Improved Environmental Status: 50 Years of Declining Fish Mercury Levels in Boreal and Subarctic Fennoscandia

Abstract

Temporally and spatially (55°-70°N) extensive records of mercury (Hg) in freshwater fish showed consistent declines in boreal and subarctic Fennoscandia. The database contains fish entries (n: pike>perch>>brown trout>roach≈Arctic charr) from 3132 lakes across Sweden, Finland, Norway, and Russian Murmansk area. 74% of the lakes did not meet the 0.5 ppm limit to protect human health. However, after 2000 only 25% of the lakes exceeded this level, indicating improved environmental status. In lakes where local pollution sources were identified, pike and perch Hg concentrations were significantly higher between 1965 and 1990 compared to values after 1995, likely an effect of implemented reduction measures. In lakes where Hg originated from long-range transboundary air pollution (LRTAP), consistent Hg declines (3-7‰ per year) were found for perch and pike in both boreal and subarctic Fennoscandia, suggesting common environmental controls. Hg in perch and pike in LRTAP lakes showed minimal declines with latitude, suggesting that drivers affected by temperature, such as growth dilution, counteracted Hg loading and foodweb exposure. We recommend that future fish Hg monitoring sampling design should include repeated sampling and collection of supporting information (pollution history, water chemistry, fish age, stable isotopes) to enable evaluation of emission reduction policies.

Percentage of methylmercury in the muscle tissue of freshwater fish varies with body size and age and among species

“…Furthermore, although several studies have estimated %MeHg in tissues of large‐bodied fish (Latif et al ; Mieiro et al ; Braaten et al ), far less is known about Hg speciation in smaller bodied species. Smaller bodied fish are subjected to different bioenergetic processes compared with their larger counterparts, and the biokinetics of Hg(II) and MeHg uptake, assimilation, and elimination can change with fish size (Dang and Wang ; Jørgensen et al ).…”

Terrestrial diet influences mercury bioaccumulation in zooplankton and macroinvertebrates in lakes with differing dissolved organic carbon concentrations

“…Thus, if MeHg bioavailability is greatly promoted under low pH in high DOC freshwaters, i.e., in our investigated humic lakes, this can significantly contribute to elevated Hg content in aquatic consumers, regardless of consumer selectivity towards different diet sources. There is also a likelihood of spurious correlations between total Hg bioaccumulation and DOC, due to a strong correlation of aqueous total Hg concentrations with DOC (in our case, r=0.90, p<0.01) (Braaten et al, 2019). Increased lake DOC also has large impacts on lake conditions, i.e., light attenuation (Ask et al, 2009) and oxygen depletion (Couture et al, 2015), which possibly affect the growth and feeding of zooplankton and macroinvertebrates on various dietary sources, and eventually Hg bioaccumulation.…”

Status and trends of mercury pollution of the atmosphere and terrestrial ecosystems in Poland

“…It was similar in the case of Scandinavian lakes, although THg concentration in this species was 4 times higher than in Poland. In the case of other predatory species, the perch (Perca fluviatilis), THg concentration measured in Sweden and Finland was over twice as high as in Poland (Miller et al 2013;Braaten et al 2019). The possible explanation for the high THg concentration in fish from Scandinavia is elevated concentration of Hg in the underlying bedrock, as its geology strongly influences the Hg level in the water and hence in aquatic fauna (Danielsson et al 2011; www.…”