THE INFLUENCE OF MACROINVERTEBRATES (FIDDLER CRABS AND PULMONATE SNAILS) ON MANGROVE PEAT TAPHONOMY AND PARTICLE SIZE, KEY LARGO, FLORIDA

Characterization of modern mangrove peat provides insight into peat taphonomy with implications for ancient wetland paleoecology and nutrient cycling in coastal wetlands. In the taphonomically active zone (TAZ), peat particle size decreases with increased decomposition and organism-peat interactions, such as detritivory and bioturbation. We surveyed macroinvertebrates and collected peat cores (15 cm) to investigate peat particle size distribution in water-logged basin and tidally influenced fringe sites in a Rhizophora (red mangrove) dominated peat swamp near Key Largo, Florida. In a m² quadrat, we estimated Uca (fiddler crabs) density by counting burrows and Melampus coffeus (detritivorous snails) density by point counts. At the fringe site, we excavated 8 peat balls from Uca burrows and sieved 3 to determine the effect of Uca burrowing on peat particle size. We found no Uca burrows at the basin site. From each core, we sieved peat samples from 0 cm, 5 cm, and 10 cm depths. We dried sieve-samples for 24 hours at 50°C and weighed them to determine peat particle size distributions at each site and in peat balls.

Macroinvertebrates varied between sites. The fringe site had abundant Uca (15 burrows/m²) and M. coffeus (61 snails/m²), both air-breathers. In contrast, Uca rarely occurred in basin sites, and flooding limited the access of M. coffeus (24 snails/m²) to the peat substrate. Although we did not survey meiofauna, basin peat contains terrestrial arthropod parts and few foraminifera whereas fringe peat commonly contains foraminifera and few terrestrial arthropod parts. Fecal pellets may be used to survey meiofauna. Basin sites have thicker leaf mats than fringe sites (basin ~ 24 stacked leaves, fringe <1). Thick leaf mats suggest a lower rate of decomposition in basin than in fringe sites; both Uca and M. coffeus contribute to the loss of the leaf mat at fringe sites. Below the leaf mat, peat from both sites has a similar particle size distribution: 89% of the particles were framework (particles with one dimension > 106 mm); 11% were matrix (particles with all dimensions < 106 mm). In contrast, 98% of particles in peat balls from Uca burrows were matrix. Peat from 5 and 10 cm depths had similar particle size distributions, suggesting that this peat has exited the TAZ. Thicker leaf mats may accumulate in the absence of fiddler crabs. Uca does not eat peat, but Uca burrowing increases the decomposition rate of peat.