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

摘要

Rapid regeneration of ~(109)Cd and ~(65)Zn from their picoplankton prey into the dissolved phase by micro-zooplankton 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 micro-zooplankton) 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 ~(109)Cd and ~(65)Zn from radiolabeled prey into the nanoplankton and microplankton did occur: ~(65)Zn was transferred 2.5 times more efficiently into the microplankton and 2.9 times more efficiently into the nanoplankton than was ~(109)Cd. Recycling of regenerated ~(109)Cd back into plankton biomass was greater than that for ~(65)Zn. Grazing by microzooplankton influenced the molecular size distribution of regenerated trace metal in the dissolved phase (77±6% ~(109)Cd < 5,000 MW; 8±24% ~(65)Zn < 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.