Effects of Vegetation Seasonality on Marsh Sediment Dynamics

Abstract:

In coastal marshes, the interaction between vegetation and sediment is complex and highly variable in time. Emergent and submergent vegetation can enhance sediment deposition by reducing flow and trapping sediment particles. However, as vegetation obstructs flow it can also restrict the sediment supply to the marsh and prevent sediment particles from depositing. Improving our understanding of which vegetation conditions favor sediment deposition in coastal wetlands is critical for successful restoration practices, such as proposed sediment diversions. In this field study, we document how sediment transport and deposition in a deltaic environment vary throughout the flood season in response to seasonal changes in vegetation growth. The study was conducted in Cubit’s Gap (Mississippi River Delta (MRD)), during Spring and Summer 2019. Changing vegetation conditions, flow, and mineral sediment accumulation rates (MSAR) were monitored in a patch that remained vegetated throughout the year, and in an emergent mudflat that was unvegetated at the beginning of the study. We quantified emergent and submerged aquatic vegetation (SAV), used large scale fluorescein and rhodamine dye injections, hydroacoustic instrumentation, and sediment traps. In both the patch and the mudflat, the lowest measured MSAR (4.4 x 10^-4 and 5.1 x 10^-4 g/cm²/day, respectively) occurred when the area was densely populated by vegetation (around June), while the highest measured MSAR (4.0 x 10^-2 and 9.0 x 10^-2 g/cm²/day, respectively) were measured when the presence of vegetation was moderate (around July). Our results have significant implications on the use of vegetation as a restoration device because operational plans can be designed to maximize mineral sediment retention based on certain vegetation conditions and time of the year.