Terrestrial Invertebrates Reflect Increases in Mercury Mobilization across a Riparian Gradient

Elevated mercury concentrations among arthropods collected from a saturated riparian zone suggest that extreme flooding increases the level of mercury mobilization. Total mercury was analyzed among Oniscidea (sow bugs; Malacostraca), Opiliones (harvestmen; Arachnida), and Succineidae (amber snails; Gastrapoda) along a riparian gradient with varying water levels, including an area flooded by a rare event. Four areas were compared: a dry upland, a flooded interface, a normal interface, and a consistently saturated wetland. Average total mercury concentration in harvestmen increased 426% in the flooded interface relative to a dry location where individuals averaged 3.21 ± 14.4 ng Hg/g dry weight. Mercury concentrations in sow bugs increased by 34% between the dry and flooded interface and reached an average of 198 ± 42.6 ng Hg/g dry weight at the wetland center, a 97% increase from the dry location concentration. Average mercury concentration in snails increased by 48% in the normal interface relative to the flooded interface (41.1 ± 15.9 ng Hg/ng dry weight), whereas snails collected within the wetland had a 23% greater mercury burden compared to those in the normal interface. Increased mercury in invertebrates is associated with increased proximity to the wetland center, and newly saturated soils mobilize mercury into invertebrates during flooding moments.