The chemical and biological properties of the water column at a Tyrrhenian site (Isola del Giglio) were studied during a 3-year period. The results highlighted the oligotrophic features of the site, characterised by quite low concentrations of organic carbon (on average DOC 102 micromol/L and POC 9 micromol/L). Relevant bacterial biomass (on average 42.1 microg C/L) and a notable activity (in terms of frequency of dividing cells, on average more than 5%) were observed. However, remarkable changes for these parameters were seasonally recorded. The cyclic occurrence, generally during the late spring-summer period, of benthic mucilage indicated that localised distrophic processes may occur. In particular, the benthic mucilage events of 2000 and 2001 were investigated, although some comparative information was available also for 1999 and 2002. The mucilage aggregates generally showed high bacterial colonisation, which have remarkable effects on the organic matter cycle both inside the aggregates and in the surrounding seawater. During the benthic mucilage development, an increase of DOC and POC concentrations was observed (up to 129 and 18 micromol/L, respectively, in June 2000 and up to 145 and 10 micromol/L, respectively, in May and June 2001) in the water column adjacent to the bottom. However, a general decrease of the trophic value of particulate matter (in terms of C/N ratio) was also observed, especially in 2000 after the disappearance of the mucilage. The available energy and organic matter during the mucilage events led to an increased presence of bacteria in the bottom waters of the Isola del Giglio, with maximum biomass values in 2001. Similarly, the replicative activity of bacteria was higher in 2001 (frequency of dividing cells about 5% vs. 3% of 2000). The lower activity of 2000, in addition to the lower trophic value of organic matter and different environmental conditions (namely lower temperature), might be involved in the persistence of mucilage in 2000 with respect to the rapid disappearance observed in 2001.