Monitoring the extension of seagrass meadows over time is of primary importance for the surveillance of marine coastal biodiversity. Here, we analyse the evolution of the meadow of Cymodocea nodosa in the Gulf of Tigullio, a coastal tract of naturalistic interest but subjected to high anthropogenic pressure. Historical maps at a scale 1:25,000 of C. nodosa meadow drawn in 1986, 1991 and 2001 were processed with GIS (Geographical Information System), using overlay vector methods. Diachronic analyses allowed the measurements of temporal changes, in term of percentage gain or loss of meadow extension, through concordance and discordance maps. A general increase in the extension of the meadow from 1986 to 1991 was evidenced, but the disparity of mapping methods (SCUBA diving in 1986, Side Scan Sonar in 1991) in the two surveys imposes caution when interpreting this result. On the other hand, the comparison of 1991 and 2001 maps, both derived from Side Scan Sonar surveys, showed a regression of the meadow of about 60% in the northern area, and modifications in the upper and lower limits of the meadow due to the impact of coastal works. C. nodosa meadow showed the only enlargement in front of the mouth of the Entella River, due to the increase in nutrient contents for rainfall in the period 1988-1994. The overall analysis evidenced a net decrease in seagrass meadow extension, an early warning of risk for marine coastal biodiversity in the Gulf of Tigullio. © 2007 Springer Science+Business Media B.V.