In vitro iron enrichment experiments at iron-rich and -poor sites in the NE subarctic Pacific

Despite evidence of Fe limitation of phytoplankton biomass in large areas of the ocean: few studies have assessed the relationships between algal stocks and Fe supply. An E-W survey transected the coastal and the open NE subarctic Pacific in May 1995, and revealed low nitrate (< 1 mu M) inshore, which increased westwards to ca. 8 mu M. Over this transect, dissolved Fe fell from > 3 nM kg(-1) inshore to < 0.5 nM kg(-1) offshore. Fluorescence indices of photosystem II quantum efficiency (F-v/F-m, measured using DCMU) increased with distance offshore, but were always submaximal, indicating physiological constraints on photosynthetic capabilities, likely nitrogen inshore and Fe offshore. Six day in vitro Fe enrichments were performed at stations that were 500 (P12, Fe-rich) and 1000 lan offshore (P26, Fe-poor). At P26, there were marked increases in NO3 uptake and in chlorophyll a in both the control and Fe enrichment (3.5 nM Fe). Changes in nitrate reductase (NR) activity paralleled changes in NO3 uptake, indicating true physiological responses. Measurements showed that controls had been contaminated with 1.8 nM Fe. However, transient increases in F-v/F-m were observed only in Fe treatments and not in contaminated controls. This may have been due to the form in which the Fe was added (chelated Fe in treatments versus probably non-chelated Fe in "controls"). The time scale for the rise and fall of F-v/F-m is similar to that reported in the Ironex experiments. At P12, Fe-enrichment did not increase NO3 uptake or NR activity, compared to the controls. Reasons for the differences in the results of enrichments at P26 and P12 are unclear. Neither Fe nor NO3 was limiting at P12, yet F-v/F-m remained sub-maximal, suggesting that non-physiological factors (e.g. grazing) were not responsible. Unlike P26, at P12, there was a relatively small "seed" population of diatoms; factors controlling diatom abundances under ambient conditions may be crucial in determining the outcome of Fe enrichments. (C) 1998 Elsevier Science B.V.