Copepod grazing and the biogeochemical fate of diatom iron

We investigated the effects of copepod grazing on fractionation of diatom cellular Fe into assimilated, dissolved, and fecal pellet pools. Grazer assimilation was only weakly dependent on prey N: Fe ratios, with assimilation efficiencies approximately tripling over a 100‐fold increase in diatom N: Fe ratios. Assimilation efficiencies strongly correlated with prey subcellular partitioning, however, exhibiting a 1 : 1 relationship to cell cytoplasmic content and a 1 : 3 relationship with diatom intracellular content. Release of Fe from copepod fecal pellets conformed with a two‐compartment loss model, with roughly 80% of associated Fe in a slowly released pool (tb_½ = 113 d). Partitioning into dissolved, assimilated, and fecal pellet pools indicated that all three are important fates for ingested cellular Fe.