Increasing nutrient inputs over the last 500 years in an Italian low-impacted seagrass meadow.

Posidonia oceanica is a seagrass endemic to the Mediterranean and it has been widely used as a bioindicator. We studied the layers of a 500-year-old matte using a multiproxy approach (δ13C, δ15N, 14C and C and N concentrations in seagrass debris) in order to evaluate the potential of P. oceanica as a long-term environmental indicator of N pollution and CO2 emissions. From 1581 to 1800, accumulation rate was ca. 0.35 cm year-1, while in the last 100 years it has amounted to ca. 0.51 cm year-1. We observed increasing δ15N values with height in the vertical matte profile, indicating an increase in anthropogenic organic N inputs over time. In contrast, no clear trend in the δ13C values was observed. This study reconstructs the long-term impact of human activities on a seagrass meadow located off the Italian coast, yielding long-term background information that can help managers to implement efficient plans.