Introduction
The tidal environment, mangroves and accommodation space
Mangrove forests are composed of diverse species of trees and shrubs that thrive in tidal zones These unique ecosystems are typically found in intertidal regions, situated between mean sea level (MSL) and high tide, with specific tidal levels varying based on the species present and their geographical location.
According to Ellison (2009), certain organisms inhabit the upper section of the tidal frame, which is defined as the region affected by tidal fluctuations This area excludes zones that are permanently submerged or only inundated during storm events.
Intertidal mudflats are dynamic environments that can rapidly change, as storms can significantly reshape muddy coastlines When mudflats reach a height above mean sea level that is suitable for mangroves, and if mangrove propagules are present, mangroves are likely to establish in these areas Once established, mangroves can alter their environment by slowing water flow and reducing wave energy, which facilitates sediment deposition and increases soil volume through their root systems This process can elevate the soil surface height, leading to a stable elevation where mangroves can thrive for extended periods, sometimes thousands of years, as seen in Twin Cays, Belize However, if soil inputs continue to exceed losses, the soil surface may rise until it reaches a point where terrestrial vegetation can outcompete mangroves.
Mangrove accommodation space refers to the difference in height between the current soil surface in a mangrove forest and the maximum potential soil surface height achievable with mangroves, influenced by soil input and loss balance or competition from terrestrial vegetation More broadly, 'accommodation space' encompasses the vertical and lateral areas available for soil expansion, considering the current soil position, tidal influences, and erosive forces It can also define the volume above the substrate that could be filled with sediment, allowing for lateral accommodation space where mangroves could thrive if sediment accumulation occurs, although this is constrained by factors like bathymetry and wave conditions While the accommodation concept is commonly utilized in geology, its application in coastal ecosystems has predominantly focused on coral reefs, with limited references to saltmarshes and mangroves.
As sea levels rise or land subsides, the accommodation space increases due to the growing difference in height between the substrate and mean sea level This expanded volume can be filled with soil, provided that soil inputs are sufficiently high.
Accommodation space is a crucial concept in sequence stratigraphy, as defined by Miall (1996) based on Jervey (1988) It refers to the space available for sediment accumulation, which is essential for sediment preservation For sediments to remain intact, there must be sufficient space below the base level, the threshold above which erosion occurs Essentially, accommodation space represents the area between the substrate level and the maximum height at which sediments can exist without being eroded.
The concept of accommodation space is crucial for mangrove ecosystems, as it allows the soil surface to rise and fill newly created areas This rise is facilitated by organic and inorganic sediment inputs, as well as subsurface roots When the mangrove soil surface increases in height, it enables mangroves to stay within their optimal tidal range, specifically between mean sea level and high tide Conversely, without this elevation, the soil could dip below mean sea level, leading to stress and potential mortality of the mangrove trees Ideally, if the change in soil surface height aligns perfectly with sea level rise, the relative height of the mangrove surface remains stable within the tidal range.
In Sections 2 to 7, we explore whether mangrove soil surfaces tend to rise in response to rises in sea level, the mechanisms underlying this, and the factors affecting it
Sea level rise
Global mean sea levels are rising due to thermal expansion of seawater and the melting of polar ice caps, contributing to eustatic sea level rise Recent satellite measurements indicate an average global sea level rise of 3.4 ± 0.4 mm/year from 1993 to 2007 Historically, sea levels have remained stable over the past 7,000 years, with a modest rise of 3 to 5 meters, equating to rates of 0.4 to 0.7 mm/year Over the last 20,000 years, sea levels have increased by over 100 meters, and they have experienced significant fluctuations over the past 250,000 years, primarily influenced by glaciation periods that caused water to be stored as ice on land.
There is significant spatial and temporal variation in eustatic sea level (Cazenave et al.,
Recent sea level trends exhibit significant spatial variation, with some regions, like parts of the Philippines, experiencing pronounced increases, while others, such as areas along the west coast of North America, are witnessing declines This regional disparity is primarily driven by differences in thermal expansion Additionally, temporal fluctuations in sea levels occur due to the temporary reorganization of ocean currents and oscillations in regional temperatures, notably influenced by phenomena like the El Niño Southern Oscillation (ENSO), which impacts extensive areas of the Pacific Ocean.
Mean sea level rise, as indicated by tide gauges, is influenced by vertical land movements such as glacial isostatic adjustments and lithospheric flexural subsidence These land level changes arise from various factors including earthquakes, tectonic activity, coastal sediment consolidation, and the extraction of natural resources like oil and water Additionally, alterations in loading on the land or sea floor, due to melting glaciers or sediment deposition in deltas, contribute to these shifts Geographically, uplift and subsidence rates differ significantly; for instance, northeast Canada experiences uplift rates of 20 to 30 mm per year, whereas areas between Greenland and northeast Canada face subsidence rates of 6 to 7 mm per year.
The interplay between eustatic and isostatic changes leads to varying rates of sea level rise along different coastlines and over time This localized change, known as Relative Sea Level Rise (RSLR), significantly impacts coastal ecosystems, including mangroves, as well as the communities residing in these areas To effectively understand how sea level changes affect mangrove surface elevation, it is essential to obtain precise local sea level measurements.
Figure 3 Global map of eustatic sea level trends between 1992 and 2012 Map and altimetry data are provided by the NOAA Laboratory for Satellite Altimetry
(http://ibis.grdl.noaa.gov/SAT/SeaLevelRise/LSA_SLR_maps.php)
Figure 4 Regional and local processes affecting the elevation of the mangrove surface relative to local mean sea level
Surface elevation change in mangroves
Elevation refers to the height of a specific point on the Earth's surface in relation to a reference datum In mangrove areas, the term "surface elevation" describes the height of the soil surface, typically measured against a local datum like mean sea level, indicating the height of the mangrove substrate.
Surface elevation change denotes the variation in the height of the soil surface over a specific time frame Typically, these changes are not aligned with a local datum due to the challenges involved in measurement.
Changes in mangrove surface elevation can occur due to various processes, categorized into surface and sub-surface processes In this report, the soil surface is defined as the boundary between the soil and the air, or water during high tide.
Surface processes in mangrove ecosystems encompass sedimentation, which involves the deposition of materials onto the soil surface, accretion, the binding of these materials in place, and erosion, the loss of surface material Understanding these processes is crucial for maintaining the health and stability of mangrove habitats.
Subsurface processes encompass various activities that take place beneath the soil surface, yet above the consolidated layer Key processes include the growth and decay of roots, soil swelling and shrinkage influenced by moisture levels, and the compaction, compression, and rebound of soils resulting from variations in the weight of overlying materials.
The schematic diagram illustrates the structure of a mangrove tree and its underlying soil, highlighting the processes of accretion, shallow subsurface changes, and deep subsidence or uplift within the soil profile It also demonstrates how these factors contribute to changes in surface elevation over time.
Sub-surface expansion, also known as shallow subsidence, occurs when changes in soil volume lead to a loss of elevation above the bedrock or consolidated layer This phenomenon is termed "shallow" to differentiate it from "deep" subsidence, which results from long-term geological processes and is factored into relative sea level rise rates.
These surface and subsurface processes are described in more detail in Section 3; the combined effect of these processes results in surface elevation change, as described in
How mangrove surface elevation varies with sea level rise
The following scenarios describe how mangrove surface elevation may change as mangroves are exposed to sea level rise (some of these are shown schematically in Figure 6):
1 In areas with very high rates of sedimentation, mangrove soil surfaces may rise at a rate which exceeds the local rate of sea level rise, such that terrestrial species invade landward areas, and progradation occurs (i.e new land is formed seaward of the current mangrove area, which mangroves then colonise); this is likely to occur around the deltas of large rivers that bring high volumes of sediment to the coast
2 Sea level rise rates may be matched by a rise in mangrove soil surface elevation, allowing mangroves to remain in the same location, possibly also colonising more landward areas if such areas have suitable substrate and topography (this scenario is illustrated in Figure 6d) An example of where this has occurred is Twin Cays in Belize, as discussed in Section 2.1
3 Mangroves soils may be unable to rise as fast as the local rate of sea level rise, resulting in death of trees in the lower areas and at the seaward edge of the mangrove area (Figure 6b) Mangroves are likely to invade landward areas which now fall within the tidal frame, providing suitable substrate and topography are present there The deeper water in mangrove areas may also allow waves to penetrate further into the mangrove area, resulting in erosion particularly at the seaward edge
The observed scenarios at a specific location are influenced by sedimentation rates and sub-surface soil inputs, alongside sea level rise Additionally, various positive and negative feedback mechanisms between sea level changes and surface and subsurface processes affecting soil volume may also contribute to these dynamics.
The elevation surplus/deficit is a valuable metric for assessing changes in mangrove surface elevation in relation to local sea level fluctuations, as defined by Cahoon et al (1995a) This measure is calculated by subtracting the rate of relative sea level rise from the change in surface elevation.
(mm/yr) (mm/yr) (mm/yr)
An elevation surplus happens when the surface elevation increases at a faster pace than sea level, whereas an elevation deficit indicates that sea level is rising more rapidly than the surface of the mangroves.
The "mangrove-surface-relative sea level rise" (MSR-SLR) is a critical measure that assesses the rate of sea level rise in relation to changing mangrove surface elevations By calculating MSR-SLR, we can effectively describe water level changes concerning the mangrove surface, which is essential for understanding the feedback mechanisms involved in these ecosystems.
Figure 6 Schematic diagram of mangroves to demonstrate tidal range, tidal frame, accommodation space, and possible scenarios following sea level rise with or without surface elevation change
Changes in water levels affect both surface and sub-surface processes, with a positive value indicating that local sea levels are rising faster than mangrove surfaces This results in deeper water over the mangrove substrate and more frequent inundation Conversely, a negative value suggests that mangrove surfaces are rising in tandem with sea level, leading to less frequent inundation.
The following section explores historical and recent evidence for mangrove surfaces keeping pace with sea level in different locations.
Can mangrove surface elevation keep pace with sea level rise?
Historical evidence
In certain regions, mangrove surface elevation has effectively matched sea level rise over thousands of years, with significant evidence found in areas like Twin Cays and the Tobacco Range Islands in Belize, where deep layers of mangrove peat exist This peat, formed from the accumulation of dead mangrove material, can reach several meters in thickness and is dated using radiocarbon techniques Notably, some peat layers have been found to be over 7,000 years old, such as a sample from Twin Cays at a depth of 8.7 meters, estimated to be between 7,430 and 7,580 years old.
The mangrove peat depth-age data from Twin Cays, Belize, as reported by McKee et al (2007), is illustrated alongside a sea-level history curve This curve is based on various studies examining the ages of mangrove peat and coral materials found at different depths throughout the Caribbean, as detailed by Toscana and Macintyre (2003).
Dept h (m et res bel o w Mean S ea Lev el)
The age of mangrove peat at various depths serves as an indicator of historical sea levels, assuming minimal compaction of peat layers Since mangroves thrive exclusively in the intertidal zone, the age of these peats has been utilized to create sea level rise curves, as evidenced by studies from Scholl (1964), Woodroffe (1990), and Toscano and Macintyre (2003) The dating of peat formed by the red mangrove is particularly significant in this research.
Rhizophora mangle, in combination with coral material formed by the reef crest coral
Acropora palmata has been utilized to develop a sea level rise curve for the Western Atlantic region, including the Caribbean, as detailed by Toscano and Macintyre (2003) Recent dating of mangrove peats from Twin Cays by McKee et al (2007) closely aligns with the original data from Toscano and Macintyre, further supporting the sea level rise findings Both datasets are illustrated in Figure 7.
Research from Twin Cays indicates that mangroves first appeared over 7,600 years ago, coinciding with a period of rapid sea level rise exceeding 3.5 mm/yr (McKee et al., 2007) Following their emergence, mangroves accumulated peat at rates of 3 mm/yr from 7,600 to 7,200 years BP, 1.3 mm/yr from 7,200 to 5,500 years BP, and 1.0 mm/yr from 5,500 to 500 years BP, effectively matching regional sea level rise rates (McKee et al., 2007) This adaptation allowed mangroves to maintain their surface elevation and survive, as failure to do so would have resulted in their submersion below sea level The lack of mangrove peat older than 7,600 years may be attributed to factors such as unsuitable substrates, adverse climatic conditions, insufficient seed dispersal, or the inability of mangroves to adapt to the higher sea level rise rates prior to that time.
Research indicates that mangroves across various regions have successfully adapted to rising sea levels over extended periods However, some mangrove areas eventually succumbed to inundation when sea level rise surpassed critical thresholds, while others transitioned to terrestrial vegetation due to increased sedimentation rates A comprehensive review of these findings can be found in Ellison (2008 & 2009).
Research indicates that mangrove peat, discovered in sediment cores from the sea bed, suggests that rising sea levels may submerge mangrove ecosystems For instance, Parkinson (1989) collected sediment cores from various sites in Florida's Ten Thousand Islands, revealing a layer of mangrove peat located beneath other sediments in open water areas up to 6 km from the coast and 5 m below mean sea level Radiocarbon dating of this peat layer showed it to be over 3,500 years old, highlighting the long-term impacts of sea level rise on mangrove habitats.
Historical records indicate that mangrove surface elevations have successfully matched rising sea levels in certain areas over thousands of years However, in some locations, this balance was only maintained for a limited time before mangroves were submerged due to a rapid increase in sea level that surpassed their elevation thresholds These thresholds differ by location and are influenced by local environmental conditions, as further explored in Section 5.3.
2 BP stands for “Before Present”, where the year 1950 A.D is taken as the reference point for “Present”
Table 1 Locations and periods where mangroves kept pace with sea level rise
Location Period during which mangroves persisted
Relative sea level rise rate that mangroves kept pace with
6 mm/yr (12 m rise in relative sea level during this period)
Mangrove swamp replaced by terrestrial vegetation after 5,500 BP as a result of sedimentary landfill
Up to 10 mm/yr Sedimentation caused mangrove forest to be replaced by freshwater wetlands
Since 7,600 years BP Up to 3 mm/yr Described in text above McKee et al., 2007
Since 2,000 years BP 0.85 to 1.1 mm/yr
Mangrove lost 26% of its area over previous century due to retreat of seaward edge
1.2mm/yr Became submerged after 5,500 years
BP with more rapid sea level rise, but re-established in new locations when rates slowed
Since 2000 years BP 1 to 2 mm/yr During rapid sea-level rise (10 mm/yr) between 4,100 and 3,700 years BP, mangrove forests retreated landwards
Recent evidence
Recent studies utilizing the Surface Elevation Table – Marker Horizon (SET-MH) methodology provide crucial insights into whether mangrove surface elevation can keep pace with rising sea levels This technique allows for the measurement of surface elevation changes over months to years, forming the basis of the findings presented in this report While other methods, such as marker horizons and radionuclide dating of sediment layers, have been employed to assess mangrove surface accretion, they fail to consider sub-surface soil volume changes, like compaction, that influence surface elevation Consequently, these alternative methods are inadequate for accurately comparing rates of surface elevation change with sea level rise.
2.2.1 Measurements made using the SET-MH methodology
Cahoon et al (2006) brought together available mangrove surface elevation change data that had been measured using the SET-MH methodology for at least a year (Cahoon and Hensel,
In 2006, data was collected from 19 geographical locations across seven countries, including the United States, Mexico, Belize, Honduras, Costa Rica, the Federated States of Micronesia, and Australia Various SET-MH stations were established in each location to investigate elevation changes in diverse forest types, such as fringe, basin, riverine, and overwash forests, as well as in different energy settings, including exposed and protected forests Overall, the analysis encompassed a total of 60 distinct settings.
3 Accretion refers to the addition of material to the soil surface, and is described in more detail in Section 3
Box 1 The SET-MH methodology
Surface elevation change is now standardly measured using the Surface Elevation Table – Marker Horizon (SET-MH) method (also called the Sedimentation-Erosion Table –
The Surface Elevation Table – Marker Horizon method integrates a marker horizon for measuring accretion with the height measurement of the soil surface above a consolidated underground layer This technique involves driving a rod or pipe into the ground until it reaches a point of refusal, establishing a reliable benchmark By utilizing this method, researchers can effectively track changes in surface elevation in relation to the underlying bedrock or consolidated layer, enhancing the accuracy of geological assessments.
1997) The combination of surface elevation change and accretion measurements allows the magnitude of sub-surface change to be calculated (described below)
The apparatus features a long pipe permanently inserted into the sediment until it reaches a stable point, serving as a reliable reference for measurements When readings are needed, a measuring device is attached to the top of the pipe This setup is designed to maintain stability over time, with any changes only resulting from geological uplift or subsidence of the underlying bedrock.
The Surface Elevation Table - Marker Horizon apparatus is illustrated schematically and in practical use in various environments In the top right image, researcher Iris Mửller measures marsh surface elevation and prepares fresh kaolinite layers as marker horizons at Cartmel Sands in Morecombe Bay Meanwhile, the bottom right image features USGS hydrologic technician Karen Balentine measuring surface elevation in a mangrove forest near Lostmans River, Everglades National Park Photos by Ben Evans capture these essential fieldwork activities.
(marsh) and USGS (mangrove; used with permission from Thomas J Smith).
Cahoon et al (2006) analyzed the relationship between surface elevation changes and long-term relative sea level rise at SET-MH sites, discovering that surface elevation typically lagged behind sea level rise, leading to an elevation deficit Their findings indicated no significant correlation between elevation change rates and relative sea level rise rates, except in embayments, where a slight increase in elevation change was observed with rising sea levels, although this result was not statistically significant (p = 0.07, n = 8).
We repeated their analysis with more recent data from 15 geographical locations (including
A study of 31 settings, utilizing data from five studies conducted between 2006 and 2011, revealed that while five sites exhibited an elevation surplus, ten sites experienced an elevation deficit in relation to relative sea level rise The average elevation surplus/deficit was -1.26 mm/yr, which was not statistically significant (t = -1.59, d.f = 14, p-value = 0.13) Surface elevation change rates ranged from -2.6 to 5.64 mm/yr, averaging 0.69 mm/yr, while relative sea level rise rates varied between -0.47 and 4.1 mm/yr, with a mean of 1.95 mm/yr These findings indicate that mangrove surface elevations are keeping pace with relative sea level rise in certain areas.
Box 1 The SET-MH methodology (continued)
The measuring apparatus features an arm connected to a reference pipe, supporting a small table that allows for the gentle lowering of nine plastic rods onto the substrate surface Measurements of the distance from the surface to the table are taken for each rod in four directions from the pipe, over time intervals spanning months to years These data points are essential for calculating the rate of change in surface elevation relative to the established benchmark.
Markers made of lighter materials like feldspar or kaolin are placed in patches of 50 by 50 cm on the sediment surface After some time, a core sample is extracted from these patches to measure the sediment depth that has accumulated This measurement allows for the calculation of the accretion rate By using the equation surface elevation change (mm/yr) = accretion (mm/yr) + sub-surface change (mm/yr), researchers can determine the rate of sub-surface change by subtracting the accretion rate from the surface elevation change.
Section 4 describes the range of measurements recorded at several different mangrove sites
The methodology for measuring surface elevation changes, initially detailed by Boumans and Day (1993), has been further refined to assess expansion in various sub-surface layers, as documented in studies by Whelan et al (2005) and Cahoon et al (2011) The USGS Surface Elevation Table website provides additional insights, while different versions of the SET-MH apparatus, including the rod SET, have been developed (Cahoon et al., 2002).
Table 2 Mangrove locations where surface elevation change has been measured and where rates of relative sea level rise are available
Surface elevation change (mm/yr)
Relative sea level rise rate (mm/yr) Source
Bay, Florida, US +0.61 to +3.85 3 2.1 McKee, 2011
Twin Cays, Belize -3.7 to +4.1 3.5 2.0 McKee et al., 2007;
Various sites on Kosrae and
Moreton Bay, Australia +1.4 to +5.9 3 2.4 Lovelock et al.,
2011a Several sites in Australia -2.6 to +5.64 3 -0.5 to +4.1 Rogers et al., 2006
Krauss et al (2010) conducted measurements of surface elevation changes over periods of 1.4 to 3 years and 5 to 6.6 years For this study, we focus on the shorter measurement period, as it aligns with concurrent assessments of accretion and sub-surface changes, which are detailed in Sections 3 and 4.
The histogram illustrates the distribution of elevation surplus and deficit values across 15 specific locations, as detailed in Table 2 and Appendix A Each location's mean values are represented, except for Kosrae and Pohnpei in Micronesia, which are analyzed as distinct locations.
Figure 9 plots these surface elevation change measurements against relative sea level rise rates as measured in nearby tide gauges (distance to tide gauges given in Appendix B) Figure
The study revealed considerable variation in surface elevation change across most sites, as detailed in Appendix A Additionally, the analysis found no significant correlation between surface elevation change and relative sea level rise, with a linear regression result of F(1,13) = 2.81 and p = 0.12.
The graph illustrates the relationship between surface elevation change and relative sea level rise across various locations The dashed line represents the scenario where surface elevation change matches the rate of sea level rise Locations above this line indicate that surface elevation is increasing at a rate faster than sea level rise, while those below the line are not keeping pace To enhance clarity, overlapping points with identical relative sea level rise values have been staggered to make the error bars more visible.
Standard errors are unavailable for elevation change measurements in Moreton Bay, Kosrae, Pohnpei, Twin Cays, Rookery Bay, and Naples Bay, Florida, due to the absence of raw data needed for their calculation in the original source papers.
2.2.2 Comparing surface elevation change data with sea level rise data
When comparing surface elevation change data with sea level rise data, several potential issues need to be taken into account, including:
Processes
Surface processes
Surface processes encompass all activities that influence the material reaching the sediment surface and its subsequent fate These processes are categorized into four main types: sedimentation, accretion, erosion, and faunal processes, which are driven by the animals inhabiting mangrove ecosystems.
Sedimentation involves the accumulation of inorganic sediments and organic matter on the soil surface, which can originate from external sources (allochthonous) or be produced within the mangrove ecosystem itself (autochthonous).
Terrigenous material, transported from land by rivers, plays a crucial role in sediment delivery, with regions like the Sundarbans receiving billions of tonnes annually from the Ganges-Brahmaputra-Meghna system (Woodroffe and Davies, 2009) Additionally, smaller rivers contribute substantial amounts of sediment, highlighting the importance of both large and small waterways in sediment transport.
Sediment is transported through creeks during high tides and deposited when these creeks overflow, with some material possibly being carried along the coast through long-shore transport, as observed along the French Guiana coast north of the Amazon delta Additionally, sediment can be advected from offshore due to wave and tidal processes, especially in macrotidal systems found in northern and north-western Australia Furthermore, significant amounts of offshore material may be introduced during major storm or tsunami events.
biologically produced, for example coral sand generated in nearby coral reef ecosystems; or
Solid calcium carbonate can be precipitated from dissolved carbonate in water, a process that contributes to the formation of calcareous muds, as seen in the Great Bahama Bank (Woodroffe and Davies, 2009).
Mangrove sediments can be classified into two primary types: carbonate and minerogenic When sediments are primarily composed of coral sands or precipitated carbonate, the mangroves are categorized as being in a carbonate setting, commonly found in regions like Florida and the Caribbean islands In contrast, when mangrove areas are dominated by mineral sediment inputs, particularly terrigenous material transported by rivers, they are classified as minerogenic settings This distinction is crucial for understanding the ecological characteristics of various mangrove environments.
Australia and south-east Asia
Autochthonous material in mangrove ecosystems consists of leaf litter, dead twigs, branches, roots, and benthic mats on the sediment surface This organic matter is integrated into the soil through bioturbation by organisms such as crabs and can also be buried under new sediment deposits The accumulation of this material is significantly affected by detritivores like crabs, amphipods, and gastropod molluscs, which play a crucial role in consuming leaf litter.
Excessive sedimentation from storms or construction activities can significantly harm mangrove trees, potentially leading to reduced vitality or even mortality, depending on the type and volume of sediment involved (Ellison, 2009) This issue is thoroughly examined in Ellison's 2009 study.
Sedimentation rates in mangroves are primarily influenced by several key factors, including the volume of incoming sediment and locally produced material, the duration of inundation that allows external materials to settle, and the conditions that determine whether particles can settle or are rapidly resuspended Key elements such as flow rates and the flocculation process of particles play a significant role in these dynamics.
Factors affecting the amount of incoming material
Proximity to a source of allochthonous material, such as a river mouth, is a key factor influencing sediment delivery to mangrove areas The transport of this sediment is influenced by water currents and flow pathways, which can vary seasonally and during storm events For instance, research by Saad et al (1999) demonstrated that in Kememan, Terengganu, Malaysia, seasonal variations significantly impacted sedimentation and accretion rates, with a notable increase to 2.6 mm/month (or 31 mm/year) during the monsoon season.
During the monsoon season, river discharge and sediment load significantly increase, leading to suspended sediment concentrations ranging from 50 to 100 ppm, in contrast to just 8 to 20 ppm outside this period Consequently, November and January experience a notable rise in sediment levels, averaging 1.2 mm/month, which translates to 14 mm/year.
Storms and hurricanes, especially their associated storm surges, can significantly transport sediment A notable instance of this occurred after Hurricane Wilma in 2005, when a mangrove area in Shark River, Florida, experienced an elevation increase of 48 mm.
In 2006, an influx of sediment resulted in 77 mm of accretion and 29 mm of shallow subsidence Cahoon (2006) highlights that sediment mobilization is typically influenced by storm intensity, the magnitude of the storm surge, and local geomorphic conditions.
Storms and waves significantly influence sedimentation processes, leading to variable outcomes in sediment deposition and erosion For instance, Hurricane Wilma caused a substantial sediment deposit of 77 mm in the mangroves of Shark River, while nearby Big Sable Creek, sheltered from the surge, received only 1 mm In marsh environments, individual storms can contribute more sediment than is typically accumulated in a year, which is vital for maintaining surface elevations in areas prone to subsidence Although the specific impact of such sediment pulses in mangroves remains unclear, it is likely to be comparable to that observed in marshes.
The amount of incoming autochthonous material is influenced by forest characteristics and local climate, with studies by Saenger and Snedaker (1993) indicating a correlation between litterfall, vegetation height, and latitude Storms can dislodge significant organic material, such as leaves and epiphytic algae, which may be washed onto the substrate However, some of this material can be carried out to sea by tidal currents, as noted by Wolanski et al (1980) in Coral Creek, Queensland, where strong outgoing tides transported leaves into tidal creeks Additionally, the accumulation of litterfall is affected by consumption by detritivores like crabs and amphipods, as well as the rates of microbial decomposition (Middleton and McKee, 2001).
Processes involved in particle settling
Subsurface processes
The literature on mangrove surface elevation change highlights various subsurface processes, which are often referred to differently by different authors This report focuses on three primary groups of these subsurface processes.
the growth and decomposition of mangrove roots and other organic matter;
The swelling and shrinkage of soils, along with the live mangrove roots embedded within them, are significantly influenced by the presence or absence of water and variations in groundwater pressure, a phenomenon known as dilation water storage.
the compaction or compression of soils, due to the sorting of particles or the weight of material above them (sediment, organic matter, or water e.g a storm surge), followed
31 in some cases by the rebound of soils when this weight is removed (e.g after a storm surge)
While the SET-MH methodology allows for the measurement of changes in sub-surface thickness, it does not provide insights into the contributions of various sub-surface processes To enhance our understanding of these processes, we rely on alternative measures, such as analyzing the mass or volume of live and dead root matter in soil samples, or by observing correlations between subsurface volume changes in different soil layers and factors like rainfall or groundwater levels Although our knowledge of these processes is expanding, there is still much to uncover regarding their functioning and their impact on surface elevation changes in mangrove ecosystems.
Mangrove soils exhibit both short-term and long-term responses to environmental factors, such as weather events and tidal fluctuations While processes like root growth, organic matter decomposition, and soil compaction influence surface elevation over many years, the immediate effects of shrink-swell and compression-rebound responses occur within hours to months Understanding these long-term processes is crucial, yet short-term events, like droughts, can significantly impact them; for instance, soil shrinkage during a drought may lower surface elevation, leading to prolonged flooding during tidal events and potentially increasing sedimentation and accretion.
The processes described below and the factors affecting their contribution to surface elevation change are summarized in Table 4
Table 4 The factors affecting sub-surface processes within mangroves, and their effects on surface elevation change These processes and factors are described in more detail in the text
The development of mangrove roots leads to an expansion of soil volume and sub-surface area, as noted by Cahoon et al (2006) and McKee (2011) In contrast, the decomposition of these roots reduces their spatial presence, leading to decreased soil volume and contributing to shallow subsidence.
Research indicates that root inputs significantly influence surface elevation changes in mangrove ecosystems A study in Twin Cays, Belize, revealed that over 50% of the variation in elevation change was attributed to root inputs, with fine roots contributing 42% and coarse roots 10%, while subsidence and accretion accounted for 36% and 2%, respectively (McKee et al., 2007) Additionally, McKee (2011) demonstrated a positive correlation between root mass accumulation and elevation change in mangrove sites across Belize and Florida Furthermore, Cahoon et al (2006) established a similar positive correlation between subsurface root production and elevation change, based on data from 18 mangrove forests with varying soil types.
Factors affecting sub-surface root growth and decomposition
Mangrove vegetation growth and decomposition, including sub-surface roots, are significantly influenced by various factors such as tree health, salinity levels, temperature tolerance, nutrient availability, tree species, and soil aeration Mangroves typically thrive in lower salinities and are sensitive to cold temperatures, while nutrient availability is affected by riverine inputs and regional geology Understanding these factors is essential for comprehending the dynamics of mangrove ecosystems.
Research highlights the critical role of tree health, particularly evident in regions affected by natural disasters like lightning strikes and hurricanes Following Hurricane Mitch in 1998, mangrove areas in Honduras that experienced tree loss due to strong winds faced significant peat collapse A study by Cahoon et al (2003a) documented a notable elevation drop of 11 mm within 18 to 33 months post-hurricane, linking this subsidence to the death and decay of mangrove roots beneath the surface.
Following Hurricane Andrew in 1992, some mangroves in southwest Florida experienced a significant elevation loss of 20 mm due to peat decomposition, as noted in unpublished data by Wanless (Cahoon, 2006) This peat collapse is likely linked to soil organic content, with higher organic levels increasing the risk of collapse after tree mortality (Cahoon et al., 2003a) Additionally, localized elevation losses of up to 60 mm in Everglades National Park can occur from gaps in the mangrove canopy caused by lightning strikes (Whelan, 2005).
Nutrient availability significantly influences root growth and decomposition in mangrove ecosystems A study in Twin Cays, Belize, by McKee et al (2007) revealed that fertilizer application impacted both the direction and rate of surface elevation changes due to its effects on root growth Specifically, the addition of superphosphate increased root accumulation in interior mangrove zones, with fine roots contributing notably to soil volume and elevation variation In contrast, applying nitrogen fertilizer (urea) led to increased root mortality and higher shallow subsidence rates in the same area The impact of nutrients on elevation and subsurface changes varied by mangrove zone; for example, nutrient addition in transition and fringing zones did not yield the same results as in interior zones In the fringing zone, both nitrate and phosphate led to shallow subsidence, while control plots exhibited sub-surface expansion A strong correlation (r = 0.94; p < 0.0001) was found between elevation change and sub-surface change, indicating that subsurface processes are critical in controlling surface elevation dynamics.
(McKee et al., 2007) It is unknown if natural variation in nutrient inputs has similar effects on root growth and decomposition
Root growth plays a crucial role in soil expansion across various soil types, including both organic and mineral soils Research by Cahoon et al (2006) revealed that root growth significantly contributes to soil expansion in 5 out of 7 mineral settings, highlighting its importance even in soils with a higher percentage of non-organic material, compared to 15 out of 17 peat settings.
Decomposition rates in mangrove soils are influenced by soil aeration, as these environments are typically anaerobic, which slows down decomposition However, air can penetrate deeper into the soil through tree roots, during periods of drying, or through the activities of burrowing invertebrates like crabs.
3.2.2 Shrink-swell of soils (dilation water storage)
Dilation water storage is the process by which soils expand or contract in response to changes in water content, leading to a shrink-swell reaction in wetland soils during flooding and drying cycles This phenomenon is influenced by factors such as osmotic pressure changes in mangrove roots and increased groundwater pressure in deeper soil layers, contributing to volume changes Additionally, soils with higher organic content may exhibit more significant dilation effects.
Cahoon and Hensel (2006) indicate that while water availability typically has temporary effects on surface elevation, permanent alterations in water flow—due to drainage, diversion, upstream dam construction, or changes in precipitation patterns linked to climate change—can significantly impact long-term surface elevation trends.
Factors affecting soil swelling and shrinkage
Soils undergo swelling and shrinking due to water presence and groundwater pressure, with changes occurring on various timescales These can range from rapid fluctuations linked to tidal levels to gradual adjustments over months or years in response to long-term climate variations, including those associated with the El Niño Southern Oscillation (ENSO).
Rogers and Saintilan (2008) measured surface elevation repeatedly over a four hour period on