the same species.

Eutrophication of coastal waters can have consequences for the growth, function and soil processes of coastal wetlands. Our aims were to assess how nutrient enrichment affects growth, biomass allocation and decomposition of plant tissues of a common and widespread mangrove, Avicennia marina, and how eutrophication drives changes in below-ground carbon sequestration. We assessed this through the measurement of above- and belowground growth and decomposition rates of plants and plant tissue in unenriched or nutrient enriched treatments. Nutrient enrichment increased biomass allocation above-ground compared to below-ground in seedlings but not in fully developed, mature trees where we observed the opposite pattern. Experiments to assess root decomposition found that 40–50% of biomass was lost within six months with little change between 12 and 18 months, indicating a high potential for accumulation of organic matter over time. We estimate root-derived carbon sequestration rates of 53, 250 and 94 g C m−2 year−1 for unenriched control, N and P enriched treatments, respectively. These results show coastal eutrophication can be beneficial and detrimental to ecosystem function of coastal plants. Eutrophication stimulates root growth in fully developed trees, increasing organic matter input to soils. Our data suggests that organic matter accumulation will increase in areas with high nutrient availability where root growth is increased and rates of decomposition are low.

Mangrove trees are rooted in saline soils which can limit their growth. Access to alternative, less saline water sources may provide important water subsidies. We assessed the hydrogen and oxygen isotopic signatures of soil porewater, groundwater and atmospheric water sources (dew and rainfall) and tree stem water from three mangroves species over two sites that varied in elevation. Although stem water isotopic signatures were most similar to porewater, variation in isotopic values indicated trees also accessed alternative water sources, the degree to which varied over sites and among species. Rhizophora stylosa had lowest values of stable isotopes among the species indicating significant groundwater utilization. In a long-term fertilization experiment we found that growth of Ceriops australis and Lumnitzera racemosa was nitrogen limited, while growth of R. stylosa was nitrogen limited to a lesser extent, suggesting groundwater may also provide nutritional benefits for R. stylosa. The uptake of alternative water sources in addition to saline porewater may improve metabolic function, differentially altering the performance of different species and over sites.

Change history: In Fig. 2b of this Letter, 'Relative wetland change (km2)' should have read 'Relative wetland change (%)' and equations (2) and (3) have been changed from 'RSLRcrit = (m × TRe) × Sed + i' and 'Sedcrit = (RSLR - i)/(m × TRe)', respectively. The definition of the variables in equation (2) has been updated. These errors have been corrected online.

Mangroves are one of the few woody ecosystems that grow in hot-arid climates. They can survive extreme conditions of low precipitation, high solar radiation, wide temperature fluctuations and hypersalinity. These unique mangroves have distinct geomorphology, hydrology, forest structure, tree physiology, and soil biogeochemistry. In this review, supported by field data from Australia and Mexico, we explore the characteristics of mangroves in arid climates of the world. These mangroves are mostly tide-dominated with freshwater flows restricted to groundwater and sporadic tropical storms. They form dense forests with stunted growth dominated mainly by trees of the genus Avicennia that co-occur with salt marshes in the high intertidal. Their soils have low nutrient and carbon concentrations, and high soil δ15 N and δ13C values compared to subhumid and humid mangroves. Mangroves in arid climates have relatively low human pressure due to sparse human settlements. Key threats to these mangroves, which often persist at the edge of their physiological tolerances, include extreme drought, reductions in groundwater inputs, altered hydrology, sea-level fluctuations and increases in nutrient loading. Restoration of mangroves in arid climates should focus on restoring their hydrology. Mangroves in arid zones are under-represented in global maps and assessment programs, as they may not be consistent with countries’ definition of “forests”. Improved global representation and understanding of the ecology of mangroves in arid climates could help sustain their valuable ecosystem services.

Coastal wetlands, such as mangrove and saltmarsh environments, can store significant amounts of soil organic carbon (SOC); however, most studies focus on tropical and subtropical environments. We assessed SOC stocks and fractions in temperate mangrove (two sites) and saltmarsh (sites SM1, SM2 and SM3) environments in southern Australia. The SOC fractions were separated according to particulate organic carbon (POC), humic carbon (HC) and recalcitrant carbon (RC) by size fractionation. Saltmarsh sites generally had the highest SOC content (up to 12.4% SOC). The POC fraction was the highest at the surface in the saltmarsh site and decreased relative to the HC and RC fractions with depth. Conversely, the proportion of POC at the mangrove sites did not decrease with depth, forming up to 76% of the SOC. The vertical displacement of soil of up to 5.8 mm year–1 at the saltmarsh sites, measured using root ingrowth bags, suggest significant contributions of POC via root materials. Retention of these POC inputs are likely to be related to waterlogging, which decreases decomposition rates – with much lower soil moisture content at SM1, where the lowest POC content occurred below the surface, compared with SM2 and SM3.

Salt marshes can play a vital role in mitigating the effects of global environmental change by dissipating incident storm wave energy and, through accretion, tracking increasing water depths consequent upon sea level rise. Atmospheric CO2 concentrations and nutrient availability are two key variables that can affect the biological processes that contribute to marsh surface elevation gain. We measured the effects of CO2 concentrations and nutrient availability on surface elevation change in intact mixed-species blocks of UK salt marsh using six open-top chambers receiving CO2-enriched (800 ppm) or ambient (400 ppm) air. We found more rapid surface elevation gain in elevated CO2 conditions: an average increase of 3.4 mm over the growing season relative to ambient CO2. Boosted regression analysis to determine the relative influence of different parameters on elevation change identified that a 10% reduction in microbial activity in elevated CO2-grown blocks had a positive influence on elevation. The biomass of Puccinellia maritima also had a positive influence on elevation, while other salt marsh species (e.g. Suaeda maritima) had no influence or a negative impact on elevation. Reduced rates of water use by the vegetation in the high CO2 treatment could be contributing to elevation gain, either directly through reduced soil shrinkage or indirectly by decreasing microbial respiration rates due to lower redox levels in the soil. Eutrophication did not influence elevation change in either CO2 treatment despite doubling aboveground biomass. The role of belowground processes (transpiration, root growth and decomposition) in the vertical adjustment of European salt marshes, which are primarily minerogenic in composition, could increase as atmospheric CO2 concentrations rise and should be considered in future wetland models for the region. Elevated CO2 conditions could enhance resilience in vulnerable systems such as those with low mineral sediment supply or where migration upwards within the tidal frame is constrained.

The external mucus surface of reef fish contains ultraviolet absorbing compounds (UVAC), most prominently Mycosporine-like Amino Acids (MAAs). MAAs in the external mucus of reef fish are thought to act as sunscreens by preventing the damaging effects of ultraviolet radiation (UVR), however, direct evidence for their protective role has been missing. We tested the protective function of UVAC's by exposing fish with naturally low, Pomacentrus amboinensis, and high, Thalassoma lunare, mucus absorption properties to a high dose of UVR (UVB: 13.4 W * m(-2), UVA: 6.1 W * m(-2)) and measuring the resulting DNA damage in the form of cyclobutane pyrimidine dimers (CPDs). For both species, the amount of UV induced DNA damage sustained following the exposure to a 1 h pulse of high UVR was negatively correlated with mucus absorbance, a proxy for MAA concentration. Furthermore, a rapid and significant increase in UVAC concentration was observed in P. amboinensis following UV exposure, directly after capture and after ten days in captivity. No such increase was observed in T. lunare, which maintained relatively high levels of UV absorbance at all times. P. amboinensis, in contrast to T. lunare, uses UV communication and thus must maintain UV transparent mucus to be able to display its UV patterns. The ability to rapidly alter the transparency of mucus could be an important adaptation in the trade off between protection from harmful UVR and UV communication. (C) 2016 Elsevier B.V. All rights reserved.

In order to understand plant responses to both the widespread phenomenon of increased nutrient inputs to coastal zones and the concurrent rise in atmospheric CO2 concentrations, CO2-nutrient interactions need to be considered. In addition to its potential stimulating effect on photosynthesis and growth, elevated CO2 affects the temperature response of photosynthesis. The scarcity of experiments testing how elevated CO2 affects the temperature response of tropical trees hinders our ability to model future primary productivity. In a glasshouse study, we examined the effects of elevated CO2 (800 ppm) and nutrient availability on seedlings of the widespread mangrove Avicennia germinans. We assessed photosynthetic performance, the temperature response of photosynthesis, seedling growth and biomass allocation. We found large synergistic gains in both growth (42 %) and photosynthesis (115 %) when seedlings grown under elevated CO2 were supplied with elevated nutrient concentrations relative to their ambient growing conditions. Growth was significantly enhanced under elevated CO2 only under high-nutrient conditions, mainly in above-ground tissues. Under low-nutrient conditions and elevated CO2, root volume was more than double that of seedlings grown under ambient CO2 levels. Elevated CO2 significantly increased the temperature optimum for photosynthesis by ca. 4 °C. Rising CO2 concentrations are likely to have a significant positive effect on the growth rate of A. germinans over the next century, especially in areas where nutrient availability is high.

Sediment retention in saltmarshes is often attributed to the presence of vegetation, which enhances accretion by slowing water flow, reduces erosion by attenuating wave energy and increases surface stability through the presence of organic matter. Saltmarsh vegetation morphology varies considerably on a range of spatial and temporal scales, but the effect of different above ground morphologies on sediment retention is not well characterised. Understanding the biophysical interaction between the canopy and sediment trapping in situ is important for improving numerical shoreline models. In a novel field flume study, we measured the effect of vegetation height and biomass on sediment trapping using a mass balance approach. Suspended sediment profilers were placed at both openings of a field flume built across-shore on the seaward boundary of an intertidal saltmarsh in the Dengie Peninsula, UK. Sequential removal of plant material from within the flume resulted in incremental loss of vegetation height and biomass. The difference between the concentration of suspended sediment measured at each profiler was used to determine the sediment budget within the flume. Deposition of material on the plant/soil surfaces within the flume occurred during flood tides, while ebb flow resulted in erosion (to a lesser degree) from the flume area, with a positive sediment budget of on average 6.5 g m−2 tide−1 with no significant relationship between sediment trapping efficiency and canopy morphology. Deposition (and erosion) rates were positively correlated to maximum inundation depth. Our results suggest that during periods of calm conditions, changes to canopy morphology do not result in significant changes in sediment budgets in marshes.

Increases in sea level are a threat to seaward fringing mangrove forests if levels of inundation exceed the physiological tolerance of the trees; however, tidal wetlands can keep pace with sea level rise if soil surface elevations can increase at the same pace as sea level rise. Sediment accretion on the soil surface and belowground production of roots are proposed to increase with increasing sea level, enabling intertidal habitats to maintain their position relative to mean sea level, but there are few tests of these predictions in mangrove forests. Here we used variation in sea level and the availability of sediments caused by seasonal and inter-annual variation in the intensity of La Nina-El Nino to assess the effects of increasing sea level on surface elevation gains and contributing processes (accretion on the surface, subsidence and root growth) in mangrove forests. We found that soil surface elevation increased with mean sea level (which varied over 250 mm during the study) and with turbidity at sites where fine sediment in the water column is abundant. In contrast, where sediments were sandy, rates of surface elevation gain were high, but not significantly related to variation in turbidity, and were likely to be influenced by other factors that deliver sand to the mangrove forest. Root growth was not linked to soil surface elevation gains, although it was associated with reduced shallow subsidence, and therefore may contribute to the capacity of mangroves to keep pace with sea level rise. Our results indicate both surface (sedimentation) and subsurface (root growth) processes can influence mangrove capacity to keep pace with sea level rise within the same geographic location, and that current models of tidal marsh responses to sea level rise capture the major feature of the response of mangroves where fine, but not coarse, sediments are abundant.

By increasing water use efficiency and carbon assimilation, increasing atmospheric CO2 concentrations could potentially improve plant productivity and growth at high salinities. To assess the effect of elevated CO2 on the salinity response of a woody halophyte, we grew seedlings of the mangrove Avicennia germinans under a combination of five salinity treatments [from 5 to 65 parts per thousand (ppt)] and three CO2 concentrations (280, 400 and 800 ppm). We measured survivorship, growth rate, photosynthetic gas exchange, root architecture and foliar nutrient and ion concentrations. The salinity optima for growth shifted higher with increasing concentrations of CO2 , from 0 ppt at 280 ppm to 35 ppt at 800 ppm. At optimal salinity conditions, carbon assimilation rates were significantly higher under elevated CO2 concentrations. However, at salinities above the salinity optima, salinity had an expected negative effect on mangrove growth and carbon assimilation, which was not alleviated by elevated CO2 , despite a significant improvement in photosynthetic water use efficiency. This is likely due to non-stomatal limitations to growth at high salinities, as indicated by our measurements of foliar ion concentrations that show a displacement of K(+) by Na(+) at elevated salinities that is not affected by CO2 . The observed shift in the optimal salinity for growth with increasing CO2 concentrations changes the fundamental niche of this species and could have significant effects on future mangrove distribution patterns and interspecific interactions.

Mangroves are distributed along tropical and subtropical riverine and coastal shores. Although mangroves are highly adapted to saline environments, maintaining water uptake under saline conditions is energetically expensive. Therefore, salinity is a lim- iting factor for mangrove growth and productivity, and access to fresh water sources, such as rainwater and groundwater, which reduce water salinity, increase mangrove ecosystem productivity. Here, we investi- gated the extent of fresh water utilization by man- groves to better predict current and future mangrove productivity. We used the abundance of 18 O isotope in stem water to assess: (1) the extent of fresh water utilization by Avicennia marina (Forssk.) Vierh across hydrological settings; and (2) whether growth, mea- sured as increments in stem circumference, is sensitive to variation in rainfall availability. The d 18 O isotopic composition of stem water indicated mangroves use both fresh and saline water sources for metabolic processes. However, our results suggest that the proportion of fresh water used by mangroves increases with the availability of fresh water. Growth of the main stems of trees was correlated with rainfall (r 2 = 0.34 and r 2 = 0.37, P = 0.001). Our results

Recent evidence indicates that climate change and intensification of the El Niño Southern Oscillation (ENSO) has increased variation in sea level. Although widespread impacts on intertidal ecosystems are anticipated to arise from the sea level seesaw associated with climate change, none have yet been demonstrated. Intertidal ecosystems, including mangrove forests are among those ecosystems that are highly vulnerable to sea level rise, but they may also be vulnerable to sea level variability and extreme low sea level events. During 16 years of monitoring of a mangrove forest in Mangrove Bay in north Western Australia, we documented two forest dieback events, the most recent one being coincident with the large-scale dieback of mangroves in the Gulf of Carpentaria in northern Australia. Diebacks in Mangrove Bay were coincident with periods of very low sea level, which were associated with increased soil salinization of 20–30% above pre-event levels, leading to canopy loss, reduced Normalized Difference Vegetation Index (NDVI) and reduced recruitment. Our study indicates that an intensification of ENSO will have negative effects on some mangrove forests in parts of the Indo-Pacific that will exacerbate other pressures.

Seagrasses sequester globally significant amounts of carbon (C), which is stored mainly in the sediment. BothC fixation by the seagrass (autochthonous) and the trapping of organic matter that is derived from outside theecosystem (allochthonous)contribute to seagrass sediment organic C (OC). However, due to limitations incurrent methods we do not yet fully understand which sources of C are most important to blue C ecosystems.We used environmental-DNA (eDNA) to identify and estimate the contribution of floral sources to seagrass sedi-ment OC sampled from three sites in Moreton Bay, Australia that differ in potential levels of autochthonous andallochthonous OC inputs. Using the plant barcode generbcLand a next-generation sequencing platform we iden-tified 150 plant operational taxonomic units in the sediment samples. We found that seagrass DNA composedon average 88% of the eDNA pool in the sediment samples. Allochtonous sources were primarily mangrove, salt-marsh, and freshwater marsh species, but the proportional contribution of other sources varied among meadows.Carbon and nitrogen (N) stable isotope mixing models suggested a lower autochthonous contribution than theeDNA data, but had high ambiguity due to indistinguishable isotopic values among some of the sources. Ourstudy shows that eDNA can be an effective tool for identifying sources to sediment OC in blue C ecosystems andcan be used in conjunction with stable isotope analysis to reduce ambiguity. Identifying the sources of OC to thesediments of blue C ecosystems is crucial for building robust and accurate C budgets for these systems. Discovery Early Career Researcher Award/[DE120101706]/DECRA/Australia UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Biología UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencias del Mar y Limnología (CIMAR)

AimTo reconstruct the historic response of two dominant mangrove species to increases in atmospheric CO2 over the past two centuries.

Mangroves are a group of highly salt-tolerant woody plants. The high water use efficiency of mangroves under saline conditions suggests that regulation of water transport is a crucial component of their salinity tolerance.This review focuses on the processes that contribute to the ability of mangroves to maintain water uptake and limit water loss to the soil and the atmosphere under saline conditions, from micro to macro scales. These processes include: (1) efficient filtering of the incoming water to exclude salt; (2) maintenance of internal osmotic potentials lower than that of the rhizosphere; (3) water-saving properties; and (4) efficient exploitation of less-saline water sources when these become available.Mangroves are inherently plastic and can change their structure at the root, leaf and stand levels in response to salinity in order to exclude salt from the xylem stream, maintain leaf hydraulic conductance, avoid cavitation and regulate water loss (e.g. suberization of roots and alterations of leaf size, succulence and angle, hydraulic anatomy and biomass partitioning). However, much is still unknown about the regulation of water uptake in mangroves, such as how they sense and respond to heterogeneity in root zone salinity, the extent to which they utilize non-stomatally derived CO2 as a water-saving measure and whether they can exploit atmospheric water sources.

The elemental composition of organisms has been linked to traits such as growth rates, through investment of phosphorus (P) in RNA (called the Growth Rate Hypothesis) and through ecological processes such as susceptibility to predation and herbivory. To test the predictions of the Growth Rate Hypothesis, we assessed the elemental composition and RNA of four phyla of common, co-occurring benthic coral reef organisms, corals, and green, brown and red macroalgae, whose patterns of dominance are predicted to be strongly influenced by differences in growth and herbivory and thus elemental stoichiometry. We assessed the importance of phylogeny and functional form, two important attributes for predicting the composition of macroalgal communities, on the variation in elemental composition. Over these widely divergent taxonomic groups of organisms, we found support for links between elemental composition and RNA in tissues. The RNA:DNA in corals was positively correlated with % nitrogen (N) and negatively correlated with carbon (C):N ratios of tissues while RNA:DNA and RNA concentrations in macroalgae were positively correlated with %N, N:P ratios and negatively correlated with C:N ratios. Corals had higher concentrations of P and lower C:P ratios than macroalgae. Among the macroalgal lineages, C:P was higher in the brown algae than in red and green algae. Overall, the variation in elemental composition of macroalgae was relatively low compared with the variation reported for terrestrial plants and similar to that in corals. Analysis of phylogenetic sources of variation (family and genus) in elemental composition and RNA found that genus accounted for a significant proportion of variation in both corals and macroalgae (15-47%), but the largest source of variation (50-97%) was unexplained by our statistical model and thus likely attributable to species and environmental factors. Although elemental composition of macroalgae did not vary in a way that was consistent with functional form models, genera with larger thalli had higher tissue N than smaller forms and more lineages with greater complexity of form had lower C:P ratios than less complex brown algal lineages. Our data indicate that the despite relatively low levels of variation in elemental composition compared with terrestrial organisms, elemental stoichiometry varied significantly among coral reef taxa and with the structure of organisms and thus may have potential for predicting variation in growth rates and patterns of consumption on coral reefs.