AME 19:149-161 (1999)  -  doi:10.3354/ame019149

ABSTRACT: 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 effect on phytoplankton. The NP enrichments resulted in the highest increases of <3 and >3 μm chlorophyll a (chl a), algal carbon production, algal abundance (especially the pennate diatom Proboscia 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 μm 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 limited 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.

KEY WORDS: Atoll lagoon · Phytoplankton · Protozoa · Nutrient limitation · Bioassays