Estimating benthic fluxes of trace elements to hypoxic coastal waters using 210Po

The activities of colloidal (10 kDa–0.45 μm) and truly dissolved (<10 kDa) 210Po and 210Pb, dissolved inorganic nutrients, and trace elements (Al, V, Co, Ni, Cu, Mo, Mn, and Fe) were measured in the artificial and saline Lake Shihwa, South Korea in July 2011. The boundary between oxic and hypoxic layers, indexed by NH4+, Mn, and dissolved oxygen, was determined by the pycnocline formed at the depth of 2–4 m. The total activities of 210Po and 210Pb in Lake Shihwa were in the range of 10.5–20.4 dpm 100 L−1 and 6.4 to 9.8 dpm 100 L−1, respectively. The proportions of the truly dissolved, colloidal, and particulate phases were, respectively, 55 ± 6%, 29 ± 5%, and 16 ± 3% for 210Po, and 48 ± 6%, 29 ± 7%, and 23 ± 4% for 210Pb. The activities of 210Po were 20–55% higher than those of 210Pb. The benthic flux of 210Po is determined on the basis of the 210Po mass balance model in the water column. Since the excess activities of dissolved 210Po relative to 210Pb showed a positive correlation with the concentrations of trace elements (except for Al), the benthic fluxes of the trace elements from bottom sediment were successfully calculated using these relationships and the 210Po fluxes. Our study highlights that 210Po can be used as a powerful tracer for estimating trace element fluxes from bottom sediment in a hypoxic environment.