A mass balance inventory of mercury in the Arctic Ocean

Abstract

Environmental context. Mercury (Hg) occurs at high concentrations in Arctic marine wildlife, posing a possible health risk to northern peoples who use these animals for food. We find that although the dramatic Hg increases in Arctic Ocean animals since pre-industrial times can be explained by sustained small annual inputs, recent rapid increases probably cannot because of the existing large oceanic Hg reservoir (the 'flywheel' effect). Climate change is a possible alternative force underpinning recent trends.Abstract. The present mercury (Hg) mass balance was developed to gain insights into the sources, sinks and processes regulating biological Hg trends in the Arctic Ocean. Annual total Hg inputs (mainly wet deposition, coastal erosion, seawater import, and 'excess' deposition due to atmospheric Hg depletion events) are nearly in balance with outputs (mainly shelf sedimentation and seawater export), with a net 0.3% year −1 increase in total mass. Marine biota represent a small fraction of the ocean's existing total Hg and methyl-Hg (MeHg) inventories. The inertia associated with these large non-biological reservoirs means that 'bottom-up' processes (control of bioavailable Hg concentrations by mass inputs or Hg speciation) are probably incapable of explaining recent biotic Hg trends, contrary to prevailing opinion. Instead, varying rates of bioaccumulation and trophic transfer from the abiotic MeHg reservoir may be key, and are susceptible to ecological, climatic and biogeochemical influences. Deep and sustained cuts to global anthropogenic Hg emissions are required to return biotic Hg levels to their natural state. However, because of mass inertia and the less dominant role of atmospheric inputs, the decline of seawater and biotic Hg concentrations in the Arctic Ocean will be more gradual than the rate of emission reduction and slower than in other oceans and freshwaters. Climate warming has likely already influenced Arctic Hg dynamics, with shrinking sea-ice cover one of the defining variables. Future warming will probably force more Hg out of the ocean's euphotic zone through greater evasion to air and faster Hg sedimentation driven by higher primary productivity; these losses will be countered by enhanced inputs from coastal erosion and rivers.

Difference of mercury bioaccumulation in red mullets from the north-western Mediterranean and Black seas

“…Thus, the lower Hg concentration recorded in the Black Sea red mullet compared to the Mediterranean species is explained most likely by the higher primary production in the Black Sea. We arrived with a similar interpretation as Outridge et al (2008) who conclude that the concentration of bioavailable Hg in seawater is incapable to satisfactorily explaining Hg variations in marine biota. They hypothesise that the rate of biological uptake and trophic transfer are the key regulators of bioaccumulation and biomagnification in marine organisms.…”

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…[31,33] Rivers provide a significant source of THg, including dissolved and particulate forms, to Arctic coasts. [34][35][36][37] In the case of the Mackenzie River, as it enters the coast during ice-free conditions (late July), processes leading to high levels of DGM in the water appear to prevail as there is a strong increase in DGM associated with plume water (Fig. 4).…”

Mercury contamination in aquatic ecosystems under a changing environment: Implications for the Three Gorges Reservoir

“…A recent study estimated that coal-fired power plants from Chongqing, Sichuan and Guizhou, where the majority of the TGR watershed is located, emit a total of 11 t/year of Hg to air in 2007 [29]. To put this number in context, the entire Arctic Ocean, which has a surface area about 9000 times greater than TGR, receives a similar amount of atmospheric Hg (8.4-98 t/year; [24]). Of course, a fraction of the emitted Hg 0 from the power plants is carried out to other regions, but a significant fraction, especially those in the form of Hg(II) (gaseous or particulate), is deposited in the region and ultimately into the reservoir.…”

“…Throughout all the three phases, biogeochemical cycling (shown as sine-wave ''noise" in Figure 2) determines the speciation, bioavailability, and uptake of the contaminant, but it is in the latter phase that these processes emerge to create a variability that is large enough to obscure downturning or up-turning trends, at least at the decadal scale if not longer [8]. Based primarily on a Hg mass balance study on the Arctic Ocean [24], we proposed at that time that Hg bioaccumulation in the Arctic marine ecosystem had reached Phase III in the past 30 years, resulting in "noise" from the processes becoming the main "signal" in the ''trend" data for Hg in Arctic marine animals during the recent decades [8]. This proposal is supported by a recent comprehensive, international assessment of Hg in the Arctic [6].…”