The abundance and activity of heterotrophic and autotrophic organisms were determined in the surface microlayer (SML) at marine and estuarine sites in the NW Mediterranean Sea from May to July 2003. At all study sites, community respiration in the SML exceeded that in the subsurface water (SSW) by factors between 1.7 and 28. The abundance of heterotrophic bacteria and the major groups of the autotrophic community displayed consistent, but low enrichment factors (EF) for the SML (mean 1.3). In contrast, EF for the abundance of heterotrophic nanoflagellates and the concentration of particulate organic carbon were, on average, 4 and 7, respectively. Community respiration rates in the SML were related to the concentration of total organic carbon; however, no relationship was detectable between community respiration and the abundance of the major groups of the heterotrophic and autotrophic community. Net community production, determined at 2 oligotrophic marine sites, displayed negative values for the SML, while in the SSW autotrophic processes balanced or exceeded heterotrophic processes. Similar rates of photochemical oxygen consumption were determined for the SML and the SSW. These results let us conclude that in situ primary production in the SML is not sufficient to support biological and photochemical mineralization at the air-water interface. We suggest that vertical transport of organic matter, mainly in colloidal and particulate form, to the air-water interface is the basis of the food web, thereby supporting net heterotrophy of the SML.