Nitrate, phosphate, and iron limitation of the phytoplankton assemblage in the lagoon of Takapoto Atoll (Tuamotu Archipelago, French Polynesia)

Algal blooms sporadically occur in atoll lagoons of the Tuamotu Archipelago (French Polynesia). The present study was conducted in the lagoon of Takapoto Atoll to investigate the roles of N. P and Fe in controlling the biomass and production of algae. Addition of P alone had no significant eifect on phytoplankton. The NP wnichments resulted in the highest increases of < 3 and > 3 pm chlorophyll a (chl a), algal carbon production, algal abundance (especially the pennate diatom Proboscla alata), and protozoan carbon production. Addition of N alone also enhanced chl a and algal carbon production, but was less effective than when N was combined with P. Dinoflagellates, which were dominated by Gymnodinium spp., showed the greatest response to the +N treatment. In one experiment, Fe addition (with EDTA) significantly enhanced chl a in the > 3 pm size fraction, and the net rates of algal and protozoan carbon production. The +Fe treatment especially enhanced the abundance of the coccolithophore Acanthoica sp. These results indicate that N followed by P limited the biomass and production of small and large algae, and that Fe may have been at times in short supply and hmited the large algae. A sudden supply of these nutrients to the lagoon, by natural phenomena, may trigger a phytoplankton outburst and change the taxonomic composition of the algal community. Production and grazing of protozoa may have been indirectly regulated by the availability of N, P and Fe, since the quantity of their algal food was controlled by these nutrients.