Seasonal Sediment Dynamics in a Constructed and Natural Tidal Marsh in the Northern Gulf of Mexico

Reduced sediment loading contributes to tidal marsh loss, making evaluations of sediment dynamics useful in assessing marsh resilience to sea-level rise. Tidal marsh construction can offset these losses, but sediment dynamics are less commonly assessed in these systems. Some studies suggest sediment dynamics should develop over time; however, these studies often focus on accumulation at a single time and/or place, without considering sediment composition (i.e., organic vs. inorganic). We compared seasonal sediment dynamics between a natural and 34-year-old constructed tidal marsh with limited hydrologic connectivity. In July 2021, we established permanent sampling points along one tidal creek in each marsh and made monthly measurements of sedimentation, organic matter accumulation, and surface scour for one year. We found that sedimentation and organic matter accumulation were lower in the constructed marsh, while surface scour was similar between sites. Additionally, the relationship between distance from the tidal creek mouth and sedimentation differed between marshes (positive in natural, negative in constructed), as did organic matter accumulation (no relationship in natural, positive in constructed). However, we found that both marshes followed similar seasonal trends in sediment accumulation (highest in summer, lowest in winter). Observed differences in sedimentation between marshes appear to be marsh-specific (due to limited hydrologic connectivity in the constructed marsh), as sedimentation rates between other natural and restored marshes in the region did not differ. Collectively, these results suggest that consideration of sedimentation rates, including spatial and temporal variation, is critical to develop adequate sedimentary dynamics in restored and constructed marshes.