Historical transformations of the saltmarshes in the six sub-basins of the Marano and Grado Lagoon (northern Adriatic Sea) were analyzed using aerial photographs (1954, 1990, 2006) and the support of historical maps and topographic surveys in case studies. Analysis of the 2006 set of aerial photographs enabled the definition of the present extent and distribution of the saltmarshes inside the lagoon (760 ha), with a total reduction in saltmarsh area of 16% (144 ha) compared to 1954. Direct human actions played a significant role in the budget, resulting in a negative balance between saltmarsh gains and losses; the total loss due to land reclamation and dredging during this period amounted to 126 ha.
After excluding the total loss due to direct human interventions, different erosional and depositional marsh types were recognized and associated with different forcing factors, based on morphological and geographical evidence. Over the 52-year period marshes were lost due to: a) drowning – the combined effects of eustatism, regional subsidence and autocompaction (102 ha); b) edge-retreat by wind wave attack (34 ha); c) erosion by vessel-generated waves (37 ha); and d) coastal dynamics and inlet migration (5.7 ha). Conversely, marshes gained in area due to: a) fluvial input (63 ha); b) tidal input (27 ha); c) paralagoonal deposition (45 ha); d) the re-opening of abandoned fish farms (18 ha); and e) the dumping of dredged material (8 ha). The relative importance of erosional versus depositional forcing factors, as well as the different rates of marsh gain and loss recorded in each lagoon basin, demonstrate that local and short-term forcing factors can obliterate or compensate the long-term ones, especially the relative sea-level rise. A test of the integrated sediment budget carried out on one third of the total lagoon (Lignano and S. Andrea basins) through a bathymetric comparison between datasets from 1964 and 2009, provided a better understanding of the relationship between the evolution of the saltmarshes and the submerged part of the lagoon. Conservation or slight expansion of the marshes inside these basins were found to be linked to an overall positive sediment budget of 61,000 m3/y. Nevertheless, significant morphological changes occurred in the submerged basin, which is affected by sustained deposition along the inner margins due to sediment supplies, by an overall erosion of tidal and subtidal flats far from the tributaries, and by an important infilling of the channels. The analyzed data, along with information available for the Venice Lagoon, highlights how the fate of open-water lagoons is to flatten whilst submerging because of the strong influence of wind waves, which tend to transform the lagoon into a marine embayment. This transgressive condition reduces, if not negates, the compensative effect of the sedimentation rate on wind-wave-induced shear stress excess, since supplies seem to contribute primarily to the morphological accommodation.
