Long-term monitoring of Nucella lapillus imposex in Ria de Aveiro (Portugal): When will a full recovery happen?

Nucella lapillus imposex has been monitored in Ria de Aveiro (Portugal) in order to understand the evolution of organotin (OT) pollution after the EU ban on OT-based antifouling systems (OT-AFS). A rapid decrease of imposex occurred, but a countercurrent tendency upsurged recently (2018-2019) with a slight increase of imposex at some sites, instead of a continuous decline towards a complete recovery. In 2018 the vas deferens sequence index ranged between 0.6 and 1.0 across stations and the percentage of females with imposex varied from 63 to 100%. In 2019 these parameters varied between 0.4 and 1.0 and 37-100%, while the concentration of butyltins (TBT, DBT, MBT) in the dogwhelk tissues were < 1 ng Sn g-1 dw. Although N. lapillus imposex levels are low they are indicative of OT pollution. The full recovery of N. lapillus imposex in the future is uncertain and a cause for concern.

Fifteen years of imposex and tributyltin pollution monitoring along the Portuguese coast.

IMO's Anti-Fouling Systems convention banned the use of organotin-based antifouling systems in 2008 as the ultimate effort to stop tributyltin (TBT) inputs into the marine environment. One of the hazardous effects of TBT is imposex (the superimposition of male sexual characters onto gastropod females), a phenomenon that may cause female sterility and the gastropod populations decline. Despite previous European Union legislation had already been shown effective in reducing the imposex levels along the Portuguese coast, this study intends to confirm these decreasing trends after 2008 and describe the global evolution in the last 15 years. Imposex levels were assessed in two bioindicators - the dog-whelk Nucella lapillus and the netted-whelk Nassarius reticulatus (Gastropoda, Prosobranchia) - in 2011 and 2014, and the results were compared with previous years. Both species showed progressive decreasing trends in imposex levels over the last 15 years; median values of the vas deferens sequence index (VDSI) fell from 3.96 to 0.78 in N. lapillus and from 3.39 to 0.29 in N. reticulatus. The temporal/spatial evolution of imposex suggests an apparent shift of TBT hotspots, being now restricted to fishing ports and marinas in detriment of large commercial harbours where TBT levels fell rapidly. Butyltins were measured in the whole tissues of N. lapillus females collected in 2014: monobutyltin (MBT) varied from < DL (detection limit: 1 ng Sn/g) to 13 ng Sn/g dw, dibutyltin (DBT) from 2.2 to 27 ng Sn/g dw and TBT from 1.5 to 55 ng Sn/g dw. Although TBT body burden has declined over time, the butyltin degradation index ([MBT]+[DBT])/[TBT] exhibited values < 1 in c. a. 90% of the sites assessed, suggesting that recent TBT inputs are still widespread in the Portuguese coast eventually due to illegal use of TBT antifouling systems and TBT desorption from sediments.

Spatial and temporal evolution of imposex in dogwhelk Nucella lapillus (L.) populations from North Wales, UK.

Nucella lapillus imposex and organotin tissue contamination were assessed, during 2006, at twenty sites in North Wales, between Anglesey and Shell Island on the Lleyn Peninsula. Vas Deferens Sequence Index (VDSI), Relative Penis Size Index (RPSI) and the percentage of affected females (%I) were used to assess imposex levels which varied between 0.5 and 3.8 for VDSI, 0.0 and 11.5% for RPSI and 49 and 100% for %I. Tributyltin (TBT) and triphenyltin (TPT) concentrations in whole tissues varied between 0.8 and 39 and 0.4 and 2.1 ng Sn/g dry weight, respectively. TBT represented the higher fraction of butyltin compounds in the tissues, suggesting that TBT inputs continue to occur. Comparisons with nineteen years of data collected during previous studies demonstrated that there had been a significant reduction in imposex levels over the last two decades following the introduction of legislative restrictions in the U.K. regarding the use of organotin based antifouling paints.