Evaluation of methods to remove ammonia interference in marine sediment toxicity tests

Ammonia naturally accumulates to high concentrations in nonsurficial marine sediments. It can also interfere and confound interpretation of toxicity from persistent anthropogenic chemicals in tests with laboratory amphipods. Renewal of overlying water has become a standardized procedure to decrease pore‐water ammonia. Our objective was to propose an alternative technique for decreasing pore‐water ammonia concentrations. Sediment samples with ammonia concentrations of 70 and 155 mg/L were purged of toxic concentrations in pore‐water ammonia using the existing procedure of performing two renewals of overlying water per day. A thin‐layer technique, which involved increasing the sediment surface area during the purging period, decreased the ammonia concentration by as much as 4.5‐fold faster than the conventional purging protocol. Minimal decreases in concentrations of polycyclic aromatic hydrocarbons, heavy metals, and total organic carbon were found among all the purging techniques. Toxicity tests with the marine amphipod Ampelisca abdita suggest that minimizing the time required to purge a sediment of ammonia is critical in maintaining the integrity of the sample. The thin‐layer purging technique appears to be an effective method of decreasing pore‐water ammonia concentrations in sediments before laboratory toxicity testing with amphipods.