The level of iron enrichment required to initiate diatom blooms in HNLC waters

The chemical speciation of iron in seawater is controlled by complexation with organic ligands, the character of which likely regulates which phytoplankton groups can readily access this resource. Evidence from the IronEx II mesoscale iron enrichment experiment provides some insight to this regulation. Dissolved iron (<0.4 μm) in the enriched surface waters during IronEx II was partitioned into colloidal (>1 kDa–0.4 μm) and soluble (<1 kDa) size fractions using cross flow filtration to better delineate iron dynamics with respect to phytoplankton growth. While dissolved concentrations in the patch increased by two orders of magnitude to 2 nM Fe, soluble concentrations only roughly doubled to 35 pM Fe, and this change occurred only during the first few days of the experiment. The subsequent decrease in soluble iron to ambient levels coincided with a dramatic increase in chlorophyll a, indicating that biological demand was responsible for the disappearance of soluble iron. This decrease also coincided with preferential drawdown of silicic acid over nitrate in the patch, indicating that diatoms were experiencing iron stress even as the bloom developed. Even so, calculations of the diffusional flux and iron uptake kinetics at the cellular level reveal that the concentrations of soluble iron during the later stages of the bloom were above that required to support continued rapid growth of the long, narrow pennate diatoms. This observation implies that the bulk of the organically bound soluble and colloidal iron was unavailable to meet the diatom growth requirements, and further that the measured increase in concentrations of strong Fe(III) complexing ligands impaired iron acquisition by diatoms. These findings indicate that excess macronutrients in HNLC waters might not be fully converted into diatom biomass even with repeated infusions of iron.