No abstract

In situ plant production is often assumed to be the major contributor to organic matter (OM) accumulation and vertical accretion in tidal marshes. Here, we evaluate the contribution of mud-associated OM in salt and brackish marshes in Louisiana. Based on 14 soil cores, the OM content of the mud fraction-i.e., any material smaller than 64 μm-was 17% AE 7% for the salt marshes and 28% AE 14% for the brackish marshes. This remains nearly uniform over the top 35 cm depth, suggesting that this material is deposited contemporaneously with the mud. The dry bulk density of the mud (300-450 kg m −3) is also much lower than what was estimated using a previously proposed two-constituent mixing model (1990 kg m −3). To reconcile this discrepancy, we developed a modified mixing model that includes mud OM and differentiates sand as a separate constituent with its high dry bulk density. The model estimates that mud contributes to~60% of the total marsh vertical accretion in Louisiana, considerably higher than the~14% estimated with the two-constituent mixing model. The result, which is a direct consequence of the relatively high porosity of mud, highlights that mud deposition is crucial for the accretion of microtidal marshes. Further, the model estimates that the mud OM constitutes~60% of the total soil OM, emphasizing that in situ plant production is not the only-and, in minerogenic marshes, not the major-contributor to OM accumulation.

Marsh edge retreat by wave erosion, an ubiquitous process along estuaries, could affect vegetation dynamics in ways that differ from well-established elevation-driven interactions. Along the marshes of Delaware Bay (USA) we show that species composition from marsh edge to interior is driven by gradients in wave stress, bed elevation, and sediment deposition. At the marsh edge, large wave stress allows only short-statured species. Approximately 17m landward, decreasing wave stress and increasing deposition cause the formation of a ridge. There, high marsh fugitive and shrub species prevails. Both the marsh edge and the ridge retreat synchronously by several meters per year causing wave energy and deposition to change rapidly. Yet, the whole ecogeomorphologic profile translates landward in a dynamic equilibrium, where the low marsh replaces the high marsh ridge community and the high marsh ridge community replaces the mid-marsh grasses on the marsh plain. A plant competition model shows that the disturbances associated with sediment deposition are necessary for the high marsh species to outcompete the mid-marsh grasses during rapid transgression. Marsh retreat creates a moving framework of physical gradients and disturbances that promote the co-existence of over ten different species adjacent to the marsh edge in an otherwise species-poor landscape.

Ecosystem connectivity tends to increase the resilience and function of ecosystems responding to stressors. Coastal ecosystems sequester disproportionately large amounts of carbon, but rapid exchange of water, nutrients, and sediment makes them vulnerable to sea level rise and coastal erosion. Individual components of the coastal landscape (i.e., marsh, forest, bay) have contrasting responses to sea level rise, making it difficult to forecast the response of the integrated coastal carbon sink. Here we couple a spatially-explicit geomorphic model with a point-based carbon accumulation model, and show that landscape connectivity, in-situ carbon accumulation rates, and the size of the landscape-scale coastal carbon stock all peak at intermediate sea level rise rates despite divergent responses of individual components. Progressive loss of forest biomass under increasing sea level rise leads to a shift from a system dominated by forest biomass carbon towards one dominated by marsh soil carbon that is maintained by substantial recycling of organic carbon between marshes and bays. These results suggest that climate change strengthens connectivity between adjacent coastal ecosystems, but with tradeoffs that include a shift towards more labile carbon, smaller marsh and forest extents, and the accumulation of carbon in portions of the landscape more vulnerable to sea level rise and erosion.