All tidal wetlands are blue carbon ecosystems.

Managing coastal wetlands is one of the most promising activities to reduce atmospheric greenhouse gases, and it also contributes to meeting the United Nations Sustainable Development Goals. One of the options is through blue carbon projects, in which mangroves, saltmarshes, and seagrass are managed to increase carbon sequestration and reduce greenhouse gas emissions. However, other tidal wetlands align with the characteristics of blue carbon. These wetlands are called tidal freshwater wetlands in the United States, supratidal wetlands in Australia, transitional forests in Southeast Asia, and estuarine forests in South Africa. They have similar or larger potential for atmospheric carbon sequestration and emission reductions than the currently considered blue carbon ecosystems and have been highly exploited. In the present article, we suggest that all wetlands directly or indirectly influenced by tides should be considered blue carbon. Their protection and restoration through carbon offsets could reduce emissions while providing multiple cobenefits, including biodiversity.

The value of ecosystem services in global marine kelp forests.

While marine kelp forests have provided valuable ecosystem services for millennia, the global ecological and economic value of those services is largely unresolved. Kelp forests are diminishing in many regions worldwide, and efforts to manage these ecosystems are hindered without accurate estimates of the value of the services that kelp forests provide to human societies. Here, we present a global estimate of the ecological and economic potential of three key ecosystem services - fisheries production, nutrient cycling, and carbon removal provided by six major forest forming kelp genera (Ecklonia, Laminaria, Lessonia, Macrocystis, Nereocystis, and Saccharina). Each of these genera creates a potential value of between $64,400 and $147,100/hectare each year. Collectively, they generate between $465 and $562 billion/year worldwide, with an average of $500 billion. These values are primarily driven by fisheries production (mean $29,900, 904 Kg/Ha/year) and nitrogen removal ($73,800, 657 Kg N/Ha/year), though kelp forests are also estimated to sequester 4.91 megatons of carbon from the atmosphere/year highlighting their potential as blue carbon systems for climate change mitigation. These findings highlight the ecological and economic value of kelp forests to society and will facilitate better informed marine management and conservation decisions.

Blue carbon drawdown by restored mangrove forests improves with age.

The restoration of blue carbon ecosystems, such as mangrove forests, is increasingly used as a management tool to mitigate climate change by removing and sequestering atmospheric carbon in the ground. However, estimates of carbon-offset potential are currently based on data from natural mangrove forests, potentially leading to overestimating the carbon-offset potential from restored mangroves. Here, in the first study of its kind, we utilise 210Pb sediment age-dating techniques and greenhouse gas flux measures to estimate blue carbon additionality in restored mangrove forests, ranging from 13 to 35 years old. As expected, mangrove age had a significant effect on carbon additionality and carbon accretion rate, with the older mangrove stands (17 and 35 years old) holding double the total carbon stocks (aboveground + soil stocks; ∼115 tonnes C ha-1) and double the soil sequestration rates (∼3 tonnes C ha-1 yr-1) than the youngest mangrove stand (13 years old). Although soil carbon stocks increased with mangrove age, the aboveground plant stocks were highest in the 17-year-old stand. Mangrove age also had a significant effect on soil carbon fluxes, with the older mangroves (≥17 years) releasing one-fourth of the CH4 emissions, but double the CO2 flux compared to young stands. Our study suggests that the carbon sink capacity of restored mangrove forests increases with age, but stabilises once they mature (e.g., >17 years). This means that by using carbon sequestration and emissions from natural forests, mangrove restoration projects may be overestimating their carbon sequestration potential.

The value of estuarine producers to fisheries: A case study of Richmond River Estuary.

Nutrient input from estuarine producers underpins coastal fisheries production and knowing which producers are the most responsible for fish diet helps effectively protect and restore coastal ecosystems. Focussing on the Richmond River in Australia as a case study, we sampled the main estuarine producers and estimated their proportional contributions of nutritional input to seven commercially important fisheries species using Bayesian isotope mixing models. We valued the dietary input of estuarine producers to the commercial fisheries by combining dietary contribution estimates with total annual catch data from commercial fishers. A conservative estimate is that estuarine producers in the Richmond River Estuary contribute at least 82 725 kg (78%) of the total annual catch of the seven commercially important fish with an estimated annual value of $AU 450 117. Sea mullet and Mud crab contributed 95% of the total catch, and 93% of the total value assigned to estuarine producers. The two highest valued estuarine producers were tidal marsh (Juncus kraussii) $AU 82 432 and seagrass (Zostera capricorni) $AU 65 423. This study demonstrates the substantial role of estuarine producers to commercial fisheries production and the fisheries economy more broadly. With large areas of estuarine producers under threat globally from land clearing for agriculture, aquaculture and urbanisation, the results presented here provide evidence to support the value of coastal habitats and benefits of their preservation and restoration.

The combined effect of short-term hydrological and N-fertilization manipulation of wetlands on CO2, CH4, and N2O emissions.

Freshwater wetlands are natural sinks of carbon; yet, wetland conversion for agricultural uses can shift these carbon sinks into large sources of greenhouse gases. We know that the anthropogenic alteration of wetland hydrology and the broad use of N-fertilizers can modify biogeochemical cycling, however, the extent of their combined effect on greenhouse gases exchange still needs further research. Moreover, there has been recent interest in wetlands rehabilitation and preservation by improving natural water flow and by seeking alternative solutions to nutrient inputs. In a microcosm setting, we experimentally exposed soils to three inundation treatments (Inundated, Moist, Drained) and a nutrient treatment by adding high nitrogen load (300 kg ha-1) to simulate physical and chemical disturbances. After, we measured the depth microprofiles of N2O and O2 concentration and CO2 and CH4 emission rates to determine how hydrological alteration and nitrogen input affect carbon and nitrogen cycling processes in inland wetland soils. Compared to the Control soils, N-fertilizer increased CO2 emissions by 40% in Drained conditions and increased CH4 emissions in Inundated soils over 90%. N2O emissions from Moist and Inundated soils enriched with nitrogen increased by 17.4 and 18-fold, respectively. Overall, the combination of physical and chemical disturbances increased the Global Warming Potential (GWP) by 7.5-fold. The first response of hydrological rehabilitation, while typically valuable for CO2 emission reduction, amplified CH4 and N2O emissions when combined with high nitrogen inputs. Therefore, this research highlights the importance of evaluating the potential interactive effects of various disturbances on biogeochemical processes when devising rehabilitation plans to rehabilitate degraded wetlands.

National scale predictions of contemporary and future blue carbon storage.

To help mitigate the impacts of climate change, many nature-based solutions are being explored. These solutions involve protection and restoration of ecosystems that serve as efficient carbon sinks, including vegetated coastal ecosystems (VCEs: tidal marshes, mangrove forests, and seagrass meadows) also known as 'Blue Carbon' ecosystems. In fact, many nations are seeking to manage VCEs to help meet their climate change mitigation targets through Nationally Determined Contributions (NDCs). However, incorporation of VCEs into NDCs requires national-scale estimates of contemporary and future blue carbon storage, which has not yet been achieved. Here we address this challenge using machine learning approaches to reliably map (with 62-72% accuracy) soil carbon stocks in VCEs based on geospatial data (topography, geomorphology, climate, and anthropogenic impacts), using Australia as a case study. The resulting maps of soil carbon stocks showed that there is a total of 951 Tg (±65 Tg) of carbon stock within Australian VCEs. Strong relationships between soil carbon stocks and climatic conditions (temperature, rainfall, solar radiation) allowed us to project future changes in carbon storage across all RCP scenarios for the years 2050 and 2090 to determine changes in environmental suitability for soil carbon stocks. Results show that soil carbon stocks in mangrove/tidal marsh ecosystems are likely to predominantly experience declines in carbon stocks under predicted climate change scenarios (19% of ecosystems area is predicted to have an increase in soil carbon stocks, while 38% of ecosystems area is predicted to have a decrease in soil carbon stocks), but a majority of seagrass area is likely to have increased soil carbon stocks (56% increase, 7% decrease). This approach is effective for developing robust national blue carbon inventories and revealing the capacity for blue carbon to help meet NDCs. The resulting spatially-explicit maps can also be used to pinpoint areas for successful blue carbon projects both now and in the future.

Planted mangroves cap toxic petroleum-contaminated sediments.

Mangroves are known to provide many ecosystem services, however there is little information on their potential role to cap and immobilise toxic levels of total petroleum hydrocarbons (TPH). Using an Australian case study, we investigated the capacity of planted mangroves (Avicennia marina) to immobilise TPH within a small embayment (Stony Creek, Victoria, Australia) subjected to minor oil spills throughout the 1980s. Mangroves were planted on the oil rich strata in 1984 to rehabilitate the site. Currently the area is covered with a dense mangrove forest. One-meter-long sediment cores revealed that mangroves have formed a thick (up to 30 cm) organic layer above the TPH-contaminated sediments, accumulating on average 6.6 mm of sediment per year. Mean TPH levels below this organic layer (30-50 cm) are extremely toxic (30,441.6 mg kg-1), exceeding safety thresholds up to 220-fold which is eight times higher when compared to top layer (0-10 cm).

Estimating blue carbon sequestration under coastal management scenarios.

Restoring and protecting "blue carbon" ecosystems - mangrove forests, tidal marshes, and seagrass meadows - are actions considered for increasing global carbon sequestration. To improve understanding of which management actions produce the greatest gains in sequestration, we used a spatially explicit model to compare carbon sequestration and its economic value over a broad spatial scale (2500 km of coastline in southeastern Australia) for four management scenarios: (1) Managed Retreat, (2) Managed Retreat Plus Levee Removal, (3) Erosion of High Risk Areas, (4) Erosion of Moderate to High Risk Areas. We found that carbon sequestration from avoiding erosion-related emissions (abatement) would far exceed sequestration from coastal restoration. If erosion were limited only to the areas with highest erosion risk, sequestration in the non-eroded area exceeded emissions by 4.2 million Mg CO2 by 2100. However, losing blue carbon ecosystems in both moderate and high erosion risk areas would result in net emissions of 23.0 million Mg CO2 by 2100. The removal of levees combined with managed retreat was the strategy that sequestered the most carbon. Across all time points, removal of levees increased sequestration by only an additional 1 to 3% compared to managed retreat alone. Compared to the baseline erosion scenario, the managed retreat scenario increased sequestration by 7.40 million Mg CO2 by 2030, 8.69 million Mg CO2 by 2050, and 16.6 million Mg CO2 by 2100. Associated economic value followed the same patterns, with large potential value loss from erosion greater than potential gains from conserving or restoring ecosystems. This study quantifies the potential benefits of managed retreat and preventing erosion in existing blue carbon ecosystems to help meet climate change mitigation goals by reducing carbon emissions.

The potential of viruses to influence the magnitude of greenhouse gas emissions in an inland wetland.

Wetlands are among the earth's most efficient ecosystems for carbon sequestration, but can also emit potent greenhouse gases (GHGs) depending on how they are managed. Global management strategies have sought to maximize carbon drawdown by wetlands by manipulating wetland hydrology to inhibit bacterially-mediated emissions. However, it has recently been hypothesized within wetlands that viruses have the potential to dictate the magnitude and direction of GHG emissions by attacking prokaryotes involved in the carbon cycle. Here we tested this hypothesis in a whole-ecosystem manipulation by hydrologically-restoring a degraded wetland ('rewetting') and investigated the changes in GHG emissions, prokaryotes, viruses, and virus-host interactions. We found that hydrological restoration significantly increased prokaryotic diversity, methanogenic Methanomicrobia, as well as putative iron/sulfate-cyclers (Geobacteraceae), nitrogen-cyclers (Nitrosomonadaceae), and fermentative bacteria (Koribacteraceae). These results provide insights into successional microbial community shifts during rehabilitation. Additionally, in response to watering, viral-induced prokaryotic mortality declined by 77%, resulting in limited carbon released by viral shunt that was significantly correlated with the 2.8-fold reduction in wetland carbon emissions. Our findings highlight, for the first time, the potential implications of viral infections in soil prokaryotes on wetland greenhouse gas dynamics and confirm the importance of wetland rehabilitation as a tool to offset carbon emissions.

Climate drives the geography of marine consumption by changing predator communities.

The global distribution of primary production and consumption by humans (fisheries) is well-documented, but we have no map linking the central ecological process of consumption within food webs to temperature and other ecological drivers. Using standardized assays that span 105° of latitude on four continents, we show that rates of bait consumption by generalist predators in shallow marine ecosystems are tightly linked to both temperature and the composition of consumer assemblages. Unexpectedly, rates of consumption peaked at midlatitudes (25 to 35°) in both Northern and Southern Hemispheres across both seagrass and unvegetated sediment habitats. This pattern contrasts with terrestrial systems, where biotic interactions reportedly weaken away from the equator, but it parallels an emerging pattern of a subtropical peak in marine biodiversity. The higher consumption at midlatitudes was closely related to the type of consumers present, which explained rates of consumption better than consumer density, biomass, species diversity, or habitat. Indeed, the apparent effect of temperature on consumption was mostly driven by temperature-associated turnover in consumer community composition. Our findings reinforce the key influence of climate warming on altered species composition and highlight its implications for the functioning of Earth's ecosystems.

More severe disturbance regimes drive the shift of a kelp forest to a sea urchin barren in south-eastern Australia.

Climate change is influencing the frequency and severity of extreme events. This means that systems are experiencing novel or altered disturbance regimes, making it difficult to predict and manage for this impact on ecosystems. While there is established theory regarding how the frequency of disturbance influences ecosystems, how this interacts with severity of disturbance is difficult to tease apart, as these two are inherently linked. Here we investigated a subtidal kelp (Ecklonia radiata) dominated community in southern Australia to assess how different disturbance regimes might drive changes to a different ecosystem state: sea urchin barrens. Specifically, we compared how the frequency of disturbance (single or triple disturbance events over a three month period) influenced recruitment and community dynamics, when the net severity of disturbance was the same (single disturbance compared to triple disturbances each one-third as severe). We crossed this design with two different net severities of disturbance (50% or 100%, kelp canopy removal). The frequency of disturbance effect depended on the severity of disturbance. When 50% of the canopy was removed, the highest kelp recruitment and recovery of the benthic community occurred with the triple disturbance events. When disturbance was a single event or the most severe (100% removal), kelp recruitment was low and the kelp canopy failed to recover over 18 months. The latter case led to shifts in the community composition from a kelp bed to a sea-urchin barren. This suggests that if ecosystems experience novel or more severe disturbance scenarios, this can lead to a decline in ecosystem condition or collapse.

Quantifying welfare gains of coastal and estuarine ecosystem rehabilitation for recreational fisheries.

Coastal and estuarine ecosystems, such as mangroves, tidal marshes and seagrass meadows, provide a range of ecosystem services, but have seen extensive degradation and decline. Effective protection and rehabilitation of coastal ecosystems requires an understanding of how efforts may improve associated ecosystem services. In this study, we present a spatially-explicit angler catch function to predict boat-based recreational catch as a function of ecosystem and angler characteristics. We developed a choice model to investigate where recreational anglers launch their boats and fish in southeast Australia. By linking the recreational catch models with a choice model, we were able to quantify welfare gains of ecosystem rehabilitation. We found welfare gains across fishing locations varied widely due to heterogeneous coverage of seagrass. The welfare gains of different fishing locations ranged from near-zero in areas of low seagrass coverage, to AU $19.18 (10% increase in seagrass area) and to AU $85.55 (30% increase) per trip in location of high seagrass coverage. Given two million fishing trips occurring per year in Port Phillip Bay, and one million in Western Port, the aggregated welfare gain could scale up to AU $6.2 million with a 10% increase in seagrass coverage, and AU $22 million per annum with a 30% increase in seagrass. We also calculated the welfare loss associated with total loss of seagrass ecosystem in each fishing location to represent the current value, which varied significantly, ranging from near-zero in some locations to AU $87.47 per trip in other locations. Over the past several decades, the bay-wide seagrass ecosystem has dropped by 36.7% in Western Port, resulting in potential welfare loss of an estimated AU $ 86.7 million per annum. Our analyses provide insightful spatial policy implications for coastal and marine ecosystem rehabilitation in the region.

Assessing passive rehabilitation for carbon gains in rain-filled agricultural wetlands.

Wetland ecosystems have a disproportionally large influence on the global carbon cycle. They can act as carbon sinks or sources depending upon their location, type, and condition. Rehabilitation of wetlands is gaining popularity as a nature-based approach to helping mitigate climate change; however, few studies have empirically tested the carbon benefits of wetland restoration, especially in freshwater environments. Here we investigated the effects of passive rehabilitation (i.e. fencing and agricultural release) of 16 semi-arid rain-filled freshwater wetlands in southeastern Australia. Eight control sites were compared with older (>10 year) or newer (2-5 year) rehabilitated sites, dominated by graminoids or eucalypts. Carbon stocks (soils and plant biomass), and emissions (carbon dioxide - CO2; and methane - CH4) were sampled across three seasons, representing natural filling and drawdown, and soil microbial communities were sampled in spring. We found no significant difference in soil carbon or greenhouse gas emissions between rehabilitated and control sites, however, plant biomass was significantly higher in older rehabilitated sites. Wetland carbon stocks were 19.21 t Corg ha-1 and 2.84 t Corg ha-1 for soils (top 20 cm; n = 137) and plant biomass (n = 288), respectively. Hydrology was a strong driver of wetland greenhouse gas emissions. Diffusive fluxes (n = 356) averaged 117.63 mmol CO2 m2 d-1 and 2.98 mmol CH4 m2 d-1 when wet, and 124.01 mmol CO2 m2 d-1 and -0.41 mmol CH4 m2 d-1 when dry. Soil microbial community richness was nearly 2-fold higher during the wet phase than the dry phase, including relative increases in Nitrososphaerales, Myxococcales and Koribacteraceae and methanogens Methanobacteriales. Vegetation type significantly influenced soil carbon, aboveground carbon, and greenhouse gas emissions. Overall, our results suggest that passive rehabilitation of rain-filled wetlands, while valuable for biodiversity and habitat provisioning, is ineffective for increasing carbon gains within 20 years. Carbon offsetting opportunities may be better in systems with faster sediment accretion. Active rehabilitation methods, particularly that reinstate the natural hydrology of drained wetlands, should also be considered.

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.

Implication of Viral Infections for Greenhouse Gas Dynamics in Freshwater Wetlands: Challenges and Perspectives.

Viruses are non-living, acellular entities, and the most abundant biological agents on earth. They are widely acknowledged as having the capacity to influence global biogeochemical cycles by infecting the bacterial and archaeal populations that regulate carbon and nutrient turnover. Evidence suggests that the majority of viruses in wetlands are bacteriophages, but despite their importance, studies on how viruses control the prokaryotic community and the concomitant impacts on ecosystem function (such as carbon cycling and greenhouse gas flux) in wetlands are rare. Here we investigate virus-prokaryote interactions in freshwater wetland ecosystems in the context of their potential influence on biogeochemical cycling. Specifically, we (1) synthesize existing literature to establish current understanding of virus-prokaryote interactions, focusing on the implications for wetland greenhouse gas dynamics and (2) identify future research priorities. Viral dynamics in freshwater wetlands have received much less attention compared to those in marine ecosystems. However, based on our literature review, within the last 10 years, viral ecology studies on freshwater wetlands have increased twofold. Despite this increase in literature, the potential implication of viral infections on greenhouse gas emission dynamics is still a knowledge gap. We hypothesize that the rate of greenhouse gas emissions and the pool of sequestered carbon could be strongly linked to the type and rate of viral infection. Viral replication mechanism choice will consequently influence the microbial efficiency of organic matter assimilation and thus the ultimate fate of carbon as a greenhouse gas or stored in soils.

Optimal soil carbon sampling designs to achieve cost-effectiveness: a case study in blue carbon ecosystems.

Researchers are increasingly studying carbon (C) storage by natural ecosystems for climate mitigation, including coastal 'blue carbon' ecosystems. Unfortunately, little guidance on how to achieve robust, cost-effective estimates of blue C stocks to inform inventories exists. We use existing data (492 cores) to develop recommendations on the sampling effort required to achieve robust estimates of blue C. Using a broad-scale, spatially explicit dataset from Victoria, Australia, we applied multiple spatial methods to provide guidelines for reducing variability in estimates of soil C stocks over large areas. With a separate dataset collected across Australia, we evaluated how many samples are needed to capture variability within soil cores and the best methods for extrapolating C to 1 m soil depth. We found that 40 core samples are optimal for capturing C variance across 1000's of kilometres but higher density sampling is required across finer scales (100-200 km). Accounting for environmental variation can further decrease required sampling. The within core analyses showed that nine samples within a core capture the majority of the variability and log-linear equations can accurately extrapolate C. These recommendations can help develop standardized methods for sampling programmes to quantify soil C stocks at national scales.

Carbon stocks, sequestration, and emissions of wetlands in south eastern Australia.

Nontidal wetlands are estimated to contribute significantly to the soil carbon pool across the globe. However, our understanding of the occurrence and variability of carbon storage between wetland types and across regions represents a major impediment to the ability of nations to include wetlands in greenhouse gas inventories and carbon offset initiatives. We performed a large-scale survey of nontidal wetland soil carbon stocks and accretion rates from the state of Victoria in south-eastern Australia-a region spanning 237,000 km2 and containing >35,000 temperate, alpine, and semi-arid wetlands. From an analysis of >1,600 samples across 103 wetlands, we found that alpine wetlands had the highest carbon stocks (290 ± 180 Mg Corg ha-1 ), while permanent open freshwater wetlands and saline wetlands had the lowest carbon stocks (110 ± 120 and 60 ± 50 Mg Corg ha-1 , respectively). Permanent open freshwater sites sequestered on average three times more carbon per year over the last century than shallow freshwater marshes (2.50 ± 0.44 and 0.79 ± 0.45 Mg Corg ha-1  year-1 , respectively). Using this data, we estimate that wetlands in Victoria have a soil carbon stock in the upper 1 m of 68 million tons of Corg , with an annual soil carbon sequestration rate of 3 million tons of CO2 eq. year-1 -equivalent to the annual emissions of about 3% of the state's population. Since European settlement (~1834), drainage and loss of 260,530 ha of wetlands may have released between 20 and 75 million tons CO2 equivalents (based on 27%-90% of soil carbon converted to CO2 ). Overall, we show that despite substantial spatial variability within wetland types, some wetland types differ in their carbon stocks and sequestration rates. The duration of water inundation, plant community composition, and allochthonous carbon inputs likely play an important role in influencing variation in carbon storage.

Spatially variable synergistic effects of disturbance and additional nutrients on kelp recruitment and recovery.

Understanding the impact of multiple stressors on ecosystems is of pronounced importance, particularly when one or more of those stressors is anthropogenic. Here we investigated the role of physical disturbance and increased nutrients on reefs dominated by the canopy-forming kelp Ecklonia radiata. We combined experimental kelp canopy removals and additional nutrient at three different locations in a large embayment in temperate southeastern Australia. Over the following winter recruitment season, Ecklonia recruitment was unaffected by increased nutrients alone, but tripled at all sites where the canopy had been removed. At one site, the combination of disturbance and increased nutrients resulted in more than four times the recruitment of the introduced kelp Undaria pinnatifida. Six months after disturbance, the proliferation of the Undaria canopy in the canopy-removal and nutrient-addition treatment negatively influenced the recovery of the native kelp Ecklonia. Given the otherwise competitive dominance of adult Ecklonia, this provides a mechanism whereby Undaria could maintain open space for the following recruitment season. This interplay between disturbance, nutrients and the response of native and invasive species makes a compelling case for how a combination of factors can influence species dynamics.