Water quality bioassays in two Bermudan harbours using the ciliate <i>Euplotes vannus</i>, in relation to tributyltin distribution

Abstract

Laboratory cultures of the ciliate Euplotes vannus were used to bioassay water samples taken from the sea surface, 0.5 m and near bottom at stations along contamination gradients in Castle Harbour and Hamilton Harbour on Bermuda. Inhibition of population growth rate was used as an index of exposure to toxic contaminants. Significant differences in growth rates from near-bottom water samples taken at two stations in Castle Harbour were reflected in differing sediment concentrations of metals and petroleum hydrocarbons. Bioassays of four Hamilton Harbour samples indicated significant differences between control and contaminated sites when results from all three depths were pooled. Highest concentrations of tributyltin (⩽ 307 ng TBT·1−1) occurred in the surface microlayer, though measurements varied widely on the two sampling occasions. At 0.5 m, concentrations were much less variable and showed a steady decline from the head of Hamilton Harbour (41 ng TBT· 1−1) seawards to the control site (0.9 ng TBT · 1−1). While these concentrations are high enough to be toxic to some species, it is known from experimental work that TBT alone could not have accounted for the depression of ciliate growth rates in these bioassays.