Limnol. Oceanogr., 44(4), 1999, 1009–1018

Dissolved iron (Fe) concentrations in the California coastal upwelling regime 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 experiments that use additions of the exogenous siderophore desferrioxamine B (DFOB, obtained from a terrestrial actinomycete fungus) to sequester ambient Fe and to markedly decrease its availability to the biota. DFOB additions resulted in artificial Fe limitation of the phytoplankton community in highFe areas of the upwelling region. Results of these ‘‘Fe-removal’’ experiments mirror those of Fe-addition experiments in low-Fe, high-nutrient, low-chlorophyll (HNLC) waters. When DFOB is added to Fe-replete waters, changes in nutrient concentrations, biomass, and other biological parameters closely resemble those seen in Fe-limited controls in HNLC areas, while the controls without 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 carbonuptake rates as well as particulate production. Diatoms and other phytoplankton bloomed profusely in unamended controls but not in Fe-limited 1DFOB bottles. Bacterial numbers and zooplankton grazing activity were also severely reduced in DFOB-addition bottles. These experiments demonstrate that artificially lowering Fe availability can induce limitation of autotrophic and heterotrophic plankton and can prevent utilization of the high ambient levels of upwelled nutrients along the California coast. Our results suggest that DFOB-bound Fe is highly unavailable to the plankton community, a result that offers researchers an important tool to use to probe the influence of Fe on biological community development in high-Fe regimes. Recent work has given us a new perspective on the biological role of iron (Fe) in coastal upwelling regimes. Hutchins and Bruland (1998), Hutchins et al. (1998), and Franck et al. (unpubl. data) demonstrated that phytoplankton in parts of the California upwelling region are Fe limited. As in previous work in open-ocean, high-nutrient, low-chlorophyll (HNLC) waters, Fe additions along the nutrient-rich but Fedepleted Big Sur coast of central California promoted the growth of large diatoms and allowed complete depletion of ambient nitrate. Fe also controlled major nutrient utilization ratios, bacterial production and abundance, and even grazing activity in these low-Fe coastal HNLC waters. Not all areas along the California coast, however, are as Fe-limited as are the waters off of Big Sur. Hutchins et al. (1998) characterized the region as a complex regional mosaic of Fe limitation and attempted to describe the continuum