Mercury in the coastal pelagic food web: phytoplankton, zooplankton and jellyfish

Four jellyfish species, ctenophoran Mnemiopsis leidyi and scyphozoan Cotylorhiza tuberculata, Chrysoara hysoscella and Rhizostoma pulmo were collected in summer of 2017 in the Gulf of Trieste (northern Adriatic Sea) and analysed for Hg and other metal(loid)s to assess their bioaccumulation and biomonitoring potential. No significant differences in Hg levels (0.06-0.22 µg/g dry mass) were observed between the studied species but all significantly concentrated Hg well above the dissolved Hg seawater levels (5 ng/L) of the gulf. The studied species have diverse diets consisting primarily of various plankton groups. C. hysoscella feeds mainly on mesozooplankton (>200 µm) R. pulmo and C. tuberculata mostly consume microzooplankton (50-200 µm) while M. leidyi preys on various organism (and particles) in the water column. In addition, C. tuberculata harbours autotrophic endosymbionts (microalgae). Considering their feeding behaviour, it appears that studied jellyfish species do not bioaccumulate Hg, nor other metal(loid)s, along the pelagic food web. Hence, the Hg levels in jellyfish are probably the consequence of the dissolved metal (passive and active) uptake. Moreover, the methodological approach analysing the jellyfish freeze-dried samples containing salt can distort the real picture and the Hg/Corg. ratio could better describe the metal level in the gelatinous organism. However, considering the high Hg bioconcentration factor (log BCF >5), jellyfish can be used aa a useful bioindicator for Hg, and other metal(loid)s, dissolved in seawater.     Keywords: Jellyfish, coastal waters, mercury, bioconcentration, contamination, bioindicator