Can copepods be limited by the iron content of their food?

Abstract

In laboratory experiments, we show that naupliar survival and egg production by the copepod Acartia tonsa was significantly lower when they were fed iron‐depleted algal cells than when they were fed a diet of Fe‐replete cells that had two orders of magnitude higher Fe levels. Naupliar survival after 1 week was reduced from 60% for copepods feeding on Fe‐replete diatoms (Thalassiosira oceanica) to 0% for those feeding on Fe‐depleted cells. The decline of egg production rate was greatest (83%) when T. oceanica was used as prey and smaller (20–30%) when the cryptophyte Rhodomonas salina and the prymnesiophyte Isochrysis galbana were used as food. The assimilation rates of Fe and C from Fe‐replete and Fe‐depleted algae were determined using radioisotopes. Egg production was hyperbolically dependent on the Fe assimilation rate (r2=0.71). The effect of Fe on copepod reproduction rates could not be explained by changes in ingestion rate and the rate of carbon assimilated, because there was no significant difference of ingestion rate and C assimilation between Fe‐replete and Fe‐depleted treatments. Iron might limit zooplankton productivity in high‐nutrient, low‐chlorophyll regions.