Regeneration, recycling, and trophic transfer of trace metals by microbial food‐web organisms in the pelagic surface waters of Lake Erie

Abstract

Rapid regeneration of 109Cd and 65Zn from their picoplankton prey into the dissolved phase by microzooplankton was observed in water sampled from the pelagic surface waters of Lake Erie (summer 1994 and 1995). Trace metals were added to grazing (lake water <210 µm) and control (lake water <0.2 µm) treatments in the form of radiolabeled Synechococcus. Picoplankton (0.2–3 µm) were grazed heavily by consumers in the nanoplankton (3–20 µm) and microplankton (20–210 µm) size classes (collectively referred to as microzooplankton) as confirmed by dilution assays used to independently measure grazing activity. Most consumed trace metals were regenerated into the dissolved phase (<0.2 µm), but some trophic transfer of 109Cd and 65Zn from radiolabeled prey into the nanoplankton and microplankton did occur: 65Zn was transferred 2.5 times more efficiently into the microplankton and 2.9 times more efficiently into the nanoplankton than was 109Cd. Recycling o:f regenerated 109Cd back into plankton biomass was greater than that for 65Zn. Grazing by microzooplankton influenced the molecular size distribution of regenerated trace metal in the dissolved phase (77±6% 109Cd <5,000 MW; 8 ± 24% 65Zn <5,000 MW). These results show that microzooplankton grazing tends to prolong the residence times of metals such as Cd and Zn in the pelagic surface waters of large lakes.