Assimilation of Fe and carbon by marine copepods from Fe-limited and Fe-replete diatom prey

Previous studies have demonstrated that growth of marine phytoplankton, bacteria and protozoa can be limited by the availability of Fe. We report evidence that the amount of Fe assimi- lated by crustacean grazers is affected by the Fe status of their prey. 59 Fe and 14 C radiotracers were used to follow the fate of Fe and carbon (C) during trophic transfer from diatoms to copepods. Fe assimilation efficiency was higher for copepods ingesting Fe-limited Thalassiosira weissflogii (17%) compared to diatoms that were not limited by Fe (10%), but assimilated Fe was lost more rapidly by copepods ingesting Fe-limited prey. Fe:C assimilation ratios were lower in copepods (5-12 µmol Fe:mol C) than the cellular ratios of the phytoplankon prey (17-35 µmol Fe:mol C), suggesting that copepods do not accumulate Fe relative to C during grazing. The largest single fate for Fe and C after grazing was regeneration to dissolved pools. Fe:C ratios in dissolved pools were approximately equal to the ratios in the original prey, but Fe:C ratios were higher in particulate pools (largely fecal pellets), which should facilitate the export of Fe from the euphotic zone relative to C. Although copepod grazing does recycle cellular Fe and C, our results indicate that grazing may also tend to enhance Fe stress for lower trophic levels by removing Fe from the euphotic zone faster than C.