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Coastal wetland ecosystems, such as salt marshes, mangroves, seagrass beds and tidal flats, are under increasing pressure from natural and anthropogenic processes shifting climatic conditions, and are declining in area and habitat quality globally. These environments provide numerous ecosystem services, including flood risk mediation, biodiversity provision and climate change mitigation through carbon storage. Hence, the need to get a deeper understanding of processes and interactions in these environments, and how these may be altered by climate change has never been greater. This is the case for ‘managed’, restored wetlands and natural systems alike.
This session will bring together studies of coastal wetland ecosystems across climates and geomorphic settings, to enhance the understanding of ecosystem service provisioning, interactions between hydrodynamics, sediment and ecology, and identify best future management practices. Studies of all processes occurring within coastal wetlands are invited. This includes, but is not exclusive to, sediment dynamics, hydrology, hydrodynamics, biogeochemistry, morphological characterisation, geotechnical analysis, bio-morphodynamics, ecological change and evolution, impact of climate change, sea level rise, anthropogenic and management implications. Multidisciplinary approaches across spatial and temporal scales are encouraged, especially in relation to global climate change. This session aims to enhance our understanding of basic processes governing coastal wetland dynamics and to propose sustainable management solutions for contemporary environmental pressures.

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Co-organized by BG4/HS13/OS2
Convener: Mark Schuerch | Co-conveners: Thorsten BalkeECSECS, Helen BrooksECSECS, Ruth Reef, Christian SchwarzECSECS
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| Fri, 08 May, 16:15–18:00 (CEST)

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Chat time: Friday, 8 May 2020, 16:15–18:00

Chairperson: Mark Schuerch
D959 |
EGU2020-2570
| Highlight
Marieke Laengner and Daphne van der Wal

Saltmarshes are known to be very important coastal ecosystems. They provide crucial functions for flora and fauna, as well as valuable ecosystem services for humankind. Many methods that are used to investigate these ecosystems are limited in space and time. Long time series of global satellite data enable to observe changes in the extent of saltmarshes on a large scale and over a long time period. We developed an unsupervised decision tree classification method in Google Earth Engine that automatically classifies satellite images into saltmarsh vegetation, mudflats, and open water. We applied the method using Landsat 5 TM data between 1985 and 2011. With this, we are able to detect trends in the seaward extent of saltmarshes globally. We reveal transitions between saltmarsh, mudflat and open water. Furthermore, we put saltmarsh habitat changes in a spatial context and couple trends in saltmarsh dynamics to environmental drivers, such as sea level rise, tidal forces, waves, and sediment availability.

D960 |
EGU2020-9659
Nicoletta Leonardi, Carmine Donatelli, Xiahoe Zhang, Neil Ganju, and Sergio Fagherazzi

Salt marshes and seagrass beds can offer sustainable coastal protection solutions and several ecosystem co-benefits. The delicate balance regulating salt marsh stability depends on several factors including the sediment added to and removed from the coastal system (Donatelli et al., 2018, 2019; Zhang et al., 2019).  Despite the importance of these sediment budget dynamics, many feedbacks between salt marsh presence and sediment availability are still unclear. Here, we use numerical models to simulate changes in depositional patterns of six estuaries along the U.S. coastline to investigate how salt marsh and seagrass beds removal and restoration can alter the sediment budget and resilience of coastal environments. 

Donatelli, C., Ganju, N.K., Kalra, T.S., Fagherazzi, S. and Leonardi, N., 2019. Changes in hydrodynamics and wave energy as a result of seagrass decline along the shoreline of a microtidal back-barrier estuary. Advances in Water Resources, 128, pp.183-192.

Zhang, X., Leonardi, N., Donatelli, C. and Fagherazzi, S., 2019. Fate of cohesive sediments in a marsh-dominated estuary. Advances in water resources, 125, pp.32-40.

Donatelli, C., Ganju, N.K., Fagherazzi, S. and Leonardi, N., 2018. Seagrass impact on sediment exchange between tidal flats and salt marsh, and the sediment budget of shallow bays. Geophysical Research Letters, 45(10), pp.4933-4943.

D961 |
EGU2020-10828
Davide Tognin, Mattia Pivato, Andrea D'Alpaos, and Luca Carniello

Coastal salt marshes are extremely important ecosystems, occupying the transitional zone between submerged and emerged environments. Since salt marshes are based on a delicate balance between hydrodynamics and sedimentary processes, their future is heavily affected by relative sea-level rise (RSLR), caused by both subsidence and eustatism. If vertical accretion is sufficient, salt marshes can keep pace with RSLR; otherwise, lack of sediment input can eventually lead to plant death and marsh drowning, transforming these landforms into tidal flats and subtidal platforms. Resuspension driven by intense meteorological events can represent an important source of sediment for salt marsh accretion in tidal environments characterized by negligible fluvial sediment supply. However, it is not yet clear what is the mutual role and relative contribution of intense storm events and fair-weather conditions in terms of sedimentation patterns. To better understand sedimentation dynamics on salt marshes, we stared a field campaign in October 2018 to measure vertical accretion rate and sediment accumulation.

In the Venice lagoon (Italy), which is the largest lagoon in the Mediterranean sea and is characterized by a semi-diurnal, microtidal regime, we selected three study areas: the San Felice and Sant’Erasmo salt marshes in the northern lagoon and the Conche salt marsh in the southern lagoon. Subsidence at all these study sites ranges between 1.0 and 2.0 mm yr-1, and the rate of sea-level rise is of about 2.0 mm yr-1, for a total rate of RSLR of about 3.0-4.0 mm yr-1. At each study area, we considered different transects, where we installed three measurement stations located respectively at 2.5 m, 7.5 m, and 27.5 m from the salt marsh margin. We equipped each station with an artificial marker horizon laid down on the marsh surface to measure the vertical accretion, and three sediment traps for measuring the short-term sedimentation. The material deposited in two sediment traps is collected monthly or after any single storm, whereas sediment deposited in the third trap is collected once a year, in order to compare sediment deposition dynamics at short (single storm event) and annual time scales. We measure accretion rate, grain size distribution, organic and inorganic content.

Short-term sedimentation displays a very high variability (0 – 320 g d-1 m-2) highlighting the importance of particularly intense storm events in resuspending and transporting sediment from tidal flats to the salt-marsh surface. In particular, during the storm events occurred in October 2018 and November 2019, sedimentation increases significantly and displays values much higher compared to fair-weather periods. According to our analysis, sedimentation grows exponentially with daily mean inundation time. Even if the inner part of the salt marsh is characterized by lower elevation and, hence, by greater inundation time, sedimentation shows smaller values compared to the salt marsh margin, since suspended material settles close to the margin and decreases towards the inner part of the marsh.

D962 |
EGU2020-11476
Angelo Breda, Patricia Saco, José Rodriguez, and Steven Sandi-Rojas

Over the last two decades, there have been important advances in eco-geomorphological modelling of coastal wetlands to predict their evolution. Different features have been incorporated into models, bust most applications still assume a constant or static sediment concentration as input representing average conditions. Such imposition is related to many constraints in obtaining a time series of total suspended matter (TSM). However, with the increasing availability of multispectral satellite products and the development of artificial intelligence algorithms, TSM data can be estimated through remote sensing. This work aims to assess the effect of using a dynamic time varying condition for the TSM input when simulating eco-geomorphological processes. We implemented a modelling framework adapted to conditions found in SE Australian estuaries, which includes hydrodynamic and sediment transport processes. Many scenarios where simulated encompassing different levels of average TSM and water levels. Our findings suggest that under low water levels and low sediment concentration, a static TSM input results in more accretion than a dynamic input. However, at higher levels and concentration, the dynamic input led to higher accretion. Predictions of vegetation distribution were not particularly sensitive to changes in TSM over time.

D963 |
EGU2020-1876
Mary Cialone and Gregory Slusarczyk

This paper will provide an evaluation of the role of coastal wetlands in flood risk mediation by performing hydrodynamic modeling of storm surge in back bays that include various configurations of wetland features. Wetland parameters varied in the research study include the elevation, shape, volume, and vegetation type (represented by the Manning’s friction coefficient) to identify the role of wetlands in reducing back bay flooding.   This information can be used to determine best future management practices for dredged material placement that will serve to maintain and restore wetlands in light of environmental pressures such as climate change, subsidence, storm-induced erosion, boat wakes, and other factors influencing coastal wetland dynamics.

Following Hurricane Sandy in 2012, the United States (U.S.) Congress authorized the large scale North Atlantic Coast Comprehensive Study (NACCS) to address the present and future flood risk to this region. Part of that study was an in-depth numerical modeling and statistical analysis using the ADvanced CIRCulation (ADCIRC) and STeady-state spectral WAVE (STWAVE) models and the Joint Probability with Optimal Sampling (JPM-OS) statistical technique. Following the NACCS, the New Jersey back bays were identified as a high-risk area requiring further in-depth analysis of the effectiveness of surge barriers and coastal wetlands to reduce water levels in the back bays during storms. This paper will discuss the analysis of a set of coastal wetland configurations in the New Jersey back bay region simulated with a set of 10 synthetic storm suite selected from the NACCS study.   Analysis of maximum surge envelopes, water level time series, and characteristics of tropical storm forcing conditions were used to evaluate and compare the effectiveness of the wetland configurations.

D964 |
EGU2020-4344
Yehui Zhong, Ming Jiang, Zhenshan Xue, Bo Liu, and Guodong Wang

Species distribution models (SDMs) are an effective tool for measuring and predicting plant response to climate change, but their application to wetland species has been relatively limited. Here, we investigate the application of SDMs to study the current and future delimitation of wetlands in the Songnen Plain, one of the densest areas of natural wetlands in China. Specifically, we focus on the iconic wetland species Phragmites australis, one of the dominant species in the Songnen plain, which has been widely used for wetland restoration efforts.

Our study has four main goals: (i) to test and improve the applicability of SDM in our study; (ii) to delimit wetland areas for prioritization; (iii) to investigate the projected change in wetland distributions under future climate change scenarios; and (iv) to identify regions that appear more (or less) stable in the face of change, and to propose areas for suitable restoration efforts with land-use.

To achieve our goals, we apply a broad variety of environmental variables using MaxEnt, to project present and future (2050s) suitable areas under two representative concentration pathways (RCP4.5 and RCP8.5). AUC (area under the curve) is used as the test measure for model evaluation. To obtain a rich representative sampling of this species’ distribution, we use field-observational records from the National Science and Technology Fundamental Research Project “Investigation on Wetland Resource of China and Its Ecological and Environmental Benefits” (2013FY111800). In addition to exploring key abiotic parameters that influence P. australis distribution, we also explore the impact of different spatial resolutions (1 km2, 250 m2, 90 m2, 30 m2) of topographic information to assess model performance.

Our results demonstrated that the performance of the MaxEnt projection of P. australis was excellent (AUC=0.922), and improved with the addition of soil, topographic and hydrological variables, but did not improve significantly with increased resolutions of topographic variables. Using the optimized model, we delimited 28,644 km2 of suitable areas and 7,959 km2 of highly suitable areas under current scenarios. The future model under RCP4.5 scenario predicted a 9.5% and 3.1% increase in the suitable and highly suitable areas, respectively. The model under RCP8.5 predicted a much smaller increase in suitable areas, and a slight reduction in highly suitable habitat compared with the current scenario. Under both future scenarios, the geographic centers of potential habitat moved toward the southeast, with the mean latitude slightly rising. Finally, we delimited 2,364 km2 of priority restoration areas under RCP4.5, including 152 km2 of paddy field, 950 km2 of dry field and 1,262 km2 of saline-alkali land. The priority areas under RCP8.5 were smaller in all three land-use types.

Our study illuminates potential priority areas of the Songnen Plain for consideration in future wetland restoration efforts. For future research, we recommend more applications of SDMs with multiple species in wetland restoration, especially over larger scales and higher resolutions.

D965 |
EGU2020-515
Joshua Kiesel, Mark Schuerch, Elizabeth K. Christie, Iris Möller, Tom Spencer, and Athanasios Vafeidis

Managed realignment (MR), a form of of nature-based coastal adaptation to reduce flood and erosion risk, involves the abandonment of existing sea defences and their relocation further inland. MR aims to (re)create intertidal habitats, such as saltmarshes, between the old and new lines of defence; as well as flood water storage. The newly created habitats dissipate wave energy and thus provide new natural coastal protection. However, the assessment of the success of MR is difficult, as restoration targets are often vague and data on project performance are scarce. The few studies that do exist show a lack of understanding about the effects of MR scheme design on high water level (HWL) attenuation and thus its coastal protection function.

Here we present the results of a 2-D hydrodynamic model, calibrated and validated against field measurements of equinoctial tides between August and October 2017, taken within, and seaward of, the Freiston Shore MR site, The Wash, eastern England. Using this model, we performed sensitivity analyses to explore whether or not, and how, the Freiston Shore MR scheme design affects HWL attenuation. For this purpose we changed the configuration of the old defence line and the breaches created within it for renewed tidal exchange and manipulated the digital elevation model of within-site topography. Specifically, we applied six scheme design scenarios (two scenarios with three breaches and varying MR areas, three single breach scenarios of different breach width and one bank removal scenario) and assessed High Water Level (HWL) attenuation rates for each scenario.

Our results show that scheme design, particularly storage area and number and size of breaches, of the Freiston Shore MR site had a significant effect on the site´s HWL attenuation capacity. When the tidal prism is varied by changing the number and size of breaches and the storage area kept constant, modelled HWL attenuation rates increased with decreasing tidal prism. However, largest HWL attenuation rates (> 10 cm km-1) were only obtained if the MR area was of sufficient size, therefore, it is only the larger sites which are exhibiting effective coastal protection. Consequently, the maximum modelled HWL attenuation rate occurred (up to 73 cm km-1) for the scenario with the largest area (142 ha).

The Mean High Water Depth (MHWD) from each of these scenarios explained most of the variation in HWL attenuation between the scenarios (R² = 0.996). This strong correlation may help to inform the construction of more efficient MR schemes with respect to coastal protection in the future.

D966 |
EGU2020-18018
Alvise Finotello, Andrea D'Alpaos, Eli D. Lazarus, Massimiliano Ghinassi, and Andrea Rinaldo

Highly sinuous meandering channels are common landforms in fluvial and coastal environments. As meanders migrate laterally, driven by sediment erosion and deposition along their outer and inner banks, respectively, they eventually cut off, leaving behind the characteristic crescent-shaped morphologies of scroll-bars and oxbow lakes. Oxbows are particularly important not only from ecological perspectives, for the diverse habitats they provide, but also because they retain signatures of the flow characteristics that shaped them, thus allowing for paleoflow reconstruction.

While alluvial plains carved by meandering rivers are littered with scars of meander cutoffs, tidal coastal settings have been perceived by geomorphologists for much of the past century as lacking morphological evidence of active meandering – even though both environments exhibit similar meander-planform dynamics and width-adjusted migration rates.

Here we analyze the planform characteristics and evolution of meander cutoffs from a variety of fluvial and tidal landscapes around the world. We combine field observations and remotely sensed data to track the abandonment of individual meander bends and the subsequent progressive infill and vegetation colonization of the meander cutoffs.

We show that tidal-meander cutoffs tend to be symmetric in planform, seldom disconnected from their parent channel, and fill up as much as 10 times more rapidly than neck cutoffs formed by meandering rivers.

We suggest that cutoffs in tidal meanders are far more widespread than previously thought, and that their supposed paucity is explained by several processes typical of tidal landscapes that collectively militate against the formation and preservation of meander oxbows after cutoff.

These results have important implications for the conservation and restoration of critically endangered coastal environments, as well as for better assessing the capacity of tidal wetlands to store large amounts of blue carbon.

D967 |
EGU2020-17985
Marie Arnaud, Paul J. Morris, Andy J. Baird, Thuong Huyen Dang, and Tai Tue Nguyen

Mangroves are hotspots of carbon sequestration, providing ecosystem services worth US$194 000 per hectare per year. In response to widespread mangrove losses, reforestation projects have been promoted. Monitoring and assessment of those projects have mainly focused on aboveground carbon stocks, but most of the carbon is found underground (as soil carbon and roots) and little is known about belowground carbon dynamics in mangroves. In particular, it is unknown how fine root production develops during the period following reforestation. A better understanding of fine root production is important, since fine root production is a major driver of soil organic matter accumulation, which allows mangroves to occupy vertical accommodation space to withstand sea-level rise. Using minirhizotrons, we characterised the variation of fine root production along a chronosequence of mangroves in the Mekong Delta in Vietnam replanted in 1978, 1986 and 1991. We found that fine root production declines with: i) mangrove age, as a result of the self-thinning processes associated with mangrove ageing; and ii) soil depth, likely due to a vertical gradient in soil nutrient availability. Our findings have important implications for understanding belowground carbon dynamics, and highlight the need to account for mangrove age when forecasting mangrove carbon dynamics and resistance to sea-level rise.

D968 |
EGU2020-6724
Maria Maza, Javier L. Lara, and Iñigo J. Losada

Although mangroves reduce annual flooding to millions of people there is not a methodology to implement these solutions and it is still difficult to estimate the protection provided by them under different environmental conditions and ecosystem properties. To move forward in the consecution of an engineering approach when implementing these solutions for coastal defense, the first step to make is to better understand and parameterize the basic physical processes involved in flow-mangroves interaction. With the aim of getting a new formulation for wave decay provided by Rhizophora mangrove forests based on flow and ecosystem properties, an experimental campaign was carried out where both wave attenuation and forces on mangrove individuals were measured under different wave conditions. Both, the hydrodynamic conditions and the mangrove forest, were scaled according to field conditions for short waves. The detailed wave attenuation and drag force measurements obtained in these experiments allowed to obtain new formulations of wave decay produced by the forest depending on the flow, i.e.: water depth, wave height and period, and on the forest characteristics, i.e.: individuals submerged solid volume fraction and density. These formulations are used to get attenuation rates under different flow and ecosystem conditions. The resultant curves provide with the wave decay produced by a specific Rhizophora forest subjected to the defined wave conditions. The forest is defined on the basis of its age, considering the differences in individual trees depending on their maturity and the density of the forest as the number of trees per unit area. Wave conditions are defined by the root mean square wave height and the peak period and water depth is also considered. The obtained curves allow to estimate the width of the forest necessary to reach a certain level of protection considering the local flow conditions and the forest age. This can assist in the inclusion of nature-based solutions in the portfolio of coastal protection measures.

D969 |
EGU2020-4583
Xudong Zhu, Zhangcai Qin, and Lulu Song

Coastal mangrove wetlands experience unique land-sea interactions including periodical tidal activity and land/sea breeze cycle. However, the influence of tidal and sea breeze activity on net ecosystem exchange of carbon dioxide (NEE) between mangrove and the atmosphere has not yet been investigated. In this study, temporal variations in mangrove-atmospheric NEE and its direct and indirect environmental controls were examined based on a three-year dataset of continuous eddy covariance and auxiliary measurements in a subtropical estuarine mangrove wetland of southeastern China. The results showed this mangrove wetland acted as a consistent carbon sink over the three-year period (mean NEE of -1233 g C m-2 year-1) with the strongest carbon sink capacity in spring, and the impacts of environmental factors on mangrove NEE varied across time scales: (1) half-hourly daytime carbon influx was regulated by photosynthetically active radiation (PAR) with down-regulation effects from high temperature and vapor pressure deficit (VPD), while half-hourly nighttime carbon efflux was dominated by air temperature with additional suppression effects from tidal inundation and rain; (2) the importance of environmental factors in controlling daily NEE decreased in the order of PAR, air temperature, sea breeze, VPD, tidal salinity, and tidal inundation; (3) the seasonality of monthly NEE was strongly regulated by tidal inundation and rain. This was the first study to examine both direct and indirect effects of tidal and sea breeze activity on mangrove NEE using long-term continuous eddy covariance measurements, and to confirm the importance of previously neglected indirect effects of tidal and sea breeze on mangrove carbon sink. Strong negative correlations between mangrove carbon sink and air temperature/tidal inundation implied that mangrove wetland could become a weaker blue carbon sink in response to global warming and sea level rise in the future.

D970 |
EGU2020-101
Gloria Reithmaier, David Ho, Scott Johnston, and Damien Maher

Most research evaluating the potential of mangroves as a sink for atmospheric carbon has focused on carbon burial. However, the few studies that have quantified lateral exchange of carbon and alkalinity, indicate that the dissolved carbon and alkalinity export may be several-fold more important than burial. This study aims to investigate rates and drivers of alkalinity, dissolved carbon and greenhouse gas fluxes of the mangrove-dominated Shark River estuary located in the Everglades National Park in Florida, USA. Time series and spatial surveys were conducted to asses total alkalinity (TAlk), organic alkalinity (OAlk), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Dominant metabolic processes driving dissolved carbon and greenhouse gas dynamics varied along the estuary salinity gradient. Dissolved carbon and greenhouse gas concentrations were strongly coupled to porewater input, which was examined using Rn-222. Shark River was a source of CO2 (92 mmol/m­2/d), CH4 (60 µmol/m­2/d) and N2O (2 µmol/m­2/d) to the atmosphere. Dissolved carbon export (DIC = 142 mmol/m­2/d, DOC = 39 mmol/m­2/d) was several-fold higher than burial (~28 mmol/m2/d) and represents an additional carbon sink. Furthermore, the estuary was a source of TAlk (97 mmol/m­2/d) to the coastal ocean, potentially buffering coastal acidification. Despite accounting for only a small share of TAlk, OAlk had a large effect on the estuarine pH. By integrating our results with previous studies, we argue that alkalinity, dissolved carbon and greenhouse gas fluxes should be considered in future blue carbon budgets.

D971 |
EGU2020-16499
Annalisa Molini, Saverio Perri, and Amilcare Porporato

Coastal wetlands represent a vital component of the global carbon cycle due to the inherent capability of sequestering carbon in both biomass and sediments. Their future ability to act as carbon sinks largely depends on how plant communities will adapt to sea-level rise, and its most direct consequences: submergence and salinization. Although tidal ecosystems can actively contrast SLR through vertical and lateral soil accretion, the impacts of salinity on soil-plant-water interactions, species succession, and ecosystem productivity remain unaccounted for in carbon budget models.

Salinity is known to limit plants capability to uptake water, and as such, it is expected to have an impact on the ecosystem productivity, the soil water balance, and water table dynamics. Here, we model the response of coastal ecosystems to salt stress, and we show that salinity and plant salt tolerance exert a dominant control on how coastal plant communities interact with water table dynamics. We demonstrate that salinization and shifts in groundwater regimes may trigger abrupt successions from the highly productive salt-sensitive species dominating freshwater coastal wetlands, to the less productive salt-tolerant species now confined within the intertidal fringe. Our theoretical results explain recent eco-hydrological patterns observed in the Florida Everglades and indicate that shifts from salt-sensitive to salt-tolerant communities could cause a drastic reduction of coastal wetland productivity.

D972 |
EGU2020-4436
Kai Xiao, Hailong Li, Alicia Wilson, Isaac Santos, and Joseph Tamborski

Abundant crab burrows in salt marshes can act as preferential water flow conduits for solute exchange by perforating muddy sediments, yet the impact of crab burrows on modulating carbon sequestration potential are not well understood. The field observations to assess how crab burrows drive carbon exchange over time scales of minutes to weeks along an intertidal marsh transect in North Inlet, South Carolina were conducted. The field analysis found that (1) continuous pore water exchange between the crab burrows and the surrounding soil matrix occurs because of tidally driven hydraulic gradients; (2) the burrow CO2 concentrations is higher than on the marsh surface because of respiration. The average gas-phase CO2 concentration in the crab burrows was approximately six times greater than ambient air. The lab analysis found that the concentrations of DIC and DOC in crab burrow porewater were lower than porewaters in the surrounding soil matrix. The porewater δ13C-DIC signatures in the crab burrows were heavier than those in the soil matrix, reflecting a mixture with seawater.

Crab burrows can influence carbon export by three pathways: gas phase CO2 release from burrows caused by the irrigation of surface water, hydraulic gradient-driven porewater exchange (PEX) and concentration gradient-driven passive diffusion transport (PDT). Among these pathways, PEX showed the dominance of the crab burrow-induced carbon export, which is at least an order of magnitude higher than the others. Crab burrow-induced carbon export from the whole intertidal transect was calculated as the sum of CO2 release and the dissolved carbon loss by PEX and PDT transport from the surrounding soil matrix to the crab burrow. The estimated C export was extrapolated to US East Coast salt marshes, which was nearly equivalent to riverine DIC flux, half of salt marsh DIC exports. These new insights underline the ecological roles of crab burrows in salt marsh carbon budgets, especially the importance of porewater exchange between crab burrows and the surrounding soil matrix.

D973 |
EGU2020-41
The Utilization of Regression and Analysis of Variance to Model Hurricane Storm Surge Sedimentation on Coastal Marshes along the Northern Gulf of Mexico Coastline
(withdrawn)
Joshua Hodge
D974 |
EGU2020-2619
Kannika Wangritthikraikul, Smith Leknettip, and Sakonvan Chawchai

Peatlands have played an important role in the global carbon cycle. Tropical peatlands are one of the largest reserves of terrestrial organic carbon. Present-day tropical peat swamp forests are, however, under the threat of anthropogenic disturbance or have already been widely degraded. In Southeast Asia, very large areas of peatland have been deforested, drained, converted to extensive and intensive agricultural land uses and exposed to regular wildfires. Khuan Kreng peat swamp is the second largest protected wetland in Thailand. Recent studies in Khuan Kreng peat swamp has focused on present day drought and forest fires, but there is still lack of paleoenvironmental data. In this study, sediment and peat sequences were collected from Forest Fire Control Station area in Khuan Kreng peat swamp, and geochemical data (loss on ignition, grain size analysis and carbonate content) were analyzed. The age of early Holocene were determined based on plant macrofossils using Radiocarbon Dating (14C). The preliminary results indicate that this area was Tidal flats. The study of sediment/peat sequences using geochemical data can improve our understanding how past environmental have affected Khuan Kreng peat swamp ecosystems.

D975 |
EGU2020-3584
Marco Ottinger, Felix Bachofer, Soner Uereyen, and Juliane Huth

Given the growing world population and rising demand for fish and seafood, aquaculture is becoming the main source of aquatic food in human consumption and a primary protein source for millions of people. Since 1990, the world aquaculture production increased from 13 to over 80 million tonnes and is currently valued at USD 231 billion. The cultivation of shrimp species in land-based ponds is one of the fastest growing food production economies and became an important industry in coastal regions, generates income and employment and contributes to food security. Shrimp farms are mainly found in low-lying coastal regions such as estuaries, bays and river deltas along the shorelines of Asia and America. Shrimp farming expanded rapidly in recent years and led to environmental degradation and conversion of valuable wetlands such as mangroves and other coastal forests. The loss of mangroves poses a major threat to coastal ecosystems and population, as mangroves provide valuable flood and coastal protection, as well as risk reduction benefits with regard to global climate change induced effects.

In this research, we use image segmentation for temporal features derived from space-borne, high-resolution synthetic aperture radar (SAR) data to extract shrimp farming ponds in coastal mangrove forest areas in Ecuador, South America. An automatic object-based image processing approach aims for the detection of rectangular shaped pond objects utilizing per-pixel median images calculated from C-band Sentinel-1 and L-band ALOS-Palsar SAR time series data. An open source connected component segmentation algorithm was used to extract and locate rectangular shrimp farms in coastal areas based on backscatter intensity and shape features. This study illustrates the opportunities by earth observation for area-wide assessments of shrimp farming activities in mangrove areas to gain more knowledge on land use dynamics with regard to global change and food security. Earth observation can effectively support the planning and management of aquaculture practices and support stakeholders, politicians, and conservationists in implementing appropriate measures in order to protect coastal environments and foster sustainable development in the coastal zone.

D976 |
EGU2020-9089
Chris Hackney, Rachael Carrie, Dao Tan Van, Joshua Ahmed, Serena Teasdale, Claire Quinn, Lindsay Stringer, Hue van Thi Le, Quang Hong Nguyen, Nga Pham Thi Thanh, and Daniel Parsons

Mangroves provide critical ecosystem services that support livelihoods and communities at the coastal margin. They are key natural flood defences to tropical cyclone driven storm surges, they store sediment that is vital for maintaining delta surface elevations in the face of rising sea levels, and transfer key nutrients to agricultural land. Over the past few decades, stressors on mangroves have increased with associated declines in global areal extent, and growing concern about their condition, including for forests that have been restored or afforested. Most remaining mangrove forests comprises a mix of ages and quality. Limited research exists exploring how differing age, structure and health of mangroves impacts sediment retention and aids the dissipation of wave and storm energy, and links these physical processes to the delivery of ecosystem services.

 

In this study, we demonstrate how mangrove age and health differentially impacts rates of sedimentation, attenuates water level and tidal propagation and aids storm energy dissipation along a section of mangrove forest in Thai Binh province on the Red River Delta in Vietnam. Data were collected over a four month period and highlight spatially variable responses to tides and the increasing influence of the nearby Thai Binh River. We show that sedimentation rates vary from 0.8 m/yr to 0.14 m/yr with increasing distance inland, whilst peak tidal range varies from 1.5 m to 0.5 m with mangrove age.  We demonstrate that these spatial patterns correlate not only to distance inland, but also mangrove age, and the provision of ecosystem services as recorded by household surveys from local communities. This highlights the need for global mangrove databases to account for mangrove quality and health data in order to capture definitively the ecological, hydrodynamic and sedimentological impacts of mangrove forests on coastal and deltaic regions.

D977 |
EGU2020-15002
Dorothea Bunzel, Yvonne Milker, Katharina Müller-Navarra, Helge Wolfgang Arz, and Gerhard Schmiedl

Salt marshes are of substantial importance for the adaptation of coastal regions to present-day and predicted future climate changes, and accompanied sea-level fluctuations. This gives the motivation to investigate salt-marsh archives from the southern North Sea region, which provide an exceptional archive to understand the response of coastal systems to climate variability of the recent past. For this study, well-stratified sediment sequences from two different salt-marsh systems were analysed by scanning XRF spectroscopy. The study sites are affected by both natural processes and anthropogenic interventions on different time scales. To address the complex interplay between storm surges, human-induced coastal management, and coupled internally forced atmosphere-ocean mechanisms during the last century, this study focuses on the ln(Zr/Rb) ratio as proxy for the relative particle-size distribution, and the Br/Corg ratio as an indicator for the marine versus terrestrial organic matter input to the sediment. Additional information about local changes in the sedimentary organic matter quality is provided by the alteration of ln(Br/Cl) ratios. The ln(Zr/Rb) records reveal periodic fluctuations at inter-annual, inter-decadal to multi-decadal time scales, suggesting a close link of sediment accretion to the atmospheric-ocean climate oscillation over the North Atlantic and Europe, which is accompanied by variations in the wind field, precipitation and river runoff. By contrast, the Br/Corg ratios exhibit a long-term increase starting from the mid-twentieth century towards recent times, resembling the observed increasing trend in North Sea storminess. Abrupt drops in the ln(Br/Cl) records coincide with relatively coarser sand layers, indicating impacts by regional storm surges during winter, while intervals of comparable higher ln(Br/Cl) values represent times of generally calm weather conditions of periods with less frequent storm surges. Our results imply that past regional to super-regional climate changes have been transferred into the sedimentary salt-marsh archives of the southern North Sea region.

D978 |
EGU2020-15346
Ben Evans, Helen Brooks, Simon Carr, Clementine Chirol, Elizabeth Christie, Matthew Kirkham, Iris Moeller, Katherine Royse, Olivia Shears, Kate Spencer, and Thomas Spencer

Salt marshes provide diverse ecosystem services including coastal protection, habitat provision and carbon sequestration. The loss of salt marshes is a phenomenon that is observable on a global scale and is of great socioeconomic concern due to the substantial benefits these environments provide. The causes of spatial variability in rates of marsh loss are inadequately understood for the purposes of predicting future ecosystem functions and distributions in the context of global environmental change.

We investigate the relationship between vegetation of different genera and the mechanical properties of the substrate. We couple in-situ and laboratory tests of substrate geotechnical properties with micro-CT imaging of undisturbed root network structures to assess the contribution of three halophytes to sediment stability. We investigate the role of Puccinellia spp., Spartina spp. and Salicornia spp. in the modification of geotechnical parameters such as critical shear strength and cohesion when compared to un-vegetated sediments. We then compare these effects between clay-rich and sandy contexts on the East and West coasts of the United Kingdom respectively.

We find that the three genera are characterised by different root network morphologies which, in part, explain the differences that we observe between the geotechnical properties of sediments colonised by these contrasting vegetation types. The presence of roots within the sediment structure increases the cohesion, as measured using a laboratory shear box test, when compared to bare sediment, with the magnitude of this effect varying by root morphology and sedimentology. In-situ shear vane tests reveal a localised spatial variability in sediment shear strength that is related to halophyte species distributions. This allows multispectral UAV imagery to be used to map species distributions and thereby infer a component of the sediment’s vulnerability to erosion that supports the prediction of future marsh distributions and, ultimately, ecosystem service provision.

D979 |
EGU2020-20537
Matthew Brolly, Isa Kwabe, Raymond Ward, and Christopher Joyce

In this study, soil sampling, vegetation analysis, and remotely sensed indices are used to devise a framework for monitoring impact of oil pollution on Mangrove forests. Mangroves are under threat from resource extraction and associated degradation. As a result of their inter-tidal location, Mangroves provide habitat for terrestrial and aquatic organisms and are important components of coastal ecosystems, providing a range of naturally available ecosystem services. Despite the widely accepted and documented range of ecosystem services provided by mangroves, they have nevertheless, experienced a worldwide degradation resulting from various anthropogenic activities including oil exploitation.

This research is conducted in the Niger Delta where the largest spatial extent of Mangrove forests in Africa is located, consisting of 7% of global stock. Hydrocarbon exploitation in the Niger Delta region is one of several resource extractions undertaken in the area and as a result associated environmental pollution has caused a drastic decline in the region’s biodiversity and ecological resources. Of interest to this study is the effect of associated oil spills on the Mangrove forest ecosystem and their detection.

This study undertook a detailed field exercise over three seasons across the Niger Delta within close proximity to recorded oil spills; as noted in the NOSDRA (National Oil Spill Detection & Response Agency) archive. Soil sampling and laboratory analyses were conducted to establish the level and nature of contamination and supported by complementary vegetation structure analysis evaluating Leaf Area Index (LAI) from ground (LAI2200C) and spaceborne (Landsat archive) systems. Levels of soil contamination were significant with respect to control areas regarding both presence and concentration of heavy metal pollutants (Cr, Mn, Fe, Zn, Pb, Al and Hg). Additionally, negative structural impacts were detected on the local soil via Bulk Density reductions, known to impact soil function, as high as 0.566 g/cm3 when comparing control Estuarine with high polluted locations, and Soil Organic Matter (SOM) reductions indicated by a mean percentage difference to the control of 11% for high polluted Fringing locations. These results highlight the immediate harm from spills, with degraded areas visually recorded and validated via ground measurements with mean LAI in high polluted Estuarine locations recording 1.8 higher. Linking vegetation structure in the Mangrove system with soil contamination allows the use of remote sensing to identify areas of degradation and subsequently to model the level and nature of contamination. The correlation between  ground and spaceborne measurements of LAI (eg. r=0.62 p<0.005 for fringing low pollution locations), allows machine learning approaches to be used to model LAI given the presence of contaminants and to provide a framework for supporting the detection and recording of areas at risk. Success will be expanded upon through use of GEDI lidar waveforms in the near future to improve the remotely derived description of forest structure.

D980 |
EGU2020-21077
Hafsah Nahrawi, Monique Leclerc, Steven Penning, Gengsheng Zhang, Navjot Singh, and Roshani Pahari

Coastal salt marshes are among the most productive ecosystems on Earth and play an important role in the global carbon cycle. The present study quantifies the net ecosystem exchange in a salt marsh dominated by large heterogeneous patches of Spartina alterniflora near Sapelo Island, GA, USA. Continuous high-frequency eddy-flux measurements were made between 2013 and 2016. The method provides both the temporally continuous and a spatial integration of the carbon exchange between the salt marsh and the atmosphere. Over the course of this multi-year study, the salt marsh ecosystem was found to be a net sink of carbon.  In 2015, annual net ecosystem exchange (-138.72 g C m2) was 55% smaller than in 2014 (-309.36 g C m-2). This can be attributed to the high temperatures and the occurrence of Proxigean Spring tides combined with persistent onshore winds arising from the indirect effect of Hurricane Joaquin. The ecosystem acted as carbon source with cumulative net ecosystem exchange value of 0.02 g m-2 day-1 during the event. In addition, both the photosynthetically active radiation and the air temperature are the main environmental drivers in the marsh. Higher air temperatures and incoming photosynthetic active radiation level limit photosynthetic activity. Further work suggests the collection of a longer record to capture the impact of climatic and other environmental variations on the strength of the carbon sink. Also, the inclusion of the lateral component of the carbon fluxes as part of the seasonal and annual budgets would also considerably augment our understanding of the functioning of the salt marsh along the southeastern coast on the US.

D981 |
EGU2020-1878
Gregory Slusarczyk and Mary Cialone

This paper will provide an analysis of the numerical modeled water levels in the vicinity of New Jersey Back Bays (NJBB) coastal wetlands in response to wave and surge forcing. The main focus of the analysis is to evaluate the contribution of the wetlands to reduce storm and flood risk, resist and recover from storms, and mitigate for degradation of the NJBB shorelines.  In order to provide information that addresses these needs, the US Army Corps of Engineers (USACE) Engineer Research and Development Center (ERDC) evaluated a set of “high” ranked Engineering with Nature (EWN)/ Natural and Nature Based Features (NNBF) measures through an application of the predictive numerical models ADvanced CIRCulation (ADCIRC) and STeady-state spectral WAVE (STWAVE) coupled via the Coastal Storm Modeling System (CSTORM-MS).

The ERDC modeling team developed a priority list of wetland configurations to evaluate, grouped into four categories: 1) Base Option designed to determine the maximum feasible benefits from a subset of NNBF measures, 2) Option 1 designed to determine how the benefits scale with NNBF size, 3) Option 2 designed to determine how the current marsh extent contributes to flood risk, 4) Option 3 designed to determine the interaction of waves with proposed NNBF measures predominantly in the Barnegat Bay area.

The above configurations were subject to wind forcing composed of a statistically-selected subset of synthetic tropical cyclones that were part of North Atlantic Coast Comprehensive Study (NACCS) storm suite. An analysis of the effectiveness of the wetland configurations was performed with respect to the following criteria: maximum surge envelopes, water level time series, and characteristics of tropical storm forcing conditions.

D982 |
EGU2020-2130
Mengfan Chu, Julian P. Sachs, Hailong Zhang, Yang Ding, Gui’e Jin, and Meixun Zhao

Mangrove systems represent important long-term sinks for carbon since they have much higher carbon burial rates than terrestrial forests or typical coastal ecosystem. However, quantifying the sources of organic matter (OM) in estuarine and coastal sediments, where mangroves occur but are not the only source of OM, is challenging due to the variety of OM sources and diverse transport processes in these dynamic environments. The sources of OM in surface sediments of two mangrove-fringed estuaries in Hainan Province, China, were investigated using the mangrove specific biomarker taraxerol and other lipid biomarkers, as well as stable carbon isotopes. Mixing models based on the concentration of taraxerol, plant wax n-alkanes and δ13 COM indicate that terrestrial non-mangrove plant OM accounted for 52-72% of the OM in the two estuaries, aquatic OM from phytoplankton and/or seagrass accounted for 8-29%, and OM from mangroves comprised 16-26% of the total. Terrestrial plants contributed 16-20% more of the OM to sediments of Bamen Bay, which is on the wetter, eastern side of Hainan Island, than to Danzhou Bay, but aquatic OM (algae plus seagrass) fraction was 17% lower than that in Danzhou Bay sediments. In both estuaries, mangrove and aquatic OM fractions increased seaward while the terrestrial OM fraction decreased. Terrestrial fraction in BMB sediments is 12% higher in summer compared to autumn, which is offset by a comparable reduction in the mangrove OM fraction, as well as higher aquatic OM fractions in both estuaries. This may be caused by enhanced river discharge, more efficient mangrove leaf litter transport offshore, and/or higher aquatic productivity. The biomarker and carbon isotope approach used here can be applied to semi-quantitatively estimate spatial and temporal variations of the sources of organic carbon in tropical estuarine and coastal sediments, a major sink for carbon in the ocean.

D983 |
EGU2020-3574
Malte Rubel, Klaus Ricklefs, Peter Milbradt, and Julian Sievers

In the awareness of the increasing conciousness regarding the sensitivity, vulnerability, and
complexity of near coastal marine ecosystems, including tidal flats, it is imperative to improve the
understanding of its individual elements. One of these elements are organisms habitating the
seabed, such as mussels.
Bivalves - specifically blue mussels (Mytilus edulis) and pacific oysters (Magallana gigas) - besides
other smaller organisms are an integral part of the seabed fauna. On the one hand they serve as a
basic food resource for a large number of higher trophic level predator. On the other hand they
affect the surface structure, stability and composition of the seabed.
To better understand the large fluctuations the mussel stocks underwent during the last decades, it is
of great benefit to know the environmental conditions of their habitats. Based on the analysis of
different physical parameters at known mussel beds, prototypical automated algorithms were
developed and used to identify other tidal flat regions with favorable conditions for epibenthic
mussels. The input parameters originate from different morphological, hydrodynamical,
sedimentological and hydrochemical numerical models. Morphological factors include
morphological activity and gradient conditions of the ground surface, hydrodynamical factors
include stream velocities, bottom shear stress, wave orbital velocities, energy of wave breaking and
duration of tidal flats falling dry during low tide, sedimentological factors include sediment
composition and hydrochemical factors include salinity. These parameters were available as
products of the mFUND project EasyGSH-DB and were supplemented with additional evaluations.
It is expected that the approach of habitat modeling will allow to determine the possibility of initial
and long-term settlements of epibenthic mussels by ruling out intertidal or subtidal seabed areas
where environmental parameter combinations do not fulfill the necessary requirements.

D984 |
EGU2020-8864
Jaume Company, Raquel Vaquer-Sunyer, Julián García-Comendador, Josep Fortesa, Aleix Calsamiglia, and Joan Estrany

Since the middle of the twentieth century, human societies has quadrupled the environmental flow of phosphorus –P, dramatically impairing freshwater and coastal marine ecosystems (1). Wetlands act as environmental buffers retaining nutrients and pollutants delivered from upstream parts of river systems. This buffer effect also avoids the conveyance of these pollutants to the marine environment preventing eutrophication of coastal marine ecosystems.

The aim of this research is to assessing P concentrations within the S’Albufera (1,708 ha), the main wetland of Mallorca (Spain) and protected by the Ramsar list of wetlands of international importance. Since the middle of the ninetieth century, it is an artificial system, in which waters are forced to take the shortest route to the sea (2). The study of accretion rates by using 137Cs measurements will be combined with the nutrient retention capacity analysing phosphorus concentrations in fine-grained sediments. Accordingly, 5 integrated sediment cores, composed by two replicas, sectioned at 5-cm intervals were collected in February 2016 in three representative hydrosedimentary environments of S’Albufera: two at the centre of the main artificial channel, one at the bank of this main channel and two within the lagoon areas. Total, inorganic and organic P were quantified in 532 samples in order to assess the nutrient dynamics into these representative hydrosedimentary environments and depths.

The 137Cs measurements depicted how the sediment cores collected in the middle of the artificial channel as well into its banks were mainly characterized by erosional processes due to the lack of 137Cs at any depth. On the other side, cores sampled at lagoon areas illustrated depositional processes with mean accretion rates since 1963 between ca. 0.139 g m-2 yr-1 to 0.169 g m-2 yr-1.

Total P concentrations were significantly higher in lagoon areas (0.082 mg P g-1 sediment) than in the sediment from banks of the main artificial channel (0.037 mg P g-1 sediment) or into the middle of this channel (0.03 mg P g-1 sediment). The same pattern was observed for organic and inorganic P with concentrations within lagoon areas of 0.039 mg P g-1 sediment and 0.043 mg P g-1 sediment, respectively. Concentrations for the banks of the main artificial channel were 0.018 mg P g-1 sediment for inorganic P and 0.019 mg P g-1 sediment for organic P. At the middle of the artificial channel, concentrations were even lower, with 0.014 mg P g-1 sediment for inorganic P and 0.016 mg P g-1 sediment for organic P.

These results elucidate that the natural maintenance of wetlands is crucial to ensure their optimal functioning as environmental buffers.

 

References

(1) Lane and Autrey, 2017. MFR. DOI: 10.1071/MF16372

(2) Lopez et al., 1996. ECSS. DOI: 10.1006/ecss.1996.0014

This work was supported by the research project CGL2017-88200-R “Functional hydrological and sediment connectivity at Mediterranean catchments: global change scenarios –MEDhyCON2” funded by the Spanish Ministry of Science, Innovation and Universities, the Spanish Agency of Research (AEI) and the European Regional Development Funds (ERDF).

 

 

D985 |
EGU2020-9248
Anna Canning and Arne Körtzinger

Wetlands are known to be significant sources for CH4, yet vary between potential sources and sinks for CO2. However, in regards to the budgets and processes, they are still considered to have high uncertainties, inconsistencies and a general lack of data overall. One key wetland region in Europe is the Danube River Delta. It is the second largest delta in Europe, consisting of the vastest compact reed bed zone in the world, intertwined with rivers, lakes and channels. It is sourced with water from a drainage basin of 817,000 km2, with the Danube River originating in Germany before travelling 2,857 km to the Black Sea. However, considering the potential pollution effects within this terminal zone, as well as the delta being one of the most important wetlands in Europe for its ecological value alone (and therefore fragile), few studies have focused on the dynamics within the carbon cycle. During 2017, three field campaigns across three seasons measured high resolution, small-scale spatial and temporal variability for pCO2, CH4, O2 and ancillary parameters within the lakes, rivers and channels with the use of a surface water flow-through package. Given the flexibility of the system, we were able to conduct day-night cycles and extensive mapping transects. We discovered day-night cycles showing significant variation of CH4 concentrations within the lakes and channels, as well ‘hot spot’ anomalies showing potential ground water sourcing and extreme CH4 concentrations flowing in from the reed beds. Although reasoning for supersaturation in surface waters are under continuous debate, we conclude a potential reason for such dynamic diel variation within the lake may be due to biomass decomposition and extensive macrophyte concentrations creating a temporarily anoxic zone during the day with mixing during the night, such as previously suggested. On top of this, with the use of discrete data collected from the same water source simultaneously, we were able to model alkalinity, dissolved inorganic carbon and pH to examine both 24 h cycles across lakes and day-night dynamics, giving an in-depth glimpse into the carbonate system. Through the extensive mapping, we successful extracted diel variations for pCO2 and the carbonate species across the lakes with the use of just day-light data, allowing for spatial and temporal variations to be distinguished. We confirm the boundaries between channels and lakes are intertwined as much as they are with the wetlands, and how small extreme anomalies can only begin to be explained with such high-resolution data, even more so in combination with modelled data.