Influence of the organic matter on the bioavailability of phenanthrene for benthic organisms

Ecotoxicological tests with benthic organisms are recommended to assess the toxicity of hydrophobic organic contaminants like polycyclic aromatic hydrocarbons that strongly adsorb on sediments. Many routes of exposure including ingestion from sediments are taken into account with bioassays. However, organic matter of the sediments can modify chemical partitioning and toxicity of these non-polar organic contaminants. The purpose of this study was to assess the influence of both quality and quantity of organic matter on the bioavailability of phenanthrene for benthic organisms, Chironomus riparius and Hyalella azteca. Formulated sediments were useful models to determine the influence of the organic matter excluding any other change. Formulated sediments with 2 to 8% total organic carbon were used with three kinds of organic matter. Acute (survival) and chronic (growth, development) endpoints were investigated. Bioassays were carried out until adult stage for both organisms. Concentrations with a significant effect compared with controls and effect concentrations for 50% of the organisms were determined. After 10 days (chironomids) or 14 days (hyalella) exposure, survival of the organisms were below 44% at 300 mg kg−1 phenanthrene (dry weight). Most of the time, toxicity observed on chironomids after 10 days was enhanced after 28 days exposure. There were, however, no significant differences for the growth of the surviving midges at any concentration. Both the quality and the quantity of the organic matter influenced acute toxicity results whereas partitioning of phenanthrene was not affected by the organic matter. The toxicity results showed that the equilibrium partitioning theory that assumes that benthic organisms are primarily exposed to contaminants via the water phase could not be applied in that case to assess the effect of phenanthrene to benthic organisms in the environment.