Inducing phytoplankton iron limitation in iron-replete coastal waters witha strong chelating ligand

Dissolved iron (Fe) concentrations in the California coastal upwelling regi me vary over two orders of magnitude (from <0.05 to >5 nM), which leads to a wide range in Fe effects on phytoplankton growth. Fe-addition experiments are appropriate to use to assess the biological role of Fe in low-Fe areas , but other methods are needed in Fe-replete regions. We present experiment s that use additions of the exogenous siderophore desferrioxamine B (DFOB, obtained from a terrestrial actinomycete fungus) to sequester ambient Fe an d to markedly decrease its availability to the biota. DFOB additions result ed in artificial Fe limitation of the phytoplankton community in high-Fe ar eas of the upwelling region. Results of these "Fe-removal" experiments mirr or those of Fe-addition experiments in low-Fe, high-nutrient, low-chlorophy ll (HNLC) waters. When DFOB is added to Fe-replete waters, changes in nutri ent concentrations, biomass, and other biological parameters closely resemb le those seen in Fe-limited controls in HNLC areas, while the controls with out DFOB behave much like HNLC Fe-addition bottles. DFOB additions in high- Fe waters greatly reduced biological Fe uptake and, consequently, nitrate, silicic acid, and carbon-uptake rates as well as particulate production. Di atoms and other phytoplankton bloomed profusely in unamended controls but n ot in Fe-Limited +DFOB bottles. Bacterial numbers and zooplankton grazing a ctivity were also severely reduced in DFOB-addition bottles. These experime nts demonstrate that artificially lowering Fe availability can induce limit ation of autotrophic and heterotrophic plankton and can prevent utilization of the high ambient levels of upwelled nutrients along the California coas t. Our results suggest that DFOB-bound Fe is highly unavailable to the plan kton community, a result that offers researchers an important tool to use t o probe the influence of Fe on biological community development in high-Fe regimes.