Sulfides as environmental stressors in Paracas Bay, Peru

Abstract

Paracas Bay, located in the Humboldt Current system, is a highly variable coastal environment where hypoxia (dissolved oxygen concentrations <2 mg L^-1) has been reported as a persistent feature of bottom conditions. In addition to hypoxia, milky water events have been reported in the bay, most likely associated with the presence of sulfides (i.e. sulfidic events), including toxic hydrogen sulfide (H₂S). This study is the first report of sulfide concentrations in the water column of Paracas Bay, with concentrations up to 6.79 μmol L^-1 measured in the bottom water layer using Diffusive Gradient in Thin film (DGT) passive samplers. Sulfides showed a marked seasonal pattern, while ENSO phases did not seem to affect their occurrence. The presence of sulfides in the water column is closely linked to hypoxic conditions. Indeed, the analysis of the relationship between the duration of severe bottom hypoxia (dissolved oxygen ≤0.15 mg L^-1) and sulfides concentrations revealed a threshold of 18 h for sulfides accumulation in the overlying waters. Also, a gradient in sulfides concentration was observed from the sediment to the upper water layer. This gradient coupled with the variability of environmental variables (temperature, dissolved oxygen and currents velocity) highlights the complexity of this system, and suggests that a combination of advective and local physical and biogeochemical processes are responsible for the sulfidic events to occur in Paracas Bay. This study provides a baseline for assessing the potential toxicity of sulfides to cultured Peruvian scallops in upwelling bays.

Keywords: Environmental monitoring; Humboldt Current system; Hypoxia; Sulfides; Upwelling bay.