Effects of Human Pressures on Ecosystem Structure, Functioning and Services in Salt Marshes

Salt marsh ecosystems are recognized as highly productive, diverse systems which provide key services to society, but are facing an increasing number of multiple human pressures resulting in global losses of these ecosystems. I aimed to assess the effects of human pressures on salt marsh status and functioning, focusing on the perennial Spartina spp. grasses across the Northern Adriatic Sea, using a combination of field observational studies, manipulation of pressures on salt marshes, and wave mesocosm experiments to measure erosion resistance. Major findings include: 1. Macrobenthos diversity and abundance in Spartina spp. patches were influenced by sediment and nutrient variables, and indirectly influenced by Spartina spp. which modified the sdiments via organic enrichment; 2. Spartina spp. below-ground biomass decreased resistance to erosion, with a more pronounced effect in sandy compared to silty sediments; 3. A heat wave that occurred in 2015 synergistically interacted with high nutrient loads to change the vegetation composition in experimentally fertilized plots, with a rapid shift from Spartina spp. grasses to Salicornia spp. succulents, and the heat wave triggered changes in sediment properties that reduced the erosion resistance of salt marsh sediments; 4. After exposure to different inundation levels through a salt marsh transplantation experiment, plant survival (indicated by % live shoot biomass) and % silt were the most significant predictors of erosion resistance. Overall, biomass and sediment properties were the main factors influencing lateral resistance to erosion in our experiments. The research in this thesis fills important knowledge gaps regarding the status of salt marshes and their functioning across lagoons of the Northern Adriatic Sea. These results help to consolidate the evidence of the important role of salt marsh vegetation in coastal protection, and demonstrate the conditions under which erosion resistance in salt marshes is most effective.