Bar-built estuaries are unique ecosystems characterized by the presence of a sandbar barrier, which separates the estuary from the ocean for extended periods and can naturally reopen to the ocean with heavy rainfall and freshwater inflows. The physical effects associated with the transition from closed to open state, specifically water mixing and sediment resuspension, often indirectly worsen water quality conditions and are suspected to drive near-annual fish kills at the Pescadero estuary in northern California. The effects of sediment acid-volatile sulfide (AVS) oxidation, specifically oxygen depletion, acidification, and metal release, are believed to aggravate water conditions for fish but remain poorly understood. We performed slurry incubations containing sediment from 4 sites in the Pescadero estuary, representing a gradient from the Pacific Ocean to freshwater tributaries. We measured near-maximum rates of aqueous hydrogen sulfide oxidation, sediment AVS oxidation, sulfate production, and acidification, as well as near-maximum release rates of iron (Fe), manganese (Mn), and zinc (Zn) to the water column. We estimated AVS oxidation rates of 8 to 21 mmol S kg d , which were 3 orders of magnitude higher than aqueous hydrogen sulfide oxidation rates, 6 to 26 μmol S kg d . We suggest that aqueous hydrogen sulfide cannot be responsible for the observed kills because of low concentrations and minimal oxidative effects on pH and metal concentrations. However, the oxidative effects of AVS are potentially severe, decreasing pH to strongly acidic levels and releasing aqueous Fe, Mn, and Zn concentrations up to 11.2 mM, 0.46 mM, and 88 μM, respectively, indicating a potential role in worsening water conditions for fish in the Pescadero estuary. Environ Toxicol Chem 2018;37:993-1006. © 2017 SETAC.

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