Assessment of inshore/offshore tributyltin pollution gradients in the northwest Portugal continental shelf using Nassarius reticulatus as a bioindicator.

Imposex and organotin tissue contamination were assessed in Nassarius reticulatus (L.) populations in the northwest Portuguese continental shelf between 2002 and 2005 over an area of 735 km2, involving 366 sampling sites. The objective was to evaluate the dispersion of tributyltin (TBT) from inshore sources inside the Ria de Aveiro estuary into the adjacent deeper sea and to assess endocrine disruption in the netted whelk N. reticulatus, using imposex as a biomarker. The highest levels of TBT tissue contamination and imposex were found in the whelks inside the Ria de Aveiro, and these declined logarithmically with distance from the mouth of this estuary. Remarkably, we found that offshore populations were also extensively affected: TBT (the dominant organotin) tissue concentration was above the detection limits at all sites where whelks were analyzed (12-356 ng TBT-Sn/g dry wt), and imposex occurred at 80% of the sampling stations, with relatively high values at some sites from the deepest area surveyed. This work shows clearly that TBT pollution is not restricted to the Ria de Aveiro but affects a significant part of the adjacent continental shelf as well. The ecological impacts of TBT pollution on offshore ecosystems are discussed.