Investigating Environmental Effects on Sediment Trapping and Binding by Microbial Mats

Polygonal microbial mats are found in in the intertidal zone of Little Ambergris Cay (LAC) in the Turks and Caicos Islands, a <2000-yr-old island composed mainly of ooid sand. They occur in locations both with and without mangroves. Mangroves play an important role in stabilizing and accumulating fine sediments, which contributes to protecting islands and coastlines as sea levels rise. Microbial mats can stabilize coarser, sand-sized sediment through trapping and binding, though it is not known how this contributes to island formation. We hypothesized that trapping and binding of sediment by microbial mats on LAC has contributed to the island’s formation during a period of net sea-level rise. We designed lab experiments to determine how quickly polygonal mats can trap and bind sediment in different experimental conditions: subtidal (fully subaqueous) without wave agitation and subtidal with wave agitation. Conditions were simulated in two different aquaria; mats were exposed to a 12-hour light cycle and submerged in artificial seawater. Ooid sand was added to the surface of replicate mat samples; mats were then incubated undisturbed for 8 hours. Ooid trapping and binding was characterized by imaging each mat hourly with a stereoscope for the next twelve hours, and thrice more in coming days. Each mat image was point counted to quantify how the proportion of ooids bound by cyanobacterial filaments increased over time. We found that in both environments, ooid loss occurred most rapidly in the first 5 hours of the experiment, and that filament growth occurred throughout the experiment. By the end of the experiment, there was more filament growth in the wave-agitated condition than the non-agitated condition. As the consequences of climate change continue to drive sea- level rise, microbial mat communities like those on LAC may be able to help protect islands by accumulating and stabilizing sediment rapidly.