Growth rate as a factor confounding the use of the dogwhelk Nucella lapillus as biomonitor of heavy metal contamination

Growth rate of individually tagged dogwhelks Nucella lapillus (L.) was measured in free-living individuals at 3 sites of differing heavy metal contamination in the Firth of Clyde, west Scotland. Condition index (CI), concentrations of metals (Cd, Cu, Pb and Zn), metallothionein (MT), RNA (the RNA/protein ratio) and glycogen were also measured. In general, the marine environments of Gourock and Largs were contaminated with significantly higher tributyltin, Pb and Zn than Loch Fyne, as indicated by the results of imposex indices, and metal concentrations in transplanted polymer-ligands (Chelex® 100) and Mytilus edulis. Further, metal concentrations of native M. edulis (Pb and Zn) and Semibalanus balanoides (Cu) from Gourock were significantly higher than those from Loch Fyne. However, metal accumulation in the dogwhelks displayed a very different pattern. At a standard size (0.5 g wet soft-body weight), N. lapillus from Largs showed higher Cd, Cu and MT in their tissues than individuals from the other 2 populations. Levels of Pb and Zn were similar among the populations despite different concentrations in Chelex and mussels. Gourock dogwhelks showed similar levels of Cu and MT but lower Cd compared to those of Loch Fyne. These differences can be attributed primarily to differences in dogwhelk growth rate between sites. Gourock individuals had a higher CI and RNA/protein ratio in the foot muscle and grew faster (especially at small sizes), resulting in a tissue-dilution effect on metal and MT concentrations. In contrast, higher levels of Cd, Cu and MT in dogwhelks from Largs can be attributed to their growth rate being relatively slow compared to the rate of metal accumulation. Slow-growing individuals in Loch Fyne had relatively high Cd, Pb Zn and MT, although Loch Fyne has been regarded as a clean reference site. Among populations, differences in growth rate may be due to differences in prey availability, predation pressure, and/or genotype. The present results demonstrate that inter-site differences in growth rate can confound the use of the dogwhelks as a biomonitor of metals.