IN-VITRO IRON ENRICHMENT EXPERIMENTS IN THE NE SUB-ARCTIC PACIFIC

There has been considerable debate about both the mechanisms controlli ng primary production, and the interpretation of data from Fe-enrichme nt experiments conducted in high nitrate-low chlorophyll (HNLC) region s. 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 influe nce 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-enri ched carboys containing mesozooplankton. Assuming similar environmenta l conditions in May 1993 and 1994, then the combined findings indicate that under ambient conditions autotrophic cells <5 mu m compose >70 % of algal biomass, primarily utilized ammonium, showed no Fe-mediated enhancement of N-normalized nitrate or ammonium uptake, and were thus unlikely to be Fe-limited. Although the coupling between grazer mortal ity and algal growth of these cells (g/mu) was ca 0.71 +/- 0.25, they grew at close to maximal rates but exhibited negligible net growth rat es, suggesting strong grazer control. Dissolved iron (DFe) supply prom oted a rapid increase in the abundance of large, initially rare, diato ms (mainly Nitzschia spp.) over 6 d, elevated diatom growth rates to c lose to their theoretical maximum and increased rates of incorporation of N-normalized nitrate and Fe by these cells over time. Unlike the s mall cells, stocks of large autotrophs also increased in the controls; DFe measurements indicate that this was probably due to inadvertent F e contamination. The development of a phytoplankton population, of a c ell size probably too large to be grazed at a significant rate by micr ozooplankton, 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 equi valent to the maximum annual in situ pelagic abundance were present. T he findings agree with the theories of others, that Fe supply rather t han grazing provides the ultimate control over the phytoplankton commu nity in the NE subarctic Pacific.