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Visual Abstract: Mangrove Sedimentation and Response to Relative Sea-Level Rise

In this visual abstract, explore sedimentation in mangrove forests and its role in mangrove response to relative sea-level rise. The technical paper is scheduled for publication in the Annual Review of Marine Science.

Karen McKee

on 21 December 2017

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Transcript of Visual Abstract: Mangrove Sedimentation and Response to Relative Sea-Level Rise

Image source: NASA
River Deltas
Mangroves occur in a variety of geomorphological settings, which determine hydrodynamic conditions, flux of sediments, and soil characteristics—all influencing the structure and function of forests.
Mekong Delta
Deltas are built from sediments carried by rivers and deposited along a continental shelf or other shallow water body. Tide, wave, and riverine hydrodynamic processes interact to shape deltas. The Mekong River, which carries an annual sediment load of 160 million tonnes, has formed a megadelta in southern Vietnam. There are extensive tide-dominated mangroves in the Can Gio region; other forests on the open coast experience high wave energy, which has created beach ridges in the past.
Northern Australia
An estuary is a semi-enclosed body of water where seawater mixes with freshwater. A series of macrotidal estuaries occurs along the northern/northwestern coasts of Australia. Examples include Darwin Harbour and the Alligator Rivers region. Organic-rich mangrove muds overlain by marine muddy sands record landward retreat of mangroves. Extensive mangrove forests developed here when sea level stabilized about 7,000 years ago, a time known as the ‘big swamp’ phase.
A lagoon is a shallow body of water separated from the ocean by a barrier of land, islands, or reef. A typical setting consists of a sandy barrier along a wave-dominated coast with mangroves developing in a sheltered back-barrier lagoon. An example is the Tabasco region in Mexico where delta distributary channels have built levees, which modified sedimentation patterns. This area contains tidally-flushed mangroves on accreting mudflats as well as in inter-distributary basins with peat buildup.
Tabasco, Mexico
Barrier Reef, Belize
In carbonate reef settings where terrigenous sediment is lacking or minimal, mangroves accumulate peat composed of decaying mangrove roots, leaves, and other organic remains. In the Belize barrier reef system, mangrove islands are underlain by thick sequences of peat up to ten metres or more. In the adjacent lagoon, mangrove peats buried beneath calcareous sands indicate that the current islands are remnants of a more extensive system that existed in the past.
Image source: Visible Earth, NASA
Image source: Visible Earth, NASA
Image source: USGS Earth Explorer
Image source: USGS Earth Explorer
At the macroscale, involving the entire geomorphological system (delta, estuary), climate and relative sea-level rise are dominant influences on mangrove structure and function.
Sea-Level Rise
At the mesoscale, are mangrove forest types associated with distinctive hydrodynamics that influence species composition and primary productivity.
At the microscale, are a range of geomorphic and biologic surface and subsurface processes affecting sediment accretion and/or peat formation.
Morphodynamics and Scale
The process of sedimentation in mangrove forests is associated with accommodation space, defined as "the space available for sediment accumulation" (Jervy 1988; http://bit.ly/1P5EDqu).

Accommodation space is a function of three processes: Sea-level change, land movement, and sediment accretion. Rising sea level and land subsidence increase accommodation space, and sediment accretion decreases it (see animation).
Sea-level Change
Land Movement
Biogenic Accretion
Ecological accommodation is "the capacity of organisms to produce and accumulate sediments" (Pomar & Kendall 2008; http://bit.ly/1CCmSIY), e.g., peat formed by mangroves and sand by calcareous algae. Biogenic accretion is an important response countering sea-level rise and subsidence in some mangrove forests.
Minerogenic Accretion
Calcareous sand
Seagrass litter
Mangrove peat
River alluvium
Sediments deposited in mangrove forests may be derived from organic or mineral sources and of autochthonous (i.e., formed in place) or allochthonous (i.e., formed elsewhere) origin. Where large rivers supply abundant terrigenous sediment, allochthonous mineral sediments can accumulate rapidly in mudbanks, which provide habitats for opportunistic colonization by mangroves. In more complex systems, sediments are redistributed by tides. In sediment-poor settings, mangroves, seagrasses, and algae may contribute to accumulation of autochthonous material.
Mangrove Sediments
Accommodation Space
Time Scale
Sedimentation Measurement Methods
The Surface Elevation Table-Marker Horizon (SET-MH) system allows simultaneous measurement of sediment accretion, elevation change, and shallow subsidence at a fine resolution (millimeters). If accretion equals relative sea-level rise (eustatic rise plus land movement), mangroves can sustain their positions in the intertidal zone.
Measuring Elevation Change in Relation to Sea-Level Rise
Short-term rates of sediment accretion can be determined by placing a marker horizon (feldspar clay, white sand) on the soil surface and measuring the deposited sediment thickness over months or years.
Summary & Conclusions
Mangroves are mostly confined to the upper portion of the tidal frame where accommodation space fluctuates with changes in sea level, land subsidence, and sedimentation.
The paleo-environmental record shows that mangroves can accommodate a range of sea-level changes by accumulating both inorganic and organic sediments.
However, human activities have disrupted the primary sedimentation processes in many locations, which has impaired the capacity of modern-day mangroves to accommodate relative sea-level rise.
Clearcutting of mangroves and dredging increase subsidence and erosion
Seawalls block movement of water and sediment
If hydrodynamic and sedimentary requirements are met, restoration and rehabilitation efforts can reverse declines in mangrove forest areas and improve the resilience of tropical coastlines to sea-level rise.
Woodroffe, C.D., K. Rogers, K.L. McKee, C.E. Lovelock, I.A. Mendelssohn, and N. Saintilan. 2016. Mangrove sedimentation and response to relative sea-level rise. Annu. Rev. Mar. Sci. 8: 243-266; doi: 10.1146/annurev-marine-122414-034025.

Read the technical paper for more information:
Atomic bomb testing and nuclear accidents (Chernobyl, Fukushima) have created anthropogenic radioisotopic markers in soil cores that can be used to measure sedimentation rates over the past 60 to 70 years.
Radiocarbon dating of mangrove wood or fossil molluscs found at depth in soil cores enable estimates of sediment accretion over thousands of years.
Visual abstract prepared by K.L. McKee
For more information about creating science visualizations: http://thescientistvideographer.com/wordpress
This review focuses on sediment accumulation and hydrodynamics in mangrove forests to better understand the response of these intertidal ecosystems to relative sea-level rise.
Mangrove forests occupy the upper intertidal zone along tropical and subtropical coastlines.
Mangrove Sedimentation and Response to Relative Sea-Level Rise
Image source: Visible Earth, NASA
River alluvium, composed of inorganic particles (e.g., silt, clay), is carried from an upstream source and deposited in a mangrove forest.
Calcareous algae (e.g.,
spp.) commonly found in or near mangrove ecosystems are major producers of sand throughout the world.
Seagrass (e.g.,
Thalassia testudinum
) litter can be carried by tides from adjacent beds and deposited in mangroves.
Peat formed from decaying remains of mangrove roots, leaves, and wood is an important sedimentary process in many mangrove forests.
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