Organic matter features, degradation and remineralisation at two coastal sites in the Ligurian Sea (NW Mediterranean) differently influenced by anthropogenic forcing

► Seawater organic matter (OM) dynamics were studied at two coastal sites (natural and urbanised). ► Low trophic value OM (high C:N ratio) accumulated at the urbanised site in summer-autumn. ► Efficient degradation and remineralisation activity coped with OM accumulation at the urbanised site. ► Allochthonous inputs and efficient OM recycling sustain microautotrophs also in summer-autumn. ► These autotrops (toxic dynoflagellates in previous years) may lead to severe dystrophic conditions.