Tidal marsh resilience to sea-level rise depends on the maintenance of surface elevations to mean sea level through both physical (i.e., sedimentation) and biological (i.e., organic matter accumulation) processes. In sediment-deprived marshes, organic matter accumulation and sediment inputs may be insufficient to maintain elevations, making them increasingly vulnerable to loss as sea level continues to rise. Discrete sedimentation events during tropical storms or floods can provide elevation capital for otherwise subsiding marshes, as well as a sediment subsidy that alleviates flooding stresses and promotes primary production. Consequently, storm-derived sediment subsidies represent an increasingly important mechanism by which tidal marshes adapt to climate change. However, biological responses to sedimentation may be non-linear, making it important to identify thresholds beyond which biological feedbacks to elevation are diminished. Through a combination of surface elevation change measurements and greenhouse experiments manipulating sedimentation, we investigated ecogeomorphic contributions to surface elevation in tidal marshes along the northern Gulf of Mexico and the mechanisms controlling biological responses to sedimentation events.
Results/Conclusions
Our 10-year record of surface elevation change in two Louisiana brackish marshes demonstrated that periods of organic matter accumulation were punctuated by pulses of storm-derived sediment input and that biological contributions to elevation were inversely related to surface accretion. Our greenhouse experiments revealed that biological contributions to elevation change vary with sedimentation, as net elevation gains following 5 cm of sediment addition were significantly lower than those in unamended controls (F1,50 = 39.73, p ≤ 0.0001). Furthermore, immediate plant responses to sedimentation included allocation to new shoot production, which preliminary data revealed increased linearly along a sediment-addition gradient (p = 0.02, R2 = 0.30). Together, these results suggest that discrete sedimentation events provide important elevation capital to otherwise subsiding marshes, but that biological contributions to elevation can vary with sediment addition. Identifying the ways in which sedimentation and subsequent biological feedbacks interact to control surface elevation maintenance is important to understand tidal marsh resilience to sea-level rise.