RESUMO
The present study aimed at creating a so called "single-site baseline" (SSB) of imposex in the purple dye murex (Bolinus brandaris) from the Ria Formosa lagoon (Algarve coast, southern Portugal), at the moment that the IMO Antifouling System Convention (banning globally the use of organotin compounds in antifouling paints on ships' hulls) entered into force (September 2008). In the context of monitoring long-term temporal trends of pollution levels, SSBs are proposed whenever it is difficult to obtain adequate numbers of bioindicator specimens in most of the study area or if significant short-term variability in imposex levels is likely to occur. In order to deal with these constraints, this study created a SSB by sampling on a monthly-basis (from October 2008 to September 2009) in a single site considered to be representative of the Ria Formosa as a whole (vicinities of the Culatra Island) and where B. brandaris is abundant all year round. The SSB provided a good characterisation of the monthly variation in the imposex indices, which ranged throughout the year between 72.7 < I% < 100; 0.08 < FPLI < 1.73; 1.20 < RPLI < 23.27; 1.29 < VDSI < 3.54. Only a SSB could have detected this large variability, which otherwise might have been unnoticed and would have produced biased temporal and spatial comparisons, leading to erroneous interpretations of environmental monitoring data. Therefore, this SSB of imposex in B. brandaris constitutes valuable reference data to assess the future trend of organotin pollution in the Ria Formosa, after the benchmark date of the complete eradication of organotin from antifouling agents was implemented worldwide.