Sediment deposition during tropical storms or floods provides direct elevation capital to tidal marshes that may enhance marsh resilience to sea-level rise. These sediment subsidies may also promote subsequent biological contributions to elevation by ameliorating flooding stress and providing nutrient amendments that stimulate plant growth. These plant responses may vary nonlinearly along a sediment addition gradient in accordance with the sediment subsidy hypothesis, where maximum plant production is expected at some modest to moderate level of sediment addition. In this study, we hypothesized that the sediment subsidy hypothesis extends to elevation responses, with a modal post-sedimentation elevation response that peaks at some intermediate sediment depth and declines with increasing sediment addition. To test this hypothesis, we established a continuous sediment addition gradient from 0 – 150 mm in a brackish marsh in southeastern Louisiana, U.S.A. Surface elevation responses were measured at 460 locations along this gradient using a modified rod-surface elevation table (rSET), yielding an unprecedented, fine-scale examination of responses to sediment deposition. Along this sedimentation gradient, we measured a suite of biological and physical variables in conjunction with elevation responses.
Results/Conclusions
In our study, elevation responses did not follow the sediment-subsidy hypothesis as we had expected. Instead, there was a continuous decline in post-sedimentation elevation gains with increasing sediment depth and a threshold slope change at approximately 34 mm, beyond which the decreasing gains in elevation slowed. While a threshold response was anticipated (motivating our methodological approach), the nature of the change in slope along the sediment addition gradient was opposite what we expected, with the most positive elevation responses observed at no or low sediment addition levels and the greatest rate of decline in elevation gains occurring between 0 – 34 mm of added sediment. Examination of the effects of sediment addition on soil physicochemistry and plant production suggest a suite of mechanisms regulating the feedbacks that contribute to elevation maintenance. Identifying these mechanisms and quantifying their contributions to elevation change has important implications for understanding tidal marsh recovery from storm events and resilience to sea-level rise.