Secular diachronic analysis of coastal marshes and lagoons evolution: Study case of the Po river delta (Italy)

Coastal lagoons and salt marshes are rapidly changing under the influence of sea level rise and human induced changes. Within this context, the proposed study describes the evolution of the lagoons of the Po Delta (Italy) and the historical transformations of the salt marshes using historical maps and aerial data from 1892 to 2018. The methodology applied provides a crucial quantification of coastal lagoon and salt marsh evolution. Image analysis shows that most of the lagoons were formed between 1892 and 1934, while the most recent are developed between 1978 and 1988. Lagoons reached their actual shape and dimensions by 1955. Lagoons present different morphological characteristics with the lagoon of Caleri having a higher morphological diversity due to the presence of salt marshes and a complex hydraulic network. Since 1988, fringing salt marshes were the dominant morphology, then marsh morphology switched to a fringing, isolated and channelized morphology. Over the last 120 years, the Po Delta lagoons have experienced a high rate of erosion and environmental degradation. Our analysis suggests that three main phases are present in coastal lagoon marsh evolution that explain the development. In the last years, those lagoons experienced a progressive reduction of fringing and isolated salt marshes. The first one (1892–1934), characterized by high fluvial sediment input and a fast seaward progression of the river mouths, corresponds to the maximum salt marsh development. The second phase (1934–1978), characterized by a negative sediment budget and human activity induced alterations (subsidence), presents a small increase of the lagoon extension associated with a reduction of the salt marsh. Finally, the third phase (after 1978), characterized by a low sedimentary budget and high human control, is characterized by a stabilization of the lagoon extension and a drastic reduction of the salt marshes.