Response scenarios for the deltaic plain of the Rhone in the face of an acceleration in the rate of sea-level rise with special attention to Salicornia-type environments

One of the most critical problems facing many deltaic wetlands is a high rate of relative sea-level rise due to a combination of eustatic sea-level rise and local subsidence. Within the Rhone delta, the main source of mineral input to soil formation is from die river, due to the low tidal range and the presence of a continuous sea wall. We carried out field and modeling studies to assess the present environmental status and future conditions of the more stressed sites, i.e., Salicornia-type marshes with a shallow, hypersaline groundwater. The impacts of management practices are considered by comparing impounded areas with riverine areas connected to the Rhone River. Analysis of vegetation transects showed differences between mean soil elevation of Arthrocnemum fruticosum (+31.2 cm), Arthrocnemum glaucum (+26.5 cm), bare soil (+16.2 cm), and permanently flooded soil (-12.4 cm). Aboveground and belowground production showed that root:shoot ratio for A. fruticosum and A. glaucum was 2.9 and 1.1, respectively, indicating more stressful environmental conditions for A. glaucum with a higher soil salinity and lack of soil drainage. The annual leaf litter production rate of the two species is 30 times higher than annual stem litter production, but with a higher long-term decomposition rate associated with leaves. We developed a wetland elevation model designed to predict the effect of increasing rates of sea-level rise on wetland elevation and Salicornia production. The model takes into account feedback mechanisms between soil elevation and river mineral input, and primary production. In marshes still connected to the river, mineral input decreased quickly when elevation was over 21 cm. Under current sea-level rise conditions, the annual amount of riverine mineral input needed to maintain the elevation of the study marshes is between 3,000 and 5,000 g m(-2) yr(-1). Simulations showed that under the Intergovernmental Panel on Climate Change best estimate sea-level rise scenario, a mineral input of 6,040 g m(-2) yr(-1) is needed to maintain marsh elevation. The medium term response capacity of the Rhone deltaic plain with rising sea level depends mainly on the possibility of supplying sediment from the river to the delta, even though the Rhone Delta front is wave dominated. Within coastal impounded marshes, isolated from the river, the sediment supply is very low (10 to 50 g m(-2) yr(-1)), and an increase of sea-level rise would increase the flooding duration and dramatically reduce vegetation biomass. New wetland management options involving river input are discussed for a long-term sustainability of low coastal Mediterranean wetlands.