Caribbean mangroves adjust to rising sea level through biotic controls on change in soil elevation

Abstract

Aim The long-term stability of coastal ecosystems such as mangroves and salt marshes depends upon the maintenance of soil elevations within the intertidal habitat as sea level changes. We examined the rates and processes of peat formation by mangroves of the Caribbean Region to better understand biological controls on habitat stability.Location Mangrove-dominated islands on the Caribbean coasts of Belize, Honduras and Panama were selected as study sites.Methods Biological processes controlling mangrove peat formation were manipulated (in Belize) by the addition of nutrients (nitrogen or phosphorus) to Rhizophora mangle (red mangrove), and the effects on the dynamics of soil elevation were determined over a 3-year period using rod surface elevation tables (RSET) and marker horizons. Peat composition and geological accretion rates were determined at all sites using radiocarbon-dated cores. ResultsThe addition of nutrients to mangroves caused significant changes in rates of mangrove root accumulation, which influenced both the rate and direction of change in elevation. Areas with low root input lost elevation and those with high rates gained elevation. These findings were consistent with peat analyses at multiple Caribbean sites showing that deposits (up to 10 m in depth) were composed primarily of mangrove root matter. Comparison of radiocarbon-dated cores at the study sites with a sea-level curve for the western Atlantic indicated a tight coupling between peat building in Caribbean mangroves and sea-level rise over the Holocene.Main conclusions Mangroves common to the Caribbean region have adjusted to changing sea level mainly through subsurface accumulation of refractory mangrove roots. Without root and other organic inputs, submergence of these tidal forests is inevitable due to peat decomposition, physical compaction and eustatic sea-level rise. These findings have relevance for predicting the effects of sea-level rise and biophysical processes on tropical mangrove ecosystems.

A global standard for monitoring coastal wetland vulnerability to accelerated sea-level rise

“…Although the extent of coastal wetlands has historically tracked rising and falling sea levels (for example, refs [13][14][15], recent transformation of the surrounding terrestrial landscape (notably by agriculture and urbanization) has often introduced embankments that constrain lateral landward wetland migration, so that rising sea levels could significantly reduce coastal habitat area 16 . Under these circumstances, a key determinant of coastal wetland vulnerability to SLR is whether the surface elevation in the intertidal zone can keep pace with rising sea level.…”

“…The RSET method has been useful not only for documenting trends in surface elevation change in specific wetlands 40 , but also for comparing rates of elevation change among hydrogeomorphic zones within a site (for example, a delta 41 ), differentiating between sites dominated by surface processes 40 versus subsurface processes 14 , documenting high variability in sediment deposition from singular storm events 44 , capturing rapid peat collapse from small-scale (for example, lightning strikes) and large-scale (for example, hurricanes) disturbances 45,46 , measuring the effects of elevated atmospheric CO 2 concentrations on surface elevation change 47 , and disentangling complex interactions among herbivory, biomass production and surface elevation change 48 . RSET data have also contributed to more applied research assessing the impacts of management practices on site-specific vulnerability to SLR, such as prescribed burning 49 , sedimentation from point-source coastal management interventions 50 , water diversion 51 , fence construction to enhance sediment deposition 52 and thin-layer deposition of dredged sediment 53 .…”

Mangroves Response to Climate Change: A Review of Recent Findings on Mangrove Extension and Distribution

“…This generally holds true for the environmental settings that dominated by rivers, tides with abundant sediment supply and waves on prograding coasts (Woodroffe 1992). However, land uplift or subsidence, groundwater influx, vegetation and soil processes, and whether the coast is prograding or eroding also play important roles in determining the extent of the landward migration (Krauss et al 2003, McKee et al 2007, Lovelock et al 2015.…”

Can Mangroves Help Combat Sea Level Rise Through Sediment Accretion and Accumulation?

“…These suspended materials settle down in the forest floor during low tide, while the velocity of waterflow is very low [2,6]. Besides, the unique and complex structure of mangrove trees having prop roots, knee roots and pneumatophore reduces the velocity of tidal flow and thus trap sediments which eventually plays key role in land building process [2,[8][9][10][11]. Pristine and undisturbed mangroves are able to trap and retain more than 80% of tidal sediments [6].…”

“…Besides, sea level rise is a severe threat for the existence of many mangroves in the coming decades [15,16]. The existence of mangroves in future will mostly depend on their accretion and retention/deposition capability of sediments to combat upcoming prediction of sea level rise [8,15,17,18]. Due to global warming, the projected rate of global sea level rise (GSLR) is expected to be 1.8 to 5.9 mm yr -1 by 2100 [19].…”