The influence of eutrophication and connected accumulation of wrack deposits on natural saltmarshes in the Venice Lagoon

The Venice Lagoon is under intense human pressure: altered hydrodynamics, fishery, boat traffic, habitat transformation, and agriculture are only some of them. As a result, the salt marshes are eroding at a concerning rate, from 170 km2 in 1901 to 47 km2 left in 2003. To mitigate erosion, a comprehensive complete knowledge about the processes involved is needed. The stability of marshland gets influenced by the plants growing on it. In particular, the reduction of below-ground biomass can lead to the destabilization of marsh sediments, with the marsh edge more likely to fall into the lagoon water and consequently eroding. The stabilizing effect of the below ground biomass can be reduced by either the reduction of plant biomass in general, or by a disbalance in the below-ground/above-ground biomass ratio. These two cases are investigated here by looking at the effect on marsh plants of wrack deposits, potentially shading ground vegetation, and at the effect of high nutrient concentration in water, potentially promoting an above-below ground biomass allocation disbalance in marsh plants. This thesis partly replicates the evaluation of marsh erosion as a consequence of a nutrient surplus as carried out in the USA. The experiments are relevant to understanding the connection between the quality of lagoon water and the erosion of marshland. Fertilization of plants in agriculture led to a concentration in the 1990s ten times higher than now present in the lagoon water. This connection however has never been tested in the Venice Lagoon. The first part of the thesis is dedicated to the influence of nutrients (nitrogen and phosphorus) on marsh plant growth. To determine if plants invest less in below-ground mass and more in above-ground growth under high nutrient conditions in water, parameters like biomass weight, root length and width, leave length and width, and weight of leave and roots were measured and compared after treating halophyte plants with nutrient-infused lagoon water for five months. Target nutrient concentrations in the treatment were estimated using historical data of the nutrient peak in the 1990s. The second part of the thesis examines the influence that coverage with organic wrack deposits has on marsh plants. Vegetated marsh areas were covered with layers of three different wrack densities, and the implicated plants were compared after one, two, and three months using the same parameters mentioned above. While marsh plants showed unclear effects connected to the nutrient treatment, the plants that were covered in algal wrack decreased significantly in coverage and biomass over time with a clear connection to the density of wrack cover.