Effect and Bioaccumulation of Mercury in Arctic Marine Plankton under Climate Change

The Arctic region is considered a major sink of anthropogenic mercury (Hg), although it has limited local emissions. This is mainly due to the transport of mercury via atmospheric and oceanic currents from the lower latitudes to the Arctic. The continuous Hg deposition and accumulation over millennials has resulted in elevated Hg concentrations, especially in the Arctic cryosphere, making it a hotspot for legacy Hg.

Given the rapid melting of the Arctic cryosphere in response to global warming, it is expected that the Hg preserved in the cryosphere will eventually be liberated into Arctic marine ecosystems, posing an unknown impact on Arctic marine biota. Indeed, future increases in Hg concentrations have been projected in Arctic marine environments. This is of particular concern as Hg, especially its organic form (methyl Hg -MeHg), has the ability to bioaccumulate and magnify in both animals and humans, imposing detrimental effects on their physiological, behavioral and reproductive success, even at minute concentrations.

Generally, Hg enters Arctic marine ecosystems in inorganic form (IHg). In the aquatic environment, IHg is acted on mainly by iron- and sulfur-reducing bacteria, converting it into MeHg, which is readily bioavailable to Arctic marine biota.

Plankton — phyto and zooplankton — are the primary constituents at the base of the Arctic marine pelagic food web, converting and transferring energy to higher trophic level organisms, including fish. Consequently, plankton plays an important role in the cycling and transfer of Hg to higher trophic-level organisms. In fact, the dietary route has been identified as a critical pathway of Hg transfer to higher trophic level organisms. Additionally, plankton are definite indicators of stressors in marine ecosystems as they can highlight subtle changes in their surrounding environment. In effect, minute changes in environmental conditions can significantly affect plankton community structure and composition. Given their role in the biogeochemical cycling of energy and Hg and sensitivity to subtle environmental changes, it is important to monitor the concentration of Hg in Arctic plankton, especially in relation to the melting of the cryosphere. This will subsequently help in developing biologically relevant parameters for modelling Hg dynamics in Arctic marine ecosystems.

Despite these needs, biomonitoring studies on Hg in Arctic marine ecosystems are scarce. Moreover, the role of Hg in shaping the community structure and composition of Arctic plankton are barely studied. Most studies are instead focused on tropical planktonic organisms.

This Ph.D. thesis quantified the concentration of THg (all Hg forms) in plankton organisms in coastal and open sea areas along the west (paper I) and east (paper II) Greenland Coast. The plankton samples were divided into three main size fractions ≥ 200, 50 – 200 and 20- 50 μm, coarsely representing mesozooplankton, microzooplankton and phytoplankton, respectively. The concentration range of Hg observed in our studies was among the highest recorded in the Arctic region (4.8 to 241.3 ngTHg (g dw)^-1). The concentration was also higher in fjords than for open sea plankton samples. The highest concentrations were observed in stations sampled during the early spring period likely indicating seasonal variation, which could be related to atmospheric mercury depletion events. However, contrary to our general hypothesis, Hg concentrations in the lower plankton size fraction were higher than concentrations in the larger plankton size fraction, resulting in biodiminishing rather than the expected biomagnification at most of the sampling stations. This may be due to the depuration ability of the larger plankton size fraction either through excretion into dissolved phase or in feacal pellets, as highlighted for tropical zooplankton.

As a consequence of the observed biodiminishing for the field studies, we sought to understand the role of Arctic zooplankton in the transfer of Hg to the deeper ocean through feacal pellet transport (paper II). To achieve this, we exposed mesozooplankton to both IHg and MeHg and measured the total Hg concentration in their tissues and faecal pellets. Our results showed that mesozooplankton exposed to IHg generally had higher Hg concentrations in their faecal pellet and generally lower concentrations in their tissues, in contrast to those exposed to MeHg, which had higher THg concentrations in their tissues than in their faecal pellets. This indicates that Arctic mesozooplankton can significantly transfer inorganic Hg to the deeper ocean while the organic mercury is retained in the organism. Hence, zooplankton plays a significant role in the cycling of mercury through depuration in faecal pellets.

Having measured the environmental concentrations of mercury (Hg) in Arctic plankton as well as the role of Arctic marine zooplankton in transferring Hg to the deeper ocean, we proceeded to quantify the concentration of IHg and MeHg that imposes lethal and sublethal effects on key Arctic zooplankton organisms (paper III). Our findings showed that the response of Arctic zooplankton was dependent on the type of Hg, with MeHg being more toxic than IHg. Also, tolerance is inversely correlated with exposure time and concentration. Further, tolerance to both Hg types was species-dependent, which may be influenced by the different mechanistic and molecular responses of the different species. The response was also related to copepod traits such as lipid sac volume, body size and life stage, with larger, lipid-rich and adult copepods being more tolerant to IHg and MeHg than smaller, lipid-poor and juvenile copepods.

In general, our results provided knowledge on the planktonic Hg concentrations in the Arctic region, the role of Arctic plankton in transferring Hg to the deeper ocean and into the food web, and how key Arctic zooplankton species respond to Hg exposure. In effect, this will aid in understanding and predicting the dynamics of Hg, which will in turn aid in environmental risk assessment in the changing Arctic.