Mercury isotopes of marine zooplankton elucidate anthropogenic mercury input and cycling in the North Pacific Ocean

Abstract

Mercury (Hg) stable isotope compositions of surface (5 meters) marine zooplankton (n = 9; 0.2 – 2 mm) from coastal North Pacific (East China Sea to China Sea), and surface, epipelagic (0-150 m), and mesopelagic (200-500 m) zooplankton (n = 19; 0.2 – > 5 mm) from the Philippine Sea located at the edge of the North Pacific Subtropical Gyre reveal information on the inputs and cycling of anthropogenically impacted Hg from the coast of China (Fig 1). Evidence of input of anthropogenically impacted Hg is provided by negative mass dependent isotopes values (MDF; δ202Hg) of coastal zooplankton (average = -0.95±0.5‰) and low mass independent isotope values (Δ199Ηg; 0.32±0.2‰), which are statistically different from surface zooplankton collected at Station ALOHA located in North Pacific Subtropical Gyre (average = δ202Hg 0.23‰; Δ199Ηg = 2.10‰; n=2 ) but overlap with the reported δ202Hg and Δ199Ηg values of coastal fish from the Chinese Bohai Sea (average; δ202Hg = -0.99±0.5‰; Δ199Ηg = 0.31±0.2‰). The Bohai Sea is surrounded by highly industrialized and densely populated cities, and has been severely contaminated by heavy metals due to anthropogenic activity. The δ202Hg values of the coastal zooplankton also overlap with the mean inorganic Hg anthropogenic river discharges (-0.49‰) estimated by a global-scale mercury isotope model. Evidence for transport of anthropogenic Hg from the coast of China to the open ocean is provided by the negative δ202Hg and low Δ199Ηg values of zooplankton collected in the Philippine Sea (average; δ202Hg = -0.37±0.1‰; Δ199Ηg = 0.55±0.2‰). Although the δ202Hg of values are less negative than the fish from the Bohai Sea and the coastal zooplankton, the values are statistically different from the recorded Atlantic and Pacific Ocean flying fish δ202Hg values (0.94±0.4‰) and epipelagic zooplankton from Station ALOHA (0.10±0.2‰). Furthermore, Δ199Ηg values of zooplankton from the Philippine sea remain significantly suppressed compared to surface marine zooplankton at Station ALOHA. Our results suggest that a significant amount of anthropogenic Hg has been transported to the Philippine Sea and likely has impacted fish used for human consumption in this area. While the Philippine sea is located at the Westerner Pacific edge of the North Pacific Subtropical Gyre, Hg stable isotopes values suggest that there is minimal transport of anthropogenically impacted Hg to the center of the Subtropical Gyre at Station ALOHA. It has been hypothesized that anthropogenic Hg from the coast of China and Japan likely accounted for a significant amount of Hg found in North Pacific Intermediate Waters at the central North Pacific Subtropical Gyre, which this is not the case based on the Hg stable isotope values of the zooplankton presented here. Our results also corroborate that the main input of Hg to the pelagic waters come from atmospheric precipitation. However, if anthropogenic emissions increase, Hg may likely start to mobilize to deeper parts of the ocean, as it has already in the Chinese coast and the Philippine sea, which is ~ 2000 km away from the Chinese coast.