Lethal and sublethal toxicity of sediment-associated fluoranthene to benthic copepods: application of the critical-body-residue approach

Abstract

Lethality occurs at tissue concentrations of organic contaminants acting via non-polar narcosis at about 2–8 μmol (g wet wt.) −1 for most vertebrate and invertebrate species tested. The range of tissue concentrations for such compounds associated with sublethal effects has yet to be determined. During this study, the lethal and sublethal toxicity of the polycyclic aromatic hydrocarbon (PAH) congener fluoranthene to benthic copepods was examined in sediment exposures. Mature female Schizopera knabeni and Coullana sp. were exposed to dosed sediment at concentrations ranging from 0 to 334 μg (g dry wt.) −1 for 10 days. Copepod survival and reproductive output were related to the [ 14C]fluoranthene concentration in the external (sediment) and internal (tissues) media. In both copepod species, fluoranthene tissue concentrations were maximal at day 1 and remained relatively constant throughout the 10-day experiment. Most of the mortality occurred between days 4 and 10. For S. knabeni, the 10-day mean lethal sediment concentration (LC 50) was 213 μg (g dry wt.) −1 (193–231, 95% confidence interval (CI)) and the 10-day median lethal tissue residue (LR 50) was 2.7 μmol (g wet wt.) −1 (2.5–2.8, 95% CI). Coullana sp. was more sensitive, with a 10-day sediment LC 50 of 132 μg (g dry wt.) −1 (88–170, 95% CI) and a 10-day LR 50 of 1.2 μmol (g wet wt.) −1 (0.8–1.6, 95% CI). Differences in sensitivity persisted even after normalization of body residues by total lipid content (LR 50=47.8 and 15.6 μmol (g lipid) −1 for S. knabeni and Coullana sp., respectively). Reproduction of S. knabeni was significantly impaired at lower concentrations in both sediment and tissue. The sediment concentration associated with a 50% reduction in S. knabeni surviving offspring in a 10-day exposure was 55 nmol (g dry wt.) −1. Tissue residues in adult females associated with this reduction were around 0.5 μmol (g wet wt.) −1, a value representing a fraction of 0.2 the 10-day LR 50. This decrease in offspring production was likely related to specific modes of action (e.g. impairment of embryonic development), rather than solely general narcosis. To directly assess the effects of narcosis (decreased activity) at sublethal tissue residues, copepod grazing rate on microalgae was measured concomitantly with sediment exposure to fluoranthene. Short-term exposures (27 h) revealed that a 50% decrease in grazing rates was associated with tissue residues of approximately 0.2 μmol (g wet wt.) −1. The use of the critical body residue (CBR) approach for toxicity assessment is supported by this study and appears to be a very promising approach for evaluating risk to benthic organisms from organic contaminants.