Influence of dissolved organic matter character on mercury incorporation by planktonic organisms: an experimental study using oligotrophic water from Patagonian lakes

Ligands present in dissolved organic matter (DOM) form complexes with inorganic divalent mercury (Hg²⁺) affecting its bioavailability in pelagic food webs. This investigation addresses the influence of a natural gradient of DOM present in Patagonian lakes on the bioaccumulation of Hg²⁺ (the prevailing mercury species in the water column of these lakes) by the algae Cryptomonas erosa and the zooplankters Brachionus calyciflorus and Boeckella antiqua. Hg²⁺ accumulation was studied through laboratory experiments using natural water of four oligotrophic Patagonian lakes amended with¹⁹⁷Hg²⁺. The bioavailability of Hg²⁺ was affected by the concentration and character of DOM. The entrance of Hg²⁺ into pelagic food webs occurs mostly through passive and active accumulation. The incorporation of Hg²⁺ by Cryptomonas, up to 27% of the Hg²⁺ amended, was found to be rapid and dominated by passive adsorption, and was greatest when low molecular weight compounds with protein-like or small phenolic signatures prevailed in the DOM. Conversely, high molecular weight compounds with a humic or fulvic signature kept Hg²⁺ in the dissolved phase, resulting in the lowest Hg²⁺ accumulation in this algae. In Brachionus and Boeckella the direct incorporation of Hg from the aqueous phase was up to 3% of the Hg²⁺ amended. The dietary incorporation of Hg²⁺ by Boeckella exceeded the direct absorption of this metal in natural water, and was remarkably similar to the Hg²⁺ adsorbed in their prey. Overall, DOM concentration and character affected the adsorption of Hg²⁺ by algae through competitive binding, while the incorporation of Hg²⁺ into the zooplankton was dominated by trophic or dietary transfer.