Limnol. Oceanogr., 44(4), 1999, 1002–1008

The consideration of iron effects on marine ecosystems has focused mainly on high-nitrate low-chlorophyll regions, but iron likely has an equally important regulatory role in coastal waters. Iron requirements of neritic phytoplankton not only are comparatively high but also differ substantially among species, so that iron fluctuations within metal-replete systems should strongly influence the composition and distribution of phytoplankton assemblages. But unlike the simplicity of testing iron effects in iron-depleted waters, ascertaining iron effects in apparently replete waters has been forestalled by a lack of experimental tools that can regulate iron availability independent of other bioactive metals in seawater. I present here the results of size-fractionated shipboard culture experiments using the fungal siderophore desferriferrioxime B (DFB) to regulate iron availability in coastal upwelling waters. Addition of excess DFB essentially eliminated 59Fe uptake by both phytoplankton and heterotrophic bacteria over a 6-h period and allowed only marginal iron uptake over 5 d of incubation. Carbon uptake by small (0.2–5.0-mm) phytoplankton was immediately curtailed upon addition of DFB, signifying a rapid onset of iron stress. In contrast, short-term (0–6 h) carbon uptake by larger (.5.0-mm) phytoplankton was not affected, indicating that larger cells contained significant iron reserves. Nonetheless, carbon assimilation was substantially lower in DFB treatments relative to the controls after 5 d of incubation. Uptake of 54Mn, 65Zn, and 57Co was not immediately affected by DFB but then slowed after 4 h and was significantly lower after 5 d, presumably because iron limitation lowered the cellular requirements for these bioactive metals. These findings demonstrate that DFB can be used to manipulate biologically accessible iron to determine how iron affects algal community structure and carbon cycling in ironreplete waters. It is now well demonstrated that low concentrations of iron play a pivotal role in controlling primary production in open ocean high-nitrate low-chlorophyll (HNLC) regions (Martin and Fitzwater 1988; Martin et al. 1990; Coale et al. 1996) and in upwelling HNLC regions along the California coast (Hutchins and Bruland 1998). Heterotrophic bacterial production in HNLC waters also can be directly limited by iron (Pakulski et al. 1996), demonstrating that iron can profoundly influence both the production and remineralization segments of the marine carbon cycle (Tortell et al. 1996). But even in regions where iron additions have little or no impact on community-level production, iron still should affect the composition of algal assemblages and the distribution of phytoplankton species because iron requirements differ dramatically among phytoplankton (and presumably bacteria) species (Brand et al. 1983; Brand 1991; Sunda and Huntsman 1995). As a result, temporal and spatial variations in the concentration and chemical speciation of iron should likely influence the character and expression of phytoplankton species succession (Wells et al. 1995). Ascertaining whether iron limits phytoplankton production in surface waters is experimentally straightforward; iron is added to on-deck incubations of the seawater, and the resultant phytoplankton response is monitored. The challenge in these experiments is the consistent elimination of iron contamination so that the controls indeed reflect ambient seawater conditions. However, it has been considerably more difficult to probe iron effects on algal composition and