Coastal Marsh Habitats Research Articles

Harmonization of Nordic coastal marsh habitat classification benefits conservation and management

Coastal marshes are productive and diverse habitats that act as essential linkages between marine and terrestrial ecosystems. Although the Nordic region comprises diverse, unique marsh habitats along the Baltic coast and the Atlantic coast of Norway, they are not widely recognized as coastal marshes, e.g., salt marshes in other parts of the world. This may be partly due to inconsistency in habitat classification, use of terminology, and the heterogeneity of these habitats across the region, which limit overviews of their ecology, distribution, and management. Here, we aim to improve the basis for assessments of Nordic coastal marshes by providing an overview of terminology, distribution, area, and management practices of these habitats across the region. We use the general term coastal marsh to cover the variety of habitat types in the study area, which are covered mainly by the European Union's (EU's) Habitats Directive Annex I habitat classes 1310 Salicornia and other annuals colonizing mud and sand, 1320 Spartina swards (Spartinion maritimae), 1330 Atlantic salt meadows (Glauco-Puccinellietalia maritimae), and 1630 Boreal Baltic coastal meadows. Based on input from national environmental officers, an expert survey, a workshop, and reviewing environmental databases, we compiled a comprehensive database on coastal marsh habitats regarding their distribution, classification, conservation status, management, and monitoring practices in four Nordic countries (Denmark, Finland, Norway, Sweden) and Germany, covering wide environmental gradients in, e.g., salinity and tidal range. Overall, we found that the four Annex I classes cover an area of ∼900 km2 in the study region, one-fourth of the total area of these habitats in the EU, with an additional ∼450 km2 of other potential coastal marsh areas and 500 km2 of coastal reed beds. Beyond the variable terminology across the region, our study revealed a gap in the European habitat classification systems concerning the diversity of the Baltic Sea coastal marshes. Creating subtype(s) for the Annex I class 1630 would help to improve the conservation of the most threatened habitat types within the class. In addition, we found pronounced differences in monitoring and managing coastal marshes among countries, indicating that these important and diverse habitats between land and sea might receive insufficient attention and protection despite the multiple ecosystem services they provide. Coordinated harmonization of habitat classifications and monitoring across the Nordic countries could increase regional-scale coordination of the management of coastal marshes and facilitate comparative studies.

Preferred atmospheric circulations associated with favorable prescribed burns in the Gulf of Mexico coast, USA

BackgroundApplication of prescribed fire in natural plant communities is an important wildlife habitat management tool. Prescribed fire managers have suggested anecdotally that changing weather patterns may be influencing the frequency of days that have optimal conditions to conduct coastal marsh burns along the US Gulf of Mexico coast. Our study objectives were to (1) determine whether the frequency of atmospheric circulation patterns associated with prescribed fire prescriptions has changed from 1979 to 2018 for the Gulf Coast and (2) identify circulation patterns preferred by land managers for implementing prescribed fire.ResultsWhile coastal marsh habitat is threatened by climate change and human-associated degradation, weather type frequency was not identified as an important factor related to the application of prescribed fire, as the frequency of weather circulation types has not changed significantly over time (p > 0.05). However, some weather circulation patterns seem more advantageous (e.g., offshore winds) or disadvantageous (e.g., wet cold fronts and high winds) for consideration by prescribed fire applicators across the Gulf.ConclusionsFurther insight into the weather conditions preferred and avoided by land managers along the Gulf of Mexico will improve prediction-based methods for identifying burn windows from weather forecasts. Land managers face many challenges in protecting coastal systems, while also reducing management conflicts (i.e., smoke transport) with local communities. Understanding how constraints such as urbanization, climate change, and sea-level rise interact to affect prescribed fire application will be an increasingly important aspect for developing successful adaptive management plans.

Improved mapping of coastal salt marsh habitat change at Barnegat Bay (NJ, USA) using object-based image analysis of high-resolution aerial imagery

Tidal wetlands are valued for the ecosystem services they provide yet are vulnerable to loss due to anthropogenic disturbances such as land conversion, hydrologic modifications, and the impacts of climate change, especially accelerating rates of sea level rise. To effectively manage tidal wetlands in face of multiple stressors, accurate studies of wetland extent and trends based on high-resolution imagery are needed. We provide salt marsh delineations for Barnegat Bay, New Jersey, by means of object-based image analysis of high-resolution aerial imagery and digital elevation models. We performed trends analyses of salt marsh extent from 1995 to 2015 and estimated drivers of marsh area change. We found that in 1995, 8830 ± 390 ha were covered with marsh vegetation, while in 2015 only 8180 ± 380 ha of salt marsh habitat remained. The resulting net loss rate of 0.37% yr−1 is equivalent to historic loss rates since the 1970s, indicating that despite regionally accelerating relative sea level rise and purported eutrophication, salt marsh loss rates at Barnegat Bay remain steady. The main drivers of salt marsh loss are excavations for mosquito control (409 ha), edge erosion (303 ha) and ponding (240 ha). Upland migration of salt marsh did not completely mitigate these losses but accounted for a gain of 147 ha of tidal marsh habitat. The methodology presented herein yielded accurate salt marsh delineations (>90%) and trend detection (85%), outperforming low-resolution wetland delineations used in coastal management. This study demonstrates the suitability of high-resolution imagery for the detection of open water features. For the purposes of salt marsh change detection and the identification of change drivers, management and conservation agencies should make use of high-resolution imagery whenever feasible.

Saving the Incredible Salt Marsh Harvest Mouse!

Did you know that one of the most unique and incredible mammals in the world is found only in the San Francisco Bay-Estuary? The salt marsh harvest mouse is an endangered species because over 90% of its coastal marsh habitat has been lost due to human development. This species can swim for over 2h, can climb using its tail like a fifth limb, can eat salty foods, and can even drink water saltier than the ocean! For decades, people thought this animal could live only in tidal marshes dominated by salt-loving plants. However, in the past few years, we have learned many new things about the salt marsh harvest mouse. For example, we now know it can live in non-tidal marshes and forage on freshwater plants. Unfortunately, the superhero abilities of this mouse will not protect it from all the threats it faces in the future, such as sea-level rise.

Projected impact of sea-level rise and urbanization on mottled duck (Anas fulvigula) habitat along the Gulf Coast of Louisiana and Texas through 2100

Coastal wetlands along the Gulf of Mexico support a wide diversity of wildlife, are important nurseries for sport and commercial fisheries, provide erosion and flood control, and serve many other ecological functions and services. These marshes have been declining in area and degrading at alarming rates since the 1930s. Effective conservation planning is vital to protect these ecosystems, but decision makers often lack knowledge of expected future conditions to strategically target conservation actions. To address this issue, we focus on a species of conservation concern, the mottled duck (Anas fulvigula), that resides year-round in the coastal marshes of the Gulf of Mexico. We used location data collected from radiomarked hen mottled ducks from 2006 to 2011 to create an ensemble model of habitat selection for 2010. We then projected future habitat states using models of sea-level rise and human development. By combining future predictions with our ensemble model, we predict future habitat for mottled ducks through 2100, in 20-year time steps beginning with 2020. Sea-level rise models predicted reductions in coastal marsh habitats and our ensemble model predict corresponding declines in overall habitat quantity and quality for mottled ducks, with the largest rate of habitat loss predicted within the Chenier Plain of Louisiana, USA at 71%. In some areas, particularly the Texas Mid-Coast, USA, future urbanization and human development is expected to reduce the ability of wetland habitat to migrate inland with rising sea-levels. Our results also highlight areas of coastal marsh particularly vulnerable to sea-level rise; and conversely, identify areas most likely to persist into the future that could be targeted for habitat conservation to help mottled ducks persist on the landscape.

Beneficial use of dredged sediment as a sustainable practice for restoring coastal marsh habitat.

Coastal Louisiana (USA) continues to sustain immense land and habitat losses due to subsidence, sea-level rise, and storm events. Approximately 65 million m3 (85 million cubic yards) of sediment is dredged annually from Gulf Coast federal navigation channels to maintain safe waterway passage. The beneficial use of these sediments continues to increase, and now this sediment is recognized as a critical resource in large-scale (estimated multibillion dollar) ecosystem restoration efforts to mitigate land and habitat losses along the US Gulf Coast. However, the documentation of restoration benefits where dredged sediments are the primary resource is lacking, which limits the potential for future applications. Therefore, this study documents the progress to restore marsh habitat and the resultant benefits in West Bay, Louisiana, and investigates how the restoration practices align with principles of the US Army Corps of Engineers (USACE) Engineering with Nature® (EWN® ) and UN Sustainable Development Goals (UN SDGs). West Bay, a 4964-ha subdelta adjacent to the Mississippi River, typifies risks of coastal land loss that also threatens the integrity of the adjacent federal navigation channel. To help restore coastal marsh habitat on a large spatial and temporal scale, the USACE constructed an uncontrolled diversionary channel from the Mississippi River and with subsequent direct and strategic placement of dredged sediment. Restoration performance was assessed through remotely sensed methods using data spanning approximately 70 years. To date, placement of dredged sediment in the bay has facilitated the creation of over 800 ha of new land in the formerly open waters of West Bay. The West Bay restoration project aligns with the principles of the EWN initiative, which supports more sustainable practices to deliver economic, environmental, and social benefits through collaborative processes and meaningfully integrates 10 of the UN SDGs designed to achieve a better and more sustainable future. Integr Environ Assess Manag 2022;18:1162-1173. Published 2021. This article is a U.S. Government work and is in the public domain in the USA.

Variation in synchrony of production among species, sites, and intertidal zones in coastal marshes.

Spatially synchronous population dynamics are important to ecosystem functioning and have several potential causes. By looking at synchrony in plant productivity over 18yr across two elevations in three types of coastal marsh habitat dominated by different clonal plant species in Georgia, USA, we were able to explore the importance of plant species and different habitat conditions to synchrony. Synchrony was highest when comparing within a plant species and within a marsh zone, and decreased across species, with increasing distance, and with increasing elevational differences. Abiotic conditions that were measured at individual sites (water column temperature and salinity) also showed high synchrony among sites, and in one case (salinity) decreased with increasing distance among sites. The Moran effect (synchronous abiotic conditions among sites) is the most plausible explanation for our findings. Decreased synchrony between creekbank and mid-marsh zones, and among habitat types (tidal fresh, brackish, and salt marsh) was likely due in part to different exposure to abiotic conditions and in part to variation in sensitivity of dominant plant species to these abiotic conditions. We found no evidence for asynchrony among species, sites or zones, indicating that one habitat type or zone will not compensate for poor production in another during years with low productivity; however, tidal fresh, brackish. and salt marsh sites were also not highly synchronous with each other, which will moderate productivity variation among years at the landscape level due to the portfolio effect. We identified the creekbank zone as more sensitive than the mid-marsh to abiotic variation and therefore as a priority for monitoring and management.

Correction to: Commodity Consistent Meta-Analysis of Wetland Values: An Illustration for Coastal Marsh Habitat

In the original article, the code used to estimate the meta-regression models (of WTP for coastal marsh habitat changes) erroneously included the natural log of nominal rather than real income as an independent variable.

Living on the Edge: Multi-Scale Analyses of Bird Habitat Use in Coastal Marshes of Barataria Basin, Louisiana, USA

Coastal marsh loss, combined with expected sea-level rise, will cause inundation and extensive shifts to vegetation and salinity regimes that may affect bird species dependent on coastal ecosystems worldwide. Within coastal-marsh habitats, birds provide key targets for coastal management goals. However, limited information on bird-habitat relationships within coastal marshes inhibits the development of restoration projects targeted to bird species. We surveyed birds bi-monthly within Barataria Basin, LA from July 2014 to December 2015 to compare their use between fresh and saline coastal marshes. Additionally, we examined habitat use at finer spatial scales to assess preference for marsh-edge microhabitats. Edge habitat supported 1.8 times more bird species (guild) richness than emergent and open-water habitat. We concluded that future modelling efforts would be improved if models incorporate edge effects for birds in coastal marshes that extend 20 m from emergent vegetation into open water, with a reduced effect if marsh types convert from fresh to saline. Our data will be useful to simulate the effects of changes in marsh type, area, and edge on habitat quality for birds in coastal Louisiana and will inform habitat restoration and management decisions aimed at optimizing bird use.

Sea-level rise thresholds for stability of salt marshes in a riverine versus a marine dominated estuary

We studied the ecological resilience of salt marshes by deriving sea level rise (SLR) thresholds in two estuaries with contrasting upland hydrological inputs in the north-central Gulf of Mexico: Grand Bay National Estuarine Research Reserve (NERR) with limited upland input, and the Pascagoula River delta drained by the Pascagoula River, the largest undammed river in the continental United States. We applied a mechanistic model to account for vegetation responses and hydrodynamics to predict salt marsh distributions under future SLR scenarios. We further investigated the potential mechanisms that contribute to salt marsh resilience to SLR.The modeling results show that salt marshes in the riverine dominated estuary are more resilient to SLR than in the marine dominated estuary with SLR thresholds of 10.3 mm/yr and 7.2 mm/yr respectively. This difference of >3 mm/yr is mainly contributed by larger quantities of riverine-borne mineral sediments in the Pascagoula River. In both systems, sediment trapping by the above-ground vegetation appears to contribute more to marsh platform accretion than organic matter from below-ground biomass based on the medians of the accretion rates. However, below-ground biomass could contribute up to 90% of accretion in the marine dominated estuary compared to only 60% of accretion in the riverine dominated estuary. SLR thresholds of salt marshes are more sensitive to vegetation biomass in the marine dominated estuary while biomass and sediment similarly affect SLR thresholds of salt marshes in the riverine dominated estuary.This research will likely help facilitate more informed decisions on conservation/restoration policies for these two types of systems in the near-term needed to minimize future catastrophic loss of these coastal marsh habitats once SLR thresholds are exceeded.

Soil surface elevation dynamics in a mangrove-to-marsh ecotone characterized by vegetation shifts

Mangrove forest encroachment into coastal marsh habitats has been described in subtropical regions worldwide in recent decades. To better understand how soil processes may influence vegetation change, we studied soil surface elevation change, accretion rates, and soil subsurface change across a coastal salinity gradient in Florida, USA, an area with documented mangrove encroachment into saline marshes. Our aim was to identify if variations in the soil variables studied exist and to document any associated vegetation shifts. We used surface elevation tables and marker horizons to document the soil variables over 5 years in a mangrove-to-marsh transition zone or ecotone. Study sites were located in three marsh types (brackish, salt, and transition) and in riverine mangrove forests. Mangrove forest sites had significantly higher accretion rates than marsh sites and were the only locations where elevation gain occurred. Significant loss in surface elevation occurred at transition and salt marsh sites. Transition marshes, which had a significantly higher rate of shallow subsidence compared to other wetland types, appear to be most vulnerable to submergence or to a shift to mangrove forest. Submergence can result in herbaceous vegetation mortality and conversion to open water, with severe implications to the quantity and quality of wetland services provided.

Partial exclusion of spawning Cyprinus carpio to improve coastal marsh habitat may come at the cost of increased carp population growth

Common Carp (Cyprinus carpio) feeding and spawning behaviours, when abundant, negatively impact shallow water habitats essential for marsh ecosystem functioning. A common coastal marsh restoration practice is to create barriers to carp movement into a marsh from connected lakes. These barriers often do not exclude all carp from a marsh, and the lower adult density and improved habitat may increase overall carp recruitment success. A population model was created for Delta Marsh on Lake Manitoba, Canada. Varying the proportion of the total carp population spawning in the marsh indicates that, with partial carp exclusion, the improvement in spawning conditions increases carp population growth rate and decreases generation time. Increasing fishing mortality can help mitigate this effect, indicating that a multi-tiered management approach is best. It is important that fish exclusion programs be implemented with awareness of this unanticipated result.

Carbon storage in the Mississippi River delta enhanced by environmental engineering

River deltas have contributed to atmospheric carbon regulation throughout Earth history, but functioning in the modern era has been impaired by reduced sediment loads, altered hydrologic regimes, increased global sea-level rise and accelerated subsidence. Delta restoration involves environmental engineering via river diversions, which utilize self-organizing processes to create prograding deltas. Here we analyse sediment cores from Wax Lake delta, a product of environmental engineering, to quantify the burial of organic carbon. We find that, despite relatively low concentrations of organic carbon measured in the cores (about 0.4%), the accumulation of about 3 T m−2 of sediment over the approximate 60 years of delta building resulted in the burial of a significant amount of organic carbon (16 kg m−2). This equates to an apparent organic carbon accumulation rate of 250 ± 23 g m−2 yr−1, which implicitly includes losses by carbon emissions and erosion. Our estimated accumulation rate for Wax Lake delta is substantially greater than previous estimates based on the top metre of delta sediments and comparable to those of coastal mangrove and marsh habitats. The sedimentation of carbon at the Wax Lake delta demonstrates the capacity of engineered river diversions to enhance both coastal accretion and carbon burial. Substantial amounts of organic carbon have been buried in the Wax Lake delta, USA, over the past 20 years, according to sediment analyses. This suggests that river diversions can lead to both coastal accretion and carbon sequestration.

Evaluating sampling strategy for DNA barcoding study of coastal and inland halo-tolerant Poaceae and Chenopodiaceae: A case study for increased sample size.

Environmental conditions in coastal salt marsh habitats have led to the development of specialist genetic adaptations. We evaluated six DNA barcode loci of the 53 species of Poaceae and 15 species of Chenopodiaceae from China's coastal salt marsh area and inland area. Our results indicate that the optimum DNA barcode was ITS for coastal salt-tolerant Poaceae and matK for the Chenopodiaceae. Sampling strategies for ten common species of Poaceae and Chenopodiaceae were analyzed according to optimum barcode. We found that by increasing the number of samples collected from the coastal salt marsh area on the basis of inland samples, the number of haplotypes of Arundinella hirta, Digitaria ciliaris, Eleusine indica, Imperata cylindrica, Setaria viridis, and Chenopodium glaucum increased, with a principal coordinate plot clearly showing increased distribution points. The results of a Mann-Whitney test showed that for Digitaria ciliaris, Eleusine indica, Imperata cylindrica, and Setaria viridis, the distribution of intraspecific genetic distances was significantly different when samples from the coastal salt marsh area were included (P < 0.01). These results suggest that increasing the sample size in specialist habitats can improve measurements of intraspecific genetic diversity, and will have a positive effect on the application of the DNA barcodes in widely distributed species. The results of random sampling showed that when sample size reached 11 for Chloris virgata, Chenopodium glaucum, and Dysphania ambrosioides, 13 for Setaria viridis, and 15 for Eleusine indica, Imperata cylindrica and Chenopodium album, average intraspecific distance tended to reach stability. These results indicate that the sample size for DNA barcode of globally distributed species should be increased to 11–15.

Subspecies delineation amid phenotypic, geographic and genetic discordance in a songbird.

Understanding the processes that drive divergence within and among species is a long-standing goal in evolutionary biology. Traditional approaches to assessing differentiation rely on phenotypes to identify intra- and interspecific variation, but many species express subtle morphological gradients in which boundaries among forms are unclear. This intraspecific variation may be driven by differential adaptation to local conditions and may thereby reflect the evolutionary potential within a species. Here, we combine genetic and morphological data to evaluate intraspecific variation within the Nelson's (Ammodramus nelsoni) and salt marsh (Ammodramus caudacutus) sparrow complex, a group with populations that span considerable geographic distributions and a habitat gradient. We evaluated genetic structure among and within five putative subspecies of A.nelsoni and A.caudacutus using a reduced-representation sequencing approach to generate a panel of 1929 SNPs among 69 individuals. Although we detected morphological differences among some groups, individuals sorted along a continuous phenotypic gradient. In contrast, the genetic data identified three distinct clusters corresponding to populations that inhabit coastal salt marsh, interior freshwater marsh and coastal brackish-water marsh habitats. These patterns support the current species-level recognition but do not match the subspecies-level taxonomy within each species-a finding which may have important conservation implications. We identified loci exhibiting patterns of elevated divergence among and within these species, indicating a role for local selective pressures in driving patterns of differentiation across the complex. We conclude that this evidence for adaptive variation among subspecies warrants the consideration of evolutionary potential and genetic novelty when identifying conservation units for this group.

Commensal Nesting ofScincella lateralis(Little Brown Skinks) inAlligator mississippiensis(American Alligator) Nests andOndatra zibethicus(Muskrat) Houses in Southwestern Louisiana

Abstract Scincella lateralis (Little Brown Skink, hereafter Skink) are ubiquitous throughout the southeastern US and primarily nest in leaf litter on the forest floor. However, their presence in coastal marsh settings necessitates their use of alternate nesting habitats. We investigated Skink nesting ecology in Alligator mississippiensis (American Alligator) nests and Ondatra zibethicus (Muskrat) houses in an intermediate/brackish coastal marsh habitat at Rockefeller Wildlife Refuge in southwestern Louisiana. For our field studies, we found and collected Skink eggs from 16 June through 5 October, and hatched eggshells from 6 July through 5 October. We found Skink eggs/eggshells in 36 of 109 active American Alligator nests (33.0%), 19 of 36 inactive Alligator nests (52.8%), and 13 of 51 Muskrat houses (25.5%). Clutch size ranged from 1 to 5 eggs (average = 3.0). The mean depth of Skink eggs was 24.6 cm in American Alligator nests and 17.1 cm in Muskrat houses. Multiple clutches commonly occurred in a struc...

Differential Expression of miRNAs Under Salt Stress in Spartina alterniflora Leaf Tissues.

Coastal marsh habitats are impacted by many factors or disturbances, including habitat destruction, pollution, and the introduction of invasive species. Spartina alterniflora (S. alterniflora) is an important invasive species, accounting for a significant proportion of the invasive plants spread around the world. Salt stress is a major environmental stress factor, which affects plant growth and development. Little information is available regarding S. alterniflora microRNAs (miRNAs) which play important regulatory roles in plant growth and development. In order to detect S. alterniflora miRNAs and determine any expression differences between S. alterniflora plants cultivated on ordinary soils from the greenhouse and salty soils from Dafeng, in Jiangsu province of China, we carried out the detection and quantification of S. alterniflora miRNAs by microarray. Among the 81 miRNAs identified as significantly down- or up-regulated under the salt stress, 21 of the miRNAs represent 8 miRNA gene families in S. alterniflora. We found that miR168, miR399, miR395, miR393, miR171, miR396, miR169, and miR164 were down-regulated under salinity stress, and 60 of the miRNAs were up-regulated, which were revealed to be induced by salt stress in plants. The identification of differentially expressed novel plant miRNAs and their target genes, and the analysis of expression, provide molecular evidence for the possible involvement of miRNAs in the process of salt response and/or salt tolerance in S. alterniflora.

Conserving freshwater biodiversity: The value, status and management of high quality ditch systems

Freshwater biodiversity is globally threatened and while most conservation efforts are focussed on natural and larger freshwater systems such as rivers and lakes, in many lowland agricultural landscapes artificial water bodies including ditches may be equally important as habitats for freshwater species. Ditches occur across the agricultural landscape but in particular, those associated with coastal and floodplain grazing marsh, have high conservation value. The importance of this habitat for rare and threatened species afforded priority status under the UK Biodiversity Action Plan is explored. The characteristics of ditches that have high conservation value are described and a set of targets against which such ditches can be assessed are presented. An analysis of the current condition of Sites of Special Scientific Interest (SSSI) for the wider coastal and floodplain grazing marsh habitat demonstrates the range of pressures affecting these sites and highlights that alongside generic freshwater issues such as eutrophication and non-native species, these sites have a unique set of pressures associated with their ongoing management and the vulnerable location of many sites at the coast. Wider conservation strategies for freshwater biodiversity in lowland landscapes across Europe need to factor in the different management requirements of artificial habitats such as ditches alongside more ambitious restoration projects for natural waterbodies. In low lying coastal areas the threat of coastal squeeze for many important grazing marshes will require a strategic approach to allow upstream migration of important biodiversity.

Artificial substrates enhance non-native macroalga and N2 production

The introduction of non-native species occurs within a context of other anthropogenic impacts: thus a holistic approach is needed to understand interactive effects. Installation of shoreline protection structures is increasing in response to rising sea levels and increasing frequency of intense storms. Shoreline hardening structures can facilitate establishment of non-native species with multiple potential consequences. We measured abundances of both native and non-native Codium, a green macroalga, on natural hard substrates (oyster reefs) and artificial substrates (bulkheads and revetments) in two estuaries and assessed the effects of each Codium species on nitrogen fixation and net N2 fluxes. Native C. decorticatum was the most abundant (86 %) Codium on oyster reefs, while exotic C. fragile dominated (99 %) artificial substrates. N2 production via denitrification was greater than nitrogen fixation for both species and the net N2 production associated with non-native C. fragile was greater than with native C. decorticatum. Comparing our results with surveys conducted in the 1940s before C. fragile had invaded, indicates that non-native Codium has outcompeted native Codium on artificial substrates, but that natural substrate provided by oyster reefs remains a refuge for native Codium. Although shoreline hardening with artificial structures can reduce ecosystem services provided by coastal marsh and other habitats, an unexpected beneficial consequence was the N2 production associated with the non-native Codium, which has the potential to mitigate anthropogenic nutrient loading. Our results illustrate that the interaction between multiple anthropogenic impacts can be positive, and indicate that non-ephemeral macroalgae could be an overlooked component in nitrogen removal from marine ecosystems by enhancing denitrification.

Mineral versus organic contribution to vertical accretion and elevation change in restored marshes (Ebro Delta, Spain)

The Ebro Delta (Catalonia, Spain) is one of the most valuable coastal zones within the Mediterranean Sea, supporting a highly productive rice agricultural system, as well as a myriad of coastal marsh habitats. However, chronic reductions of fluvial sediments coupled with accelerated relative sea-level rise have created an environment where approximately half of the Ebro Delta is now vulnerable to flooding impacts. To assess relative sea-level rise (RSLR) mitigation options through marsh restoration within abandoned deltaic rice fields, we established the experimentally restored marshes spanning three years. We used two freshwater input type treatments (riverine irrigation and rice field drainage water) and three water level treatments (10, 20 and 30cm deep). Our hypotheses were that: (1) vertical accretion and elevation change in oligohaline restored marshes would be primarily controlled by organic contributions under sediment-deficit conditions, and (2) both vertical accretion and elevation change would demonstrate higher rates compared with predicted RSLR in the Ebro Delta (5–8mmyr−1). Vertical accretion had higher mean values in both water type treatments (11.5 and 15.5mmyr−1) than elevation change (9.1 and 8.8mmyr−1). Vertical accretion (but not elevation change) was significantly higher in drainage water treatment receiving greater sediment mineral input, which caused higher surface soil mineral content. Conversely, experimentally restored marshes closer to rice fields in both water type treatments had greater elevation change (11.3 and 17.8mmyr−1) than vertical accretion (8.3 and 15.1mmyr−1) due to higher belowground biomass because of high weed colonization by Paspalum distichum L. These results showed that vertical accretion and elevation change were generally controlled by mineral contribution, although fast growing, ruderal plant species such as P. distichum can play a significant role in marsh elevation via root growth. The results supported the hypothesis that restored marshes using either water type promote marsh elevation gains higher than predicted RSLR at least during the initial marsh development (3 years). This study indicates that the use of agricultural runoff as a primary source of sediment, nutrient, and freshwater is beneficial for marsh restoration projects focused primarily on mitigating RSLR. This research also highlights how nuisance species such as P. distichum can play a key role in mitigating RSLR impacts when inexpensive and effective measures are needed to promote marsh elevation as the primary restoration goal.