Assimilation of metals in marine copepods and its biogeochemical implications

Abstract

Laboratory experiments employing gamma-emitting radiotracers assessed the retention efficiency of ingested metals in the calanoid copepod Anomalocera patersoni and the retention of excreted metals in fecal pellets. Adult copepods ingested the haptophyte Isochrysis galbana labeled with Io9cd , "Z n, 2 0 3~g , " Â ¥ ' "~ and '-"Pu at a rate of 3.79 mg algal C g-I ammal dry wt h-I. Average retention efficiencies were 30 % for Cd, 48 % for Zn, 21 O/O for Hg, 4.5 % for Am, and 0.8 % for Pu. Algal cells resuspended into unlabeled seawater retained metals to varying extents, with depuration curves conforming to a 2-compartment model; Cd was lost most rapidly. The fecal pellets produced by animals feeding on radioactive I. galbana lost essentially all of their l o g c d , "~n , ~m , and 2 3 7~u to unlabeled seawater within 1 d. ^Hg was lost most slowly, again conforming to a 2-compartment model; its retention half time was about 25 d in the slowly exchanging pool. Bacterial activity did not appear to have an effect on metal retention in the fecal pellets. The retention efficiencies suggest that Cd, Zn, and Hg should be recycled by copepods in surface waters as part of the organic cycle in the sea, while the transuranic elements should be defecated and removed from surface waters by sinking biogemc debris.

Grazer-mediated regeneration and assimilation of Fe, Zn and Mn from planktonic prey

“…They reported typical assimilation efficiencies for elements such as Am and Pu ranging from less than 1 % to a few percent. Values obtained in the present set of experiments for the biologically required, particlereactive element Fe were about an order of magnitude higher than this, although not as high as values for major nutrient elements such as phosphorus ( 7 2 % ; Reinfelder & Fisher 1991). Assimilation efficiencies for Zn in this set of experiments ranged from 12 to 56%; Fisher et al (1991) reported a value of 48% for this metal.…”

“…Values obtained in the present set of experiments for the biologically required, particlereactive element Fe were about an order of magnitude higher than this, although not as high as values for major nutrient elements such as phosphorus ( 7 2 % ; Reinfelder & Fisher 1991). Assimilation efficiencies for Zn in this set of experiments ranged from 12 to 56%; Fisher et al (1991) reported a value of 48% for this metal. Mn was least efficiently assimilated in the experiments presented here (2 to 10%), a range of values only slightly higher than literature values for non-essential particle-reactive metals.…”

“…Efficiencies of Fe assimilation in these experiments ranged from 7 to 22%, higher than values for several non-required particle-reactive elements measured by Fisher et al (1991) andReinfelder &Fisher (1991). They reported typical assimilation efficiencies for elements such as Am and Pu ranging from less than 1 % to a few percent.…”

“…Jumars et al (1989) suggested that carbon is rapidly lost from egested fecal pellets because a large proportion is present in soluble forms within the pellet matrix. Fisher et al (1991) observed a rapid diffusion of Zn, Cd, Am and Pu from copepod fecal pellets, and suggested that a similar 'pore water' loss mechanism might be responsible. The increases in soluble Fe, Mn and Zn obsewed in the experiments presented here could be partially due to this type of remineralization mechanism, especially if acidic digestive processes have a significant influence on metal speciation prior to egestion.…”

“…Efficient grazer assimilation of elements ingested from prey is another mechanism which tends to extend the residence time of nutrients in surface waters (Fisher et al 1991). Assimilated elements may later be excreted, making them available to support future production.…”