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 'OgCd 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 < 2'10 brn) and control (lake water CO.2 pm) treatments in the form of radiolabeled Synechococcus, Picoplankton (0.2-3 pm) were grazed heavily by consumers in the nanoplankton (3-20 pm) and microplankton (20-2 10 pm) 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 (x0.2 pm), but some trophic transfer of 'OgCd 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 logCd. Recycling of regenerated logCd 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% log Cd <5,000 MW; 8f24% "5Zn ~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.The ultimate geochemical fate of particle-reactive trace metals in pelagic zones of lakes is controlled by the vertical flux of metal in the water column, where metal loss is governed by the sinking of particulate matter to the underlying sediment (Fig. 1A). Metals may become associated with this particulate matter by various sorptive processes (scavenging) such as adsorption, co-precipitation, and, in the case of organisms, cellular internalization. Various inorganic surfaces, such as calcite, clays, and iron and manganese oxyhydroxides, may be involved in scavenging, but in the pelagic environment, biological surfaces are considered to dominate scavenging processes (Sigg 1994;Murray 1987;Morel and Hudson 1985). This is particularly true in the pelagic surface waters of the Laurentian Great Lakes of North America, which are effectively isolated from sediment influences during thermal stratification (Eadie and Robbins 1987).Among the biological particles, picoplankton (bacteria, cyanobacteria, and algae; 0.2-2 pm) are ideally suited to the scavenging of trace metals (Fisher 1985) owing to their rapid growth rates and high surface area-to-volume ratios.

Individual responses of trace-element assimilation and physiological turnover by the marine copepod Calanus sinicus to changes in food quantity

“…Biologically-mediated regeneration is critical to metal cycling in marine and freshwater systems (Hutchins et al 1993, Hutchins & Bruland 1994, Twiss & Campbell 1995, Twiss et al 1996, Wang & Fisher 1998b. The present results have demonstrated that the physiological turnover of assimilated trace metals in Calanus sinicus is relatively independent of algal diet or the amount of food ingested, similar to the results of previous studies on other copepods .…”

Diurnal variations of trace metals and heterotrophic bacterioplankton concentration in a small boreal lake of the White Sea basin

“…These are the biologically important elements such as Mn, Co and Mo, and also Cd. The latter is known to substitute Zn in the carbonic anhydrase of diatoms in case of Zn limitation conditions (Lane et al, 2005) and both are significantly affected by the trophic transfer along the foodwebs (Twiss et al, 1996). Peak concentrations of these elements occurred during the second night of monitoring.…”

Accumulation of trace elements in a marine copepod

“…The release rates of metals from decomposing fecal material have been quantified for a variety of conditions and can affect the rates of metal remineralization during their descent in the water column (Lee andFisher 1992, 1994;Reinfelder et al 1993;Wang et al 19966). Regeneration of assimilated trace elements by zooplankton grazing can significantly affect their geochemical cycling and residence times and can affect their bioavailability to phytoplankton (Hutchins and Bruland 1994;Lee and Fisher 1994;Twiss and Campbell 1995;Twiss et al 1996;Wang et al 19966). However, mechanistic and quantitative studies of zooplankton regeneration have been limited.…”