Coastal inorganic mercury time series reveals interannual and seasonal variability driven by regional climate factors

Inorganic mercury (iHg) is an anthropogenic pollutant that forms monomethylmercury, a neurotoxicant affecting human health through seafood consumption. Despite iHg emissions reductions, the impact on oceanic concentrations remains unclear due to limited long-term data. Here, we present a four-year weekly time series of oceanic iHg concentrations at Scripps Pier in La Jolla, California, capturing interannual and seasonal variability. Interannual variability is driven by wet season precipitation, with wet years exhibiting sevenfold higher iHg concentration variance than dry years, potentially linking to El Niño-Southern Oscillation. Seasonally, precipitation and wave dynamics influence iHg inputs, with wet seasons driven by precipitation and runoff and dry seasons by upwelling. These parameters informed a model built to reconstruct a 20-year record of iHg concentrations, suggesting a long-term decline of 0.005 pM yr ⁻¹ due to climate-driven effects alone. This study highlights challenges in detecting long-term trends and emphasizes the need for sustained monitoring of oceanic iHg.