Elevated iron to nitrogen recycling by mesozooplankton in the Northeast Atlantic Ocean

Low dissolved iron (DFe) concentrations limit primary production in most high-nutrient low-chlorophyll (HNLC) regions. Increased recycling of iron (Fe) relative to nitrogen (N) by zooplankton may help to sustain phytoplankton production in these conditions. We concurrently determined rates of DFe and ammonium (NH4+) recycling by natural mesozooplankton communities in HNLC conditions of the Northeast Atlantic. NH4+ excretion remained constant and ranged between 14.2–54.1 nmol NH4+ mg dry weight−1 h−1. Fe recycling ranged between 6–138 pmol DFe mg dry weight−1 h−1 during the first hour and decreased thereafter, reflecting the transition from the loss of phytoplankton-derived Fe to basal DFe excretion. Mesozooplankton-driven nutrient recycling was estimated to support 6–59% and <1–13% of the respective phytoplankton requirements for DFe and N; DFe:N regeneration ratios were 5–26 times larger than those required by phytoplankton. Our data suggest that Fe recycling by grazing organisms has the potential to reduce the intensity of HNLC conditions.