Tidal Salt Marshes Research Articles

Protecting People and Property While Restoring Coastal Wetland Habitats

Flood mitigation and protection of coastal infrastructure are key elements of coastal management decisions. Similarly, regulating and provisioning roles of coastal habitats have increasingly prompted policy makers to consider the value of ecosystem goods and services in these same decisions, broadly defined as “the benefits people obtain from ecosystems.” We applied these principles to a study at three earthen levees used for flood protection. By restricting tidal flows, the levees degraded upstream wetlands, either by reducing salinity, creating standing water, and/or by supporting monocultures of invasive variety Phragmites australis. The wetlands, located at Greenwich, NJ, on Delaware Bay, were evaluated for restoration in this study. If unrestricted tidal flow were reestablished with mobile gates or similar devices, up to 226 ha of tidal salt marsh would be potentially restored to Spartina spp. dominance. Using existing literature and a value transfer approach, the estimated total economic value (TEV) of goods and services provided annually by these 226 ha of restored wetlands ranged from $2,058,182 to $2,390,854 y−1. The associated annual engineering cost for including a mobile gate system to fully restore tidal flows to the upstream degraded wetlands was about $1,925,614 y−1 resulting in a benefit-cost ratio range of 0.98–1.14 over 50 years (assuming no wetland benefits realized during the first 4 years). Thus, inclusion of a cost-effective mobile gate system in any engineering design to improve long-term flood resilience in the region would produce dual benefits of protecting people and property from major storms, while preserving and enhancing ecosystem values.

Temperature sensitivity of anaerobic CO2 production in soils of Phragmites australis marshes with distinct hydrological characteristics in the Yellow River estuary

Temperature sensitivity (Q10) is important to reveal carbon decomposition responding to climate change. It’s remains limited to understand how Q10 of anaerobic soil organic matter (SOM) decomposition is regulated by soil property in various wetlands with distinct hydrological characteristics. In the present study, samples of soil at the depths of 0–10 cm, 10–20 cm and 20–30 cm were collected in three typical Phragmites australis marshes, including a freshwater marsh (FPa), a no-tidal salt marsh (NTPa) and a tidal salt marsh (TPa), in the Yellow River estuary. The soil samples were incubated at 10 °C, 20 °C and 30 °C, respectively, to determine the rates of anaerobic CO2 production, Q10 values, and their relations to soil properties. Over 70-d incubation, temperature rise significantly increased the mean rates by 70%–136% (from 10 °C to 20 °C) and 64%–142% (from 20 °C to 30 °C) among the marshes, with the mean Q10 values ranging from 1.61 to 2.66. The rates of CO2 production and Q10 values were significantly affected by wetland type and soil depth. Among marshes, the Q10 of the FPa and NTPa soils was higher than that of the TPa soil (for 0–10 cm and 10–20 cm but for 20–30 cm); along soil depths, the Q10 of the top soil was higher than that of the subsoil (for FPa and NTPa but for TPa). Q10 was affected by total organic carbon (TOC), salinity and pH, and the effects were different with marshes. TOC was a main factor regulating Q10 for the freshwater and the no-tidal salt marshes, while salinity and pH were main factors for the tidal salt marsh. The findings highlight an interactive mechanism of soil property and tidal underlying the response of SOM decomposition to temperature change in estuarine wetlands.

Mapping coastal salt marshes in China using time series of Sentinel-1 SAR

Salt marshes provide crucial ecological functions and services and are experiencing rapid losses and degradation under global climate changes and high-intensity human activities in coastal zone. However, mapping salt marsh distributions and compositions with high accuracy on the national or global scale remains challenging. Here, we used Sentinel-1 time-series data and knowledge-based automatic decision tree classifiers to produce a 10-m map of the salt marshes in China coastal zones. We established annual synthetic aperture radar (SAR) composite features to address seasonal variations and tidal dynamics in salt marshes and created five partitioned-region classifiers according to the climatic and environmental characteristics. In total, 1729 Sentinel-1 images in 2019 and 3418 in situ sites were included for the classification. The results showed that (1) the annual mean SAR composite is more stable and practical than the seasonal composite for salt marsh classification along the China coastal zones in terms of these features addressing variations in tidal fluctuation, climate zones, and salt marsh compositions; (2) SAR surface roughness can effectively separate salt marshes into sparse density and red phenotype salt marshes; (3) the total area of Chinese coastal salt marshes was 127,477.37 ha with an overall accuracy of 87.30%, it included 48.30% Spartina alterniflora, 29.19% Phragmites australis, 13.76% Suaeda spp. and 8.75% Scirpus spp., and more than 85% of salt marshes were distributed from Shandong Province to Zhejiang Province. This study showed the potential application of time-series Sentinel-1 SAR data, annual composite data and knowledge-based classifiers for large-scale coastal zones, and these data will be valuable for coastal ecological restoration and sustainability management.

Biophysical drivers of net ecosystem and methane exchange across phenological phases in a tidal salt marsh

Salt marshes are large carbon reservoirs as part of blue carbon ecosystems. Unfortunately, there is limited information about the net ecosystem (NEE) and methane (CH4) exchange between salt marshes and the atmosphere to fully understand their carbon dynamics. We tested the influence of biophysical drivers by plant phenological phases (i.e., Greenup, Maturity, Senescence and Dormancy) on NEE and CH4 exchange in a grass-dominated temperate tidal salt marsh. We used three years of data derived from eddy covariance, PhenoCam (to measure vegetation phenology), and ancillary meteorological and water/soil variables. Overall, NEE showed significant differences among all phenological phases (p < 0.05), while CH4 exchange had significant differences among all phases except for Greenup and Dormancy. Net CO2 uptake was higher across Maturity (-61 g C-CO2 m2), while CO2 emissions were higher during Dormancy (182 g C-CO2 m2). The lower but constant CO2 emissions during Dormancy overshadowed the CO2 uptake during the growing season and contributed to >72% of the annual CO2 emissions in this ecosystem. Net CH4 emissions were higher during Maturity (3.7 g C-CH4 m2) and Senescence (4.2 g C-CH4 m2). Photosynthetically active radiation (PAR) substantially influenced (r2 > 0.57) daytime NEE across phenological phases, but a combination of variables including water table level (WTL), water temperature and atmospheric pressure were relevant to explain CH4 exchange. The study site was an overall net carbon source to the atmosphere with annual emissions of 13-201 g C-CO2 m−2yr−1 and 8.5-15.2 g C-CH4 m−2yr−1. Our findings provide insights on: a) the role of plant phenological phases on ecosystem-scale CO2 and CH4 fluxes; b) challenges for modeling ecosystem-scale CO2 and CH4 fluxes in salt marshes; and c) the potential net loss of carbon to the atmosphere that should be considered for carbon management and accounting in these ecosystems.

Artificial modification on lateral hydrological connectivity promotes range expansion of invasive Spartina alterniflora in salt marshes of the Yellow River delta, China

‘Invasibility’, or the extent to which a habitat is prone to being invaded by plants, is a measure of the resistance of that ecosystem to biological invasion: a limited extent represents abiotic conditions unsuitable for invasion by invasive species; however, human activity can change that and make a habitat prone to rapid invasion. Field surveys and greenhouse experiments were carried out to explore, using spatial analysis, how a strong invader, namely Spartina alterniflora, is assisted by such activities as constructing levees and digging trenches, ditches, and pits in a tidal salt marsh. These activities changed the lateral hydrological connectivity of a salt marsh. The invasibility was then estimated based on the probability of seed dispersal and retention using the classical probabilistic method, and the rate of seedling emergence using threshold analysis. Changes in lateral hydrological connectivity led to more seeds of the invading species being retained, especially in high marshes, and promoted the emergence of its seedlings by making the soil more moist and less saline. The results suggest that changes in the lateral hydrological connectivity in a salt marsh can make it more prone to being invaded. The results have important implications for the control of invasive plants by limiting human activity and thereby regulating lateral hydrological connectivity in coastal ecosystems.

Gastropods Diversity in Thondaimanaru Lagoon (Class: Gastropoda), Northern Province, Sri Lanka

Thondaimanaru lagoon (TL) is one of the three lagoons in the Jaffna Peninsula, Sri Lanka. TL (N-9.819584, E-80.134086), which is 74.5 Km2. Fringing these lagoons are mangroves, large tidal flats and salt marshes. The present study is carried out to assess the diversity of gastropods in the northern part of the TL. The sampling of gastropods was performed by using quadrat method from July 2015 to June 2016. Different sites were selected and rainfall data, water temperature, salinity of the water and GPS values were collected. Collected gastropod shells were classified using standard taxonomic keys and their morphological as well as morphometrical characteristics were analyzed. A total of 23 individual gastropods were identified from the lagoon which belongs to 21 genera of 15 families among them 11 gastropods were identified up to species level.

Stable isotope signatures reveal the significant contributions of microphytobenthos and saltmarsh-driven nutrition in the intertidal benthic food webs

A tidal saltmarsh supporting marine ecosystem services becomes more recognized, but an understanding of its trophic dynamics is far limited. We investigated the benthic food-web dynamics in Ganghwa tidal flat, a representative tidal saltmarsh of South Korea. The stable carbon and nitrogen isotope ratio values were analyzed for the sixteen species of macrobenthos and their potential food sources such as microphytobenthos (MPB) and various organic matters. A year-round survey was conducted in the three sites at Ganghwa showing the different types of sediment bottom (sandy mud or mud) and/or vegetation (Phragmites or Suaeda). In general, the isotopic signature fairly well demonstrated that trophic structure is primarily influenced by sediment mud content and/or dominated vegetation. Among the four feeding types of consumers (filter feeder; deposit feeder; omnivore; carnivore), the deposit feeders such as mollusk, arthropod, and annelid showed increased dietary dependency on MPB at bare sites. Whilst they actively utilized 13C-depleted organic matters at vegetated sites. Meantime, significantly enriched stable nitrogen isotopic ratio of macrobenthos was evidenced in fall, which reflected the seasonal variation of food sources and physiological processes in survival strategy. Overall, the stable isotopic approach was useful to address the key factors (in)directly influencing the benthic food web structure and its functioning in a typical tidal saltmarsh of the Yellow Sea.

Mapping properties of vegetation in a tidal salt marsh from multi-spectral satellite imagery using the SCOPE model

ABSTRACT Quantitative remote sensing of vegetation biophysical and biochemical parameters through the inversion of physical models is of great significance for monitoring vegetative growth, ecosystem function, and the global carbon and nitrogen cycles. However, mapping and comparing vegetation parameters in a tidal saltmarsh based on multi-spectral satellite imagery has been rarely documented. In this study, a coupled model (Soil-Canopy-Observation of Photochemistry and Energy fluxes, SCOPE) was used to quantitatively invert the spatial patterns of key parameters such as the leaf area index (LAI), leaf chlorophyll content (C ab), and fraction of absorbed photosynthetically active radiation (fPAR) of salt marsh vegetation from Land Remote-Sensing Satellite 8 (Landsat-8), Sentinel-2A, and RapidEye multispectral satellite imagery. The results show that RapidEye outperformed Landsat-8 and Sentinel-2A in the retrieval of LAI and C ab, with higher coefficients of determination (R 2 = 0.539 and 0.488, respectively) and lower normalized root mean square errors (NRMSE = 0.32 and 0.28, respectively). Without the red-edge band, the R 2 between the modelled and observed LAI and C ab for RapidEye decreased by 27.83% and 5.33%, respectively, while the NRMSE increased by 3.13% and 46.43%, respectively, highlighting the important role of the red-edge band in vegetation parameter retrieval. Tidal flooding resulted in poor retrieval results from Sentinel-2A. However, the accuracy of C ab inversion was significantly improved by removing the sampling sites near tidal trenches or in low-lying areas, indicating that tidal inundation exerted a notable effect on the retrieval of saltmarsh vegetation parameters when using multi-spectral images. In addition, the spatial pattern of the simulated fPAR from RapidEye is consistent with that of the gross primary productivity. This study provides an alternative method for estimating ecosystem functions in a tidal saltmarsh wetland, as well as for large-scale and high-precision monitoring of wetland restoration that is beneficial for the ecological protection of coastal zones.

An Eddy Covariance Mesonet For Measuring Greenhouse Gas Fluxes in Coastal South Carolina

Coastal ecosystems are vulnerable to climate change and have been identified as sources of uncertainty in the global carbon budget. Here we introduce a recently established mesonet of eddy covariance towers in South Carolina and describe the sensor arrays and data workflow used to produce three site-years of flux observations in coastal ecosystems. The tower sites represent tidal salt marsh (US-HB1), mature longleaf pine forest (US-HB2), and longleaf pine restoration (replanted clearcut; US-HB3). Coastal ecosystems remain less represented in climate studies despite their potential to sequester large amounts of carbon. Our goal in publishing this open access dataset is to contribute observations in understudied coastal ecosystems to facilitate synthesis and modeling analyses that advance carbon cycle science.

Morphological characterization of ponds and tidal courses in coastal wetlands using Google Earth imagery

Ponds and tidal courses are significant landforms that frequently arise in marshes and tidal flats environments. An understanding of their development and permanence is relevant to determine future dynamic processes that alter tidal flats and salt marshes environments, such as changes in the sea level, increase in the wave activity, and some other variations associated to the climate change. Direct access for monitoring in these regions is complex, extremely expensive and not always feasible. Remote sensing imagery represents a monitoring alternative, but requires the research of specific image processing procedures to extract the information concerning to these environmental studies. In this work, we developed a methodology for assessing the relevant morphological parameters of ponds and tidal courses using Google Earth imagery. An automatic classifier identifies these landforms as such (accuracy over 86%), producing a shape descriptors dataset. Then, ponds and tidal courses in tidal flats are morphologically characterized, and their behavior is compared to the surrounding environment. Subsequent analysis found significant differences in morphological characteristics that arise independently of the marsh environmental conditions. The evidence suggests that the evolution processes of the depressions in salt flat environments are clearly different in comparison with salt marshes environments. In salt marshes, the permanence and evolution of the depressions is related to the age of marshes, whereas in tidal flats the dynamic processes and sediment input have influence on depressions evolution.

Tidal effects on ecosystem CO2 exchange in a Phragmites salt marsh of an intertidal shoal

Understanding the mechanisms and controlling factors of ecosystem CO2 exchange in tidal wetlands is of great benefit for research concerning the global carbon cycle and climate change. In spite of this, the multiple controls of ecosystem-atmosphere CO2 exchange in coastal wetlands subject to subdaily tidal flooding have yet to be adequately addressed. In this study, we investigated the tidal influence on ecosystem CO2 exchange in a Phragmites salt marsh of an intertidal shoal in the Changjiang estuary, based on eddy covariance (EC) measurements. The results revealed that the study area acted as a strong sink for atmospheric CO2 (net ecosystem exchange, NEE = -901 g C m−2) in 2018. Photosynthetically active radiation (PAR), air temperature (Ta), and vapor pressure deficit (VPD) were major drivers of NEE on both diel and multi-day scales. The tides, along with the bio-meteorological variables, strongly affect ecosystem photosynthesis (gross primary production, GPP) and ecosystem respiration (Reco) in the tidal wetland, especially on the multi-day scale. Regardless of which flux partitioning method was utilized, tidal inundation generally imposed inhibitory effects on GPP, which were directly attributed to tidal water level (TWL) and salinity. The daytime data-based estimates of Reco was also suppressed on average under the tidal inundation condition when Ta was higher relative to the non-inundation condition, reflecting the influence of TWL on Reco and the reduced sensitivity of Reco to Ta. We observed that NEE responded positively or negatively to tidal flooding, depending on the magnitude of tidal suppression on GPP and Reco. When Ta was roughly between 28 and 32°C and PAR was > 1200 μmol m−2 s−1, GPP was suppressed by tides more than Reco during the early and rapid vegetative stage, while during the peak vegetative stage, the opposite was true. This study not only shows the unique impact of tidal salt marsh wetlands on carbon uptake, but it also represents an example of a coastal wetland in which tidal inundation promotes the net uptake of CO2.

Emerging salt marshes as a source of Trichoderma arenarium sp. nov. and other fungal bioeffectors for biosaline agriculture.

AimsSustainable agriculture requires effective and safe biofertilizers and biofungicides with low environmental impact. Natural ecosystems that closely resemble the conditions of biosaline agriculture may present a reservoir for fungal strains that can be used as novel bioeffectors.Methods and ResultsWe isolated a library of fungi from the rhizosphere of three natural halotolerant plants grown in the emerging tidal salt marshes on the south‐east coast of China. DNA barcoding of 116 isolates based on the rRNA ITS1 and 2 and other markers (tef1 or rpb2) revealed 38 fungal species, including plant pathogenic (41%), saprotrophic (24%) and mycoparasitic (28%) taxa. The mycoparasitic fungi were mainly species from the hypocrealean genus Trichoderma, including at least four novel phylotypes. Two of them, representing the taxa Trichoderma arenarium sp. nov. (described here) and T. asperelloides, showed antagonistic activity against five phytopathogenic fungi, and significant growth promotion on tomato seedlings under the conditions of saline agriculture.Conclusions Trichoderma spp. of salt marshes play the role of natural biological control in young soil ecosystems with a putatively premature microbiome.Significance and Impact of the StudyThe saline soil microbiome is a rich source of halotolerant bioeffectors that can be used in biosaline agriculture.

Review of Coast and Marine Ecosystems in Temperate Australia Demonstrates a Wealth of Ecosystem Services

Temperate Australia has extensive and diverse coast and marine habitats throughout its inshore and offshore waters. The region includes the southernmost extent of mangroves, over 500 estuaries and coastal embayments, home to extensive meadows of seagrasses and tidal saltmarsh. In areas of hard substrate, rocky reefs are abundant and productive with large forests of macroalgae. Coastal regions can be densely populated by humans and often habitats can be degraded, polluted or lost, while some remain relatively isolated and pristine. These habitats provide services to society including provision of food, regulate our climate through sequestration of carbon, treating our waste and protecting our shorelines from damage from storms. Coastal areas are culturally importantly hubs for recreation and tourism. Habitat mapping demonstrates diverse habitats throughout temperate Australia, but a formal investigation of services provided by these habitats has been lacking. This review of ecosystem services provided by coast and marine environments throughout temperate Australia reveals vast and productive ecosystems that provide multiple ecosystem services, substantial value to the Australian economy and contribute to the health and well-being of people who live in, visit of benefit from services or products from these regions. Some of these are considered within traditional economic metrics such as provision of wild catch fisheries, but this review demonstrates that regulation and maintenance services including waste treatment and protecting shorelines from extreme events are under recognised, and their value is substantial. However, consistent with many locations globally, coast and marine habitats are under threat from increasing development, sewage, agricultural, industrial discharges, urban runoff and climate change. Resultantly, temperate Australian coast and marine habitat extent and condition is generally declining in many regions, putting the provision of services and benefits to the community at risk. Continued degraded or lost habitats indicate current management frameworks are not capturing the full risk from development and there are winners and losers in trade off decision making. Incorporating ecosystem services in decision making may allow an integrated approach to management, and acknowledgement of services provided could prevent habitats from being undervalued against economic and social interests, a practice that often results in environmental degradation.

Seasonal changes in river-tide dynamics in a highly human-modified estuary: Modaomen Estuary case study

Many estuaries worldwide are subject to significant human interventions, such as dam construction, channel deepening, and land reclamation. However, the impact of these interventions on the spatiotemporal river-tide dynamics is insufficiently understood. In this study, we explored the seasonal changes in river-tide dynamics in the Modaomen Estuary of the Pearl River, where a significant increase in tidal range by approximately 0.14 m on average was observed after substantive channel deepening mainly due to human-induced sand excavations. Using the double mass curve approach with regard to the river discharge and the absolute value of tidal damping rate (or residual water level slope), we identified three stepwise phases (i.e. 1960–1990 representing the pre-human intervention period, 1991–2000 representing the transitional period, and 2001–2016 representing the post-human intervention period) corresponding to high to low residual water level slopes, and qualitatively matched the river-tide dynamics shift observed in the Modaomen Estuary. It was shown that the increase in tidal range was primarily due to the reduction in residual water level slope (and thus the weakening of the tidal damping effect) caused by the channel deepening along the Modaomen Estuary (with length of 124 km) where the residual water level slope was decreased by 46% from the pre-human period (1960–1990) to the post-human period (2001–2016), while the absolute value of the tidal damping rate was decreased by 32% due to the overall impacts from human interventions. Based on the regression coefficients obtained from the double mass curve approach, we showed that the river-tide dynamics in terms of tidal damping rate and residual water level slope were more sensitive to human interventions during the dry season than during the wet season, owing to the combined impacts of riverbed deepening and freshwater regulation by dam's operation. The underlying mechanism could be primarily attributed to the considerable seasonal variation in the lateral storage area (such as tidal flats or salt marshes). The results also showed that larger freshwater discharge is required to alter the critical value of maximum tidal damping (indicating the strongest damping effect along the channel) owing to the increase in tidal range due to substantial riverbed deepening and hence significant increase in channel storage. The method proposed in this study to quantify the overall impacts of human interventions on seasonal changes in river-tide dynamics is particularly useful for setting scientific guidelines for sustainable water management and conservation planning for engineering works in the Modaomen Estuary and other estuaries with significant human interventions.

Nitrogen input weakens the control of inundation frequency on soil organic carbon loss in a tidal salt marsh

The soil carbon (C) sequestration capacity of salt marshes is of considerable importance with respect to the mitigatiing the potentially detrimental consequences of global climate change. Given that tidal salt marshes are subjected to periodic cycles of inundation, it is assumed that soil C cycle in these marshes is expected to be controlled to varing extent by changes in soil moisture and salinity induced by the alternating patterns of drying and rewetting. In addition, with increases in the extent and severity of nitrogen (N) eutrophication becomes serious, we predicted that soil organic carbon (SOC) loss in tidal salt marshes is likely to be highly responsive to increased N loading. In this study, we conducted a two-factor mesocosm experiment (simulated inundation and N input in a tidal salt marsh) to determine the interactive effects of N eutrophication and inundation frequency on CO2 and CH4 emissions and dissolved organic carbon (DOC) contents. Our results showed that increased inundation frequency led to lower levels of CO2 emission but greater CH4 emission, whereas N input weakened the effects of changes in inundation frequency on CO2 and CH4 emissions. Moreover, N input was found to modify CO2 and CH4 emission in response to variations in soil moisture. We also observed an enhancement of soil DOC loss in response to increasing inundation frequency, and that DOC loss in the surface soil was considerably greater than that in subsurface soil. Further, as inundation frequency increased, we detected changes in the relationship between soil DOC and CO2 and CH4. Our findings highlighted that vertical variation in soil moisture induced by inundation is a key factor controlling SOC loss in tidal salt marshes, although N input can weaken this control.

Time‐lagged impacts of extreme, multi‐year drought on tidal salt marsh plant invasion

AbstractClimate change is projected to increase the frequency of extreme drought events, which can have dramatic consequences for ecosystems. Extreme drought may interact with other stressors such as invasion by non‐native species, yet little research has explored these dynamics. Here, we examine the physical mechanisms and temporal scale underlying a dieback of an invasive non‐native plant, Lepidium latifolium, in tidal salt marshes of the San Francisco Bay, California, USA, during an extreme, multi‐year drought occurring from 2012 to 2015. Using generalized additive mixed models (GAMMs), we explored the relationship between eight years of estuarine salinity data and five years of L. latifolium density data from three marshes spanning a gradient of salinity across the San Francisco Bay. We found a significant time‐lagged (3 yr) effect of estuarine salinity on L. latifolium density, with high salinities preceding reductions in L. latifolium densities and low salinities preceding increases. The most dramatic change in stem density, a 54% reduction in 2015, was preceded by a salinity increase of 43% from 2011 to 2012. We found the L. latifolium decline was driven by impacts on mature, rather than young, plants. Additionally, we tested the importance of local precipitation in driving L. latifolium densities in a one‐season rain exclusion experiment. We found 100% exclusion of precipitation during one rainy season (January–mid‐May) did not have a significant impact on densities of mature stands of L. latifolium. Our finding that estuarine salinity was a key driver of L. latifolium invasion dynamics suggests sea level rise, like extreme drought, may hinder L. latifolium invasion, as it will also raise estuarine salinities. Further, our study highlights the importance of temporal lags in understanding climate change impacts on biological invasions, which has received very little study to date.

Mechanisms of enhanced methane emission due to introduction of Spartina anglica and Phragmites australis in a temperate tidal salt marsh

Introductions of exotic plants are widespread ecological threats disturbing the carbon and nutrient cycles in global coastal wetlands. Previous studies have found that the invasion has enhanced methane emission through increased primary production and a changed methanogenic pathway. However, the previous studies did not determine the depth profiles of the changes while the depth in the soil profile to which the disturbance propagates is one of the critical issues to be addressed. In this study, we illustrated the depth profiles of the effect of the introductions of S. anglica and P. australis on the potential methane production, soil biogeochemistry, and soil microbial community which play a substantial role in enhanced methane emission due to the invasions. The in situ field measurements and anaerobic incubation were conducted to determine the mechanistic differences of depth profiles. Both introductions of S. anglica and P. australis increased methane emission through different microbial mechanisms depending on depth profile. Potential methane production was stimulated in shallow and deep soil layer of S. anglica and P. australis-introduced marshes, respectively. S. anglica increased the contribution of methylotrophic methanogenesis in top soil layers while P. australis decreased competitive inhibition by sulfate reducers in all soil layers. These results emphasize the importance of below-ground microbial activity and vertical distribution in carbon cycles in tidal marshes. Further plant invasion studies should focus on depth profiles of microbial processes, as their distribution and activity vary substantially across soil depths.

Saltmarsh Expansion in Response to Morphodynamic Evolution: Field Observations in the Jiangsu Coast using UAV

Dai, W.Q.; Li, H.; Chen, X.D.; Xu, F.; Zhou Z.; and Zhang, C.K., 2020. Saltmarsh expansion in response to morphodynamic evolution: Field observations in the Jiangsu coast using UAV. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 433–437. Coconut Creek (Florida), ISSN 0749-0208.Most studies on the biological features and the morphodynamic indications of saltmarshes focused on individual plant groth, such as the stem height and the biomass. However, the understandings of the the planemetric patterns are limited. In this study, UAV (Unmanned Aerial Vehicle)-based observations are conducted in six saltmarsh areas along the Jiangsu coast, China. 3D point clouds, consequent DEM (digital elevation model) and colour orthomosaics are generated using the SfM (Structure from Motion) techniques. We track the variations in the locations of the saltmarsh front as well as the sizes and the distributions of patchs. The saltmarshes are distributed as a whole piece in the landside, while they turn to scattered patches to the sea. We find the locations of the saltmarsh front as a function of bed elevations and season. The area of a single patch is between 0 and 90 m2. The number of the patches increases then decreases from landside to the seaside. For tidal flats in progradation, saltmarsh slowly expands seaward following the sediment accretion. This study highlights the abilities of the drone and the SfM technologies to measure the salt marsh distributions, and further to analyze the geometric characteristics and biomorphodynamics of tidal saltmarsh flats.

Spatial and temporal heterogeneity of geochemical controls on carbon cycling in a tidal salt marsh

Tidal salt marsh ecosystems store copious amounts of carbon (C) within sediments. In order to predict how these C stores may be affected by environmental change, it is critical to assess current CO2 and CH4 production and efflux from these ecosystems. Production and efflux of these greenhouse gases (GHGs) are governed by coupled geochemical, hydrological, physical and biological processes in sediments that are sensitive to local conditions, which can result in large spatial and temporal heterogeneity of GHGs dynamics within the ecosystem. To understand how the drivers of GHGs dynamics vary across salt marsh ecosystems, we coupled solid-phase geochemistry to measurements of porewater chemistry (to ∼1 m), CO2 and CH4 production in sediments and efflux to the atmosphere in a temperate tidal salt marsh for over one year to capture seasonal patterns within two vegetation zones of the marsh landscape that have distinct biogeochemical and hydrologic conditions: Tall Spartina (TS) and Short Spartina (SS). The SS vegetation zone experienced nearly constant inundation, low redox values (−200 to 200 mV), porewater pH 6–7 that did not vary with depth or time, an enrichment of pyrite and goethite with depth and up to 3 mM porewater sulfide. In contrast, the TS vegetation zone on the natural levee proximal to a tidal channel experienced large water level oscillations due to spring-neap tides that resulted in variable but higher redox values (0–700 mV), porewater pH 6–7 at depth but surface (0–3 cm) as low as 4 in the spring, an enrichment of ferrihydrite and a depletion of pyrite at ∼30 cm, and up to 0.8 mM ferrous Fe in porewater. At 50–56 cm, solid phase analyses (STXM-NEXAFS) revealed differential C speciation between the two vegetation zones, with stronger C-Fe spatial association at TS and stronger C-Ca co-association at SS despite both having similar soil pH of 3–4. These results suggest that soil pH may not be strongly predictive of C-mineral control in flooded marsh sediments. Both vegetation zones showed consistent CO2 and CH4 emissions from sediments to the atmosphere throughout the study period with TS having ∼60% higher median CO2 and SS having ∼55% higher median CH4 efflux. Using depth profiling, unexpectedly high concentrations of CO2 (>200 μM) and CH4 (>200 μM) were observed at depths 50–75 cm at both zones that were higher for SS in these sulfate-rich (up to 17 mM) sediments, which suggests methylotrophic methanogenesis occurs deep within the profile of salt marsh sediments away from the tidal channel. Moreover, if we extrapolate our median depth values of CH4 and CO2 to the 5.3 Mha of global salt marshes, this could account for a conservative estimate of ∼70 Gg of unaccounted C stored in gaseous form (i.e., CH4 and CO2) in marsh sediments, which should be considered when attempting to understand the current patterns and future responses of carbon dynamics from these ecosystems.

A UAV-Based Dye-Tracking Technique to Measure Surface Velocities over Tidal Channels and Salt Marshes

An accurate description of hydrodynamic processes in coastal wetlands is needed to improve their management and conservation. As a consequence, higher knowledge of the connected morphological and ecologic processes is achievable. However, it is very costly to collect spatially distributed values of flow velocities over tidal channels and intertidal areas by means of in situ sensors. Also, when deploying sensors, humans perturb the ecosystem, which takes time to recover. In this study, a new low-cost unmanned aerial vehicle (UAV)-based method to measure surface velocities is proposed and validated. The study area is a salt marsh system on the southeast coast of Little Sapelo Island, Georgia, USA. Two unmanned aerial vehicles were used in the survey. A first UAV released a fluorescent dye tracer on marshes and tidal creeks, while a second one tracked its movement by collecting RGB images. Flow direction and magnitude were calculated from the images using a newly developed algorithm. A comparison with velocities measured with an acoustic Doppler current profiler confirmed the effectiveness of the method. Our results indicate that the calculated flow field is affected by vegetation, marsh morphology, and marsh width. In particular, a non-negligible velocity component perpendicular to the creek axes is detected both in ebb and in flood. Our technique proves to be an effective, non-intrusive, low-cost way to survey the two-dimensional hydrodynamics on salt marsh environments at a km scale. Collected data would be beneficial for calibrating and validating numerical models with accurate water flux information.