Surface Elevation Table-marker Horizon Research Articles

Living root contributions dominate vertical accretion, but not carbon burial, in two SE Australian tidal wetlands

Organic matter derived from allochthonous and autochthonous sources makes an important contribution to the accumulation and burial of “blue carbon” in tidal wetlands. Organic matter accretion is also a mechanism by which tidal marshes and mangroves adjust vertically to the pressure of sea-level rise, through feedbacks between marsh autotrophic productivity and hydroperiod. However, the separate contributions of inorganic matter, detrital organic matter and living roots to marsh accretion have rarely been documented. We used a network of Surface Elevation Table-Marker Horizon (SET-MH) stations SE Australia as a benchmark against which the accumulation of inorganic, organic and living root material was measured. Established in 2000–2002, the SET-MH stations allowed for sampling accretion and elevation gain in mangrove and saltmarsh in fluvial and marine sand geomorphic settings. We found living root material to be the dominant contributor to the volume of accretion above feldspar marker horizons and elevation gain over the 20-year experimental period. However, detrital organic matter, being of lower bulk density than living roots, contributed most to carbon burial. Higher rates of accretion and elevation gain were associated with higher contributions of pore volume, and lower contributions of inorganic matter. The rate of subsidence increased with accretion and was lower where inorganic matter contributions to mass were higher. The results highlight the importance of biological and hydrological contributions to accretion and elevation gain in a region subject to riverine minerogenic inputs.

Vertical Accretion Trends in Australian Tidal Wetlands

Australian tidal wetlands differ in important respects to better studied northern hemisphere systems, an artefact stable to falling sea levels over millennia. A network of Surface Elevation Table-Marker Horizon (SET-MH) monitoring stations has been established across the continent to assess accretionary and elevation responses to sea-level rise. This network currently consists of 289 SET-MH installations across all mainland Australian coastal states and territories. SET-MH installations are mostly in mangrove forests but also cover a range of tidal marsh and supratidal forest ecosystems. Mangroves were found to have higher rates of accretion and elevation gain than all the other categories of tidal wetland, a result attributable to their lower position within the tidal frame (promoting higher rates of accretion) higher biomass (with potentially higher rates of root growth), and lower rates of organic decomposition. While Australian tidal marshes in general show an increase in elevation over time, in 80% of locations, this was lower than the rate of sea-level rise. High rates of accretion did not translate into high rates of elevation gain, because the rate of subsidence in the shallow substrate increased with higher accretion rates (r2 = 0.87). The Australian SET-MH network, already in many locations spanning two decades of measurement, provides an important benchmark against which to assess wetland responses to accelerating sea-level rise in the decades ahead.

Soil Elevation Change in Mangrove Forests and Marshes of the Greater Everglades: A Regional Synthesis of Surface Elevation Table-Marker Horizon (SET-MH) Data

Soil Elevation Change in Mangrove Forests and Marshes of the Greater Everglades: A Regional Synthesis of Surface Elevation Table-Marker Horizon (SET-MH) Data

Hurricane Sandy Effects on Coastal Marsh Elevation Change

High-magnitude storm events such as Hurricane Sandy are powerful agents of geomorphic change in coastal marshes, potentially altering their surface elevation trajectories. But how do a storm’s impacts vary across a large region spanning a variety of wetland settings and storm exposures and intensities. We determined the short-term impacts of Hurricane Sandy at 223 surface elevation table–marker horizon stations in estuarine marshes located across the northeast region of the United States by comparing post-storm surface elevation change with pre-storm elevation trends. We hypothesized that the storm’s effect on marsh elevation trends would be influenced by position relative to landfall (right or left) and distance from landfall. The structural equation model presented predicts that marshes located to the left of landfall were more likely to experience an elevation gain greater than expected, and this positive deviation from pre-storm elevation trends tended to have a greater magnitude than those experiencing negative deviations (elevation loss), potentially due to greater sediment deposition. The magnitude of negative deviations from elevation change in marshes to the right of landfall was greater than for positive deviations, with a greater effect in marshes within 200 km of landfall, potentially from the extent and magnitude of storm surge. Overall, results provide an integrated picture of how storm characteristics combined with the local wetland setting are important to a storm’s impact on surface elevation, and that the surface elevation response can vary widely among sites across a region impacted by the same storm.

A Tropical Cyclone-Induced Ecological Regime Shift: Mangrove Forest Conversion to Mudflat in Everglades National Park (Florida, USA)

The ecological effects of tropical cyclones on mangrove forests are diverse and highly location- and cyclone-dependent. Ecological resistance, resilience, and enhancement are terms that describe most mangrove forest responses to tropical cyclones. However, in the most extreme cases, tropical cyclones can trigger abrupt and irreversible ecological transformations (i.e., ecological regime shifts). Here, we examine a cyclone-induced ecological regime shift that occurred in Everglades National Park (USA), where forest mortality and peat collapse due to a powerful tropical cyclone (the 1935 Labor Day Hurricane) led to the conversion of mangrove forests to mudflats and an estimated elevation loss of approximately 75 cm. We investigated soil elevation change measured in these mangrove forests and adjacent mudflats during a twenty-year period [1998–2018] using Surface Elevation Table-Marker Horizon (SET-MH) methods. This period encompasses the effects of Hurricanes Wilma (2005) and Irma (2017). We also used historical sea-level rise rates and future sea-level rise scenarios to estimate surface elevation changes in the past (1930–1998) and to illustrate elevation gains needed for these ecosystems to adapt to future change. Collectively, our findings advance understanding of the long-term effects of cyclone-induced ecological regime shifts due to forest mortality, peat collapse, and conversion of mangrove forests to mudflats.

Applications and utility of the surface elevation table–marker horizon method for measuring wetland elevation and shallow soil subsidence-expansion

Byrnes et al. (Geo-Marine Letters 39:265–278, Byrnes et al. 2019) present subsidence data for Barataria Basin located south and west of New Orleans in coastal Louisiana to better inform wetland protection and restoration planning by the Louisiana Coastal Protection and Restoration Authority. They measured subsidence using geodetic GPS elevation surveys of rod benchmarks, similar to the rod benchmarks of the surface elevation table–marker horizon (SET-MH) method used to measure surface biophysical processes influencing elevation dynamics and shallow subsidence (i.e., subsidence occurring above the base of the rod) in coastal wetlands. Byrnes et al. (Geo-Marine Letters 39:265–278, Byrnes et al. 2019) argue that (1) SET-MH measures should not be included in subsidence measures because subsidence is a purely geologic process, separate from biophysical processes occurring in the active marsh zone, (2) shallow subsidence measured by the SET-MH method in deep Holocene sediments are not valid because of downdrag on the rod, and (3) high spatial variability of wetland surface processes precludes the ability to make meaningful estimates of subsidence using the SET-MH method. This reply paper presents an extensive summary of the peer-reviewed literature that refutes all three of these claims and demonstrates that it is not only reasonable but also essential to apply the SET-MH method to obtain a complete as possible assessment of surface elevation dynamics to inform coastal wetland restoration and management planning in Barataria Basin and other coastal wetlands worldwide.

The Effect of Vegetation on Surface Elevation in Coastal Mangrove Areas

Guo, Y.; Chen, Y.; Liao, B.; Huang, B.; Wu, F., and Jiang, Z., 2020. The effect of vegetation on surface elevation in coastal mangrove areas. Journal of Coastal Research, 36(3), 600–607. Coconut Creek (Florida), ISSN 0749-0208.Rising sea levels pose a serious threat to global coastal mangroves. The ability of surface elevation to change synchronously with rising sea level is the key to the survival of mangroves. Mangrove vegetation can have a variety of effects on surface and subsurface processes; therefore, it has an important role in surface elevation changes. In this study, rod surface elevation table–marker horizon (RSET-MH) technology was used to study changes in surface elevation and accretion between nonvegetated and vegetated mangrove areas at three different sites. The results showed that the existence of vegetation had a significant effect on elevation changes, with increased surface elevation in vegetation areas observed to be significantly greater than those in nonvegetation areas. In addition, the subsurface changes in vegetated area showed expansion, whereas the nonvegetation area showed subsidence. Surface accretion showed variable trends in different research sites, although these could be related to their geographical location and thus be affected by many factors. Moreover, there was a significant correlation between density of vegetation and elevation/subsurface changes (r = 0.763, p < 0.05; r = 0.714, p < 0.05 respectively), and there were no significant correlations between accretion and vegetation characteristics. The results indicate that mangrove vegetation has a positive effect on the increase of surface elevation, and vegetation density may be a key factor because of the high biomass and carbon storage rate in high-density areas. This study provides additional data with which to explore the elevation variation in mangroves and a scientific basis and technical support for strengthening wetland protection.

Soil Structure and Its Relationship to Shallow Soil Subsidence in Coastal Wetlands

Accelerating sea-level rise poses a threat to mangroves and salt marshes. Sediment accretion on the soil surface and belowground root production is proposed to increase soil elevation enabling these intertidal habitats to maintain their position relative to mean sea level. However, shallow soil subsidence is frequently observed, which may cancel out or even outweigh the positive effects of surface accretion and root production on soil elevation gains. The processes that lead to shallow soil subsidence have not often been investigated. Here, we used computed tomography (CT) imaging to determine the volume of coarse pores in the top 20 cm of 30-cm-deep soil cores collected from mangroves and salt marshes within Moreton Bay, eastern Australia. The four field sites have long-term observations of variation in soil surface elevation gain, surface accretion rate, and shallow subsidence rate using the rod surface elevation table-marker horizon (RSET-MH) method. Over our sites, we found that the rates of shallow soil subsidence were positively related to the volume of soil coarse pores and soil clay and silt content rather than to the volume of fine pores or total porosity or other soil properties. In turn, the volume of soil coarse pores was positively related to the volume of animal burrows and root mass. Fine-pore volume and total porosity of soils were negatively associated with soil bulk density, which was in turn negatively related to soil organic matter content. Our results indicate influences of differently sized soil pores in shallow soil subsidence and suggest that animal burrowing may play a role in shallow soil subsidence.

The Long-Term Effects of Hurricanes Wilma and Irma on Soil Elevation Change in Everglades Mangrove Forests

Mangrove forests in the Florida Everglades (USA) are frequently affected by hurricanes that produce high-velocity winds, storm surge, and extreme rainfall, but also provide sediment subsidies that help mangroves adjust to sea-level rise. The long-term influence of hurricane sediment inputs on soil elevation dynamics in mangrove forests is not well understood. Here, we assessed the effects of sediment deposition during Hurricanes Wilma (2005) and Irma (2017) on soil elevation change at two mangrove forests located along the Shark and Lostmans Rivers in Everglades National Park. We used surface elevation change data from a 16-year period (2002–2018), measured with the surface elevation table-marker horizon (SET-MH) approach. At the Shark River mangrove forest, we used marker horizons and a combination of deep, shallow, and original SETs to quantify the contributions of four soil zones to net soil elevation change. Rates of elevation change were greatly influenced by storm sediments. Abrupt increases in elevation due to sediment inputs and subsurface expansion during Hurricane Wilma were followed by: (1) an initial post-hurricane period of elevation loss due to erosion of hurricane sediments and subsurface contraction; (2) a secondary period of elevation gain due primarily to accretion; and (3) an abrupt elevation gain due to new sediment inputs during Hurricane Irma. Our findings suggest that elevation change in hurricane-affected mangrove forests can be cyclical or include disjunct phases, which is critical information for advancing the understanding of wetland responses to accelerated sea-level rise given the expectation of increasing storm intensity due to climate change.

High surface elevation gains and prediction of mangrove responses to sea-level rise based on dynamic surface elevation changes at Dongzhaigang Bay, China

Mangrove forest responses to sea-level rise (SLR) are likely misestimated in many studies because current assessment methods often assume a constant rate of surface elevation change along the intertidal profile. Most studies also neglect the ecogeomorphic feedback process known to accelerate soil building with SLR. Here, an empirical, dynamic model that accounts for the above processes is constructed for the intertidal zone of a mangrove bay in Hainan, China. The model allows us to explore the response of a tide-dominated mangrove forest to different rates of SLR. Rod surface elevation table-marker horizon (RSET-MH) data (2015–2018) indicate rather high rates of surface accretion (13.3–57.9 mm yr−1) and surface elevation changes (7.9–35.5 mm yr−1). Surface accretion and surface elevation change decreased exponentially with increasing ground elevation. Our model showed that SLR (at rates of 4 and 16 mm yr−1) would not lead to the loss of mangrove cover but that it could alter mangrove species zonation along the intertidal profile by 2100 due to a rich sediment supply and ecogeomorphic feedback. Importantly, rapid SLR and landward barriers can pose a considerable threat to the high-intertidal community by causing increased inundation of the high-intertidal zone, subsequent shifts in species zones, and the loss of mangrove biological and structural diversity along the intertidal profile. Overall, this study predicts probabilities of habitat suitability for mangrove species under SLR and expands our current understanding of the impacts of SLR on tide-dominated mangrove forests in sediment-rich systems. Incorporation of extensive, long-term RSET data into this kind of dynamic model could contribute to the better prediction of mangrove responses to SLR at large scales in the future.

Observations of Surface and Subsurface Processes on a Saltmarsh in the Central Jiangsu Coast (China)

Jin, C.; Gong, Z.; Geng, L.; Zhao, K.; Xu, B.B., and Coco. G., 2018. Subsurface processes in salt marsh of central Jiangsu coast(China). In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 296–300. Coconut Creek (Florida), ISSN 0749-0208.Studies of tidal flats morphodynamics tend to ignore subsurface processes like bioturbation or underground water dynamics so that changes in the surface elevation are always considered to be the result of sedimentation/erosion on the soil surface. However, at times, subsurface variations can be as large as elevation changes driven by sedimentation and so control the evolution trend of surface elevation. In this study, four observation sites were set up on the salt marsh in the central Jiangsu coast (China). A three-year field observation campaign has been carried out studying seabed elevation changes. The Surface Elevation Table-Marker Horizon (SET-MH) technique was applied to measure surface elevation and net sedimentation. The subsurface variation was derived from the difference between the surface elevation and the sedimentation. Results show that the changes in surface elevation are not equal to the sedimentation and that subsurface process, responsible for such deviation, are strongly affected by storm surge. Storm surges increase the underground water content leading to the observed expansion of the subsurface soil. The expansion results in the rapid increase of surface elevation and it takes almost one year to recover to the surface elevation before the storm season.

Differential in surface elevation change across mangrove forests in the intertidal zone

A better understanding of surface elevation changes in different mangrove forests would improve our predictions of sea-level rise impacts, not only upon mangrove species distributions in the intertidal zone, but also on the functioning of these wetlands. Here, a two-year (2015–2017) dataset derived from 18 RSET-MH (rod surface elevation table-marker horizon) stations at Dongzhaigang Bay, Hainan, China, was analyzed to investigate how surface elevation changes differed across mangrove species zones. The current SET data indicated a rather high rate (9.6 mm y−1, on average) of surface elevation gain that was mostly consistent with that (8.1 mm y−1, on average) inferred from either the 137Cs or 210Pb dating of sediment cores. In addition, these surface elevation changes were sensitive to elevation in the intertidal zone and differed significantly between the two study sites (Sanjiang and Houpai). Mangrove species inhabiting the lower intertidal zone tended to experience greater surface elevation change at Sanjiang, which agrees with the general view that sedimentation and elevation gains are driven by elevation in the intertidal zone (i.e., greater when positioned lower in the intertidal profile). However, at Houpai, both surface elevation change and surface accretion showed the opposite trend (i.e., greater when positioned higher in the intertidal profile). This study's results indicate that the pattern of surface elevation changes across the intertidal profile maybe inconsistent due to intricate biophysical controls. Therefore, instead of using a constant rate, models should presume a topography that evolves at differing rates of surface elevation change in different species zones across the intertidal profile when predicting the impacts of sea-level rise on mangrove distributions.

Will fluctuations in salt marsh-mangrove dominance alter vulnerability of a subtropical wetland to sea-level rise?

To avoid submergence during sea-level rise, coastal wetlands build soil surfaces vertically through accumulation of inorganic sediment and organic matter. At climatic boundaries where mangroves are expanding and replacing salt marsh, wetland capacity to respond to sea-level rise may change. To compare how well mangroves and salt marshes accommodate sea-level rise, we conducted a manipulative field experiment in a subtropical plant community in the subsiding Mississippi River Delta. Experimental plots were established in spatially equivalent positions along creek banks in monospecific stands of Spartina alterniflora (smooth cordgrass) or Avicennia germinans (black mangrove) and in mixed stands containing both species. To examine the effect of disturbance on elevation dynamics, vegetation in half of the plots was subjected to freezing (mangrove) or wrack burial (salt marsh), which caused shoot mortality. Vertical soil development was monitored for 6years with the surface elevation table-marker horizon system. Comparison of land movement with relative sea-level rise showed that this plant community was experiencing an elevation deficit (i.e., sea level was rising faster than the wetland was building vertically) and was relying on elevation capital (i.e., relative position in the tidal frame) to survive. Although Avicennia plots had more elevation capital, suggesting longer survival, than Spartina or mixed plots, vegetation type had no effect on rates of accretion, vertical movement in root and sub-root zones, or net elevation change. Thus, these salt marsh and mangrove assemblages were accreting sediment and building vertically at equivalent rates. Small-scale disturbance of the plant canopy also had no effect on elevation trajectories-contrary to work in peat-forming wetlands showing elevation responses to changes in plant productivity. The findings indicate that in this deltaic setting with strong physical influences controlling elevation (sediment accretion, subsidence), mangrove replacement of salt marsh, with or without disturbance, will not necessarily alter vulnerability to sea-level rise.

Assessing coastal wetland vulnerability to sea-level rise along the northern Gulf of Mexico coast: Gaps and opportunities for developing a coordinated regional sampling network

Coastal wetland responses to sea-level rise are greatly influenced by biogeomorphic processes that affect wetland surface elevation. Small changes in elevation relative to sea level can lead to comparatively large changes in ecosystem structure, function, and stability. The surface elevation table-marker horizon (SET-MH) approach is being used globally to quantify the relative contributions of processes affecting wetland elevation change. Historically, SET-MH measurements have been obtained at local scales to address site-specific research questions. However, in the face of accelerated sea-level rise, there is an increasing need for elevation change network data that can be incorporated into regional ecological models and vulnerability assessments. In particular, there is a need for long-term, high-temporal resolution data that are strategically distributed across ecologically-relevant abiotic gradients. Here, we quantify the distribution of SET-MH stations along the northern Gulf of Mexico coast (USA) across political boundaries (states), wetland habitats, and ecologically-relevant abiotic gradients (i.e., gradients in temperature, precipitation, elevation, and relative sea-level rise). Our analyses identify areas with high SET-MH station densities as well as areas with notable gaps. Salt marshes, intermediate elevations, and colder areas with high rainfall have a high number of stations, while salt flat ecosystems, certain elevation zones, the mangrove-marsh ecotone, and hypersaline coastal areas with low rainfall have fewer stations. Due to rapid rates of wetland loss and relative sea-level rise, the state of Louisiana has the most extensive SET-MH station network in the region, and we provide several recent examples where data from Louisiana’s network have been used to assess and compare wetland vulnerability to sea-level rise. Our findings represent the first attempt to examine spatial gaps in SET-MH coverage across abiotic gradients. Our analyses can be used to transform a broadly disseminated and unplanned collection of SET-MH stations into a coordinated and strategic regional network. This regional network would provide data for predicting and preparing for the responses of coastal wetlands to accelerated sea-level rise and other aspects of global change.

Measuring, mapping and modelling: an integrated approach to the management of mangrove and saltmarsh in the Minnamurra River estuary, southeast Australia

Mangrove and saltmarsh ecosystems appear particularly vulnerable to the impacts of climate change, and their effective management will require forecasts of how these wetland habitats are likely to respond to sea-level rise through the twenty-first century. We describe a preliminary study of a small stand of mangrove and saltmarsh that involves measuring of elevation change and accretion, mapping of wetland communities, and modelling of their potential response to sea-level rise. The wetland occurs on the banks of the Minnamurra River estuary in southern New South Wales and has been the focus of several studies over recent decades. The research includes empirical measurements of sedimentation at sites in both mangrove and saltmarsh vegetation using the surface elevation table-marker horizon technique. This is a site at which mapping has been undertaken to delineate the extent of each vegetation community from a time-series of aerial photographs using geographical information systems; the gradual incursion of the mangrove, Avicennia marina, into more landward saltmarsh communities, observed over past decades for many systems in southeastern Australia, has continued into the twenty-first century. The observed patterns of change are compared with simulations of how this wetland system might respond to future sea-level rise, adopting several different approaches and the upper and lower bounds of Intergovernmental Panel on Climate Change sea-level rise projections. The model results show considerable variability in response depending on the parameters adopted. We advocate the need for the integration of these three approaches, measuring, mapping and modelling, as a basis for future management and adaptation. Our study demonstrates the considerable opportunities to refine the data input and model outputs as part of adaptive management, as more sophisticated technologies and data become available.

Surface elevation change in natural and re-created intertidal habitats, eastern England, UK, with particular reference to Freiston Shore

Quantification of processes contributing to overall surface elevation change is critical to the accurate assessment of saltmarsh sedimentary status, particularly when predicting system stability in relation to accelerated sea level rise. Rates of short-term (up to 5.5 years) surface elevation change and accretion on a temperate saltmarsh were measured at an open coast managed realignment (habitat creation) trial, and the surrounding intertidal zone using surface elevation table–marker horizon (accretion plate) methodologies. Mean surface elevation changes on vegetated saltmarsh control sites, at some distance from the hydrodynamic disturbance of breaches in a former sea defence line, showed rates of change compatible with marsh surface dynamics seen elsewhere in the region, exceeding rates of regional relative sea-level rise. Within the managed realignment, sites close to the breaches showed exceptionally high rates of both accretion and surface elevation change, most likely as a result of high localized sediment supply from breach and channel enlargement and the presence of surfaces left unnaturally low in the tidal frame. Positive surface elevation change on the landward side of the breaches reached >30.4 mm a−1, up to one hundred times greater than rates of surface elevation change at locations 1 km from the breach entrances. Shallow sub-surface processes, or ‘surface subsidence’, was an important process on natural marshes outside the managed realignment but not seen within the site, due to the consolidated nature of the underlying substrate. The Freiston Shore managed realignment site shows remarkably similar time variation in mean elevation to that of the longer record from the Tollesbury managed realignment site, Blackwater estuary, Essex coast, UK. It is not clear how long it will take for re-created intertidal habitats at both Freiston Shore and Tollesbury to approach an ‘equilibrium’ elevation within the tidal frame but modelling suggests that in natural wetlands on the coast of eastern England this process takes at least 150 years.

Sediment infilling and wetland formation dynamics in an active crevasse splay of the Mississippi River delta

Crevasse splay environments provide a mesocosm for evaluating wetland formation and maintenance processes on a decadal time scale. Site elevation, water levels, vertical accretion, elevation change, shallow subsidence, and plant biomass were measured at five habitats along an elevation gradient to evaluate wetland formation and development in Brant Pass Splay; an active crevasse splay of the Balize delta of the Mississippi River. The processes of vertical development (vertical accretion, elevation change, and shallow subsidence) were measured with the surface elevation table–marker horizon method. There were three distinct stages to the accrual of elevation capital and wetland formation in the splay: sediment infilling, vegetative colonization, and development of a mature wetland community. Accretion, elevation gain, and shallow subsidence all decreased by an order of magnitude from the open water (lowest elevation) to the forest (highest elevation) habitats. Vegetative colonization occurred within the first growing season following emergence of the mud surface. An explosively high rate of below-ground production quickly stabilized the loosely consolidated sub-aerial sediments. After emergent vegetation colonization, vertical development slowed and maintenance of marsh elevation was driven both by sediment trapping by the vegetation and accumulation of plant organic matter in the soil. Continued vertical development and survival of the marsh then depended on the health and productivity of the plant community. The process of delta wetland formation is both complex and nonlinear. Determining the dynamics of wetland formation will help in understanding the processes driving the past building of the delta and in developing models for restoring degraded wetlands in the Mississippi River delta and other deltas around the world.