Measurement and distribution of interstitial and overlying water ammonia and hydrogen sulfide in sediment toxicity tests

Hydrogen sulfide and ammonia are naturally occurring and anthropogenically enriched components of marine sediments which may occur in concentrations toxic to marine organisms. In this study, ammonia and hydrogen sulfide were measured in interstitial and overlying water at the initiation and termination of solid phase toxicity tests using amphipods (Rhepoxynius abronius). Interstitial hydrogen sulfide concentrations were generally higher than overlying concentrations, presumably because hydrogen sulfide that diffused from sediments was quickly volatilized in the overlying water. Unionized ammonia concentrations were higher in interstitial water at test initiation, but were often higher in overlying water after 10 day exposures. The pH was higher in overlying water samples than in interstitial samples, and contributed to increased proportions of unionized ammonia in the overlying water. Concentrations of hydrogen sulfide and ammonia above the effects threshold for Rhepoxynius occurred more predominantly in the interstitial water. Eight of twelve treatments with high hydrogen sulfide concentrations, and five of eight treatments with high ammonia concentrations, had significant toxicity. Based on these results we suggest measuring initial and final interstitial concentrations of hydrogen sulfide and ammonia, and initial and final concentrations of overlying ammonia. A precise method of sulfide measurement in seawater is presented, based on techniques and equations from the literature.