Mesocosm study of Mytilus edulis larvae and postlarvae, including the settlement phase, exposed to a gradient of tributyltin

In a mesocosm study, the effects of a gradient of tributylin (TBT) (nominal TBT concentrations of 0.3, 2.3, 18.5, 146, and 1150 ng Sn liter-1) on Mytilus edulis larvae and postlarvae, including the settlement phase, were investigated over 15 days. Effects of TBT on mortality, growth measured as increase in shell length, shell dimensions, and settlement were evaluated. In general, mortality was high in all mesocosms including the control; during the first 24 h of the experiment, mortality was 63% at the highest TBT concentration (26% in the control). An LC50 (24 h) of 254 ng Sn liter-1 was estimated. The mortality rate of larvae/postlarvae increased 40% at 18.5 ng Sn liter-1 (0.41 day-1) compared with the control (0.30 day-1). For postlarvae, the growth rate decreased with increasing TBT concentration. The mean shell length at 2.3 ng Sn liter-1 was significantly reduced in comparison to the control on Day 15. Then EC10 (15 days) for shell growth was estimated to be 5.4 ng Sn liter-1. This is the lowest effect concentration ever reported in the literature. For postlarvae, shell dimensions in terms of shell length-shell width relations were affected by TBT at 1150 ng Sn liter-1, because the reduced growth led to the failure of adult mussels to secret a dissochonch shell. During the first days of exposure, the settlement monitored on polyethylene settling strips was stimulated by TBT, after which the settlement decreased due to the high mortality. Only a small portion of the population survived to the end of the test period. By comparison of the biotic conditions (in terms of larval abundance and particle concentration reflecting larval food) in the control mesocosm with those in the cove where the experiment was conducted, it was concluded that the mesocosm had successfully simulated the field conditions.