Toxicity of the crude oil water-soluble fraction and kaolin-adsorbed crude oil on Daphnia magna (Crustacea: Anomopoda)

Crude oil is a complex mixture of hydrocarbons entering aquatic environments from accidental or normal marine and transportation activities. Toxicologic crude oil analysis is usually performed on the basis of the water-soluble fraction. However, this yields only a partial estimate of the damage caused by these contaminants because a substantial hydrophobic amount can be adsorbed onto suspended solids (biotic and abiotic), which directly affects filter-feeding species and permits bioaccumulation through trophic relationships. This study determined the acute toxic damage sustained after 48 hours caused by seven types of crude oil from Tabasco, Mexico on the cladoceran Daphnia magna. Comparisons were documented based on the responses of D. magna from application of the water-soluble fraction and exposure to entire crude oil samples adsorbed on kaolin clay. Oil-sorbed kaolin was more toxic than the water-soluble fraction in acute exposure. This confirms that tests of the water-soluble fraction tend to underestimate the toxic damage that can be produced in natural environments. Furthermore, chronic toxicity (21 days) was evaluated for crude oil samples adsorbed on kaolin at sublethal concentrations as established from Application Factors (AF) criteria. Results showed that in most cases, it is impossible to predict safe concentrations on the basis of LC₅₀ values because samples with lower acute toxicity exercised a greater influence on D. magna reproduction and survival when subjected to chronic exposure.