Tidal Restriction Research Articles

Seas the opportunity: multi-criteria decision analysis to identify and prioritise blue carbon wetland restoration sites

IntroductionThe emergence of voluntary carbon markets is creating new opportunities to sustainably finance Natural Climate Solution (NCS) projects. In Australia, the federal government recently enacted the Tidal Restoration of Blue Carbon Ecosystems Methodology Determination 2022 (Tidal Reconnection Method), whereby restoration activities that reintroduce tidal flows to allow the re-establishment of coastal wetland (blue carbon) ecosystems, through the removal or modification of a tidal restriction, can be used to gain and sell Australian carbon credit units. Australia has the highest net blue carbon wealth in the world, with 5%–11% of global carbon stocks, yet there is currently a lack of large-scale feasibility assessments and supporting methodologies to identify and prioritise sites with the greatest potential for NCS project implementation to help inform investment decisions.MethodsIn this study, we applied a spatial Multi-Criteria Decision Analysis (MCDA) to identify, map, and prioritise potential sites for blue carbon coastal wetland restoration in South Australia that meet criteria outlined in the Tidal Reconnection Method. This study compared information on 1) predicted flooding extent following tidal reconnection and under sea level rise (SLR; present-day, 2050 and 2,100); 2) project implementation complexity (e.g. who possesses land tenure); and 3) carbon sequestration potential through predicted area of vegetation change under the above SLR scenarios. ResultsOur results identified 64 sites of interest, of which 32 received an overall “high” prioritisation score of 3 or more out of 5. This equates to approximately 21,114 ha of high priority potential blue carbon restoration sites.DiscussionThe MCDA enables development of a portfolio of viable restoration projects through a rapid “desktop” prioritisation of sites of interest, which can then guide investment in further detailed cost/benefit feasibility assessments. This study demonstrates an adaptable MCDA approach to map potential NCS projects at meaningful spatial scales and in-line with carbon market-based opportunities.

Mapping tidal restrictions to support blue carbon restoration

Blue carbon ecosystems (BCEs), encompassing mangroves, saltmarshes, and seagrasses, are vital ecosystems that deliver valuable services such as carbon sequestration, biodiversity support, and coastal protection. However, these ecosystems are threatened by various anthropogenic factors, including tidal restrictions like levees, barriers, and embankments. These structures alter the natural seawater flow, often converting coastal ecosystems into freshwater environments. Identifying tidal restrictions and assessing their suitability for tidal restoration in areas amenable for coastal management is a crucial first step to successfully restore BCEs and the associated ecosystem services they provide, i.e., managed realignment. This study presents a novel approach for detecting tidal restrictions in the state of Victoria, Australia, using high-resolution LiDAR data, geospatial analysis techniques, and a multi-criteria scoring system. Our model successfully identified 90 % of known tidal restrictions from an existing dataset, while also detecting an additional 118 potential tidal restrictions, representing a 35 % increase. The model performance analysis revealed trade-offs between precision, recall, and noise ratio when using different noise reduction thresholds, highlighting the importance of selecting an appropriate threshold based on project objectives. The multi-criteria scoring system, which considered factors such as proximity to BCEs and current land use, enabled the selection of tidal restrictions based on their hydrological suitability for restoration. The results of this study have significant implications for BCE restoration efforts not only in Victoria, but more broadly across Australia and globally, providing a systematic approach to identifying and targeting areas with the greatest potential for successful restoration projects. While the approach is low-cost and user-friendly, it is dependent on the availability of LiDAR data for the study area. This can make it accessible to researchers and practitioners worldwide, allowing for its adaptation and application in diverse regions to support global efforts in restoring BCEs through tidal restoration.

Fantastic wetlands and why to monitor them: Demonstrating the social and financial benefit potential of methane abatement through salt marsh restoration.

Salt marsh restoration has the potential to reduce greenhouse gas emissions thereby providing an opportunity for blue carbon crediting, but implementation has been limited to date because of insufficient data and validation. In this paper, we demonstrate the potential scale of methane emissions that could be avoided if salinity-reducing impairments are mitigated by applying findings from six salt marsh restoration sites in Massachusetts combined with a previously demonstrated application of the salt marsh salinity-methane relationship. We used calculations of these avoided emissions to estimate the social benefit of salt marsh restoration by calculating avoided costs. Of the six sites selected, restorations at two sites were successful in improving salinity which we used as a methane proxy. Our approach and findings demonstrate the potential benefits in developing consistent accounting methodologies to better track, prioritize, and implement wetlands restoration strategies to mitigate methane emissions and contribute toward state-level emissions reduction targets across some of the 475 Massachusetts salt marches with an existing tidal restriction. We found the potential for $12 -$26M in added social benefit from 176 tons of avoided methane across 932 hectares of degraded salt marsh in Massachusetts. A significant limitation in estimating benefits, however, is the lack of coordinated, widespread monitoring strategies to infer methane and other GhGs at scale. While not insurmountable, these challenges will need to be addressed for GhG emissions reduction and/or sequestration through salt marsh restoration to be accepted as an effective strategy. We conclude that while carbon crediting may offer benefits to marsh restoration and state greenhouse gas emissions reduction targets, there remain significant limitations because of a lack of project monitoring and data validation. In the worst case, this could result in the offsetting of actual greenhouse gas emissions with credits that are supported by indirect and less-than-rigorous monitoring data.

Tidal restriction does not always supplant local conditions shaping estuarine plant communities

Salt marshes are a common feature of temperate estuaries and represent the world's most abundant wetland type. They also harbour very productive plant communities and provide numerous ecosystem services. In an anthropized setting, we aimed at describing the effect of tidal restriction on the diversity patterns of vegetation communities, and to link them to the physical and chemical parameters of the estuary's oligohaline marshes.The tidal marshes of the Seine Estuary in northern France display long-lasting human activities that have shaped the ecological processes of the river Seine and its associated marshes. One of the estuary's distinctive features is a dyke on the north shore, isolating the wetlands behind it from direct tidal regimes. These areas are submitted to flooding through water gates, but tidal restriction is known to alter ecosystem processes and to possibly modify vegetation communities. Over the course of three years, we have surveyed the vegetation communities and soil parameters of three levels along the topographical gradients of two sites under tidal restriction and one site under tidal influence, with cases of mowing and grazing.Overall species richness increased along the elevation gradient, as abiotic constraints are lifted. Decomposition of species turnover (beta diversity) revealed different replacement dynamics, with tidal restriction being associated with species replacement rather than abundance difference. Tidal restriction was also associated with higher temporal stability. Canonical Correspondence Analysis between vegetation and soil surveys revealed that topography, soil pH, conductivity and soil organic carbon explained most of the differences between vegetation communities. Higher elevation vegetation plots converged, regardless of tidal restriction or land-use, while lower elevation plots diverged between study sites.We found that tidal restriction attenuated the abiotic constraints on one site, while creating new constraints linked to freshwater retention on the other site. In addition, tidal restriction was associated with longer elevational gradients compared to the connected site. Tidal restriction might therefore be a valuable management tool for estuarine marshes, as well for restoration purposes as for the mitigation of future effects related to climate change induced sea level rise.

Port Accessibility Depends on Cascading Interactions between Fleets, Policies, Infrastructure, and Hydrodynamics

Reducing waiting times is crucial for ports to be efficient and competitive. Important causes of waiting times are cascading interactions between realistic hydrodynamics, accessibility policies, vessel-priority rules, and detailed berth availability. The main challenges are determining the cause of waiting and finding rational solutions to reduce waiting time. In this study, we focus on the role of the design depth of a channel on the waiting times. We quantify the performance of channel depth for a representative fleet rather than the common approach of a single normative design vessel. The study relies on a mesoscale agent-based discrete-event model that can take processed Automatic Identification System and hydrodynamic data as its main input. The presented method’s validity is assessed by hindcasting one year of observed anchorage area laytimes for a liquid bulk terminal in the Port of Rotterdam. The hindcast demonstrates that the method predicts the causes of 73.4% of the non-excessive laytimes of vessels, thereby correctly modelling 60.7% of the vessels-of-call. Following a recent deepening of the access channel, cascading waiting times due to tidal restrictions were found to be limited. Nonetheless, the importance of our approach is demonstrated by testing alternative maintained bed level designs, revealing the method’s potential to support rational decision-making in coastal zones.

A variable neighborhood search heuristic for real-time barge scheduling in a river-to-sea channel with tidal restrictions

The scheduling of barge transportation in narrow channels is critical to transportation efficiency, especially when the tide exists and affects the channel navigation of barges. Barge movements in channels are frequently correlated with other activities (e.g., berth allocation) and must thus be considered concurrently. To enhance transportation efficiency, effective scheduling algorithms are required. In this study, we examine the barge scheduling problem arising from a river-to-sea channel with tidal restrictions in the real world, where barges’ loading and unloading operations are placed at both ends of the navigation channel and must be planned and coordinated with their channel movements. To reduce the overall completion time for all barges, we develop a mixed integer linear programming model with several valid inequalities. To satisfy the second-level computational efficiency requirement, a straightforward yet efficient variable neighborhood search algorithm is proposed. Real-world case studies based on a barge transportation project in Boffa, Guinea are conducted. It is demonstrated that the proposed approach is applicable for efficient decision-making and effective in assisting barges to avoid missing feasible tide windows by slightly altering the first-come-first-serve schedule of the barges. With the proposed approach, the size and speed of the barge fleet can be optimized, and the tidal influence is also evaluated quantitatively.

Tidal restriction likely has greater impact on the carbon sink of coastal wetland than climate warming and invasive plant

Tidal restriction likely has greater impact on the carbon sink of coastal wetland than climate warming and invasive plant

High sedimentation rates lead to rapid vegetation recovery in tidal brackish wetland restoration

IntroductionTidal wetland restoration in the Bay of Fundy involves restoring tidal hydrology to sites with tidal restrictions. Most have focused on salt marsh sites close to the mouth of estuaries, but there are also many tidally restricted wetlands closer to the freshwater end of tidal rivers. Recovery of salt marsh vegetation has been rapid in past projects, but little is known about sediment and vegetation dynamics post restoration in tidal brackish or freshwater environments.MethodsWe implemented tidal wetland restoration projects on two tidal rivers near the inland limit of saltwater. Hydrological restoration involved breaching (St. Croix) or realigning agricultural dykes (Belcher Street). We monitored hydrology, sediment accretion and vegetation at replicated plots on restoration sites and nearby reference tidal marshes; and conducted habitat mapping and elevation surveys using drones.ResultsAfter re-establishing tidal flow, sediment accretion was very rapid, leading to a deep layer of new sediments. Plant colonization at both sites resulted in a high diversity of halophytes in the first 2 years post restoration, but the St. Croix site transitioned to freshwater wetland species dominating by the fifth year post- restoration. The Belcher St. site has a mix of freshwater and brackish wetland species after the fourth-year post-restoration.DiscussionHigh suspended sediment concentrations at both sites suggest that each site was positioned closed to the estuarine turbidity maximum within its river. Tidal wetland restoration at the head of estuaries may benefit from the large ecological disturbance associated with rapid sediment accretion, providing a productive substrate with little competition from prior vegetation. However ultimate vegetation patterns may take longer to develop as elevation gains alter tidal flooding frequency. Low salinities suggest that the physical disturbance of sediment burying prior vegetation is the main mechanism creating a clean slate for plant recolonization, rather than mortality of terrestrial vegetation due to salt water. The majority of elevation change was due to allochthonous sediment deposition, with belowground processes playing a minor role. The wetlands restored showed substantial net elevation gains in the first years following tidal hydrological restoration, but long-term monitoring is required to track their overall resilience in the face of sea level rise.

Tidal restrictions in a central Californian estuarine system are associated with higher mean pH, but increased low-pH exposure

Coastal acidification is an emerging concern in estuaries impaired by nutrient pollution. In addition to rising levels of atmospheric CO2 which drives ocean acidification, high nutrient inputs to coastal areas can amplify heterotrophic metabolism, raise water column CO2 levels, and exacerbate pH declines. This study focuses on how a third anthropogenic stressor, tidal restriction, shapes effects of coastal acidification. Tidal restrictions associated with installation of gates that reduce tidal flow to a portion of an estuary are a common impact to coastal landscapes and can negatively affect water quality. This study examined pH in locations subject to varying levels of tidal restriction across a series of interconnected central California estuaries, whose waters are nutrient-impaired due to surrounding agriculture, and where 50% of the system is affected by tidal restrictions. Mean and variance of pH differed based on the level of tidal restriction. Sites lacking tidal restrictions had the lowest mean pH (7.98) but the least pH variance (0.07), and the most infrequent exposure to low pH (<7.0) conditions. In contrast, sites with minimal tidal exchange had the most exposure to low pH conditions, although mean pH levels were greater (8.08), because they also saw greater pH variance (0.46). Our results suggest that tidal restrictions alter pH levels and affect the resilience of estuaries to coastal acidification.

Tidal Restriction Leads to Enhanced Methane Emissions in Tropical Australia

Tidal Restriction Leads to Enhanced Methane Emissions in Tropical Australia

Vegetation Zonation Predicts Soil Carbon Mineralization and Microbial Communities in Southern New England Salt Marshes

Coastal marshes are important blue carbon reservoirs, but it is unclear how vegetation shifts associated with tidal restoration and sea level rise alter soil microbial respiration rates and bacterial community composition. Within 20 Connecticut salt marshes (10 without tidal restrictions, 10 tidally restored), we sampled three vegetation zones dominated by Spartina alterniflora (short-form, < 30 cm tall), S. patens, and Phragmites australis to estimate microbial respiration rates (SIR, substrate-induced respiration; carbon mineralization), root zone bacterial 16S rRNA genes, and a suite of plant and soil characteristics. Carbon density was greater in unrestricted marshes than tidally restored marshes and was the only parameter that differed among sites with varying restoration histories. We observed strong differences among vegetation zones, with vegetation being a top predictor of both SIR and carbon mineralization. Electrical conductivity (EC) was also a top predictor for SIR, and we observed strong, positive correlations between EC and both metrics of microbial respiration, with elevated rates in more frequently inundated S. alterniflora than P. australis zones. We also observed distinct root zone microbial communities associated with vegetation zones, with greater abundance of sulfate-reducing bacteria in Spartina spp. zones. Our findings suggest that dominant salt marsh vegetation zones are useful indicators of hydrologic conditions and could be used to estimate microbial respiration rates; however, it is still unclear whether differences in microbial respiration and community composition among vegetation zones are driven by plant community, environmental conditions, or their interactions.

Changes to spotted turtle (Clemmys guttata) habitat selection in response to a salt marsh restoration

Development of coastal New England has led to the replacement of up to 37% of salt marshes with degraded freshwater wetlands, primarily through tidal restrictions. Removing these restrictions to restore salt marsh ecology would improve water quality, increase flood and storm protection, nutrient filtration, erosion control, and carbon sequestration. However, such restorations replace functional freshwater wetlands and could potentially impact freshwater species. Freshwater-dependent wildlife species such as the spotted turtle (Clemmys guttata), may be especially vulnerable to rapid changes in habitat resulting from tidal reintroduction due to their high site fidelity and limited ability to disperse quickly. Conservation of genetically and physically isolated spotted turtle populations as well as the restoration of salt marshes to mitigate climate change impacts are both high priorities on Nantucket Island. These two priorities conflicted in a project to restore tidal hydrology to an impounded freshwater marsh known to host a robust spotted turtle population. We evaluated changes to spotted turtle home range size and location and habitat use in response to salt marsh restoration over eight years. Home range size did not change but the location of home ranges shifted into bordering wetlands landward of the tidal salt water influence. Spotted turtles selected remaining freshwater marshes and shrub swamps while avoiding developed land and areas of establishing salt marsh within areas that had previously been high quality habitat. This study suggests prioritizing conservation of wetlands adjacent to planned salt marsh restoration to provide habitat for freshwater species to migrate.

Reclamation-induced tidal restriction increases dissolved carbon and greenhouse gases diffusive fluxes in salt marsh creeks

Intertidal creeks play an important role in transporting nutrients between coastal ecosystems and ocean. Reclamation is a predominant anthropogenic disturbance in coastal regions; however, the influence of reclamation on carbon and nitrogen species and greenhouse gas (GHG) fluxes in creek remains unclear. In a subtropical salt marsh of eastern China, the seasonal patterns of dissolved carbon (DOC, DIC, CO2, and CH4) and inorganic nitrogen (NH4+-N, NO2−-N, and NO3−-N and N2O) species, and the diffusive fluxes of CO2, CH4, and N2O, were compared between the natural tidal creeks and the reclaimed creeks. Due to notably changed hydrological and biological conditions in the reclaimed creeks, concentrations of all dissolved carbon species, NH4+-N and NO2−-N increased significantly by 60.2–288.2%, while NO3−-N and N2O decreased slightly, compared to the natural tidal creeks. DIC and NO3−-N were the primary components of the total dissolved carbon and inorganic nitrogen in both creek types; however, their proportions decreased as a result of elevated DOC, CO2, CH4, NH4+-N, and NO2−-N following reclamation. Significantly higher global warming potential (0.58 ± 0.15 g CO2-eq m−2 d−1) was found in the reclaimed creeks, making them hotspot of greenhouse effects, compared to the natural tidal creeks. Our results indicated that changes in flow velocity, salinity, Chlorophyll a, and pH were the main factors controlling the dissolved carbon and nitrogen and consequent GHG emissions, due to reclamation. This study is helpful in understanding of carbon and nitrogen sink-source shifts resulting from land use changes in coastal wetlands.

Impacts of tidal restriction caused by embankments on the plastic growth responses of Bolboschoenus planiculmis in Korea

Tidal restriction caused by embankments can have large impacts on estuarial ecosystems. It changes the tidal regimes, which influence the growth and development of marsh plants. Although large-scale tidal freshwater marshes are negatively or positively affected by the embankments especially around the Yellow Sea in Korea, little is known regarding how the tidal restriction changes the plastic growth responses of marsh plants, which may cause a population decline in the field. Therefore, we evaluated the impacts of tidal restriction on the growth, plasticity in morphology, reproduction, and biomass allocation of the important but vulnerable marsh plant Bolboschoenus planiculmis in Korea. These responses of B. planiculmis in tidal marshes and tide-restricted marshes were compared. The tide-restricted marshes experienced deeper water (9 cm) and greater dominance of other species (51%) than the tidal marshes (respectively 0 cm, 20%). Populations in the tide-restricted marshes revealed a lower density (128 ramets m−2), decreased sexual reproduction ratio (6%), and higher short rhizome density ratio (17%) with a rapid increase in allocation to below ground biomass compared to the tidal marsh populations (respectively 1046 m−2, 35%, and 0.3%). A redundancy analysis indicated that B. planiculmis with a low density in the tide-restricted marshes developed more short rhizomes and allocated more energy to below ground biomass in order to persist in the original habitats where neighbor species dominated. On the other hand, the tidal marsh populations with higher densities invested more energy to sexual reproduction and developed more long rhizomes to expand into new areas. In this paper, tidal restriction caused the dominance of neighbor species such as Phragmites australis in standing water and negative impacts on B. planiculmis. These results indicate that the impacts of tidal restriction caused by artificial structures should be minimized by maintaining natural tides for sustainable management of B. planiculmis.

Plant biomass and rates of carbon dioxide uptake are enhanced by successful restoration of tidal connectivity in salt marshes

Salt marshes, due to their capability to bury soil carbon (C), are potentially important regional C sinks. Efforts to restore tidal flow to former salt marshes have increased in recent decades in New England (USA), as well as in some other parts of the world. In this study, we investigated plant biomass and carbon dioxide (CO2) fluxes at four sites where restoration of tidal flow occurred five to ten years prior to the study. Site elevation, aboveground biomass, CO2 flux, and porewater chemistry were measured in 2015 and 2016 in both restored marshes and adjacent marshes where tidal flow had never been restricted. We found that the elevation in restored marsh sites was 2–16 cm lower than their natural references. Restored marshes where porewater chemistry was similar to the natural reference had greater plant aboveground biomass, gross ecosystem production, ecosystem respiration, as well as net ecosystem CO2 exchange (NEE) than the natural reference, even though they had the same plant species. We also compared respiration rates in aboveground biomass (AR) and soil (BR) during July 2016, and found that restored marshes had higher AR and BR fluxes than natural references. Our findings indicated that well-restored salt marshes can result in greater plant biomass and NEP, which has the potential to enhance rates of C sequestration at 10-yrs post restoration. Those differences were likely due to lower elevation and greater flooding frequency in the recently restored marshes than the natural marsh. The inverse relationship between elevation and productivity further suggests that, where sea-level rise rate does not surpass the threshold of plant survival, the restoration of these salt marshes may lead to enhanced organic and mineral sedimentation, extending marsh survival under increased sea level, and recouping carbon stocks that were lost during decades of tidal restriction.

Multiple factors contribute to the spatially variable and dramatic decline of an invasive snail in an estuary where it was long-established and phenomenally abundant

Boom-bust dynamics of invasive species have long intrigued scientists and managers alike, but quantification of such dynamics, let alone their causes, is rare. We documented the decline of a previously prolific invasive mudsnail, Batillaria attramentaria, at Elkhorn Slough estuary in central California, USA. The mudsnail was the most abundant epibenthic invertebrate in the estuary, maintaining very high densities for many decades before declining heterogeneously throughout the estuary over the past decade, decreasing in density by three orders of magnitude at some sites. We used field and laboratory experiments to test several possible mechanisms for its demise. We show that the crab Pachygrapsus crassipes can prey heavily on Batillaria. We detected high dissolution rates of Batillaria shells, and we measured greater predation rates on tethered snails with dissolved versus intact shells. Warm water temperatures and high water levels coincided with the period of most dramatic Batillaria declines (2013–2015). Localized water impoundments appear to buffer environmental drivers of the decline because Batillaria remained abundant at sites with artificial tidal restriction, while the population crashed at one site after full tidal exchange was restored. We also investigated trematode parasite prevalence and molluscicide applications to the surrounding watershed as possible causes of mudsnail declines, but they had little explanatory power. Our findings illustrate the potential for population crashes even for long-established introduced species at pest levels of abundance, and demonstrate that such declines can exhibit spatial heterogeneity. Both of these results highlight the value of investigating population dynamics of invaders across multiple temporal and spatial scales.

Ghost forests of Marco Island: Mangrove mortality driven by belowground soil structural shifts during tidal hydrologic alteration

Land use changes often create in situ stress and eventual mortality in mangroves as unsuitable hydroperiods are created through tidal flow alterations. Here, we document mangrove forest and soil structural changes within transects established in tidally restricted areas on Marco Island (Collier County, Florida, USA), which has broad swaths of dead-standing or unhealthy mangroves (“ghost forests”). Transects (N = 4) were arranged to include full canopy, transitional, and open canopy (dead) forests, and compared to nearby reference forests. Aboveground and belowground carbon (C) stocks (Total C) ranged from 288 to 304 Mg C ha−1 on full canopy, transitional, and reference sites, which did not differ from each other. However, Total C was lower for dead sites (233 Mg C ha−1) dictated entirely by differences in aboveground C (live and dead trees, downed wood); no differences were found among forest condition in belowground C stocks. This belowground C has been persistent in the soil for 85 years since initial tidal restriction. Nevertheless, hydrologic rehabilitation has the potential to increase total C stocks on dead sites by 70–110 Mg C ha−1. Collapse of the soil surface by 6–8 cm just under the active root zone in chronically stressed mangroves was evident within the bulk density profiles from transitional versus dead sites, suggesting that surface elevation loss as root turnover ceases may work correlatively with chronic stressors (anoxia, P limitation) to affect rapid mortality of forests over short periods of time many years after stress initiation. Hydrologic rehabilitation of stressed or denuded mangroves must also include an understanding of how these soil processes might be re-established.

Tidal Hydrology and Salinity Drives Salt Marsh Vegetation Restoration and Phragmites australis Control in New England

The Medouie Creek wetland complex on Nantucket Island, MA was historically one contiguous salt marsh. Diking in the 1930s caused tidal restriction, creating a freshwater wetland colonized by Phragmites australis. To restore salt marsh habitat, the tidal restriction was removed and tidal salt water hydrology reestablished. Soil pore water salinity increased rapidly through the site with the majority of the marsh exhibiting salt marsh hydrology and higher salinities (25-30 ppt) eight years post-restoration. Extensive dieback of freshwater vegetation facilitated fairly rapid colonization by salt marsh vegetation, particularly adjacent to the culvert and marsh ditches. A non-metric multidimensional scaling (nMDS) analysis demonstrated that vegetation community composition was driven primarily by soil pore water salinity and marsh surface elevation. Eight years post-restoration, areas under 4 m elevation were dominated by salt marsh species while areas over 4 m retained freshwater vegetation. Unlike some other salt marsh restoration projects, salt marsh vegetation communities successfully established at Medouie primarily due to hydrological alteration and without the need for active revegetation or other restoration methods. P. australis stands were also significantly decreased. Given appropriate marsh conditions, altering hydrology alone to mimic functioning salt marsh hydrology may effectively drive vegetation succession and lead to ecologically functional salt marshes.

Incomplete tidal restoration may lead to persistent high CH4 emission

AbstractSalt marshes provide a range of critical ecosystem services including high rates of carbon sequestration. Recent recognition of the importance of these ecosystems has led to an interest in restoring marshes that have been degraded by human activities. Tidal restriction from coastal development can alter salt marsh plant and animal communities as well as marsh biogeochemistry. However, much less is known about how tidal restriction, and subsequent tidal restoration, may alter greenhouse gas emissions. Since salt marshes typically have lower emissions of methane than freshwater wetlands, tidal restoration that increases the salinity in a restricted marsh could theoretically improve the net global warming potential of a marsh by lowering methane emissions. In this study, we examined the emissions of carbon dioxide, methane, and nitrous oxide at four temperate salt marshes, three of which had been restricted and subsequently restored. Greenhouse gas fluxes were poorly correlated with environmental parameters (biomass, temperature, moisture, sediment organic content, porewater salinity and pH, etc.) normally considered to be primary drivers of salt marsh biogeochemical processes. Carbon dioxide and nitrous oxide emissions and uptake were not influenced by the restriction and restoration history of the marshes. Despite comparable salinity, methane emissions in one partially restored marsh were 25 times higher than those in unimpacted reference sites 13+ years after restoration. However, methane emissions from the other restored sites were equal to or significantly lower than those from unimpacted reference sites. This study highlights that we cannot assume recovering ecosystems will function like unimpacted ones, especially if restoration is incomplete.

Effects of Hypoxia on Fish Survival and Oyster Growth in a Highly Eutrophic Estuary

Human land use activities around estuaries can result in high levels of eutrophication. At Elkhorn Slough estuary, a highly eutrophic California estuary, we investigated the effects of impaired water quality on two stress-tolerant estuarine species, a common fish, the staghorn sculpin, Leptocottus armatus and a foundational invertebrate, the Olympia oyster, Ostrea lurida. We caged the two indicator species at six wetlands with different levels of water quality impairment, four of which had restricted tidal flow. We also recorded water quality parameters simultaneously at all sites using YSI sondes, and sampled nutrients and chlorophyll-a monthly, building on the National Estuarine Research Reserve System-wide Monitoring Program. We found that the monitored environmental variables predicted ecological responses by the indicator species. In particular, we found that the duration and severity of hypoxia were negatively correlated with fish survival and oyster growth. Further, our results corroborate previous studies that artificial tidal restriction leads to increased hypoxia stress. We conclude that large diurnal fluctuations in dissolved oxygen and extended nighttime hypoxia can have lethal and sub-lethal effects even on stress-tolerant organisms in the estuary. While laboratory experiments have often shown such effects, it is relatively rare to demonstrate negative effects of oxygen variation with in situ experiments, which provide stakeholders with concrete evidence for impaired water quality at local wetlands. Tidally restricted sites, which experience the largest fluctuations in dissolved oxygen and longest periods of hypoxia, harbor conditions harmful to vertebrates and invertebrates in the estuary. Reversing the anthropogenically induced low oxygen levels, by restoring more natural tidal exchange and by decreasing agricultural runoff, could improve the survival and growth of important estuarine organisms.