Abstract
Total mercury (THg) and methylmercury (MeHg) concentrations were analyzed in zooplankton (≥450 and ≥850 µm size fractions) collected seasonally over 6 years in Lake Maggiore (Northern Italy), characterized by a legacy mercury contamination. Analysis of δ 15N and δ13C stable isotopes was carried out to trace how taxa with different trophic levels and carbon sources contributed to mercury concentrations and trends. THg ranged between 44–213 µg kg−1 d.w. and MeHg 15–93 µg kg−1 d.w., representing 24–61% of THg. Values showed strong seasonal variations, with peaks in winter, due to the high biomass of predator taxa (Bythotrephes longimanus, Leptodora kindtii) and of Daphnia longispina-galeata gr. A positive correlation between THg and MeHg and δ15N signature was observed. D. longispina-galeata gr. prevailed in both size fractions, substantially contributing to THg and MeHg concentrations. Δ13C signature was strictly bound to lake thermal circulation dynamics. Mercury stock in the zooplankton compartment ranged between 19–140 ng THg m−2 and 6–44 ng MeHg m−2 for the ≥450 µm size fraction and between 2–66 ng THg m−2 and 1–7 ng MeHg m−2 for the ≥850 µm fraction, with the highest values in spring when zooplanktivorous fish actively prey in the pelagic zone. The results highlighted the crucial role of zooplankton as a repository of mercury, easily available to higher trophic levels.
Highlights
Accepted: 18 February 2022Mercury is a contaminant of concern in freshwater environments because it is very toxic and persistent and effectively accumulates in food chains [1]
This paper highlights the crucial role of zooplankton as repository of mercury pollution in Lake Maggiore
Mercury transfer in the pelagic pathway is generally prevalent over the benthic one, the dynamics of mercury concentrations in zooplankton may help in understanding why the Hg levels of fish in Lake Maggiore still exceed the Environmental Quality Standard for biota, even if the main sources of the contaminant have been reduced and/or removed
Summary
Mercury is a contaminant of concern in freshwater environments because it is very toxic and persistent and effectively accumulates in food chains [1]. Even if concentrations in the water may be very low, i.e., in the order of a few ng L−1 [5,6,7,8], values in biota may reach high levels due to bioaccumulation and biomagnification along the trophic chains [9,10,11] This is frequently observed in many fish species, even where point sources of mercury are not present [12,13], reaching levels considered a threat to wildlife and human health through fish consumption [2,14,15]
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