Evaluating the Performance of Diffusive Gradients in Thin Films for Predicting Ni Sediment Toxicity

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Abstract
Diffusive gradients in thin films (DGTs) rapidly measure labile fractions of metal and are promoted as an assessment tool for bioavailability. Using macroinvertebrate community composition as a response, this study compared the predictive ability of DGT-measured Ni with acid volatile sulfide (AVS) and organic carbon (OC) corrected Ni [(SEMNi–AVS)/fOC] and total Ni concentrations. In two experiments, sediments were amended with Ni and placed within either a streamside mesocosm or deployed in situ. DGT-measured Ni concentrations (CDGT) increased with increasing total Ni, were greater at depth, and decreased over time. Relationships between Ni CDGT and sediment geochemistry indicated a shift in Ni partitioning from AVS-bound to Fe- and Mn-associated Ni. In both experiments, DGT-measured Ni poorly predicted the invertebrate response to metal, whereas models that included total Ni or (SEMNi–AVS)/fOC effectively predicted the invertebrate response for the streamside mesocosm and in situ experiments, respectively. CDGT overestimated the available Ni fraction, possibly due to sampling either nonbioavailable solid-phase Ni or Ni irrespective of cations competing at the biotic ligand. We suggest that CDGT cannot replace (SEMNi–AVS)/fOC for predicting invertebrate response to sediment Ni, and greater understanding of metal species lability to DGTs is needed before assuming equivalence between bioavailable and DGT-labile metals in sediments.

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