Soil accretionary dynamics, sea-level rise and the survival of wetlands inVenice Lagoon: A field and modelling approach

Over the past century, Venice Lagoon (Italy) has experienced a high rate of wetland loss. To gain an understanding of the factors leading to this loss , from March 1993 until May 1996 the soil accretionary dynamics of these we tlands were studied. Vertical accretion, short term sedimentation, soil ver tical elevation change and horizontal shoreline change were measured at sev eral sites with varying sediment availability and wave energy. Short term s edimentation averaged 3-7 g dry m(-2) day(-1) per site with a maximum of 76 g m(-2) day(-1). The highest values were measured during strong pulsing ev ents, such as storms and river floods, that mobilized and transported suspe nded sediments. Accretion ranged from 2-23 mm yr(-1) and soil elevation cha nge ranged from -32 to 13.8 mm yr(-1). The sites with highest accretion wer e near a river mouth and in an area where strong wave energy resuspended bo ttom sediments that were deposited on the marsh surface. A marsh created wi th dredged spoil had a high rate of elevation loss, probably due mainly to compaction. Shoreline retreat and expansion of tidal channels also occurred at several sites due to high wave energy and a greater tidal prism. The cu rrent rate of elevation gain at some sites was not sufficient to offset rel ative sea-level rise. The results suggest that reduction of wave energy and increasing sediment availability are needed to offset wetland loss in diff erent areas of the lagoon. Using the data collected as part of this project , we developed a wetland elevation model designed to predict the effect of increasing rates of eustatic sea-level rise on wetland sustainability. The advantage of this model, in conjunction with measured short-term rates of s oil elevation change, to determine sustainability is that the model integra tes the effects of long term processes (e.g. compaction and decomposition) and rakes into account feedback mechanisms that affect elevation. Specifica lly, changes in elevation call result in changes in allogenic sediment depo sition, decomposition and autogenic primary production. Model results revea led that, given the Intergovernmental Panel on Climate Change (IPCC) 'best estimate' eustatic sea-level rise scenario of 48 cm in the next 100 years, only one site could maintain its elevation relative to sea level over the n ext century. Under the IPCC 'current conditions' scenario of 15 cm in the n ext 100 years, four of seven sites remained stable. This work demonstrates that more accurate predictions of the future of coastal wetlands with risin g sea level will be obtained with a combination of short-term measurements of accretion and soil elevation change and long-term modelling. (C) 1999 Ac ademic Press.