There has been considerable debate about both the mechanisms controlling primary production, and the interpretation of data from Fe-enrichment experiments conducted in high nitrate-lotv chlorophyll (HNLC) regions. This paper presents results of 3 in vitro Fe-enrichment experiments performed in May 1993 and May 1994 at Ocean Station Papa (OSP) in the NE subarctic Pacific. Expt 1 (May 1993) considered jointly the influence of both Fe supply and microzooplankton herbivory on algal stocks, while the second (May 1993) investigated the influence of Fe supply on the partitioning of C, N and Fe between algal size classes. The third study (May 1994) monitored changes in phytoplankton stocks in Fe-enriched carboys containing mesozooplankton Assuming simllar environmental conditions in May 1993 and 1994, then the comblned findings indicate that under ambient conditions autotrophic cells < S pm compose >70'% of algal biomass, primarily utilized ammonium, showed no Femediated enhancement of N-normalized nitrate or ammonium uptake, and were thus unlikely to be Felimited. Although the coupling between grazer mortality and algal growth of these cells ( g / p ) was ca 0.71 k 0.25, they grew at close to maximal rates but exhibited negligible net growth rates, suggesting strong grazer control. Dissolved iron (DFe) supply promoted a rapid increase in the abundance of large, initially rare, diatoms (mainly Njtzschla spp.) over 6 d, elevated diatom growth rates to close to their theoretical maximum and increased rates of incorporation of N-normalized nitrate and Fe by these cells over time. Unlike the small cells, stocks of large autotrophs also increased in the controls; DFe measurements indicate that this was probably due to inadvertent Fe contamination. The development of a phytoplankton population, of a cell size probably too large to be grazed at a significant rate by microzooplankton, resulted in a decoupling of herbivory and algal growth. In addition, mesozooplankton herbivory appeared unable to prevent the accumulation of these large cells; in May 1994, Fe supply prompted an 8-fold increase in algal stocks over 6 d in carboys where grazers equivalent to the maximum annual in situ pelagic abundance were present. The findings agree with the theories of others, that Fe supply rather than grazing provides the ultimate control over the phytoplankton community in the NE subarctic Pacific.

Emiliania huxleyi, a small oceanic coccolithophore, was isolated from the NE subarctic Pacific and maintained under oceanic conditions. Coccolith-forming cultures were grown with either NO3-or NH,' as the primary nitrogen source. Fe-stress was induced experimentally, and physiological parameters including metal quotas (Fe, Mn, Zn, Cu) were measured for both NO3--and NH,'-grown cells to determine whether ~t was advantageous for the cells to grow on NH,' rather than NO3-under Fe-stressed conditions The parameters used to observe the cell's physiological status were specific growth rate M), cell volume (CV), carbon (C), chlorophyll a (chl a), and nitrogen (N) per cell volume.

The effect of iron (Fe) on the sinking rate of a n oceanic diatom Actinocyclus sp. and an oceanic coccolithophore Emiliania huxleyi, both isolated from the subarctic Pacific, was examined in natural oceanic seawater. The Fe status of the diatom had a dramatic effect on its sinking rate, causing a 5 times increase from 0.17 to 0.93 m d-' from Fe-replete to Fe-stressed conditions. In contrast, Fe had no effect on the sinking rate of the oceanic coccolithophore, whlch maintained its sinking rate at 0.12 m d-' The cell volume of the d~a t o m decreased slightly under Fe-stressed conditions, but the cell volume of the coccolithophore decreased substantially (4tiC%) under Fe-stressed conditions. The effect of nitrogen source (nitrate vs a m m o n~u m ) on the chlorophyll a (chl a), carbon ( C ) , and nitrogen ( N ) quotas of the oceanic dlatom Act~nocyclus sp. was also examined. Under Fe-stressed conditions when the energystress on the cells is the greatest, ammoni.urn-grown cells appeared to have a physiological advantage over nitrate-grown cells In this oceanic diatom. Ammonium-grown cells were able to maintain normal N and C quotas under Fe-stress, whereas nitrate-grown cells were not, resulting in an 80% reduction In N cell-' for nltrate grown cells under FP-stress. Also, In vlvo f1uo1-escence:chl a increased and chl a C decreased more drastically for nitrate-grown cells under Fc-stress than for ammonium-grown cells, indicating that nitrate-grown cells under Fe-stress are less capable of trapping and utilizing light energy. These findings support theoretical predictions based on Fe and energy requirements for nitrate versus ammonium utilization. Metal quotas (Fe, Mn, Zn) were measured simultaneously using cold-metal techniques to determine the metal content of the cells. There were no significant differences in metal to carbon ratios between nitrate and ammonium-grown cells under Fe-replete conditions. Under Festressed conditions, nitrate-grown cells had significantly higher Mn:C and significantly lower Zn:C ratios than ammonium-grown cells, but there was no observed difference in Fe quotas. In this study we observed that 2 different species of phytoplankton from the subarctic Pacific responded physiologically differently to s~milar Fe conditions. Our results suggest that the sol~tary, centric, 20 to 60 pm diameter oceanlc dlatorn would have a higher sinking rate than the oceanlc coccolithophore in the subarct~c Pacific, perhaps havlng implications for biogenic fluxes to depth. Moreover, our data indicate that thls diatom is probably utll~zing ammonium to mcet its nltrogen requlrements in situ under the low Fe conditlons found In the northeast subarctic Pacific. A c~~~~o c~c~u s s~.appears incapable of effectively chang111g its cell volume to help alleviate Fe-(and other nutnent) stress, whereas the coccolithophore can reduce ~t s cell volume substantially, allowing it to reduce its requlrements for N, C, and Fe. These physiological results help to explain phytoplankton c o m p o s~t~o n dynamics in the ...