Nickel toxicity was examined in an artificial streamside mesocosm using a flow-through design with natural stream water with two nickel spiked sediment types (low acid volatile sulfide (AVS), high AVS).  Indigenous and surrogate organism responses were tested against sediment in situ during weeks 1, 2, 4, and benthic communities were sampled during weeks 4, 7, and 17.  Chironomus tentans and Psephenus herricki had lower survival in the highest nickel concentration (1030 mg/kg), and the benthic communities responses were negatively correlated with increasing Total Ni and SEM-AVS/ƒOC in the streamside mesocosm.  Using diffusive gradients in thin films (DGTs), Ni appears to be fluxing out of both sediment types, and partitioning into the periphyton layer.  Nickel spiked sediments were placed in situ for 4 d at Greenville Creek, OH, USA, and Isonychia spp. lengths, and Anthopotamus verticis weights were significantly different in the highest concentration (521 mg/kg) compared to the reference sediment (p < 0.05).  Anthopotamus verticis were tested in a 7 d lab nickel sediment test and exhibited an EC₅₀ of 1462 mg/kg.  A 7 d water-only nickel test showed Isonychia spp. survival and weights, and A. verticis lengths responded negatively to the highest concentrations (9500 µg/L) (p < 0.05).  Toxicity of nickel is being affected by Ni-periphyton partitioning, Ni bioavailability factors (hardness, pH, organic carbon, AVS), and Ni flux in sediments in situ.