Elevated mercury accumulation and inefficient biomagnification in subterranean streams: Insights from karst cave ecosystems.

Karst cave ecosystems, characterized by their isolation, absence of light, and limited food resources, represent unique environments where mercury (Hg) accumulation and biomagnification remain largely unknown. This study investigates the bioaccumulation and biomagnification of total mercury (THg) and methylmercury (MeHg) within aquatic food webs of karst cave stream ecosystems in Southwest China. The results indicated significantly elevated Hg concentrations in cave-dwelling organisms, with THg levels ranging from 104 to 1732 ng g-1 and MeHg levels ranging from 22.0 to 624 ng g-1. Notably, basal species, such as mayfly larvae, displayed concentrations of THg and MeHg that were 1.5-12.3 times greater than those found in surface freshwater ecosystems. Paradoxically, the biomagnification of Hg was less efficient, with trophic magnification slope (TMS) and trophic magnification factor (TMF) values being up to 20.7 and 3.69 times lower, respectively, compared to surface freshwater ecosystems. The high Hg accumulation and low trophic transfer efficiency in cave organisms may be attributed to the unique characteristics of karst caves, including low light, low dissolved oxygen, low temperature, food web structures, physiological and ecological traits of cave-dwelling organisms, and high baseline Hg levels. These findings provide unique insights into Hg cycling in subterranean ecosystems, revealing cryptic Hg reservoirs with critical implications for implementing of the Minamata Convention regarding Hg inventory completeness in karst terrains.