Effects of two oil dispersants on phototaxis and swimming behaviour of barnacle larvae

The effects of two oil dispersants (Vecom B-1425 GL and Norchem OSD-570) mixed with diesel oil on the survival and behaviour of the stage II nauplii of the barnacle Balanus amphitrite were investigated. The 24 and 48-hour LC₅₀ values for Vecom B-1425 GL:diesel mixture were 514 and 48 mg l^-1 respectively, while respective values for Norchem OSD-570:diesel mixture were 505 and 71 mg l^-1. Under sublethal concentrations, increased levels of the dispersant:diesel mixtures caused a reduction in phototactic responses. Balanus amphitrite nauplii failed to exhibit phototactic responses when exposed to Vecom B-1425 GL:diesel mixtures of 400 mg l^-1 and higher for 24 hours. A longer exposure time of 48 hours further reduced the Lowest Observable Effect Concentrations (LOECs) to 60 mg l^-1. The LOECs for Norchem OSD-570:diesel mixtures under exposure periods of 24 and 48 hours were 400 and 80 mg l^-1 respectively. The curvilinear velocities (VCL) and straight-line velocities (VSL) of the stage II nauplii ranged from 0.7-1.1 and 0.2-0.4 mm s^-1 respectively. Increased concentrations of dispersant:diesel mixtures caused a significant change in the curvilinear and straight-line velocities. Both oil dispersants, dispersant:diesel mixtures of 20 to 40 mg l^-1 caused significant increases in VCL, but no significant change in VSL. Dispersant:diesel mixtures of 100 mg l^-1 and higher resulted in a reduction in VSL for both dispersants.