Chronic toxicity and physical disturbance effects of water- and oil-based drilling fluids and some major constituents on adult sea scallops (Placopecten magellanicus)

Adult sea scallops, Placopecten magellanicus, were exposed in the laboratory under environmentally representative conditions to different types and concentrations of used operational drilling fluids and their major constituents. Threshold waste concentrations causing reductions in somatic and/or reproductive tissue growth are: greater than 10 mg l−1 for used water-based mud (WBM); 2 mg l−1 for bentonite; and less than 0.5 mg l−1 for barite and used oil-based mud (OBM). Chronic exposure to OBM caused high mortalities at concentrations as low as 1.0 mg l−1. Non-nutritious particles in the food supply (all wastes) and chemical toxicity (OBM and perhaps barite) affected the growth rate and survival of sea scallops by altering physiological state (scope for growth) and nutritional condition (O:N ratio). The value of scope for growth (SFG) calculations for assessing the relative chronic toxicity of the drilling wastes was demonstrated by the close relation observed between SFG and actual growth measurements. These results show that chronic intermittent exposure of sea scallops to dilute concentrations of operational drilling wastes, characterized by acute lethal tests as practically non-toxic, can affect growth, reproductive success and survival.