EFFECTS OF NITROGEN AND IRON ENRICHMENTS ON PHYTOPLANKTON COMMUNITIESIN THE NORTHWESTERN INDIAN-OCEAN

Nutrient-enrichment bottle experiments in the northwestern Indian Ocea n surface waters were conducted to investigate phytoplankton growth fo llowing enrichments with either NH4+, NO3-, Fe or Fe + NO3-. Stimulati on of phytoplankton growth could be achieved by the addition of either NH4+ or NO3- under the ambient Fe concentrations, but the most signif icant increases in Chl a, POC, and cell densities were observed in the Fe + NO3--amended culture. Iron addition caused more rapid responses of phytoplankton growth in the Fe + NO3- treatment than those in the N O3- and NH4+ treatment. However, the Fe-enrichment treatment revealed minimal growth of phytoplankton because of severe major nutrient defic iency and was similar to the control treatment. Increases in the cell density of diatoms and spherical phytoplankton cells (< 10 mu m) were significant in the NH4+-enriched samples, whereas NO3- enrichment alon e had little effect on the diatoms. Simultaneous addition of Fe and NO 3- stimulated maximal growth of phytoplankton, in particular in diatom s, coccolithophorids and Phaeocystis type colonies. However, the domin ance of coccolithophorids and Phaeocystis type colonies in the Fe + NO 3- treatment may be interpreted as resulting from Si-limitation. The h igh N/P ratio for phytoplankton nutrient uptake in the N-amended cultu re indicates the possibility of some P-limited growth. From these resu lts, we conclude that in the northwestern Indian Ocean, Fe and major n utrients are co-limiting phytoplankton production during the northeast monsoon. Iron appeared to affect the ability of phytoplankton to resp ond quickly to transient nutrient inputs.