Several recent studies report that low parts per billion (ppb) concentrations of petroleum polycyclic aromatic hydrocarbons (PAH) are toxic to marine fish embryos and that crude oil toxicity increases as it weathers. Such claims for Pacific herring embyros derive from two experiments by Carls et al. (1999) in which herring eggs were exposed to seawater passed through gravel coated with artificially weathered Alaska North Slope crude oil. The experiments differed in the extent of weathering of the oil on gravel. Carls et al. reported that developmental abnormalities in herring embryos occur during chronic exposure to PAH levels as low as 0.4 ppb in seawater passed through the oiled gravel. Earlier studies had shown that effects are observed at low PAH levels only when oil droplets or films adhered to the herring eggs. To better understand Carls et al. experiments, we examined effluent from a gravel bed prepared following Carls et al. and report that ammonia, sulfides, and oil droplets were present in the effluent from oiled gravel generators that were shut down between two 16-day trials (as was done by Carls et al.). Oil droplets (0.5 to 1 mm) were intermittently present in effluent from oiled gravel generators even when the flow was continuous. Two hours after restarting flow, low dissolved oxygen, ammonia, and sulfides were present in the generators and in the effluent. Droplets, ammonia, and sulfides all induce developmental abnormalities of the types seen by Carls et al. The presence of ammonia and sulfide in the effluent after shutdown is a laboratory artifact and constitutes clear evidence of anaerobic biodegradation of the oil on gravel. Evidence of anaerobic biodegradation suggests that the exposure regime of Carls et al. did not effectively simulate field conditions. Our results demonstrate that the presence of confounding toxicants in the Carls et al. experiments cannot be dismissed. There is no basis to conclude that aqueous exposure to low ppb PAH levels affects herring eggs or that weathering increases oil toxicity to fish eggs without additional experiments that specifically account for the potential confounding factors and all chemicals in effluents from oiled gravel columns.

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