Across vast stretches of the open ocean, the growth of phytoplankton, and therefore the productivity of entire marine food webs, is limited by scarce iron. Researchers from Hokkaido University and the University of Tokyo have added a missing piece to the ocean's iron story, reporting what they describe as the first measurements worldwide of iron stable isotope ratios in individual zooplankton species rather than mixed net hauls.
Using a trace stable isotope method built on the double-spike technique, the team measured iron concentrations and isotope ratios species by species. The results show that species-separated zooplankton carry lower iron requirements than bulk net samples suggest, and their isotope ratios are significantly lower. Conventional bulk sampling, in other words, obscures species-specific signatures, which may explain discrepancies between earlier studies.
Published in ACS ES&T Water, the findings open a pathway to tracing how iron moves from seawater through zooplankton and onwards to fish, a chain that bulk measurements could never resolve. Since iron availability governs how much carbon the ocean's biological pump can draw down, a sharper species-level accounting of its cycling matters both for fisheries science and for understanding the ocean's role in the global climate system.