Grain-Trapping by Microbial Mats—A Key Mechanism of Sediment Accumulation in Mangroves?
Abstract
In tropical and subtropical regions, mangroves play an important role in protecting coastlines by creating emergent land via trapping of fine sediment and the accumulation of mangrove peat at rates that can outpace sea-level rise. An additional but little studied mode of sediment accumulation that can occur in coastal regions is grain-trapping by microbial mats—layered communities of microorganisms, often stratified by metabolism, which experimental studies have demonstrated can rapidly sequester sand- to pebble-size grains. Although microbial mats commonly co-occur with mangroves, previous studies of grain-trapping by microbial mats have primarily focused on applications as biosignatures for the ancient rock record, while the potential contributions of microbial mats to sediment accumulation and surface elevation in modern environments have not been evaluated. We hypothesized that microbial mats can augment grain-trapping effects of mangroves, promoting the accumulation of coarse sediment. To test this hypothesis, we designed a series of field experiments to characterize the mechanism(s) and pace of grain-trapping by modern microbial mats on Little Ambergris Cay in the Turks and Caicos Islands. We also examined the shallow stratigraphic record for evidence of microbial mat grain-trapping via sedimentology and micro computed tomography (microCT) imaging; characterized the microbial community using 16S amplicon sequencing; and measured critical shear stresses of microbially-stabilized sediment surfaces. Experimental results demonstrated that microbial mats were capable of trapping sediment over timescales as short as 24 hours, with some sand-size grains fully surrounded by cyanobacterial filaments after several days. Electron microscopy suggested that the efficacy of microbial grain-trapping was affected by the surface roughness of the particles. Microbial mats on Little Ambergris Cay increased critical shear stress by a factor of 10-20 over non-microbially-stabilized sediment surfaces and stratigraphic observations were consistent with microbial mats having played a role in the accumulation of sand-size sediment on Little Ambergris Cay. These findings suggest that microbial mats could contribute substantially to the stabilization of coarse sediment in mangrove systems.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMEP23B..03T
- Keywords:
-
- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 1625 Geomorphology and weathering;
- GLOBAL CHANGE;
- 5210 Planetary atmospheres;
- clouds;
- and hazes;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 5415 Erosion and weathering;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS