Biophysical controls on accretion and elevation change in Caribbean mangrove ecosystems

Habitat stability of coastal ecosystems, such as marshes and mangroves, depends on maintenance of soil elevations relative to sea level. Many such systems are characterized by limited mineral sedimentation and/or rapid subsidence and are consequently dependent upon accumulation of organic matter to maintain elevations. However, little field information exists regarding the contribution of specific biological processes to vertical accretion and elevation change. This study used biogenic mangrove systems in carbonate settings in Belize (BZ) and southwest Florida (FL) to examine biophysical controls on elevation change. Rates of elevation change, vertical accretion, benthic mat formation, and belowground root accumulation were measured in fringe, basin, scrub, and dwarf forest types plus a restored forest. Elevation change rates (mm yr⁻¹) measured with Surface Elevation Tables varied widely: BZ-Dwarf (−3.7), BZ-Scrub (−1.1), FL-Fringe (0.6), FL-Basin (2.1), BZ-Fringe (4.1), and FL-Restored (9.9). Root mass accumulation varied across sites (82–739 g m⁻² yr⁻¹) and was positively correlated with elevation change. Root volumetric contribution to vertical change (mm yr⁻¹) was lowest in BZ-Dwarf (1.2) and FL-Fringe (2.4), intermediate in FL-Basin (4.1) and BZ-Scrub (4.3), and highest in BZ-Fringe (8.8) and FL-Restored (11.8) sites. Surface growth of turf-forming algae, microbial mats, or accumulation of leaf litter and detritus also made significant contributions to vertical accretion. Turf algal mats in fringe and scrub forests accreted faster (2.7 mm yr⁻¹) than leaf litter mats in basin forests (1.9 mm yr⁻¹), but similarly to microbial mats in dwarf forests (2.1 mm yr⁻¹). Surface accretion of mineral material accounted for only 0.2–3.3% of total vertical change. Those sites with high root contributions and/or rapid growth of living mats exhibited an elevation surplus (+2 to +8 mm yr⁻¹), whereas those with low root inputs and low (or non-living) mat accumulation showed an elevation deficit (−1 to −5.7 mm yr⁻¹). This study indicates that biotic processes of root production and benthic mat formation are important controls on accretion and elevation change in mangrove ecosystems common to the Caribbean Region. Quantification of specific biological controls on elevation provides better insight into how sustainability of such systems might be influenced by global (e.g., climate, atmospheric CO₂) and local (e.g., nutrients, disturbance) factors affecting organic matter accumulation, in addition to relative sea-level rise.