Comparing Biological Iron Uptake and Particle Association Kinetics across Different Systems of the North Atlantic

Cycling of Fe in surface waters is driven by a range of processes which drive exchanges between the major biological, particulate, colloidal and ligand associated pools. Experimental determination of the kinetics of these exchange processes in natural systems is complicated by their variety and the potential for interactions between the pools, alongside the practical constraint imposed by the requirement to make tracer measurements at low (i.e. truly trace) levels. Here we utilise high specific activity (effectively carrier free) 55Fe to directly measure the kinetics of particle association and biological uptake across a variety of subsystems in the North Atlantic including: high particle loading shelf waters, high Fe low latitude (sub-)tropical waters and seasonally Fe limited high latitude waters. The data reveal rapid exchange between dissolved, colloidal and particle associated pools under the utilised experimental conditions, with the majority of the added tracer becoming associated with the non-biological particulate pool within hours across all systems. In contrast biological uptake rates were highly variable, with synthesis of results across the multiple systems revealing patterns of uptake which appeared consistent with measured or expected Fe limitation status. Further comparing results with simple first order modelling of ligand exchange kinetics we suggest that bioavailability may play a key role in the development of Fe limitation following the spring bloom in the high latitude North Atlantic.