INSIGHTS INTO THE RATES AND MECHANISMS OF MICROBIAL MAT SEDIMENT TRAPPING AND BINDING VIA FIELD AND LAB EXPERIMENTS

Little Ambergris Cay (Turks and Caicos Islands) is a young (2,000 years old) island that has formed emergent land during a period of sea level rise. With oceans rising rapidly many coastal environments are at risk; what factors does Little Ambergris Cay possess that make it resilient to sea level rise? Microbial mats and mangroves cover most of the intertidal area in the interior lagoon of Little Ambergris Cay; we hypothesize that they both play an important role in sediment stabilization in this setting. We designed a series of field and lab experiments to characterize the rates and mechanisms of trapping and binding by the microbial mat communities. Field experiments characterized how sediment trapping and binding were influenced by mat type and environment. Lab experiments were designed to document the timescales of sediment trapping and binding with varying sediment types and bed shear velocity. For the field experiments, ooids were added onto microbial mat, incubated in the field for 24 hours, and then collected to assess how much sediment remained and characterize filament growth. Replicate pieces of polygonal mat, flat mat, and flat mat with Batophora growth were incubated in either a high energy environment (tidal channel) or a low energy environment (interior lagoon protected by mangroves). We compared before and after images to characterize how much of the added sediment remained at the end of the experiment. For the lab experiments, mats were placed in a flume with a known amount of sediment to observe trapping and binding during active sediment transport at different bed shear velocities. Trapping and binding were characterized by measuring the mass of sediment not attached to the mat and by comparing before and after images of the mat. The results of our experiments demonstrate that microbial mats on Little Ambergris Cay can trap and bind a substantial amount of the sand deposited on them on timescales as short as a tidal cycle in the field and the lab. This amount of sediment is much more than is delivered during fair weather conditions, but is comparable to the thickness of sediment delivered by storms. Storm frequency and rapid sediment binding by microbial mats could enable fast but intermittent sediment accumulation, possibly contributing to the island's resiliency against sea-level rise.